THE STRUCTURAL CHEMISTRY OF EUXENITE, .FERGUSONITE AND RELATED OXIDES

Thesis submitted for the Degree of DOCTOR OF PHILOSOPHY to the University of London by VASANT VIRUPAX DESHPANDE, M.Sce,Ph.D.(Bom.),A.R.I.C.

(JULY 1961) ABSTRACT.

Euxenite (YNbTiO6) and fergusonite (YNbO4) have been synthesized from their component oxides. X-Ray investigation showed that they are identical in structure with the natural metamict minerals. The composition range of existence of euxenite has been investigated by the study of the ternary system Y203-Nb205-Ti02 and also by

substituting the cations TO+, ThYt-, U114- 9 0e4+ and NO+ and studying the resulting solid solutions. In the course of this study the new compounds NdNbTi06, NdTaTiO6, CeNbTiO6 and CeTaFe06 have been isolated, and their X-ray structure data are given. Cation substitution of Ta5+, Ce"- and in the fergusonite structure has also been investigated, and solid solution studies are reported. A new compound CeTa04 has been isolated, and its X-ray data are given. The systems Nb205-Ti029 Zr02-Nb205, Zr02-Ta205 and Zr02-Ti02 have been studied in a preliminary manner and the existence of the compounds, ZrTiO4, 6Zr029 Nb205 and 6Zr02,Ta205 has been confirmed. In the system Th02-Ti02 a new compound ThTi20e, with a complex structure has been prepared; its unit•-cell dimensions could not be determined.

A brief preliminary study of the systems Ce02-Ti02, Be0-Nb2059 Be0-Ta208 and U308-Ti02 is also reported. ACKNOWTEDGEMENTS.

I am very grateful to my supervisor Dr. A. J. E. Welch for his kindness, advice, and guidance throughout the course of this work. I thank Dr. D. F. Evans and Dr. L. Pratt, for their helpful discussions. I also thank my colleagues in Room 78, Dr. B. D. Joyce, Dr. B. E. Baughan, Messrs. P. E. D. Morgan, M. J. Gregory, A. C. Skapski, G. C. Nicholson, R. A. Brown and Miss T. Nyein for their cooperation. I gratefully acknowledge the award of a Central Overseas Scholarship by the Govt. of Maharashstra and Govt. of India, which made this study possible. Finally I thank Mrs. Y. Dolejsi for her help in the laboratory.

Inorganic Chemistry Research Laboratories, Imperial College of Science and Technology, London, S.W.7. CONTENTS. PAGE

CHAPTER I. INTRODUCTION. 1 1. The Metamict Minerals. 2 2. Euxenite. 3 3. Fergusonite and Formanite. 4. The System Ti02-Nb205. 13 5. The System Zr02-Ti02, 14 6. The Systems Zr02-Nb205 and Zr02-Ta205. 15 7. The System Th02-Ti02. 16 8. The System Ce02-Ti02 16 9. The Systems Be0-Nb205 and 17 Be0-Ta205. 10. The System U308-Ti02. 17

CHAPTER II. EXPERIMENTAL TECHNIQUES. 18 1. Materials. 19 2. Preparation of Oxide Mixtures. 20 3. Furnace Equipment. 21 4. Method of X-Ray Powder Photography. 22

CHAPTER III. EXPERIMENTAL RESULTS. 23 PART I. 1. Synthetic Euxenite. 24 2. The Ternary System Y203-Nb205-Ti02. 24 3. The System YNbTi06-YTaTi06. 28 4. The System YNbTi06-ThTip6. 30 5. The System YNbT106-ThIleNb06. 33 6. The System YNbTi06-CeNbTi06. 36 7. The System YNbTi06-CenFe06. 38 Page 8. The System YNbTi06-NdNbTi06. 41 9. The System YNbTi06-UTi206. 43 10. The System YTaTi06-NdTaTi06. 46 11. The System CeNbFe06-CeTaFe06. 48 PART II. THE FERGUSONITE SERIES. 12. Synthetic Fergusonite. 50 13. The System YNb04-YTa04. 52 14. The System YNb04-CeNb04. 54 15. The System YNb04-ThTiO4. 56 16. The System YNI004-UTiO4. 58 17. The System YTa04-CeTa04 . 61 PART III. 18. The System Nb205-Ti02. 63 19. The System Zr02-Ti02. 65 20. The Systems Zr02-Nb205 sand 67 Zr02-Ta205. 21. The System Th02-Ti02. 70 22. The System Ce02-T102. 72 23. The Systems Be0-Nb205 and 74 Be0-Ta206. 24. The System U308-Ti02 75

CHAPTER IV. DISCUSSION. 78

APPENDIX (Containing X-ray Powder Data). 92

REFERENCES 119 CHAPTER 1.

INTRODUCTION. 2.

1. THE AETAMICT MINERALS.

Radioactive minerals are of general interest because they frequently contain uranium and related elements, but they hold a special interest mineralogically because they include most of the minerals classed as metamict". The possession of this characteristic is probably the main reason why certain of these minerals have not been effectively studied previously by X-ray diffraction methods. Probably these minerals were originally crystalline, but they have been transformed into a glass- like, isotropic mass with a conchoidal fracture, although the original external forms of the crystals have often remained unchanged. The term "metamict' was introduced by BrAger (1893). Characteristic metamict minerals are orthite, gadolinite, samarskite, euxenite, polycrase, polymignite, aeschynite, fergusonite, and yttrotantalite. Although various theories of the metamict state in minerals have been put forward from time to time

(e.g. by Goldschmidt and Thomassen (1924), Mugge (1922), and Vegard (1927)), it is now generally accepted that the disordering of the crystal lattice arises from atom 3.

displacements due to radioactive disintegration of constituents of the mineral. All minerals that are commonly metamict have fairly complex compositions, involving a large amount of isomorphous substitution. They usually contain, among other elements, metals of the lanthanide group, uranium, or thorium. A characteristic of these minerals is their isotropy, as well as the lack of short X-ray diffraction lines.

2. EUXENITE.

On account of its structural interest, this rare mineral was selected for synthesis and further study. The name of the mineral is derived from a Greek word meaning "friendly to strangers, hospitable", in allusion to the rare elements that it contains. It is found in granite pegmatites, sometimes in large amounts, associated with biotite, muscovite, ilmenite, monazite, xenotime, zircon, beryl, magnetite, garnet, gadolinite, blomstrandine and less frequently, thorite, uraninite, betafite, and columbite. Discolorations and radial fractures are frequently noted in the matrix of the crystals or masses of euxenite and other metamict or highly radioactive minerals. '4.

Known occurences of euxenite are predominantly in Norway, Sweden, Madagascar, Congo Republic, Brazil, and Canada. Euxenite is ordinarily metamict (Goldschmidt and Thomassen, 1924), in common with most rare-earth oxides of niobium, tantalum, and titanium and it becomes crystalline only on ignition. Arnott (1950), working on mineral euxenite, ignited it at 800°C and found that the material was cryptocrystalline. He was, therefore, unable to obtain single crystals of the mineral. X-Ray powder photographs taken of the mineral after heating at 400° to 800°C showed considerable variations, chiefly in the presence or absence of particular lines. However, samples heated at 1000°C and above gave similar diffraction patterns. Faessler (1942) found that the crystalline structure of the mineral gadolinite is regained rapidly by heating at 800°C, and regained more slowly by heating at low temperatures. In the case of euxenite the uniformity of structure after heating at 1000°C suggests that the temperature of rapid recovery of the crystal structure lies between 800° and 1000°C, the variations at low temperature may then be the result of incomplete recovery of the crystalline structure when the sample is heated. Euxenite is an oxide or titanate-columbate of the type AB206 (Machatschki, 1929) with A = Y, Ce, Ca, U, Th. Ti, Nb, Ta, Fes+. The predominant constituents are yttrium, calcium, titanium, niobium and tantalum. The high titanium end member is called polycrase and the high niboium-tantalum member euxenite. The normal member of the series contains more niobium than tantalum although tantalum varieties are not uncommon. The ratios Ti o Nb + Ta lie between 2 : 3 and 3 : 1. Some reported analysis include up to 9% of A1203 and as much as 21% of Si02, but these constituents do not occur in appreciable amounts in homogeneous samples. The morphological similarity of euxenite and polycrase was first recognized by Scheerer (1847) and the chemical relationship by BrAger (1906). According to the morphological evidence, these minerals crystallige in the dipyramidal class of the orthorhombic system and have an axial ratio of a o b o c = 0.3789 : 1 : 0.3527 (Palache, Berman and Frondel, 1944). They show two 6.

kinds of habit, prismatic and massive, showing no crystal faces. They are black with a brilliant sub- metallic or somewhat vitrous lustre, giving a brown streak. Their hardness is 5.5 - 6.5 and density 5.10 - 5.40.

X-Ray Structure Analysis of Euxenite. Arnott (1950) carried out an X-ray analysis of mineral euxenite after igniting it at 1000°C. He indexed the powder pattern on the basis of orthorhombic symmetry, with the unit cell dimensions a = 5.53 A, b = 14.60 A, and c = 5.17 A. Euxenites obtained from different sources gave slightly different lattice dimensions. Using the above values for the unit cell dimensions, the spacings of all possible planes were calculated down to a spacing of 2.00 A. By comparing these possible reflections with those actually observed in the powder pattern, Arnott derived the probable space-group of euxenite as either Pccn or Pcmn. Both these space-groups satisfy the observed reflections on the X-ray powder photograph. Alexandrov and Pytenko (1959) calcined samples of euxenite obtained from different sources at 800°C to crystallize them, and obtained X-ray powder 7.

photographs of the products. Some of the samples calcined at 800°C gave unsatisfactory powder photographs and had to be heated at 1100°C to develop good crystallinity. The X-radiograms of these samples were different from those of samples crystallized at 800°C in containing extra lines which could not be indexed from the orthorhombic unit cell of euxenite. These, however, fit a cubic unit cell of fluorite type with a = 5.07 - 5.-13 A. The content of this cubic component was particularly high in samples which were originally highly hydrated and decomposed. The cubic component proved to be a pyrochlore with a disordered cation distribution. The

'a' value measured for the pseudo-cell was half that of the pyrochlore. The cell dimensions for euxenite were given as a = 5.56, b = 14.75, c -= 5.17 A. Hardly any work has been done on the structural chemistry of euxenite. Structural relationships between euxenite-polycrase on one hand and fergusonite- formanite on the other are of interest, and one ultimate aim of the present work was to clarify these relation- ships. Attempts were, therefore, made to prepare a synthetic euxenite which could then be used for further studies in this field. 8.

3. FERGU8ONITE AND FORL1ANITE.

The mineral fergusonite, named after R. Ferguson, occurs in nature only in the metamict state. It is found in granite pegmatites, particularly those rich in rare earths, niobium, tantalum, and beryllium. It is associated with zircon, biotite, magnetite, monazite, gadolinite, orthite, euxenite, and other rare-earth minerals. It is obtained chiefly from Norway, Ceylon, Africa, Japan and U.S.A. The mineral readily alters by hydration without loss of external form. The change is accompanied by a decrease in density, by a change in colour, and by loss in crystallinity. Fergusonite is essentially an.. oxide of yttrium, erbium, niobium and tantalum, with the type formula AB04, where, A = Y, Er, Ce, La, Di, WI+, Zr, Th, Ca, Fe2+ B = Nb, Ta, Ti, Sn, W. A complete solid solution series exists between fergusonite in which B is predominantly niobium and the almost pure tantalum member, formanite. The lustre of fergusonite is externally dull and submetallic and the colour varies from grey or yellow to brown. The metamict material crystallizes 9.

on ignition (Goldschmidt and Thomassen, 1924). The predominant oxides in fergusonite are those of yttrium, niobium, and tantalum. The ideal formula is Y(Nb, Ta)04. It is reported that ionic substitution can take place; lanthanides, uranium, zirconium, thorium or ferrous iron can partially replace yttrium, and titanium, tin and tungsten can partially replace niobium and tantalum. Barth (1926) was the first to investigate the structure of fergusonite. He fused fergusonite in an oxy-acetylene torch and obtained a powder X- radiogram of the product. The diffraction lines were sharp. He observed that fergusonite crystallizes at 400°C, but the crystal particles are too small to give good X-ray photographs, unless crystallization is completed at 800° - 1000°C. Barth synthesized artificial fergusonite according to a method described by Goldschmidt (1925). A ground mixture of yttrium oxide and niobium pentoxide, in equimolecular proportion, was pressed into a pellet and then fired in an oxy-acetylene torch. The X-ray powder diagram of the product, YNb04, showed that the structure of this compound was practically identical with that of natural fergusonite. YTa04 was synthesized 10.

in the same way. This was also identical in structure with natural fergusonite. Barth found that the powder X-radiograms of YNb0,i , YTaO4, and natural fergusonite could be indexed on the basis of a tetragonal unit cell, with the parameters listed in Table I.

TABTR I.

a c c/a 1. Fergusonite (Natural) 7.74A 11.31A 1.46 2. YNbO4 (Synthetic) 7.76A 11.32A 1.46 3. YTaO4 7.75 A 11.41A 1.47

Although all the reflections recorded by Barth (1926) can be indexed satisfactorily on the tetragonal unit cell, the highest reflection recorded corresponds to a Bragg angle of 37° and only fourteen reflections are given. Even if this cell is correct, the lattice dimensions are not likely to be very accurate, because no high angle lines were obtained. The molecular weight of the "yttrium oxide" used for preparing the sample was 288; the actual molecular weight of yttrium oxide is 225.8. The oxide must, therefore, have contained heavier rare-earth impurities. 11.

Berman (1955), working on fergusonite obtained from different sources, found that the samples begin to crystallize when heated at 400°C; if the material is heated between 400° and 800°C for some time, a crystalline structure is obtained which can be indexed on the basis of a tetragonal unit cell (a = 5.18 A, c = 5.48 A). If the sample is heated at a still higher temperature the fergusonite crystallizes with a different structure, identical with that formed when equimolecular proportions of yttrium oxide and niobium pentoxide are sintered or fused together. Berman, therefore, believes that there are two crystalline forms of fergusonite. It is of interest to note, however, that if the a° value of the ignited fergusonite (a = 7.76 A) is divided by the square root of two, the result is nearly identical with the c° value of the smaller unit cell (a = 5.48 A). This suggests that some simple structural relation may exist between the two polymorphs, Komkov (1957) also found fergusonite to be polymorphic. He observed that the commonly known form is isostructural with Scheelite, V+ replacing Ca2+ in eight—fold coordination groups, and N1061- replacing W6+ in four—fold coordination groups. ThelBeudo— 12.

tetragonal unit cell has the dimensions a = 3.15 kx, co = 10.89 kx, a/c = 2.11 and density = 5.82. When the pseudo-tetragonal fergusonite is heated above 90000 it changes into monoclinic YNb041 unknown as a mineral species but identical with the synthetic YNb04 prepared by Barth. It has the cell dimensions a = 5.05 kx; b = 10.89 kx; c = 5.27 kx; p = 85°30'; a a b o c = 0.464 : 1 : 0.484. On this basis the probable space-group is 12, with four molecules in the unit cell. The polymorphic change tetragonal to monoclinic occurs over an ill-defined temperature range between 75000 and 900°C. Wilson (1958), working in this laboratory, tried to prepare single crystals of the synthetic fergusonite, in order to establish its crystal structure. He obtained twinned crystals, but no satisfactory single crystals. On the basis of an X- ray examination of a twin, Wilson was able to index the powder photograph of YNb04 on an orthorhombic unit cell with a = 8.226"; b = 10.548; c = 10.948 A. This literature survey shows that insufficient work has/been done in the structural chemistry of fergusonite, and that further studies are necessary. 13.

4. THE SYSTEM Ti02-Nb205.

A search through the literature revealed no detailed study of this system. Durbin and Harman (1952) reported some fusion data; all the samples prepared by them betweem 1 : 4 and 4 : 1 compositions began melting between 1400° and 1500°C. Only one compound TiO2Nb206, was reported; later Durbin, Wagner and Harman (1952), by X-ray examination, confirmed the identity of Ti021\11205. A suspected

compound, 2TiO2Nb205, was proved to be a mixture containing a rutile phase. Roth and Coughanour (1955), working on this system, prepared mixtures of the compositions

T102:9Nb2059 Ti02:4N102059 Ti02:3Nb205, T102 g2Nb205, 2Ti0:3Nb205 and Ti02:Kb206. These were heated at 1350°-1400°C for three hours and quenched in air. X-Ray analysis of these samples suggested that only TiO2Tb206 and TiO23Nb206 occur as true compounds. Since doubt still exists regarding the compounds in this system, an investigation of it was undertaken. 14.

5. THE SYSTEM Zr02-Ti02.

A liquidus curve for this system was published by von Wartenberg and Gurr (1931). It showed a eutectic point at about 80 mol. % TiO2 and 1760°C. Blissem, Schusterius and Ungewiss (1937) in an X-ray study of the system noted solid solution formation but found no compounds. Sowman and Andrews (1951) published a phase diagram for the system Zr02-Ti02; it again showed partial solid solution, but no compounds. The eutectic was located at 45-50 wt. % TiO2 and 1600°C. Later Sowman in a personal communication reported that at the 1:1 composition, after heat treatment, either a distorted form of zirconium oxide or a new compound is obtained. Brown and Duwez (1952) proposed a diagram indicating the existence of the compound ZrO2Ti02. Roth, Coughanour and DeProsse (1954) found that the compound ZrO2Ti02 melts incongruently at about 1820°C. The eutectic point is located at about 80 mol. % TiO2 and 1760°C. Extensive solid solution was found at higher temperatures. The compound Zr027i02, when cooled slowly from 1600°C to room 15.

cell temperature, has the followinG/dimensions: a = 4.806; b = 5.032; c = 5.447 A. If the compound is quenched from 1600°C the c-axis is found to have a larger value, whereas the a and b axes remain almost unchanged. The parameters of the quenched sample are: a = 4.802; b = 5.034; c = 5.483 A. Some doubt evidently still exists as to the formation of the compound ZrO2Ti02, so it was decided to attempt to prepare this compound at a temperature lower than those mentioned by previous authors.

6, THE SYSTEMS Zr02-Nb205 and Zr02-Ta205.

Durbin and Harman (1952) reported some fusion data for the system Zr02-Nb205. Compositions higher in niobium oxide are reported as melting between 1400° and 1450°C. Durbin, Wagner and Harman (1952) observed solid solution of zirconium oxide in niobium oxide. X-Ray diffraction studies also showed that cubic zirconium oxide was stabilized, indicating some solid solution of niobium oxide in zirconium oxide. They report no compounds. Roth and Coughanour (1955) also investigated this system and reported one compound, 6Zr02,Nb205. Data from powder photographs indicated it to have an orthorhombic unit cell with a = 4.964; b = 5.120; c = 5.289 A. The X-ray diffraction pattern of this compound was very similar to that of the cubic zirconium oxide solid solution observed by Durbin et al (1952). However, the solid solution formation was not thoroughly studied. It was, therefore, decided to prepare these compounds and investigate the solid solutions. As the Nb5+ and TO+ ions have similar ionic radii, the tantalum compound was also prepared and studied.

7. THE SYSTEM Th02-Ti02.

Thorium oxide was found by Goldschmidt and Thomassen (1923) to have a fluorite structure with lattice dimension a = 5.61 A. Although structure studies have been carried out on a number of binary oxide systems in which thorium oxide is one component, the system Th02-Ti02 has not been studied. This system was, therefore, investigated.

8. THE SYSTEM Ce0 2-Ti0 •

A number of workers have investigated systems with cerium oxide and a trivalent or tetravalent oxide, but no work has been done on the system 0e02-Ti02 17.

or on any other system of cerium oxide with the oxide of a tetravalent metal of small ion size. Haayman (1948) reported the dielectric constant of "CeTiO4" as 40, but no details of this supposed oxide are given. The system Ce02-Ti02 has, therefore, been investigated briefly.

9. THE SYSTEMS Be0-Nb208 and Be0-Ta208.

No literature data exist on these systems, on which some preliminary work was therefore carried out.

10. THE SYSTEM U308-Ti02.

In a study of a number of uranium oxide phase equilibrium by Lambertson and Mueller (1955), ,it is stated that U308 and titanium dioxide do not react at a temperature up to 1000°C. A brief study of the system was made in order to verify this report. L8

CHAPTER II.

EXPERIMENTAL TECHNIQUES. 19.

1. MATERIALS.

The following compounds were used in the experimental work: 1. Yttrium oxide, Thorium 'oxide - Thorium Ltd., London. 2. Niobium pentoxide, Tantalum pentoxide, and Zirconium oxide (high purity grades) - Murex Ltd. 3. Titanium dioxide (high purity) - Laporte Titanium Ltd. 4. Ferric oxide (pure) - Ltd. 5. Ceric oxide - Johnson-Matthey and Co. Ltd.

6. Beryllium oxide - B.D.H. 7. Uranyl nitrate - AnalaR. Niobium pentoxide, tantalum pentoxide, ferric oxide, titanium dioxide and zirconium oxide were ignited in platinum crucibles over a blast "Meker" burner before use and kept in a desiccator over silica gel. The other materials were analyzed by ignition of suitable weighed quantities in platinum crucibles for their moisture content, for which corrections were applied where necessary. The metal oxide contents of the nitrate solutions used in some cases were determined by evaporation of a known to quantity, ignition of the nitrate/oxide, and weighing 20.

of the oxide, all in platinum vessels.

2. PREPARATION OF OXIDE MIXTURES.

For convenience and on account of the high cost of the rare—earth oxides, only about 0.5 g. of each mixture was prepared. • Each of the constituent oxides was weighed accurately in a weighing bottle and then transferred carefully, by means of a camel— hair brush, into an agate mortar. The mixture was ground for about twenty minutes to ensure effective mixing; small particle size and thorough mixing facilitate solid—state reactions. The mixture was pressed into a pellet in a mounting press, in a die of 3/16" diameter at a pressure of 6000 p.s.i. Pressed pellets were preferable to loose powders, for they were easier to handle, occupied less volume, and allowed the reacting particles larger interfacial area for reaction. Some of the more refractory ceramic oxides like thorium oxide and zirconium oxide, which were heated at high temperatures in their preparation, were often found to react incompletely. In order to facilitate complete reaction, solutions of nitrates prepared from these oxides were taken and analyzed by weight, and the required quantities of 21.

solution were transferred to a platinum dish. The nitrate solutions were evaporated under an infra- red lamp and the residue was converted into oxides by heating in a platinum crucible to about 800°C. The oxide mixture was then ground and used for the preparation of mixtures, as described above. It was found that this method of preparation produced more crystalline fired products than the previous one and better X-ray films resulted.

3. FURNACE EQUIPMENT.

The various mixtures of oxides were fired either in a muffle furnace heated electrically by means of "CRUSILITE" silicon carbide rods (for temperatures up to 1400°C), or (for higher temperatures) in an electric tube furnace heated by hydrogen- protected molybdenum-wire elements. All firings were carried out in air, and firing periods were usually about sixteen hours. After firing, the products were cooled during several hours to about 600°C and then removed from the furnace. Firing temperatures were generally measured by means of platinum/platinum-rhodium thermojunctions connected to suitably matched temperature indicators; in a few cases temperatures in the higher ranges were 22, measured by means of a disappearing-filament optical pyrometer.

4. METHOD OF X-RAY POWDER PHOTOGRAPHY.

X-Ray powder photographs were taken and interpreted by standard methods. Copper radiation (filtered through nickel foil) was generally used, with 9- and 19-cm. Unicam cameras, for conventional X-radiograms; iron-filtered cobalt radiation proved more suitable in a few cases, particularly for samples containing iron. The need to record low-angle lines accurately, and to detect the smallest possible quantities of any phases additional to the main constituent led to extensive use of a Guinier focussing camera. This employed copper radiation monochromatized by means of a bent quartz plate (see d'Eye, 1960). The Guinier photographs showed more sharply resolved lines and a much reduced general background intensity, and they were used as much as possible. New phases were charabterized by their

d spacings", calculated by standard methods. Whenever possible, unit cell dimensions were determined by inspection and indexing of sin2G values found from the photographs. 23.

CHAPTER III.

EXPERILENTAL RESULTS. 24,

PART I.

1. SYNTHETIC EUXENITE. The difficulties of studying metamict euxenite have already been discussed in the Introduction, where the absence of structural data on the mineral has also been noted. Synthesis of a pure oxide phase corresponding with euxenite, YNbTiOs, was therefore attempted. The necessary quantities of yttrium oxide, niobium pentoxide, and titanium dioxide, were accurately weighed, ground together, pressed into a pellet, and fired at 1400°C in air for about 16 hours. The product was examined by X-ray diffraction. The powder photograph of this synthetic material was found to be identical with that of mineral euxenite (Arnott, 1950). The X-ray powder data are given in full in the Appendix. The X-radiogram of the synthetic material has been indexed on the basis of an orthorhombic unit cell with the dimensions: a.= 5.550 ± 0.005; b = 14.625 - 0.005; c = 5.175 ± 0.005 A.

2. THE TERNARY SYSTEM Y203-Nb206-T102. Euxenite (type formula YNbT106) and fergusonite (type formula YNb04) belong to two distinct 25.

species of metamict minerals, found in nature in pegmatites along with other rare-earth minerals. Although euxenite has orthorhombic symmetry, fergusonite has tetragonal symmetry (Barth 1926). Dana (1944) has pointed out certain evidence of crystallographic relationships between the two structures. It is to be noted that both have a cation to oxygen anion ratio of 1 : 2 and. are in this respect reminiscent of columbite; they differ from columbite, and from rutile (to which the columbite structure is closely related) in containing the relatively large yttrium or lanthanide cation. A short study of the ternary system Y203- Nb205-Ti02 was undertaken as a preliminary to a better understanding of any structural interconnection between YNbO and YNbTiO® phases. Such a study was expected to disclose the approximate composition range of existence of each phase, and the extent of any solid solubility between them. Suitable composition in the ternary system (see Fig. 1) was therefore selected, and samples prepared and fired; the products were then identified by X-ray examination. The mixing and pressing were carried out by normal methods, and firing was for 16 hours at 1380°C. The results are Nb205

Y203 Ti02 Fig. 1. THE SYSTEM Y203-Nb205-TiO2. E = euxenite range. E 1 N = euxenite + Nb2TiO7. E 4 P = euxenite + pyrochlore. E 4 F = euxenite + fergusonite. 26,

summarised in Table II. The main conclusion from Table II is that the composition range over which euxenite exists is relatively restricted. A second phase (of composition fully consistent with the direction of change) appears as soon as the proportion of a main component is changed to a moderate extent. Compositions 5 and 6 can be reproduced in mixture of YNbTiO6 (euxenite) and Y2Ti207 (pyrochlore), and no deviations from these stoichiometric compositions need be assumed in the products. The same is true of compositions 7 and 8, which can be made up from YNbTiO6 and YNbO4 (fergusonite), but the appreciable changes of lattice constant with composition do indicate some mutually consistent deviations from the stoichiometric compositions of these phases; these may deserve further study. The situation is different with compositions 1, 2, 3,and 4, because these cannot be accounted for as mixture of stoichiometric YNbTiO6 and Nb2Ti07. The phase Nb2TiO7 is a new oxide, and other measurements on the Nb205.13TiO2 system (see Table XVIII) show that the Nb:Ti ratio in it does not deviate substantially from 2:1. The results in Table II (compositions 1-4) TABLE II Composition in the system Y203-Nb205-Ti02.

Composition IY202(g)Nb205(01 Ti02(g) Colour Phases present in the product

1.Y08NbTi05.7 0.1490 0.2193 0.1318 White Euxenite (a=5.542;b=14.546;c=5.175) + Nb2Ti07(a=11.492;b=10.796;c=9.956)

2.Y0,6NbTi05.4 i0.1207 10.2369 0.1424 VI Euxenite (a=5.541;b=14.5491c=5.172) + Nb2Ti07(a=11.492;1=10.794;c=9.958)

3.Y0.4NbT105.1 0.0875 0.2576 0.1549 11 Euxenite (a=5.54103=14.546;c=5.174) + Nb2Ti07(a=11.494;1=10.794;c=9.955)

4.Y02NbTiO4.8 0.0480 0.2823 0.1697 11 Euxenite (a=5.540;b=14.548;c=5.173) + Nb2Ti07(a=11.490;b=10.794;c=9.952)

5.YNb0.7Ti05 _ 25 j0.1976 0.1625 0.1390 11 Euxenite (a=5.54500=14.571;c=5.140) + Pyrochlore (a=10.070)

6.YNb0.42iO4.5 0.2295 0.1080 0.1624 1i Euxenite (a=5.51)1;b=14.570;c=5.158) + Pyrochlore (a=10.068)

7.YNbTi0 . 805 . 6 0.1822 0.214.6 0.1032 1? Euxenite (a=5.525;b=14.598;c=5.178) + Fergusonite (a=8.176;b=10.533; c=10.926)

8.YNbTi0 .605 .2 0.1922 0.2263 0.0816 11 Euxenite (a=5.557;b=14.609;c=5.198) + Fergusonite (a=8.175;b=10.532; c=10.924) 28.

therefore indicate that when yttrium is deficient, the euxenite phase can take up more titanium, a point of some interest. The lattice constant variations for the euxenite phase (Table II) are very small. There is no evidence, apart from this incorporation of excess of titanium, that euxenite gives structures with large proportions of defects.

3. THE SYSTEM YNbTi06-YTaTi06.

Tantalum is usually present in mineral • euxenite in solid solution, replacing niobium. This indicates that the expected ionic substitution of tantalum for niobium occurs in the euxenite lattice. In order to confirm that a complete range of solid solutions exist, the system YNbTi06-YTaTiO6 was investigated. Samples of selected compositions intermediate between the two end-members were prepared from the constituent oxides by the usual technique and fired at 1400°C for about 16 hours, in air. The results of the X-ray examination of the products are given in Table III. The results in Table III show that only one phase, euxenite, is observed in this series. This is expected, since the ionic) radii of niobium (0.69 A) and TABLE III The system YrnTi06-YTaTi06.

Composition ! Y203(g) ! Nb205(g)! Ta205(g )1 Ti02(g) I Colour Phases present

1. YN1D0.8Ta0 ,2Ti06 0.16)1)1 0.1548 0.0643 0.1163 Pale pink Euxenite (a=5.547;b=14.622;c=5.187)

2. YNb0. 6Ta0.4Ti06 0.1564 0.1104 0.1224 0.1106 77 Euxenite (a=5.5111I;b=14.570;c=5.181)

3. YNb04Ta0.6Ti06 0.1491 0.0702 0.1751 0.1055 )7 Euxenite (a.5.543;b=14.556;c=5.174)

4. YN100.2Tac.8Ti06 0.1425 0.0335 0.2231 0.1008 If Euxenite (a=5.542;b=14.546;c=5.169)

5. YTaTiO6 0.1365 _. 0.2670 0.0965 White Euxenite (a.5.541;b=14.533;c=5.163) 30.

tantalum (0.68 A) are similar. The solid-solution range appears to be complete. The relatively small progressive decrease in the lattice constants as tantalum is substituted for niobium (see Table III, last column) corresponds approximately with the difference between the ionic radii. It is noteworthy, however, that the fall in the b dimension is considerably greater than that in the a and c dimensions, an indication that there may be differences of bond distribution between the oxygen polyhedra surrounding niobium and tantalum, respectively. The X-ray powder data for the compound YTaTi069 which have not previously been recorded, are given in the Appendix.

4. THE SYSTEM YNbTi06-ThTiP6.

Thorium is frequently found in association with minerals such as euxenite, and it is of particular interest to discover how the thorium is incorporated in the structures of such minerals, presumably by some normal mechanism, of solid solution. The thorium ion, 11:1÷9 is similar in size to the yttrium ion, Y3+, and it could presumably replace it without undue distortion of the structure; some 31.

compensating valency change would be necessary, however, to maintain the charge balance. The replacement YNbTiO6 ThT1206 would point the necessary compensation, Th4+ + Ti4+ replacing the two ions y3+ + N1051-, which are comparable in size. ThTi206 appears to exist (see Section 21, below) as a distinct compound, although it does not apparently possess the euxenite structure. In view of these considerations, the series YNbTi06-ThTi206 was examined. Compositions between the end-members were prepared by the technique described earlier and fired at 138000 for about 12 hours, in .air. After firing the samples were removed, and cooled, and subjected to X-ray examination. The results are given in Table IV. The results show that at least some thorium can enter the euxenite lattice. At the composition

Y Th 06 0.85 0.15Nb0.85Ti1.15 the only phase observed was euxenite. The lattice constants of this phase are approximately less than those of euxenite itself, a particularly large decrease being noted in the b dimension. On further increasing the proportion of thorium, the compound ThTi206 appeared as a second phase, and at the composition Y0.4Th0.6Nb0.4Ti1.606 TABLE IV The system YNIDTi06-ThTi206.

Composition !Y203(g) :Th02(g) INb2050Ti I Colour; Phase present 02(g) , , 1.Yo.85T110.15Nb0.85Ti1.150J 0.1409 0.05829 0.1659 0.1350 white Euxenite (a=5.521;b=14.599;c=5.175)

2. Yc .7Thc 3Nb0.7Ti1 ,306 0.1113 0.1115 0.1310 0.1462 " Euxenite (a=5.519;b=14.598;o=5.175) + ThTi206

3. Y0,5Tho.5n05Ti1.506 0.07531 0.1761 0.0886 0.1599 " Euxenite (a=5.516;b=14.596;c=5.172) + ThTi206

4. Yc.4Th0.6no.4Ti1.604 0.05872 0.2061 0.0691 0.1662 " Euxenite (trace) + ThTi206

5. Yc,2Th0.8N130.2Ti1.806 0.0279 0.2613 0.0328 0.1778 " ThTi206 33 4

the ThT1206 lines were predominant and the euxenite lines were very faint. At the composition

Y0.2Th 0.8Nb 0.2 Ti1.8 Os the euxenite phase disappeared, showing some solid solubility of yttrium and niobium in ThTi206. It is evident that only limited solid solution takes place in this series. The compound ThTi206 was prepared from its constituent oxides, and the X-ray data are given in full in the Appendix.

5. THE SYSTEM YNbTi0e-ThFeNb06.

In this system the replacement of Y3+ + Ti' ions by Th4+ + Fe3+ in the euxenite lattice was investigated. Samples were prepared from component oxides and fired at 1380°0 for about 16 hours, in air. After cooling, an X-ray examination of the products was carried out. The results are given in Table V. It is seen from Table V that Th41- and Fe3+ cannot be introduced in the euxenite lattice to any appreciable extent. Even on incorporation of 10% of Th4+ and Fe3+ions (in the composition Y Th Ti 0.9 0.1 0.9Fe 0.1 Nb0 6 )' euxenite is completely transformed into fergusonite. This result is of considerable interest as it shows a relationship' between the euxenite and fergusonite series of minerals. TABLE V The system YNbTi06-ThFeNb06.

Composition Y203(g/Th02(g)Ti02(g1Fe203(g)Nb206(g)Colour: Phases present 1. Yo 9Tho 1Ti0 9Fe0 11Th06e0.1491 0.03870.1055 0.0117 0.1950 Straw Fergusonite (a,=-8.215;b=10.550;c=10.937) 2.Y0.8Th0.2Ti0.8Fe0.2Nb 06 0.1269 0.0742 0.0897 0.0224 0.1867 Pale- Fergusonite ( • . 1 brown (a=8.216;b=10.50;c=10.935) + FeNb04(trace) 3.Y 0. 7Th0. 3Ti0. 7Fe0 ,33tb06 0.1065 0.1068 0.0753 0.0323 0.1790 Fergusonite (a=8.214;b=10.550;c=10.934) + FeNb04(trace) Yo 6Tho 4Ti0 , 6Fel.4Nb06 0.0877 0.1368 0.0620 0.0414 0.1721 TI Fergusonite (a.8.215;b=10.5480.934) + FeNb04 !).1- Y0.4Th0.6Tio.4Fec .eNb06 0.0542 0.1903 0.0383 0.0576 0.1595 Dark- Fergusonite brown (a=8.213;b=10.548;c40.934) + FeNb04 + Th02 . Yo.r,Tho 7Tio.3Fe0.7N1306 0.0392 0.2142 0.0277 0.0648 0.1539 8 Fergusonite (a=8.210;b=10.547;c=0.933) + FeNb04 + Th02 7. Yc,. 2Thb. 6 T1-0. 2Fe0. 6Nb06 0.0252 0.2365 0.0178 0.0716 0.1467 Fergusonite(trace) + FeNb04 + Th02 3. Y,.1Th0. 9Ti0.1Fe0 9N1006 0.0122 0.2574 0.0086 0.0779 0.1438 rt . FeNb04 + Th02 ThFeNb06 0.2769 - 0.0838 0.1393 Grey- FeNb04 + Th02 brown 35.

No detectable solid solution formation occurs in the system. As the proportion of Th4+ and Fe3+ is increased to the composition Y0.8Th0.2Ti0.8Fe0.2Nb061 a few additional lines of another phase appear; this has been identified as ferric niobate (FeNb04). The lattice constants of the fergusonite phase remain constant. The same two phases (fergusonite and FeNb04) are observed up to the composition

Y0.4Th 0.6Ti 0.4Fe 0.6 Nb06 . A third phase, thorium oxide, then appears. These three phases are observed up to the composition Y0.1Th0.9T10.1Fe0.9Nb06, at which the fergusonite phase disappears. The interesting feature of the appearance of fergusonite in this system is that the ideal formula of fergusonite (YNb04) indicates a 1:1 ratio of large and small cations. From the results in Table V it is clear that the fergusonite phase must contain some relative excess of small cations. The significance of this will be considered in the Discussion. The FeNb04 phase identified in this system has been prepared in this laboratory by Wilson (1958) and indexed on the basis of an orthorhombic unit cell. The lattice dimensions are a = 9.30; b = 5.627; c = 5.005 A. 36.

6. THE SYSTEM YNbTi06-CeNbTi06.

Analysis of mineral euxenite always shows considerable amount of cerium in solid solution. The possible substitution of cerium for yttrium in euxenite therefore merits structural study. Such a study would also reveal the possible extent of solid solution. Several compositions were therefore prepared between the end members YNbTiO6 and CeNbTiO6 and fired at 1380°C for about 16 hours in air. The samples were cooled and submitted to X-ray examination. The results are given in Table VI. It is seen from Table VI that cerium (presumably as Ce3+) can be substituted for Y3+ in the euxenite lattice to a limited extent. The euxenite phase obtained at composition 1, which is apparently homogeneous, has a unit cell appreciably larger than that of YlibTi06 , on account of incorporation of cerium. This is expected, since the ionic radius of CO+ (1.04) is considerably larger than that of Y'+ (0.93). However, as the proportion of cerium is increased to the composition Y0.4Ce0.6NbTi06, the euxenite lattice can no longer accommodate more cerium, and the lattice breaks down. A new phase, which has TABLE VI The system YlibTi06-CeNbTi06.

Composition Y203(g) Ce02(g) i Nb205(g) 1 Ti02(g) 1 Colour Phases present r 1.Y 0. 8Ce0, 2NbTi06 0.13W1 0.0512 0.1977 0.1189 Brown Euxenite (a=5.564;b=14.652;e=5.198)

2.1 .0.6Ce0, 4NbTiO6 0.0978 0.0994 0.1919 0.1153 ?T Euxenite (a=5.595;b=14.680•c=5.210) + CeNbTiO6 (traoa) 3.Yo. 4Ce0.61TbTiO6 0.0633 0. )1)18 0.1864 0.1120 Dark- CeNbTiO6 brown (a=5.913;b=114.076;c=8.619)

4.Y0.2Ceo.81lbTi06 0.0307 0.1877 0.1812 0.1089 TT CeNbTiO6 (a=5.91303.14.074;c=8.619) 5.CoNbTiO 6 - 0.2283 0.1763 0.1065 li CeNbTiO6 (a=5.914;b=14.075;c=8.620) — 14. 71 — 5. 61 —

5. 21 14. 70 — 5. 60

14.69

5.20— 14. 68 — 5. 59

14.67 — 5.58 — A. in t 5.19- .14. 66 — taa s on. c e 14. 65 — 5.57 — ic t t La

5. 18— 14. 64 —

5. 56 —

14. 63

5. 17— 14. 62 — 5. 55• Ce Ce O. 0 Ce O. 2 0. 4 c b a Proportion of Cerium

Fig. 2. THE SYSTEM YNbTiO6--CeNbTiO6. 38.

been identified as CeNbTi06, then appears. On further substitution only this phase is observed until the whole of Y" has been replaced by Ceal-; it is noteworthy that in this phase very little change of unit cell size accompanies substitution of Y" for 0e3+. There are no reports in literature concerning this compound, which has an orthorhombic unit cell with a = 5.914; b = 14.075; c = 8.620,A. The full X-ray data for this compound are given in the Appendix. A plot of lattice constants against compo- sition for this system is given in Fig. 2. It is observed from this plot that Vegard's law is not obeyed. Since, some CeNbTiO6 was detected at the Ce0.4" composition (see Fig. 2), the three lattice constant points shown should really appear at somewhat low cerium content.

7. THE SYSTEM YNbTi06-CeNbFe06.

This system was studied to see whether Y3+ and Ti4+ could be replaced by Ce4+ and Fe". This study is analogous to that of the system YNbTi06- ThFeNb06. As Th4+ and Ce44- ions have similar ionic radii, similar results might be expected for these two systems always provided that cerium remains in the quadrivalent state. 39.

Compositions were prepared intermediate between the two end members and fired at 138000 for 16 hours, in air. The samples were cooled (also in air) and submitted to X-ray investigation. The results obtained are summarized in Table VII. The results prove to be similar to those obtained in the system YNbTi06-ThNbFe06. When Ce" and Fe31- are substituted for Y3-1- and Ti'+, the euxenite lattice breaks down at a quite low level of substitution, and a fergusonite phase is obtained. This phase is observed over most of the composition range, but at high CO+ + Fe+ content the second phase is ferric oxide, not ferric niobate (cf. YNbT106-ThNbFe06 ). Some rutile appears at inter-. mediate compositions. The results again show that a fergusonite phase is obtained with a substantial predominance of small over large cations. When the smaller cations are rejected as a second coexisting phase, this contains no niobium as d major constituent. For some structural reason, Ce4+ and Nb5+ appear to be more compatible with each other in a fergusonite structure than Th41- and Nb6-1-, an interesting feature of the stability relations of the system. The present system discloses a much wider variation in the lattice constants TABLE VII The system YNbTi06-CeNbFe06.

Composition Y203(0Ce02(0pb205(giTi02(g e203(01 Colour Phases present 1 1. Yo . 8Ceo 21\113Tio 8Feo. 2 6 1 0.1338'0.0509 0.1968 0.0946 0.0236q Brown Fergusonite (a=8.187;b=10.546;c=10.970 2• Yo . 8Ceo ..1_NbTio. 8Feo .408 0.0969 0.09851 0.1901 0.0685 0.0457 Greenish Fergusonite -brown (a=8.210;b=10.555;c=11.018)\I + Rutile (trace) 3.Y 0 ,40e0.6NbTie,45ec.60 0.0625 0.1429 0.1839 0.0442 0.0664 11 Fergusonite (a=8.265;b=10.672;c=11.095) + Rutile 1 4. Y0.2ce0.8nTio 2Fe0,806 0.0302 0.1845 0.1781 0.0211 0.0857 11 Fergusonite (a=8.262;b=10.670;c=11.095), + Fe203 5.CeNbFe0 6 0.2235 0.1725 - 0.1038 Greenish Fergusonite -black (a=8.260;b=10.670;c=11.092) Fe203 41.

of the fergusonite phase, the larger 00+ ion bringing about an increase in unit-cell size. It must not necessarily be assumed that cerium is present only as 00+ in these structures, although some change of composition or some intrusion of lattice defects would be involved in any reduction of cerium to the Ce'+ state. Experimental distinction between Ce4+ and 00+ would be extremely difficult in a system of the kind now being studied.

8. THE SYSTEM YNbTiO6-NdNbTi06. • In this system the substitution of Nd3-1- for Y3+ was investigated. It would be expected from ionic size considerations (Y31- = 0.93 A; Ne+ = 0.99 A) that a continuous series of solid solutions would exist over the whole composition range. Accordingly, samples were prepared having appropriately spaced compositions, and fired at 140000 for about 12 hours. After cooling, the samples were subjected to X-ray analysis. The results are given in Table VIII. Table VIII shows that only limited solid solution occurs in the system. A single euxenite phase, with increasing lattice dimensions, is observed up to the composition Yo.eNd0.4NbTi06. The euxenite TABLE VIII The system IlibTi06-NdNbTi06.

Composition I Y203(g) Nd203(g) ! N10205(g) Ti02(g)1 Colour! Phases present

1.Y o, 8Nd0,2NbTi06 0.1341 0.0499 0.1973 0.1186 Light Euxenite pink (a=5.564;b=14.643;c=5.198) 2.Y 0. 6Ndo .4NbTiO6 0.0973 0.0967 0.1910 0.1148 t, Euxenite (a=5.604;b=14.663;c=5.204) 3• Y0.4Ndo.enTi06 0.0629 0.1406 0.1851 0.1113 Pink NdNbTi06 (a---5.9130)=13.849; c=8.621) 4.Y 0. 2Nd0.8NbTiO6 0.0305 0.1819 0.1796 0.1079 Dark NdNbTiO6 pink (a.5.913;b=13.849;c=8.620)

5.NdNbT10 6 - 0.2207 0.1743 0.1048 tt• NdNbTiO6 (a=5.913;b=13.8Li9;c=8.621) 5.21 - - 14.66

5.60

5.20 - - 14.65 5.59

5.58

-14.64

5.57

5.18 - - 14.63 5.56

5.17 - -14.62 5.55 Nd O. 0 Nd O. 2 Nd 0.4 Proportion of Neodymium Fig. 3. THE SYSTEM YNbTiO6 --NdNbTi06. 43.

lattice can accommodate Nd3+, but only to a limited extent; on further substitution the lattice breaks down. The structure then changes into a new ortho- rhombic phase, called NdNbTiO8 in Table VIII. This appears to be homogeneous over the remainder of the composition range, but its lattice dimensions show no appreciable change. This compound has not so far been described; the structure has been indexed on the basis of an orthorhombic unit cell with a = 5.913; b = 13.849; c = 8.261 A. Full X-ray powder data for this compound are given in the Appendix. A plot of euxenite phase lattice-constants against composition is given for this system in Fig. 3. It is observed from this plot that Vegard's law is not obeyed.

9. THE SYSTEM YNbTi06-UTi206.

Uranium is often found associated with mineral euxenite. A substitution readily allowing quadrivalent uranium to be introduced into the euxenite lattice would be Y3+ + Nb5+-a U4+ + Ti'+ obtained in the series YNbTi08-UTi206. The ionic radii of Y3-1- and U4+ are similar (Y3-1- = 0.93 A; U4+ = 0.97 A), as are those of N135+ (0.69 A) and Ti."- (0.68 A), so it is logical to expect extensive solid solution formation in this system. Compositions were prepared between the two end members (YNbTIO6 and UTi206 ) and fired at 140000 for about 18 hours, in air. The samples were cooled and an X-ray study was made of the products. The results are summarized in Table IX. The results show that some uranium can enter the euxenite lattice by the proposed mechanism. A homogeneous solid solution is obtained up to the approximate composition U0.2 1.0.8n0.8Ti1.2°6* The lattice constants of the euxenite phase decrease slightly over this range. The decrease in lattice parameters is scarcely expected, because the U4+ cation (radius 0.97 A) is larger than the Y3+ cation (0.93 A), and the substitution of Ti'+ (0.68 A) for Nb6+ (0.69 A) should have a relatively much smaller effect. It is possible that at least some of the uranium is present as U64- cations, which are appreciably smaller; the consequent increase in the oxygen content of the solid would not have been detected under the conditions of the experiments.

At the composition UOo3Y0.7 0.7Ti1.306' a few additional lines corresponding to a new phase, TABLE IX The System YNbTi06-UTi206.

Composition i U308(g)IY203(g) 1Tb 205(g) Ti02(g) Colour . Phases present

1. U0.1Y0. 9Nb0,9Til ,106 10.0890 0.3213 0.3782 0.2779 Brown Euxenite (a=5.507;b=14.518;c=5.151)

2.U0,21.0.8E10,8T11.206 0.1620 0.2606 0.3068 0.2766 Dark Euxenite brown (a=5.500;b=14.491;c=5.145)

3. U0.3/-0,7no.7Ti1.306 0.2372 0.2226 0.2620 0.283 2 Brown- Euxenite black (a=5.495;b=14.471;c=5.128) + UNb208(trace)

Fig. 4. THE SYSTEM YNbTiO6--UTi2O6.

5. 18 5. 58

14.63 — 5.57

5. 17 — 5. 56

5. 55 5. 16 — 14. 59 —

5. 54 A.

in 5. 15 ts tan s 14.55 — 5. 53 on c e ic tt 5. 14 5.52 La

'14.51 - 5.51

5. 13 — 5. 50

5. 12 _ 14.47 — 5.49 I

c b a U0. 0 U0. 1 UO. Proportion of uranium L16.

UNb208, were identified. At this composition the limit of solid solution has evidently been passed. This compound UNb208 has been prepared in this laboratory by Joyce (1960), who also gives powder diffraction data for it; the structure is evidently complex, and the unit cell could not be determined. A plot of lattice constants against composition is shown in Fig. 4. Vegard's law is not obeyed.

10. THE SYSTEM YTaTiO6-NdTaTiO6.

In an earlier section (YNbTiOs-NdNbTiOs), it was found that only limited solid solution occurs between the yttrium and neodymium end-members, the neodymium compound being new. It therefore became of interest to study the corresponding system containing tantalum instead of niobium. Appropriate compositions were therefore prepared between YTaTiOs and NbTaTiO6 and fired at 138000 for about 16 hours, in air. The samples were cooled and examined by means of X-rays. The results are given in Table X. The results are closely comparable with those for the corresponding niobium system. A new phase, NdTaTiO6, corresponding in structure with NdNbTiOs, was obtained. Inspection of the results (Tables VI TABLR X The system YTaTi06-NdTaTi06.

I Composition . Y 28(g)0 ,a1203(e) Ta205(g) Ti02(g) Colour Phases present, 1 i 1 11. Y0. 811do_2TaTiO6 0.1063 0.0396 0.2600 0.0940 Light- Euxenite i pink (a=5.550;b=14.612;c=5.175) i2. Y0 eNd0.4TaTiO6 0.0777 0.0772 0.2534 0.0916 if Euxenite I (a=5i548;b=14.612;c=5.173) I + NdTaTi06(trace) 1 ;3. Yo.4(1d 0. 6TaTi06 0.0505 0.1130 0.2471 0.0893 Bluish NdTaTiO6 1 -pink (a=5.906;b=13.833;c=8.619) i4. Y02Ndo. 8TaTi06 0.0246 0.1469 0.2411 0.0872 ?, NdTaTiO6 1 (a=5.90801=13:834;c=8.617) '5. NdTaTiO6 - 0.1793 0.2354 0.08516 n NdTaTiO6 1 (a=5.909;b=13.834;c=8.618) 48.

and X) suggests that the ranges of solid solution near the end-member compositions are more restricted in the tantalum case. The X-ray pattern of the NdTaT1O6 has been indexed on the basis of an orthorhombic unit cell with a = 5.909; b = 13.834; c = 8.618A.. Fuller X-ray data for this compound are given in the Appendix.

11. THE SYSTEM! CeNbFe06-CeTaFe08.

In a preliminary study on CeTaFe06 it was found to have a pyrochlore structure. It was there- fore of interest to study the system CeTaFe06- CeNbFe06. Intermediate compositions in the above range were prepared, and fired at 138000 for about 16 hours, in air. The samples were cooled and the usual X-ray examination was carried out. The results are summarized in Table XI. The euxenite phase does not appear in the series. Fergusonite appears, along with ferric oxide, at the composition CeNb Ta Fe06. On increasing 0.8 0.2 the proportion of tantalum (e.g. at the composition 6 CeNb0.6 Ta0.4 Fe0 ) a few lines corresponding to a new phase appear, with fergusonite lines. This new phase becomes more prominent as substitution proceeds. TABLE XI The system CeNbFe06-CeTaFe06.

Composition Ce02(g) N10205(g) Ta205(g)! Fe203(g)i Colour; Phases present

1.CeNb 0.8Ta0.2Fe06 0.2737 0.1320 0.0548 0.0993 Black Fergusonite (a=8.243;b=10.691;c=11.281) + Fe203 2. CeNb0. 6Ta0. 4Fe06 0.2048 0.0948 0.1051 0.0951 Fergusonite (a=8.242;b.10.690;c=11.280) + CeTaFe06(trace) 3.CeNb 0.4a0. 6Fe06 0.1965 0.0607 0.1513 0.0913 Violet Fergusonite black (a--.8.240;b=10.688;c=11.280) + CeTaFe06(a7-.10.355) 4.CeNb 0. 2Ta0. 8Fe06 0.1889 0.0291 0.1940 0.0878 rt CeTaFe06 (a=10.356)

5. CeTaFe06 0.1819 0.2335 0.0845 Black CeTaFe06 (a=10.358) 50.

At the composition CeNb0.4Ta0.6Fe06 the two phases still occur, but at compositions containing more tantalum only CeTaFe06 is observed. CeTaFe06 is a new compound, not so far reported in literature. The X-ray powder data are given in the Appendix.

PART II. THE FERGUSONITE SERIES,

12. SYNTHETIC FERGUSONITE.

Barth (1926) prepared synthetic fergusonite (yttrium niobate, YNb04 ) by mixing equimolecular proportions of yttrium trioxide and niobium pentoxide and heating at 800°C. The product was submitted to X-ray powder analysis. This synthetic material was almost identical with mineral fergusonite. The X- radiogram was indexed on the basis of tetragonal symmetry with the unit cell dimensions a = 7.76 and c = 11.32 A. YTa04 was also prepared and shown to be isomorphous with YNbO4. In the present study YNbO4 was prepared from the component oxides by firing at 1400°C for about 20 hours, in air. A Guinier photograph was taken of the product. It was indexed on the basis of ortho- 51.

rhombic symmetry. The cell dimensions are given in Table XII and compared with those of other workers. TABU1 XII. Lattice Dimensions of Fergusonite, YNb04.

Barth (1926) Komkov (1957) Wilson (1958) Present work

Tetragonal Monoclinic Orthorhombic Orthorhombic a = 7.76 A a = 5.05 kx a = 8.226 A a = 8.179 A c = 11.32 A b = 10.89 kx b = 10.548 A b = 10.531 A c = 5.27 kx c = 10.948 A c = 10.927 A p = 85°.30'

Although previous workers have indexed synthetic fergusonite on different symmetries, it was found in the present work that the powder photograph could be completely indexed only on the basis of the orthorhombic unit cell. It is possible that the use of a focussing camera, giving better resolution of low-angle lines, has afforded some advantage over previous workers. The results of Wilson (1958) are in quite good agreement with those now obtained. Complete X-ray powder data are given in the Appendix. 52.

13. THE SYSTEM YNb04-YTa04.

From ionic size considerations, YTa04 is expected to be isomorphous with YNb04, and a complete series of substitutional solid solutions should exist between them. In order to verify this, suitable compo- sitions intermediate in the range were prepared and fired at 1400°0 for about 12 hours, in air. The samples were cooled and an X-ray examination was carried out. The results are given in Table XIII. The fergusonite phase is the sole product throughout the series; its lattice parameters change slightly with composition, on account of the slightly different ionic radii of Nb54- and TO+. The results confirm that YTa04 is isomorphous with YNb04 and that the expected complete series of solid solutions exist between them. Ferguson (1955) prepared the compound YTa04 by heating equimolecular proportions of Y203 and Ta205 in the cratered electrode of a carbon arc. The temperature of firing was about 2000°C. X-ray examination of the product indicated that the symmetry of YTa04 was not tetragonal, as assumed by Barth, but tetragonal with a monoclinic distortion; the reported TABLE XIII The system YNb04—YTa04.

Composition Y203(g) Nb205(g) Ta205(g) Colour Phases present

1. INb0 .8Ta0204 0.2143 0.2018 0.0838 Pink Fergusonite (a=8.178;b=10.526;c=10.922)

2. /1110, 6Ta0. 40.1 0.2008 0.1418 0.1572 it Ferausonite (a--78.176;b=10.521;c=10.913)

3. YNb0.4Ta0.604 0.1890 0.0890 0.2219 If Ferausonite (a78.178;b=10.523;0=10.915)

4. YNbo . 2Tao . 804 0.1785 0.0420 0.2794 Light Fergusonite pink (a=8.180;b.10.525;c=10.917) 5. YTa04 0.1691 — 0.3308 White Fergusonite (a=8.195;b=10.559;c=10.905) 54.

unit cell parameters were a = 5634; b = 10.94;

p = 950 3t . = 5.07 A; 0

14. THE SYSTEM YNb04-CeNb046

The introduction of cerium into the euxenite lattice has already been described. In view of the occurrence of cerium in fergusonite it was of interest to examine the direct substitution of cerium for yttrium, and the system YNb04-CeNb04 was examined with this in mind. Samples of appropriate intermediate compositions were prepared and fired at 1300°C for about 18 hours, in air. The results of X-ray examination of the products are given in Table XIV. The results show that only one phase of fergusonite structure is observed up to the composition, Y0.5Ce2.5Nb3012. The lattice parameters increase quite rapidly as cerium is introduced; this is expected because the ionic radius of CO+ (1.07 A) is considerably greater than that of Y3-1- (0.93 A). On further increasing the proportion of cerium, however, a few lines due to Nb206 also appear. This finding was not investigated further, but it suggests that the stoichiometric phase CeNb04 may not exist, possibly on account of oxidation of part of the cerium to the Ce4+ state. TABLE XIV The system YM04,-CeNb04.

Nb 0 Phases present Composition 1 Y203(g) Ce02(g) 2 5(g) Colour

1. Y2.5Cep .5Nb3012 0.1849 0.0563 0.2612 Yellow- Fergusonite • green (a=8.196;b=10.630; c=11.013) 2. Y2CeNb3012 0.1432 0.1091 0.2527 Yellow-Fergusonite green (a=8.218;b=10.691; c=11.081) 3. Y1,5Ce1.51113012 0.1040 0.1585 0.2448 Dark Fergusonite green (a=8.207;b=10.733; a-11.141) 4. Yi_25001.75N133012 0.0853 0.1820 0.2410 1? Fergusonite (a=8.246;b=10.747; c=11.173) 5. YCe013012 0.0672 0.2049 0.2372 Dark Fergusonite green (a=8.252;b=10.756; c=11.184) 6. 0 3012 0.0496 0.2274 0.2337 Fergusonite Yc.75 e2.28N10 (a=8.265;b=10.765; c=11.202) 7. Yo.5Ce2.5n3012 0.0326 0.2486 0.2303 11 FergusOnite (a=8.270;b=10.774; c=11.220) 8. Y0_25Ce2.751,103012 0.0161 0.2694 0.2269 Fergusonite (a=8.269;b=10.774; c=11.220) + Nb205(trace) 9. Ce3Nb3012 j 0.2897 0.2237 Fergusonite (a=8.269;b=10.770; c=11.220) + Vb205(trave) 8. 3

a

8.2 a

8. 1

10. 8

10.7

A.

in 10. 6 ts tan s 10. 5 con ice tt 11 3 Fig. 5. THE SYSTEM YNbO4--CeNbO4. La

11.2

11. 0

Proportion of cerium in (Ce, Y)Nb04 .

.10. 9 Ceo. 67 Ce 0. 17 Ce0. 34 Ce O. 50 Ce 0. 63 56.

A plot of lattice parameters against composition is shown in Fig. 5. It is seen from this that Vegard's law is not obeyed.

15. THE SYSTEM YNbOv-ThTiq. In the earlier studies on euxenite it was observed that thorium can, at least to a small extent, enter into solid solution in the euxenite structure. Thorium also occurs in fergusonite in association with rare earths, and a similar test of thorium substitution in fergusonite was therefore of interest. The substitution Y31- + Nb5+ Th4+ + Ti4+ again appeared to offer a likely mechanism for introduction of thorium. Samples at appropriate compositions in the System YNbOt-ThTiOt were therefore prepared from the component oxides and fired at 1400°C for about 16 hours, in air. The results of X-ray study of the products are given in Table XV. Clearly some thorium can be substituted (presumably for yttrium) in the fergusonite lattice; substitution occurs up to the approximate composition

Y2ThTiNb2012. The cell dimensions of the fergusonite phase increase with the thorium content; this is expected because Th4+ ion is larger than the Y3+ ion.

TABLE XV The system YNb04-1hTiO4.

Composition Y203 (g) !Th02(g) d Ti02(01Nb205(g)i Colour! Phases present

1.Y2,5Th0. 5Ti0. 511b2.5012 0.1794 0.0839 '0.0254 0.2112 White Fergusonite (a=8.214;b.10.559;c=10.972) 2.Y2ThTiNb2012 0.1351 i0.1580 0.0478 0.1590 VI Fergusonite (a=8.238;b=10.630;c=11.037) + ThTi206(trace) 3.Y1. 5Thl.5T1-1.5Nb1.5012 0.0957 i0.2240 0,0677 0.1127 VI Fergusonite (a=8.161;b=10 630;c=11.111) + ThTi206 4.YTh 2Ti2Nb012 0.0604 0,2828 0.0855 0.0711 YP Fergusonite (a=8.155;b=10.631;c=11.114) + ThTi206 5. 1.0.75Th2,25T12 25n0.75012 Fergusonite 0.3099 0.0937 0.0519 tP (a=8.154;b=10.633;c=11,115) + ThTi206 6.Yo 5Th2 5T-2.0,81 5N10 012 10.0287 0.3358 0.1015 0,0338 VW Fergusonite (trace) ThTi206 7.Th3Ti 3012 0.3838 0.1161 it ThTi206 (complex) Fig. 6. THE SYSTEM YNbO4--ThTiO4.

8.3 10.7- 11.05 —

11.00 —

10.6— 8.2

10.95 —

10.90 — 10.5— 8.1 Th 5' h b a h0.0 0.17 O. 33 Proportion of thorium in (Th, Y)(Ti, Nb)04 58.

The simultaneous substitution of Ti4+ (radius 0.68A) for the larger Nb5+ ion (radius 0.70A) evidently has a smaller effect on the unit cell size, and the net effect of substitution is an increase in cell dimensions. At the composition, Y2ThTiNb2012 a few lines corresponding to a new phase, ThTi206, appear. As the proportion of Th4+ and Ti4+ was further increased, fergusonite and ThTi206 phases were observed in the expected proportions, up to the composition

Y0.75Th2.25Ti2.25 0.75°12° At the composition T0.5Th2.5T12.5Nb0.5012 the ThTi206 phase was predominatt and only a trace of fergusonite was observed. The compound ThTi206 has already been referred to earlier; X-ray powder data for it are given in the Appendix. A plot of lattice constants against composition for the fergusonite phase is given in Fig. 6. Vegard's law is not obeyed.

16. THE SYSTEM YNbO4-UT1O4. Uranium is also found in solid solution in mineral fergusonite. The substitution Y54- + NY' .... U4+ + Ti4+, analogous to the thorium substitution just discussed, appeared to provide an acceptable 59. mechanism for introduction of uranium. The system YNb04-UTiO4 was therefore selected for study. Compositionsin this range were prepared and fired at 138000 for 12 hours, in air. The results of X-ray analysis of the products are set out in Table XVI. The results show that U4+ behaves similarly to Th4+; limited solid solution (up to a composition approaching U-u 61-2.4n2.4T1-0.6 0,2) is again observed. At this composition lines of a second phase, later identified as UNb20e, are observed. The unit-cell size of the fergusonite phase falls as uranium and titanium are introduced; this may reflect the fact that the U4+ ion (radius 0.97 A) is appreciably smaller than Th4 (radius 1.02 A), but the possibility must be borne in mind that some of the uranium may be in the U64- state. This point was not investigated further. The presence of U8+ would, of course, involve compensating composition changes of some kind (e.g. inclusion of interstitial oxygen). The new phawe observed in these studies, UNb208, has been prepared in this laboratory by Joyce (1960) and has a complex structure of which the unit- cell could not be determined. TABLE XVI The system YNb04-UTi0.1.

\I Composition !U308(g) Y203kgiif N1020s(g) Ti02(g)i0olour 1 Phases present 1 1 1.11 0 .27.2.8n2.8Tio.2012 0.0740 0.4169 0.4906 0.0210 Yellowish Fergusonite -green i (a=8.164;b=10 ... 5020!,10.891)

i 2.U-).4Y2.6N102.6T10.4012 0.1Wil 0.3767 0./11115 0.0410 Dark 1 Fergusonite green 1 (a=8.159;b=10.)93;c=10.882)

ft 3. 110-61-2.4n2.41110.60±2 0.2105 0.3387 0.3987 0.0599 Fergusonite (a=8:155ib.10.490;c=10.880) + UNb208 (trace)

0 10.53 — 8. 19

10. 93 —

10. 92 — 10. 52 — 8. 18

A.

10. 91 — in

ts 10. 51 — 8. 17 n ta ns o

c 10. 90 — ice tt La 10. 50 — 8. 16

10.89 —

I 10.88 — 10.49 — 8.15 1 U0.00 U 0.2 U0.4 c a Proportion of uranium

Fig. 7. THE SYSTEM YNb04 — UTiO4. 61.

A plot of lattice constants against compo- sition is shown in Fig.7. Vegard's law is evidently not obeyed in this system.

17. THE SYSTEM YTa04-CeTa04. ti YTa04 is isomorphous with YNb04. In the YTa04-CeTa04 system ionic replacement of Y3+ by CO+ was studied. Owing to the larger size of the Ce3+ ion, only partial solid solution would be expected in this system. As usual, samples were prepared with compositions intermediate between the end members and fired at 13800C for 18 hours, in air. The X-ray results obtained from the products are summarized in Table XVII. The results clearly show that CO+ can replace Y3+ in the fergusonite lattice only to a limited extent. On increasing the proportion of cerium, lines belonging to a new phase quickly appear. The broad effect of cerium substitution in this system resembles that in the system YNbTi06-CeNbTiOs, discussed earlier. The new phase has been identified as CeTa04. The powder photographs of this compound have been indexed on the basis of tetragonal symmetry TABLE XVII The system YTa04-CeTa04.

Composition Y203(g) 1 Ce02(g) Ta205(g)I Colour Phases present

1.Y 2.5Ce0, 5Ta301 0.1374 0.0418 0.3226 Pale- Fergusonite yellow (a=8.196;b=10.639;c=11.014) 2.Y 2CeTa3O12 0.1072 0.0817 0.3147 9? Fergusonite (a=8.198;b=10.638;c=11.014) + CeTa04 (trace) 3.Y 1. 5Cel 5Ta30,2 0.0785 0.1197 0.3073 Pale- Fergusonite green (a=8.194;b=10,635;c=11.015) + CeTa04 (a=11.056;o=10.286) 4.YOe 2Ta3012 0.0511 0.1559 0.3001 I? Fergusonite (a=8.193;b=10.635;c=11.012) + CeTa04 (a=11.056;c=10.288) 5.Y0. 50e2.5Ta3012 0.0249 0.1904 0.2933 Dark- Fergusonite (trace) green + CeTa04 (a=11.058;c=10.288) 6.Ce 3Ta3012 0.2235 0.2868 IT CeTa04 (a=11.060;c=10.290) 63.

with the cell dimensions a = 11.060; c = 10.296 A. Complete X-ray data are given in the Appendix.

TART III.

18. THE SYSTEM Nb205-Ti02. Various workers (see Introduction) have studied this system. Durbin and Harman (1952) found only one compound in it, whereas Roth and Coughanour (1955) reported two compounds, Nb205,TiO2 and 3Nb205,Ti02. As considerable uncertainty exists on the formation of these compounds, it was decided to re-examine the system, which has obvious connections with the newer systems studied in this Thesis. Selected compositions in the system were therefore prepared from the component oxides and fired at 1400°C for 20 hours, in air. The results of X-ray study of the products are summarized in Table XVIII. Only one compound, viz. Ti02,Nb205(Nb2Ti07) could be confirmed. This was found to have ortho- rhombic symmetry with the cell dimensions a = 11.493; b = 10.798; c = 9.954 A. Complete X-ray powder data are given in the Appendix. TABLE XVIII The system Nb205-T102.

Reaction I Nb205(g) T102(g) Colour Phases present

1. Nb205 + T102 0.3845 0.1155 White Nb205,T102 (a=11.49303=10.798;c=9.954)

2. 2R-10205 + T102 0.4348 0.0652 ft n205,1102 (a=111,494;3=10.796 ;C=9•954) + Nb205

3. N1205 + 2T102 0.3123 0.1877 il Nb205,T102 (a.11.49303=10.799;c=9.956) + Rutile

4. 3Nb205 + T102 0.45h)i 0.0455 II Nb205,T102 (a=11.494;3=10.796;c=9.953) + Nb2O5 65.

There was no evidence of any other compound in the system, the only phases other than NbTi07 being niobium pentoxide and rutile,

19. THE SYSTEM Zr02—Ti02. According to a recent survey very few binary systems of which Zr02 is a component have been studied. A brief survey of the literature has been attempted in the Introduction. The form of Zr02 stable at room temperature is monoclinic, and its crystal structure has been studied by several workers. There is, however, few agreements between the various sets of lattice parameters reported in the literature (Table XIX). The lack of agreement may be attributable to impurities, particularly llf02, which is isomorphous with Zr02.

TABLE XIX. 0 Lattice. Parameters of Zr02 (in A).

Monoclinic Tetragonal Ruff and Cohn and Duwex and Ruff and Ebert Tolk8dorf Odell Ebert (1929) (1930) (1950) (1929) a 5.174 5.21 5.16 5.074 b 5.266 5.26 5.25 c 5.308 5.37 5.29 5.160 P 80048' 80052' 80010' 66.

The crystal structure of the high-temperature fcrm of Zr02 is tetragonal with an axial ratio of 1.07. The extensive work of Geller and Yavorsky (1945) indicates that the transformation from monoclinic to tetragonal takes place over a wide range of temperature. The beginning of transformation is near 1000°C, as determined by dilatometric and thermal analysis methods. The high temperature tetragonal form can be described as having a deformed fluorite structure in which the c-axis is slightly longer than the other two axes. The unit cell of the monoclinic form has also been described as a distorted tetragonal cell obtained by rotating the c-axis until the angle between a and c axes is about 80°, and then making small changes in the lengths of the three axes. Na'ray-Szabo (1936) suggests that the structure is actually more complicated. The intensity measurements of Yardley (1926) show that the oxygen atoms deviate considerably from their positions in the structure of the tetragonal form. A third allotropic form of Zr02 has been reported by Cohn (1935), who claims to have produced it by prolonged heating above 1900°C. The structure is described as hexagonal with a = 3.60 A; c/a = 1.633. 67.

The earlier literature survey describes that doubt still exists as to the formation of the compound Zr02,Ti02. Attempts were therefore made to prepare this compound. Equimolecular proportions of Zr02 and TiO2 were mixed together and fired at 140000 for about 20 hours, in air. The X-ray photograph of the product showed two phases, monoclinic Zr02 solid solution and Zr029TiO2 (ZrTiO4). The same sample was fired again at 1660°C for about 10 hours, in air. The X-ray powder. photograph of the product showed only one phase, Zr02,Ti02, for which full X-ray data are given in the Appendix. ZrTiO4 was found, by indexing of the powder photograph to have orthorhombic symmetry with a = 4.800 ; b = 5.008; C = 5.451 A.

20. THE SYSTEMS Zr02-Nb205 and Zr02-Ta205. Roth and Coughanour (1955) investigated the system Zr02-Nb205 and reported only one compound, 6Zr02,Nb206, which is orthorhombic, with a = 4.964; b = 5.120; c = 5.289 A. Limited solid solution was found between 6Zr02,Nb205 and Nb205. On account of possible structure relation with the oxides•already examined, a brief study was made of the solid 68.

solutions and of the supposed compound 6Zr02,Nb205. Compositions corresponding with solid solutions between 6Zr02,Nb205 and Nb205 were prepared from the oxides and fired at 1400°C for 16 hours, in air. X-ray powder photographs of the products showed that reaction was not complete, so all the samples were refired at 1660°C for 10 hours, in air. The results of X-ray analysis of the products are given in Table XX. These results indicate that the homogeneity range of 6Zr029 Nb205 is much smaller than the work of Roth and Coughanour (1955) suggests. They place the solubility limit at 62 mol % Zr029 whereas (Table XX) the present work shows Nb205 to be present even at 80% Zr02. It is possible that the use of focussing-camera techniques has allowed smaller quantities of admixed Nb205 to be detected. The other results of Roth and Coughanour are fully confirmed, and no compound ether than 6Zr02,Nb205 has been found. The full X-ray data for the compound .) 6Zr02,'Nb205 are given in the Appendix. The corresponding tantalum compound, 6Zr02,Ta205 was also prepared by exactly similar methods, and was found on X-ray examination to have TABLR XX The system Zr02-Nb205.

Reaction Zr02(g) Nb205(g) ' Colour Phases present

1.Zr0 2(62 mol %) + Nb205 (38 mot %) 0.2153 0.2847 White 6Zr02,Nb 205 (a=4.910 ;b=5.103 ;c=5,286) + Nb205

PP 2.Zr0 2(68 mol % + N13205(32 mol %) 0.2481 0.2519 6Zr02,Nb 20 5 (a=4.912 0)=5.104 ;c=5,287) + Nb20, 3.Zr0 2(74 mol %) + Nb205(26 mol %) 0.28!4 0.2156 ft 6Zr02,Nb 20, (a=4,912 ;b=5.102 ;c=5.288) + Nb205 4.Zr0 2(80 mol %) + Nb205(20 mol %) 0.3248 0.1752 6Zr02,Nb 205 (a=4.915 ;10=5.105 ;c=5.288) + Nb205( trace)

6Zr02 + Nb205 0.3677 0.1322 6Zr02,11b 206 (a=4.935 ;b=5.095;c=5.258) 70.

the same structure as the niobium compound. X-Ray powder data for 6Zr02,Ta205 are also given in the Appendix. The lattice constants of 6Zr02,Ta205 are slightly lower than those of the niobium compound, which is expected. The measured lattice dimensions of both compounds are given in Table XXI, with those reported by Roth and Coughanour. TABLE XXI. (All lattice dimensions in A)

Authors ZrTiO4 6Zr02,Nb205 6Zr02,Ta205

Roth and a = 4.802 a = 4.964. a = 4.961

Coughanour b = 5.034 b = 5.120 b = 5.117

c = 5.483 c = 5.289 c = 5.278

Present a = 4.800 a = 4.935 a = 4.929 work b = 5.008 b = 5.095 b = 5.089 c = 5.451 c = 5.258 c = 5.252

21. THE SYSTEM Th02-Ti02. The need to study this system (on which no information appears in the literature) arose from the appearance of oxide phases containing thorium and titanium during the investigations reported earlier in this Thesis. Suitable compositions were prepared from TABLE XXII The system Th02-T102.

Composition j Th02(g) 1 Ti02(g) Colour Phases present

1. ThTi206 0.3115 0.1885 White ThTi206 (complex)

2. Th1. 5Ti1,506 I0.3840 0.1160 1? ThTi206 + Th02

3. Tho.sTi2.506 0.1990 0.3010 If ThTi206 + TiO2 72.

the oxides and fired at 1400°C for 15 hours, in air. The products were examined by taking X-ray powder photographs; the results are given in Table XXII. Only one compound was identified, viz. ThTi206. The Guinier photograph was, however, too , complex to index. The "d" values of this compound are listed in the Appendix. Thorium oxide and rutile, respectively, appeared when the Th02:Ti02 ratio was increased or decreased from 1:2.

22. THE SYSTEM Ce02-Ti02. Apart from a report of the dielectric constant of CeTi0,1 (Haayman, 1948) no work on this system appears in the literature. It became of interest in the present work when oxides containing cerium and titanium needed to be identified in some of the systems reported on earlier in this Thesis. Samples of appropriate compositions were prepared from the oxides and fired at 1380°0 for about 6 hours, in air. X-hay examination of some of the products showed that reaction was not complete. The samples were, therefore, crushed and refired at 1400°C for 10 hours, in air, after which complete reaction appeared to have occurred. The results of X-ray powder study of the products are given in Table XXIII. TABLE XXIII The system Ce02-Ti02.

Composition Ce02(g) Ti02(g) 1 Colour Phases present

1. Ce1.8Tio. 204 0.4754 0.0245 Pale Fluorite (a.--,5.407) pink

2. Cel. 8Tio .404 0.4480 0.0519 fl Fluorite (a=5.389) + CeTiO4 (trace) .

3. ee1...1210.804 0.4170 0.0829 Ochre Fluorite (a=5.372) + . CeTiO4 (trace)

4. Cel. 2Tio,804 0.3818 0.1181 II Fluorite (a=5.371) + CeTiO4 (trace)

5. CeTiO4 0.3414 0.1585 Grey Fluorite (a=5.370) + CeTiO4 brown (a=10.701;b=11.498;c=9.490) 6.Ce 0. 8Ti1.404 0.2400 0.2599 II Fluorite (a=5.368) + CeTiO‹, (a=10.700;b=11.496;c=9.488)

7. Ceo ,2Tii. 804 0.0965 0.4034 VI Rutile + CeTiO4 (a=10.700;b=11.494;c=9.485) 74.

The variations of lattice constant recorded in Table XXIII tend to show (contrary to previous finding) that equilibrium had still not been attained in the reaction products. Nevertheless, in view of the complex nature of the system and the valency- change possibilities interest in it, no further study was attempted. It is evident that a solid solution of fluorite structure containing an appreciable proportion of titanium is formed in this system. There is also evidence of a compound of orthorhombic structure, provisionally designated CeTi0,1, but this composition is by no means established. The system deserves more detailed study. X-Ray powder data for "CeTiO4" are given in the Appendix.

23. THE SYSTEMS Be0-Nb206 and Be0-Ta205.. There is very little literature describing systems containing Be0 and other oxides; and complex oxides (other than silicates) containing beryllium have not often been reported. The Be0-Zr02 system was studied by Ruff, Ebert and Stephen (1929). They reported that no mixed crystals and no compounds were found in the system. 75.

Cocco (1958) studied the systems Be0-Ti02 and Be0-Zr02. He fired the component oxide mixtures at 1200 - 1700°C and on X-ray analysis found that no compound was formed in either system. The systems Be0-Nb206 and Be0-Ta205 have apparently not been studied. A brief preliminary investigation of these systems was therefore carried out. Appropriate oxide mixtures were prepared and fired at 1660°C for 10 hours, in air. The result of X-ray analysis of the products are summarized in Table XXIV. Only component oxide lines were present in the powder photographs. It was concluded that no reaction occurs between the component oxides under the conditions used.

24. THE SYSTEM U305-Ti02. Lambertson and Mueller (1955) studied this system and reported no compound formation in it. It was considered useful to verify this report. Equimolecular proportions of U308 and TiO2 were mixed and fired at 1400°C for about 16 hours, in air. X-Ray examination of the cooled sample showed that no reaction had taken place. The sample was refired at 1660°C for about 10 hours, in air. TABLE XXIV The systems Be0-N10205 and Be0-Ta205.

Reaction Be0(g) Nb205(g) I Ta205(g) Colour Phases present

1. Be0 + Nb205 0.0429 0.4569 - White Be0 + Nb205 (no reaction)

2. 2Be0 + Nb205 0.0791 0.4208 - TT Be0 + Nb205 (no reaction)

3. Be0 + Ta205 0.0267 - 0.4732 PI Be0 + Ta205 (no reaction)

4. 2Be0 + Ta205 0.0508 - 0.W191 ti Be0 + Ta205 (no reaction) 77,

X-Ray examination of the refired product showed only lines o; the component oxides. It was concluded that no reaction takes place between the oxides under the conditions of these experiments. 78.

CHAPTER IV. DISCUSSION. 79.

THE CRYSTAL STRUCTURE OF EUXENITE.

Cell Dimensions. The powder pattern of synthetic euxenite was satisfactorily indexed on the basis of an ortho- rhombic unit cell. The cell dimensions of the synthetic material are compared with those of samples of mineral euxenite in Table XXV.

TABLE XXV.

Material Unit cell dimensions Author a

1.Euxenite Mineral (from Mattawan, Ont.) 5.520 A 14.57 A 5.166 A Arnott (1950) 2.Euxenite Mineral (from Sabine, Ont.) 5.552 A 14.16 A 5.194 A Arnott (1950) 3. Euxenite Alexandrov and Mineral 5.560 A 14.75 A 5.17 A Pytenko (1959) 4. Euxenite (synthetic) 5.550 A 14.625 A 5.175 A Present studies

Derivation of Space Group. The systematic absences of reflections from the powder photograph of euxenite (see Appendix) indicate the space-group to be either Pccn or Pcmn; distinction between these two possibilities requires further evidence. 80.

The law of Donnay and Harker (1937) may be used to decide, from the morphology of the mineral, which of these two space-groups is more likely. By this means Arnott (1950) selected the space-group Pcmn as the more probable. Prolonged efforts were made to establish the structure of euxenite by trial and error methods, on the basis of the available powder data. These proved unsuccessful; the unit cell is too large for the structure to be solved without considerable additional information, preferably from single- crystal studies. The lack of single-crystals prevented any further work in this direction, and inability to establish the structure of euxenite in some detail was reluctantly accepted.

THE COMPOSITION RANGE OF EMENITE. The range of existence of the euxenite phase was studied by investigating the ternary system Y203-Nb205-Ti02 and also by selected cation substitutions in the lattice. 1. The Ternary System Y203-Nb205-Ti02. A study of part of this system was undertaken to establish, if possible, the relationship between fergusonite (YNb04) and euxenite (YNbTi06). 81.

The results obtained show conclusively that the range of existence of euxenite is very limited, and on altering the ratios of cations to anion additional phases are obtained. There was evidence of some deviation from stoichiometry in the euxenite and fergusonite phases, in the changes in the lattice constants of both phases. In the compositions which produced euxenite and Nb2Ti07, there were no appreciable changes in the euxenite lattice constants, showing that stoichiometric excesses of niobium are not accommodated to any appreciable extent. However, the incorporation of excess of the smaller titanium atoms, yttrium being deficient, appears to be possible; these titanium atoms may occupy the lattice sites formerly occupied by yttrium, or they may be in interstitial positions in the euxenite lattice. 2. Substitution of Tantalum. The isomorphous substitution of TO+ for Nb5+ in the euxenite lattice yielded no surprising results. The results obtained clearly show that only a single euxenite phase is observed throughout the substitution, a result not unexpected because the sizes of the ions are similar. 82,

The observed changes in lattice constants, although small, are greater than those expected from the difference in ionic sizes. Data for other oxides suggest that the Nb61- ion is slightly larger than the Ted' ion, or at best that the Ta-O bond distance in oxide structures is somewhat less than the Nb-O distance. This is evidently a case in which different degrees of covalency, as well as ion-size effects, influence the observed bond distances. In euxenite the introduction of tantalum in a quite complex bond system causes an unusual change in the dimensions of the structure, but the exact nature of this change cannot be determined until the structure itself has been more fully )established. 3. Substitution of Thorium. The results obtained on substitution of Th4+ for Y31- (with compensating replacement of Nb51- by Ti)44 show that partial solid solution can occur in this system. Approximately 15% of Th41- ions can be introduced into the euxenite lattice. The lattice constants of the euxenite phase decrease as introduction of thorium proceeds. Since Th4+ is larger than Y3+ and Nb5+ and Ti'l+ are of comparable size, an increase in the lattice constants would be expected. Apparently bond type, rather than ionic radius is more 83,

important in determining the structural effects of the composition changes. 4+ and Fe 4. Substitution of Th 34-. • A study of the substitution of Th'+ for Y3+ and of Fe3+ for Ti4+ was also made. The results obtained show that the euxenite phase cannot accommodate both these ions to any extent, and that their introduction even at quite a low level of substitution transforms the structure into fergusonite. The introduction of iron into the structure is evidently the cause of the appearance of fergusonite. The thorium substitution (with compensating addition of titanium) did not produce a fergusonite phase in the previous system. The fergusonite structure, it would seem, allows the iron atoms to adopt a bond distribution which is more favourably related to the electron configuration than the bond distribution allowed by the structure of euxenite. 5. Substitution of Cerium. Only partial solid solution occurs in the system YNbTi06-CeNbTi06. The appreciable increase in lattice constants observed when cerium is substituted for yttrium can be attributed to the higher ionic radius of cerium. The plot of lattice constants against 84.

composition (Fig. 2) shows that Vegard's law is not obeyed. This may be due to the distortion of the coordination polyhedra of the larger (yttrium or cerium) ion as the substitution proceeds; crystal- field effects would be expected to be different for the Y3-1. and Ce3+ ions, and their bond arrangements in identical oxygen-ion environments may be different. It is also possible, particularly at higher cerium contents, that at least part of the cerium is oxidised to the quadrivalent state; this would obscure the linear relation of lattice constants to composition normally found when simple ionic substitution occurs without associated effects. 6. Substitution of Cerium and Iron. The substitution of cerium and iron in the euxenite lattice yielded results similar to those for Th4+ and Fe34", described in Section (4) above. Any attempt to substitute appreciable proportions of these ions destroys the euxenite lattice and transforms it into fergusonite. The pronounced effect of iron, commented on earlier, is again evident, and a similar explCmation probably applies. It is noteworthy, both in this system and in the corresponding system obtained with thorium, that fergusonite structures (type formula YNbal, with equal numbers of large and small cations) are actually obtained when small cations (in this case Fe3+ and Nb6+) are present in substantially larger numbers than large cations (V+, Th4+, Ce4+). The mechanism of substitution allowing this needs investigation when the detailed crystal structure of fergusonite has been established. 7. Substitution of Neodymium. Solid solution occurs in the system YNbTi06-NdNbTiO6 until about 40% of Ne+ is substituted for V+. The lattice constants of the euxenite phase increase slightly as Nd3+ ions are introduced, presumably on account of the higher ionic size of Nd3+. The ionic radius difference between the two ions is evidently sufficient to prevent continuous solid solution between YNbTiO6 and NdNbTi06 , and to produce a distinct structure, not immediately related to that of euxenite, in NdNbTi06. The plot of lattice constants against composition (Fig. 3) again shows that Vegard's law is not obeyed, and the anomalous variation can be attributed to the different electronic configurations of the yttrium and neodymium ions. 8. Substitution of Uranium. Results obtained in the system YNbTiO6-UTi206

86.

show that about 20% of uranium can enter the euxenite lattice. The lattice constants of the euxenite phase decrease as uranium is introduced. The valency relations normally found with uranium are such that the simple replacement Y3+ + + Ti4+ (uranium remaining quadrivalent) is unlikely to occur in this system. The substitution mechanism may be expected to be more complex and to involve some oxidation of uranium to the sexavalent state. This probably accounts for the decrease of lattice constants as the uranium content increases, a change scarcely expected in the light of the ionic radius relation between Y31- (0.92 A) and U -1. (0.97 A). 9, The System YTaTi06-NdTaTi06. The results obtained in this system were similar to those for the analogous system YNbTiO6- NdNbTi06. The lattice dimensions of the new compound, NdTaTi0e, isolated in this system are similar to those of NdNbT106, and (not unexpectedly) the two compounds appear to be isostructural. 10. The System CeNbIle06-CeTale06. Particularly interesting results were obtained in this system. No euxenite phase was detected at any composition, but a fergusonite phase (already seen to 87.

be favoured when iron is present) appeared at the higher niobium contents. This broke down, however, and was replaced by a pyrochlore structure when apprecialbe substitution of tantalum for niobium was attempted. The change-over suggests for the first time that a fairly close structural relationship may exist between pyrochlore (cubic) and euxenite (orthorhombic), and that the detailed bonding differences between niobium and tantalum, resulting from their slightly different electron configurations, are sufficient to change the stability relations of the structures.

FERGUSONITE.

Artificial fergusonite has been synthesized and shown to be identical in structure with the mineral. The Space Group of fergusonite could not be established from systematic extinctions in powder photographs. The present studies confirm the conclusions of Berman (1955) and Komkov (1957) that the fergusonite lattice is not tetragonal as assumed by Barth (1926), but orthorhombic and pseudo-tetragonal. Cation Substitution in Ferffsonite. The range of existence of fergusonite was investigated by attempting the substitution of 88,

appropriate cations in the fergusonite lattice and studying the phases developed. 1. Substitution of Tantalum. A complete series of substitutional solid solutions exists throughout the range YNb04-YTa04. This result is in conformity with the very similar ionic sizes of N136-4- and TO+. Ferguson (1955) observed that the compound YTa04 had a distorted tetragonal unit cell and was probably monoclinic. However, in the present study, it was found that all the reflections could be satisfactorily indexed on the basis of an orthorhombic unit cell. 2. Substitution of Cerium. The substitution of cerium> in the fergusonite lattice occurs only to a limited extent, and the lattice constants of the fergusonite phase increase rapidly with substitution of cerium on account of the higher ionic radius of the cerium ion. Vegard's law (see Fig. 5) is again not obeyed, probably for the reasons already given in Section 5 ahove. 3. Substitution of Thorium. Fergusonite solid solutions again our only to a limited extent, and the lattice constants increase 85.

as thorium is introduced. This can be explained on the basis of higher ionic size of Th'+. 4. Substitution of Uranium. Similar results to those in the YNbTi06- UTi206 system were obtained in the system YNb0I-UTiO4, for similar reasons. Limited solid solution occured, and the lattice constants of the fergusonite phase decreased on introduction of uranium. 5. The System YTa04-CeTa04. This system was analogous to the YNbTi06-0eNbTiO6 system discussed earlier. Only partial solid solution was observed, the lattice constants of the fergusonite phase increasing with introduction of cerium.

MISCELLANEOUS RELATED SYSTEMS.

(a) THE SYSTEM Nb205-Ti02. A brief survey of the literature dealing with this system is attempted in the Introduction. Only one compound,- Nb205,Ti02, was reported by Durbin et al,(1952), but Roth and Coughanour (1955) reported two compounds, Nb205,Ti02 and 3Nb2059 Ti02. In the present studies, only one compound, Nb2059Ti02, is confirmed. It has an orthorhombic unit cell with the dimensions a = 11.493 ± 0.006, 96.

b = 10.798 ± 0.006, c = 9.954 ± 0.006 A. (b) THE SYSTEM ZrOa-Ti02. In the present investigation, only one compound, ZrTiO4, could be isolated in this system. It is orthorhombic, with the unit-cell dimensions a = 4.800 ± 0.005, b = 5.008 ± 0.005, c = 5.451 ± 0.005 A. (c) THE SYSTEMS Zr0 -Nb 05 and Zr02-Ta 05. Roth and Coughanour (1955), who investigated the system Zr02-Nb205, report only one compound 6Zr02,Nb205 (orthorhombic, a = 4.964, b = 5.210, = 5.285 A). In the present investigation, the existence of only one compound, 6Zr02,Nb205, was confirmed and lattice constants in agreement with those of Roth and Coughanour (1955) was obtained. Similar studies were made in the system Zr02-Ta205 and in this case again only one compound, 6Zr02,Ta205, was isolated. Solid solution studies made between 6Zr02,Nb205 and Nb206 reveal that the solid solution range extends only to 80 mol.-% of Zr02. At lower zirconia content, niobium pentoxide separates. These results are contrary to those of Roth and Coughanour (1955), who report that the solid solution range extends 91.

down to 62 mol.-% of ZrO2. Similar results were obtained in the system. Zr02-Ta205. (d) THE SYSTEM Th02-Ti02. In this system, one new compound, ThTi05, was isolated. Unfortunately, the Guinier photograph of this compound was complex and could not be indexed. (e) THE SYSTEM Ce02-Ti02. There was evidence in this system of the existence of a solid solution of fluorite structure containing an appreciable amount of titanium, and also of a new compound designated CeTiO4, having ortho- rhombic symmetry. The products were not fully reacted, and further study of this system will be necessary at higher temperatures. (f) THE SYSTEMS Be0-Nb205 and Be0-Ta205. The brief investigation of these systems showed that no reaction occurs between the component oxides. This may be due to the low reactivity of beryllium oxide. (g) THE SYSTEM U50e-Ti02. lamberton and Mueller (1955) investigated this system and reported no compound formation. The present studies confirm their results.

•

92.

APPENDIX (Containing Tables 1-15 of X-Ray Powder Data).

APPENDIX TABLE SPECIMEN 1 YNbT1O6 2 YTaTiO6 3 YNb04 4 YTa04 5 ZrTiO4 6 6ZrOapb205 7 6Zr02Ta205 8 NdNbTIO6 9 NdTaTiO6 10 CeTaFe06 11 CeTa04 12 CeNbTiO6 13 CeTiO4 14 Nb2TiO7 15 ThTi2O6

In these Tables: I 777 visually estimated relative intensity of line, on scale 1-10; measured Bragg angle for line; d - corresponding interplanar spacing, in A; hkl = indices of crystal plane responsible for reflection. All the details given relate to Guinier photographs obtained with copper Ka radiation. 93.

Table 1. X-Ray Powder Data for YNbTi06.

9 Sine® Sin29 . hkl (obs.) (calc.) 2 8.53 5.19 0.0220 0.0221 110 5 12.16 3.66 0.0444 0.0443 130 0.0442 111 0.0444 040 5 13.24 3.364 0.0524 0.0527 121 10 14.98 2.98 0.0668 0.0665 131 4 16.14 2.77 0.0773 0.0772 200 1 17.06 2.63 0.0861 0.0859 141 2 17.23 2.58 0.0887 0.0888 002 0.0883 220 0.0888 150 1 17.56 2.552 0.0910 0.0915 012 3 18.41 2.44. 0.0997 0.0999 022 0.0994 201 0.100a 060 1 19.51 2.306 0.1115 0.1109 112 0.1105 221 0.110D 151 1 20.49 2.20 0.1226 0.1217 240 2 20.76 2.17 0.1256 0.1244 231 2 21.42 2.11 0.1333 0.1331 132 1 22.36 2.02 0.1447 0.1439 241 1 22.76 1.774 0.1527 0.1525 142 2 23.42 1.938 0.1580 0.1583 052 6 24.03 1.891 0.1658 0.1660 202 6 24.95 1.825 0.1780 0.1771 222 0.1776 152 0.1772 260 0.1777 171 0.1779 080 94.

Table 1, continued.

Sin29 Sine® hkl (obs.) (calc.)

6 25.75 1.773 0.1888 0.1888 062 6 26.59 1.72 0.2004 0.1994 261 1 27.23 1.68 0.2093 0.2102 242 4 28.03 1.64 0.2208 0.2094 181 0.2218 113 0.2209 331 3 28.65 1.605 0.2298 0.2302 123 2 29.04 1.585 0.2356 0.2355 252 0.2356 271 4 29.57 1.56 0.2436 0.2442 133 1 30.89 1.50 0.2636 0.2625 302 0.2636 143 4 31.14 1.49 0.2674 0.2667 082 0.2667 191 2 31.57 1.47 0.2741 0.2737 360 1 31.96 1.455 0.2785 0.2780 0,10,0

CRYSTAL SYSTEM ORTHORHOMBIC. LATTICE CONSTANTS : a = 5.550 ± 0.005 A. b = 14.62510.005 A. c = 5.175 ±0.005 A. STRUCTURE TYPE : EUXENITE. 95.

Table 2. X-Ray Powder Data for YTaTiO6. Sin2G Sin2G hkl (obs.) (calc.)

4 8.56 5.17 0.0221 0.0224 110 5 12.21 3.64 0.0448 0.0446 130 0.0444 111 0.0450 040 5 13.27 3.35 0.0527 0.0529 121 1C. 15.00 2.976 0.0670 0.0669 131 6 16.14 2.77 0.0773 0.0774 200 4 17.08 2.63 0.0863 0.0866 141 5 17.31 2.59 0.0885 0.0892 002 0.0886 220 0.0897 150 4 17.59 2.55 0,0913 0.0920 012 3 17.77 2.524 0.0931 0.0926 051 4 18.41 2.44 0.0997 0.0997 201 4 18.52 2.425 0.1009 0.1004 022 0.1013 060 1 19.44 2.313 0.1107 0.1113 112 0.1109 221 0.1120 151 1 19.71 2.283 0.1137 0.1145 032 1 20.49 2.20 0.1226 0.1224 240 1 20.69 2.18 0.1249 0.1250 231 2 21.38 2.113 0.1330 0.1338 132 2 22.34 2.026 0.1445 0.1447 241 2 23.03 1.968 0.1521 0.1536 142 3 23.46 1.934 0.1584 0.1575 170 0.1595 052 4 24.05 1.889 0.1661 0.1662 202 96.

Table 2, continued.

Sin2G Sin2G hkl (obs.) (calc.)

6 24.99 1.823 0.1785 0.1779 222 0.1770 310 0.1789 152 0,1795 171 0.1787 260 6 25.52 1.788 0.1856 0.1854 320 5 25.81 1.769 0,1895 0.1905 062 8 26.59 1.721 0.2004 0.1993 311 0.1995 330 0.2010 261 2 28.08 1.636 0.2215 0.2217 181 0.2228 113 0.2216 331 2 28.65 1.605 0.2298 0.2313 123 1 29.09 1.583 0.2364 0.2369 252 0.2376-= 271 2 29.61 1.558 0.2442 0.2453 133 2 30.89 1.499 0.2636 0.2634 302 6(d) 31.19 1.486 0.2682 0.2679 262 0.2693 082 0.2695 191 3 31.90 1.457 0.2792 0.2781 203 0.2798 281

CRYSTAL SYSTEM : ORTHORHOMBIC LATTICE CONSTANTS : a = 5.541 ± 0.005 A. b =14.523 ± 0.005 A. c = 5.163 ± 0.005 A. STRUCTURE TYPE o EUXENITE. 97.

Table 3. X-Ray Powder Data for YNb04.

Sin29 Sin2G hkl (obs.) (calc.)

2 8.14 5.43 0.0119 0.0119 002 2 9.35 4.74 0.0264 0.0264 021 10 14.34 3.11 0.0613 0.0608 212 1 14.93 2.99 0.0662 0.0663 023 10 15.12 2.95 0.0682 0.0681 032 2 15.91 2.81 0.0752 0.0751 123 4 16.39 2.73 0.0797 0.0797 004 5 17.03 2.63 0.0858 0.0857 040 4 17.95 2.50 0.0950 0.0946 140 1 19.09 2.35 0.1070 0.1063 ' 321 1 19.67 2.29 0.1134 0.1143 142 2 20.37 2.21 0.1212 0.1205 214 3 20.95 2.15 0.1279 0.1279 034 2 22.64 2.00 0.1482 0.1478 151 1 22.94 1.97 0.1520 0.1524 411 1 23.66 1.92 0.1610 0.1601 205 8 23.96 1.90 0.1648 0.1654 044 6 24.58 1.851 0.1731 0.1727 035 1 25.09 1.81 0.1798 0.1793 006 1 25.43 1.79 0.1843 0.1846 016 4 26.03. 1.75 0.1932 0.1929 060 116 4 28.01 1.64 0.2206 0.2201 216 5 28.35 1.62 0.2278 0.2275 036 260 1 28.79 1.60 0.2320 0.2323 511 5 29.40 1.57 0.2410 0.2420 502 424 98.

Table 3, continued.

I 9 d Sin2G Sin29 hkl (obs.) (calc.) 2 29.65 1.556 0.2448 0.2457 245 3 29.79 1.549 0.2470 0,2472 512 3 30.80 1.503 0.2622 0.2630 236 1 31.02 1.49 0.2656 0.2654 027 3 31.51 1.47 0.2732 0.2732 064 146 1 32.29 1.44 0.2855 0.2849 217 1 33.29 1.40 0.3014 0.3011 137 2 34.39 1.36 0.3192 0.3197 363 065 2 35.85 1.314 0.3430 0.3430 080 1 36.88 1.282 0.3601 0.3609 551 622 218 2 37.80 1.259 0.3756 0.3759 552 1 38.37 1.24 0.3858 0.3859 623 2 38.73 1.23 0.3914 0.3914 417 1 39.28 1.22 0.4011 0.4006 455 2 39.60 1.208 0.4063 0.4071 446 1 39.97 1.99 0.4127 0.4122 109 2 40.54 1.185 0.4225 0.4222 374 380 1 41.11 1.172 0.4323 0.4314 184 2 42.44 1.140 0.4554 0.4554 456 1 43.01 1.13 0.4653 0.4661 418 1 44.33 1.10 0.4883 0.4882 258 319 99.

Table 3., continued.

CRYSTAL SYSTEM a ORTHORHOMBIC. LATTICE CONSTANTS : a = 8.179 ±0.005 A. b = 10.531 ± 0.005 A. c = 10.927 i 0.005 A. STRUCTURE TYPE : FERGUSONITE. 100,

Table 4. X-Ray Powder Data for YTa0.1.

G d Sin29 Sin26 hkl (obs.) (calc.)

2 8.16 5.43 0.0196 0.0200 002 4 9.36 4.73 0.0264 0.0263 021 10 14.31= 3.116 0.0611 0.0607 212 10 15.28 2.93 0.0695 0.0685 032 4 16.48 2.714 0.0805 0.0800 004 4 16.99 2.636 0.0854 0.0852 040 0.0849. 310 4 17.95 2.499 0.0950 0.0941 140 1 18.95 2.391 0.1054 0.1052 312 1 19.78 2.276 0.1145 0.1144 142 3 21.11 2.138 0.1297 0.1292 034 1 22.54 2.008 0.1469 0.1467 323 0.1471 151 5 23.80 1.908 0.1628 0.1619 402 5 23.99 1.894 0.1652 0.1662 314 0.1649 340 0.1665 044 5 24.70 1.842 0.1746 0.1754 144 4 26.34 1.735 0.1969 0.1969 061 5(d) 28.08 1.635 0.2215 0.2210 162 1 28.63 1.606 0.2295 0.2008 036 1 28.85 1.595 0.2328 0.2316 511 5(d) 29.43 1.566 0.2415 0.2415 502 3(d) 29.89 1.544 0.2484 0.2476 262 0.2498 254 4 31.07 1.491 0.2663 0.2678 316 2 31.78 1.462 0.2774 0.2770 146 1 32.13 1.447 0.2829 0.2838 326 101.

Table 4, continued.

d Sin29 Sin29 hkl (obs.) (calc.)

2 35.72 1.319 0.3408 0.3407 080

1 36.97 1.280 0.3616 0.3605 208

1 37.48 1.265 0.3702 0.3702 182

1 38.46 1.238 0.3868 0.3868 083

CRYSTAL SYSTEM : ORTHORHOMBIC. LATTICE CONSTANTS : a = 8.195 ± 0.006 A b = 10.559 ± 0.006 A. c = 10.905 ± 0.006 A. STRUCTURE TYPE e FERGUSONITE. 102.

Table 5. X-Ray Powder Data for ZrTiO4.

Sin% Sin% hkl (obs.) (calc.)

4 12.42 3.58 0.0462 0.0458 101 10 15.35 2.91 0.0700 0.0695 111 3 16.44 2.72 0.0801 0.0800 002 5 17.95 2.499 0.0950 0.0948 020 3 18.79 2.389 0.1037 0.1032 200 2 20.37 2.212 0.1212 0.1206 120 3 21.11 2.138 0.1297 0.1295 112 1 22.04 2.053 0.1408 0.1406 121 5 24.70 1.842 0.1746 0.1748 022 4 25.40 1.795 0.1840 0.1832 202 5 26.46 1.730 0.1985 0.1980 220 4 27.12 1.69 0.2078 0.2069 212 2 28.60 1.608 0.2291 0.2295 113 6 31.87 1.458 0.2788 0.2780 222 2 32.79 1.44 0.2933 0.2933 032 2 33.25 1.404 0.3006 0.3006 123

CRYSTAL SYSTEM : ORTHORHOMBIC LATTICE CONSTANTS : a = 4.800 ± 0.005 A. b = 5.008 ± 0.005 A. c = 5.451 ± 0.005 A. 103.

Table 6. X-Ray Powder Data for 6Zr029 Nb208.

I 9 d Sin% Sin29 hkl (obs.) (calc.)

3 12.42 3.583 0.0462 0.0459 101 10 15.25 2.93 0.0691 0.0688 111 5 17.06 2.626 0.0861 0.0860 002 4 17.61 2.546 0.0915 0.0915 020 4 18.23 2.462 0.0978 0.0976 200 2 19.25 2.336 0.1088 0.1089 012 2 21.75 2.078 0.1373 0.1375 121 6 24.88 1.83 0.1770 0.1776 022 6 25.40 1.795 0.1840 0.1836 202 6 25.79 1.770 0.1893 0.1892 220 3 27.02 1.695 0.2064 0.2065 212 3 27.81 1.650 0.2176 0.2179 103 5 29.34 1.57 0.2401 0.2408 113 5 30.11 1.535 0.2517 0.2520 131 5 30.96 1.496 0.2647 0.2640 311 3 31.62 1.468 0.2749 0.2752 222 1 32.70 1.425 0.2919 0.2921 032 2 33.77 1.385 0.3090 0.3095 123 2 37.25 1.271 0.3663 0.3664 040 0.3669 140 1 38.73 1.230 0.3914 0.3904 400 1 39.97 1.199 0.4131 0.4131 410 2 40.63 1.184 0.4240 0.4240 133 1 41.39 1.165 0.4372 0.4360 313 1 42.03 1.150 0.4482 0.4472 331 2 42.99 1.1296 0.4651 0.4645 214

CRYSTAL SYSTEM : ORTHORHOMBIC LATTICE CONSTANTS : a = 4.935 ± 0.006 A. b = 5.095 ± 0.006 A. c = 5.258 ± 0.006 A. 104.

Table 7. X-Ray Powder Data for 6Zr029Ta205.

Sin2G Sin29 hkl (obs.) (calc.) 3 12.42 3.58 0.0462 0.0460 101 10 15.25 2.93 0.0691 0.0689 111 5 17.06 2.627 0.0861 0.0862 002 4 17.61 2.541 0.0915 0.0918 020 4 18.22 2.463 0.0978 0.0978 200 2 19.28 2.333 0.1091 0.1091 012 2 21.79 2.075 0.1378 0.1378 121 6 24.92 2.825 0.1778 0.1780 022 6 25.40 1.795 0.1840 0.1840 202 6 25.79 1.769 0.1893 0.1896 220 2 27.05 1.693 0.2068 0.2069 212 3 27.85 1.648 0.2182 0.2183 103 5 29.38 1.570 0.2408 0.2413 113 5 30.13 1.532 0.2520 0.2525 131 5 30.93 1.498 0.2642 0.2645 311 3 31.64 1.467 0.2752 0.2758 222 1 32.70 1.425 0.2919 0.2927 032 2 33.81 1.384 0.3097 0.3101 123 2 37.29 1.270 0.3670 0.3672 040 0.3677 014 2 38.69 1.232 0.3907 0.3912 400 0.3916 140 1 40.06 1.197 0.4143 0.4141 410 1 40.67 1.183 0.4247 0.4248 133 1 41.39 1,162 0.4372 0.4369 313 1 42.00 1.151 0.4477 0.4481 331 1 43.01 1.1292 0.4653 0.4658 214 105.

Table 7, continued.

CRYSTAL SYSTEM : ORTHORHOMBIC LATTICE CONSTANTS : a = 4.929 ± 0.005 A. b = 5.089 ± 0.005 A. c = 5.252 ± 0.005 A. 106.

Table 8. X-Ray Powder Data for NdNbTi08.

Sin20 Sin20 hkl (obs.) (calc.)

4 8.14 5.43 0.L00 0.0201 110 4 10.09 4.39 0.0307 0.0294 120 1 10.27 4.31 0.0318 0.0320 002 2 11.02 4.03 0.0365 0.0351 012 1 13.13 3.52 0.0516 0.0521= 112 4 14.50 3.076 0.0627 0.0614 122 8 14.87 3.001 0.0659 0.0666 140 10 15.18 2.942 0.0686 0.0680 200 3 16.06 2.785 0.0766 0.0775 050 4 16.83 2.66 0.0838 0.0844 023 1 17.49 2.562 0.0903 0.0890 103 1 18.41 2.44 0.0997 0.0999 033 1 18.48 2.43 0.1004 0.1002 202 1 20.01 2.252 0.1171 0.1169 133 2 20.51 2.198 0.1227 0.1217 043 1 20.97 2.152 0.1281 0.1286 160 0.1280 004 2 21.95 2.060 0.1397 0.1400 203 0.1404 024 3 22.48 2.014 0.1462 0.1455 250 3 23.28 1.949 0.1562 0.1561 310 1 23.71 1.915 0.1617 0.1610 301 1 24.37 1.866 0.1703 0.1690 170 1 24.67 1.846 0.1743 0.1734 321 2 25.84 1.766 0.1899 0.1890 331 2 26.48 1.727 0.1988 0.1984 080 3 27.00 1.696 0.2061 0.2055 054 4 29.06 1.582 0.2360 0.2356 342 107.

Table 8, continued.

I 9 d Sin20 Sin29 hkl (obs.) (calc.) 2 30.05 1.537 0.2508 0w2511 090 1 30.29 1.526 0.2544 0.2530 333 1 30.98 1.494 0.2650 0.2646 360 1 31.64 1.467 0.2752 0.2751 410 0.2746 343

CRYSTAL SYSTEM : ORTHORHOMBIC LATTICE CONSTANTS : a = 5.913 ± 0.005 A. b = 13.849 ± 0.005 A. c= 8.621 ± 0.005 A. 108.

Table 9. X-Ray Powder Data for NdTaTi06.

sin% sin% hkl (obs.) (calc.) 2 8.14 5.43 0.0200 0.0201 110 2 10.09 4.395 0.0307 0.0294 120 1 10.24 4.33 0.0316 0.0320 002 1 11.03 4.027 0.0366 0.0359 031 0.0351 012 0.0374 121 2 11.94 3.724 0.0428 0.0444 022 4 14.50 3.076 0.0627 0.0615 122 8 14.86 3.002 0.0658 0.0667 140 10 15.18 2.942 0.0686 0.0680 200 2 16.06 2.785 0.0766 0.0776 050 4 16.83 2.66 0.0838 0.0844 023 2 17.49 2.562 0.0903 0.0891 103 1 18.52 2.425 0.1009 0.1001 202 0.1014 123 1 19.23 2.336 0.1085 0.1096 052 1 19.97 2.255 0.1226 0.1217 043 1 21.01 2...15 0.1285 0.1280 004 0.1288 160 1 21.91 2.065 0.1392 0.1401 203 0.1404 024 4 22.48 2.014 0.1462 0.1457 250 2 23.26 1.95 0.1559 0.1560 034 0.1562 310 1 23.69 1.916 0.1614 0.1618 301 0.1608 162 1 23.81 1.907 0.1629 0.1642 311 1 24.37 1.866 0.1703 0.1691 170 109.

Table 9 (contd.)

d sin2G sin2G hkl (obs.) (calc.)

1 24.81 1.835 0.1760 0.1771 171 1 25.22 1.807 0.1816 0.1811 330 1 25.86 1.766 0.1902 0.1898 243 1 26.50 1.726 0.1991 0.2001 005 0.1987 080 4 26.96 1.699 0.2055 0.2057 054 4 29.07 1.584 0.2361 0.2350 342 3 30.02 1.538 0.2503 0.2498 045 0.2505 090 1 30.57 1.513 0.2587 0.2595 091

CRYSTAL SYSTEM o ORTHORHOMBIC. LATTICE CONSTANTS o a = 5.909 t 0.005 A. b = 13.834 ± 0.005 A.. c = 8.618 ± 0.005 A. 110.

Table 10. X-Ray Powder Data for CeTaFe0e.

I 9 d Sin% Sin20 hkl (obs.) (calc.) 3 7.43 5.96 0.0166 0.0166 111 3 14.27 3.125 0.0608 0.0609 311 10 14.93 2.99 0.0664 0.0664 222 5 17.31 2.588 0.0885 0.0886 400 8 24.86 1.832 0.1767 0.1772 440 8 29.50 1.563 0.2425 0.2437 622 2 31.03 1.493 0.2658 0.2680 444 1 36.54 1.292 0.3545 0.3548 800 2(d) 40.44 1.187 0.4208 0.4210 662 2(d) 41.75 1.563 0.4434 0.4430 840

CRYSTAL SYSTEM a CUBIC. LATTICE CONSTANT a = 10.358 ± 0.005 A. Table 11. X-Ray Powder Data for CeTa04.

Sin2G Sin20 hkl (obs.) (calc.)

2 5.63 7.85 0,0096 0.0096 110 8 11.39 3.90 0.0390 0.0384 220 2 12.76 3.49 0.0488 0.0480 310 8 14.31 3.12 0.0611 0.0604 222 1 14.79 3.02 0.0651 0.0652 302 10 16.15 2.77 0.0774 0.0768 400 2 16.60 2.75 0.0816 0.0816 410 1 19.96 2.25 0.1166 0.1180 422 2 23.10 1.96 0.1539 0.1536 440 3 23.26 1.95 0.1560 0.1567 205 5 23.82 1.91 0.1631 0.1632 530 1 26.04 1.75 0.1928 0.1920 620 2 26.18 1.74 0.1946 0.1948 602 4 28.22 1.63 0.2236 0.2239 335 3 28.86 1.59 0.2330 0.2335 425 1 31.85 1.46 0.2715 0.2784 730 2 33.86 1.382 0.3105 o.aan7 605 1 34.82 1.35 0.3260 0.3264 820 2 36.24 1.30 0.3497 0.3502 830 2 38.50 1.24 0.3875 0.3888 900 1 39.69 1.205 0.4079 0.4080 842

CRYSTAL SYSTEM o TETRAGONAL. LATTICE CONSTANTS : a = 11.060 ± 0.006 A. c = 10.296 ± 0.006 A. 112.

Table 12. X-Ray Powder Data for CeNbTi06.

sin2G sin2G hkl (obs.) (calc.)

4 8.16 5.43 0.0201 0.0200 110 1 9.19 4.82 0.0255 0.0250 101 4 10.15 4.37 0.0311 0.0320 002 4 10.95 4.055 0.0361 0.0370 121 0.0350 031 0.0350 012 1 13.06 3.41 0.0511 0.0520 112 8 14.43 3.09 0.0621 0.0610 122 10 14.75 3.025 0.0648 0.0650 140 10 15.05 2.96 0.0674 0.0680 200 4 15.99 2.796 0.0759 0.0760 201 0.0750 013 4 16.63 2.69 0.0820 0.0830 051 2 17.45 2.57 0.0899 0.0890 103 1 18.16 2.472 0.0971 0.0970 142 1 18.47 2.432 0.1003 0.1000 202 2 19.55 2.301 0.1120 0.1120 222 1 19.80 2.273 0.1147 0.1160 240 2 20.85 2.164 0.1267 0.1270 232 0.1280 004 1 21.72 2.08 0.1369 0.1370 143 4 22.25 2.034 0.1434 0.1430 213 .3 23.19 1.955 0.1550 0.1550 071 0.1560 310 3 23.58 1.925 0.1600 0.1610 301 5 24.19 1.879 0.1679 0.1670 233 1 24.63 1.848 0.1737 0.1730 321 1 25.65 1.779 0.1874 0.1880 312 113.

Table 12 (contd.).

I 9 d sin2G sin29 hkl (obs.) (calc.) 1 25.79 1.77 0.1893 0.1880 331 1 26.29 1.739 0.1962 0.1970 322 0.1960 172 5 26.80 1.708 0.2033 0.2030 015 1 27.62 1.660 0.2149 0.2150 253 1 28.05 1.637 0.2212 0.2200 115 1 28.60 1.608 0.2291 0.2290 125 0.2280 350 4 28.86 1.595 0.2329 0.2330 342 1 29.29 1.573 0.2394 0.2410 182 1 29.73 1.552 0.2458 0.2470 272 1 30.03 1.538 0.2503 0.2510 091 2 30.93 1.498 0.2642 0.2650 145 1 33.57 1.392 0.3057 0.3050 106 ..1 34.37 1.364 0.3186 0.3190 291 ...1 34.94 1.344 0.3281 0.3280 441 1 35.83 1.315 0.3427 0.3430 292 2 0 39.46 1.211 0.4040 0.4030 414

CRYSTAL SYSTEM a ORTHORHOMBIC LATTICE CONSTANTS e a = 5.914 ± 0.005 A, b =14.075 ± 0.005 A, c = 8.620 ± 0.005 A. Table 13. X-Ray Powder Data for CeTi0.1.

I 9 d Sin29 Sin29 hkl (obs.) (calc.)

1 7.41 5.98 0.0166 0.0163 111 4 9.38 4.724 0.0266 0.0264 002 1 10.41 4.26 0.0327 0.0319 211 5 13.03 3.42 0.0508 0.0513 310 4 13.40 3.324 0.0537 0.0534 311 1 14.91 2.993 0.0662 0.0669 032 3 1;.57 2.87 0.0720 0.0720 040 2 16.30 2.744 0.0788 0.0786 041 0.0777 312 2 17.86 2.512 0.0940 0.0943 411 1 18.57 2.419 0.1014 0.1012 420 1 19.90 2.262 0.1159 0.1153 114 1 21.18 2.132 0.1306 0.1315 510 1 22.71 1.994 0.1490 0.1501 432 2 24.06 1.889 0.1662 0.1671 160 2 24.35 1.867 0.1700 0.1695 015 0.1704 324 3 27.32 1.678 0.2107 0.2107 135

CRYSTAL SYSTEM : ORTHORHOMBIC. LATTICE CONSTANTS : a = 10.701 ± 0.006 A. b = 11.498 ± 0.006 A. c = 9.490 ± 0.006 A. 115.

Table 14. X-Ray Powder Data for Nb2Ti07. Sin29 Sin2G hkl (obs.) (calc.)

2 8.64 5.126 0.0227 0.0231 210 2 8.78 5.045 0.0233 0.0240 002 10 12.05 3.69 0.0436 0.0444 022 1 12.35 3.60 0.0458 0.0458 030 8 13.04 3.42 0,0509 0.0504 130 8 13.19 3.36 0.0521 0.0515 311 2 13.49 3.30 0.0544 0,0540 003 2 15.62 2.86 0.0725 0.0720 400 5 16.28 2.75 0.0786 0.0780 401 2 16.38 2.73 0.0796 0.0789 123 3 16.85 2.66 0.0840 0.0831 411 1 17.86 2.51 0.0940 0.0945 303 1 19.48 2.31 0.1112 0.1104 332 0.1101 142 1 19.62 2.29 0.1127 0.1125 500 4 22.04 2.05 0.1408 0.1405 333 4 22.27 2.033 0.1436 0.1420 432 6 23.92 1.899 0.1644 0.1644 531 1 25.63 1.78 0.1871 0.1881 160 0.1860 602 0.1860 153 1 26.00 1.76 0.1922 0.1911 612 1 26.30 1.74 0.1963 0.1959 035 1 26.64 1.72 0.2011 0.2016 260 1 27.00 1.696 0.2061 0.2064 622 4 27.62 1.66 0.2149 0.2139 225 0.2160 206 0.2160 603 116.

Table 14, continued.

Sin29 Sin2G hkl (obs.) (calc.) 1 28.38 1.62 0.2261 0.2256 710 1 29.75 1.55 0.2463 0.2460 551 1 30.00 1.54 0.2500 0.2496 712 2 33.50 1.395 0.3046 0.3040 073 0.3045 117 1 37.00 1.28 0.3622 0.3625 570 1 37.66 1.260 0.3733 0.3729 381 1 39.49 1.210 0.4046 0.4044 481 1 39.74 1.204 0.4087 0.4099 128

CRYSTAL SYSTEM : ORTHORHOMBIC. LATTICE CONSTANTS : a = 11.493 ± 0.006 A. b = 10.798 0.006 A. c = 9.954 ± 0.006 A. 117.

Table 15. X-Ray Powder Data for ThTi208

I 9 d Sin2G (obs.)

2 7.26 6.10 0.0160 6 9.40 4.715 0.0267 2 10.09 4.398 0.0307 2 10.41 4.263 0.0327 8 12.84 3.47 0.0494 5 13.21 3.371 0.0522 1 13.32 3.345 0.0531 1 13.52 3.297 0.0547 10 13.91 3.205 0.0578 1 14.03 3.178 0.0588 1 14.21 3.138 0.0603 4 14.66 3.044 0.0640 4 15.40 2.90 0.0705 6 16.05 2.786 0.0765 6 16.16 2.768 0.0775 5 17.58 2.55 0.0912 6 18.23 2.644 0.0979 3 18.70 2.403 0.1028 2 19.35 2.324 0.1098 2 19.62 2.295 0.1127 2 19.85 2.268 0.1153 1 20.81 2.168 0.1262 2 21.18 2.132 0.1306 2 21.50 2.101 0.1343 4 22.04 2.053 0.1408 4 22.16 2.042 0.1422 6 23.09 1.963 0.1538 118.

Table 15, continued.

I Sin219 (obs.)

3 23.50 1.931 0.1590 4 23.88 1.903 0.1639 4 23.97 1.896 0.1650 1 24.50 1.857 0.1720 1 25.14 1.812 0.1804 3 25.93 1.761 0.1912 3 26.45 1.73 0.1984 2 26.98 1.698 0.2058 6 27.35 1.676 0.2111 2 27.75 1.653 0.2168 2 28.14 1.632 002225 2 28.30 1.623 0.2248 1 28.51 1.612 0.2277 2 28.73 1.602 0.2310 1 28.99 1.588 0.2349 1 29.16 1.580 0.2375 2 29.65 1.556 0.2448 1 30.64 1.510 0.2597 1 31.15 1.488 0.2676 1 31.68 1.468 0.2749 1 32.04 1.451 0.2814 1 32.83 1.420 0.2939 5 33.65 1.389 0.3070 5 37.13 1.275 0.3644 3 38.30 1.242 0.3841 1 38.70 1.232 0.3909 1 39.59 1.208 0.4062 1 41.48 1.163 0.4387 4 42.75 1.1347 0.4608 1 43.48 1.1194 0.4735 3 44.37 1.1015 0.4890 119.

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