Revista Latinoamericana de Metalurgia y Materiales, Vol. 7, Nos. 1 & 2 (1987) 21

Formatíon of Close-Packed.Ordered A(B1-xB'x)a - type Intermediate Phases me Metallic ABa-AB'3 Pseudobinary Alloy System

g-Am Cho*, B. C. Giessen** andN.J. Grant

Depal''tment of Metallurgy and Materials Scíence, Massachusetts Institute of Technology, Cambridge, Massachusett. 39, U.S.A. , I I

The terminal solid solubility Iimits, the structure types, and the composition Iimits of occurence of close-packed ordered A(B¡-xB'x>S- type intermediatephases in thepseudobinary systerns: TaRbs - TaNis, Ta1rs -TaPt¡¡,YPda -YA~, YPt¡¡- MoPt¡¡,ZrPt¡¡- MoPt¡¡and SiNie - NbNi;.. were studied by means ofX-ray diffraction and, in part, by metallography. Oftftese, the last three systems do not forro interroe- diate phases. The number of intermediate phases encountered in the first three systems are respectively five, three and two. The,sys- tem, YPdg - YA~, showed an unknown phase above about 80 atomic percent YA~. As the compcsítíons near to the ríght end components of the systems, the structure of the ordered elose-packet ABg- type atomic layers transform forro type ABgSh(O) to type ABsSh(1/2) without having a transition type layer between them. Although the stability Iimits of the intermediate phases are not corre-

lated with the number of total electron in dos electron shells 01' the average group number (AGN) in transition metals, the sequence oí

occurrence of phases ofthe types, A(B,,-.B'.)8(l)Sh(O), is such that with increasing AGN 01' number oftotal dos electrons in alloys the equilibrium phases havean increasing proportion of h-Iiks packing in the stacking sequence ranging from 0% to 100% h-líke.

lI'RODUCI'ION diate phases, their structures and stability limits, lattice parameters, reduced axial ratios, average atomic volu- In recent years many binary transition-metal sys- mes, and the correlation between the structures and the have been scanned for intermediate phases at eer- percentage of the h-like packing of the structures for the - compositions [1, 2] or have been studied over the following pseudobinary systems: TaRba - TaNis, Talr, - plete composition ranges [3-8]. It was found that for TaPtg, YPda - YAgg, YPtg - MoPtg,ZrPtg - MoPtg and SiNis e combinations of elements A(Ti, V, Cr groups) and - NbNia· Co and Ni groups), close-packed ordered intermediate occur preferentially around simple stoichiome- EXPERIMENTAL DETAILS - composition, such as AB, ~B5' A~, ABa and AB4' A on alloy chemistry and stability of transition metal The purity of the starting materials used in this ys between the metal s inGroup V or VI and Groups investigation was above 99.90% except niobium, whose VIII A, VIII B, VIII e is given by Haworth and purity was 99.82%. The metals¡ were weighed to one- e-Rothery [9]. Alloy chemistry of transition metal s tenth milligram with an analytical balance to fix the desi- reviewed by Nevitt [10], an overview ofintermetallic red alloy composition and the mixtures were melted and pounds has appeared in the book edited by West- remelted under standard-grada argon at 40 mm Hg on a k [11] and the alloy phase stability criteria in gene- water-cooled copper hearth in a non-consumable tungs- has appeared in the recent review book by Giessen ten electrode arc-melter. During the melting operation a ]. The most numerous of the close-packed phases is titanium gettering botton was used. All alloys employed d at the ideal composition ABa. The AB3 interme- in this work experinced weightlosaes les s than about 1,0 phases are also formed by elements other than percent by w ight except the alloys of the system YPda - :ransition metal combinations [10]. YAgg; silver lost was approximately 6%, and yttrium 2%. Many types of ABa intermediate phases in the com- Appropriate excess amounts of silver and yttrium were tions of various types of component metallic ele- added during the alloy preparation. The alloys, except !:lIIDts have recently been determined. Such combina- for the YPda - YA&! system, were annealed in a tantalum - ns are; rare earth-transitian metal s [13-15], rare tube electric furnace under a' dynamic vacuum of less h-common metal s [16-18], transition-comman me- than 2.5 X 10-5 mm Hg. The alloys ofYPda - YA&! system [19, 20], transition-semimetals [21], tans uranic- were annealed in a Vycor tube to prevent silver loss in the mmon metal s [22], etc. Since the finding of an off- alloys during the annealing process. Stress-relieving ichiometric AB3 - type close packed phase; Nb was done in the powdered state at the annealing tempe- 6 Rh.9(jNblO)g in the binary Nb-Rh system [5], necessity of ratures under less than 8 X 10- mm Hg vacuum and erlension of the study to the regian ofthe ternary or qua- quenched. N o sintering occured during any str.ess-relief. zemary system involved immediately adjacent to the A more complete description of the procedure used pseudobinary has arisen. The present work is a portion appears in reference [4] and the typieal heat treatments he pragram conducted in this laboratory. This paper are listed in Table 1. AII specimens (as-cast, as-annealed, escribes the experimental investigation on interme- and stress-relieved) were X-ray scanned. Crystal struc-

• Now at Instituto Venezolano de Investigaciones Científicas and Universidad Central de Venezuela, Caracas, Venezuela. Now at Department of Chernistry, Northeastern University, Boston, Massachusetts, U.S.A. 28 lA,tinAmerican Journal of Metallurgy and Materials, Vol. 7, Nos. 1 & 2 (1987) ture and lattice parameterdeterminations were made layers, triangular [2, 25] and rectangular [23, 26] orde- using a GE-XRD5 difractometer utilizing filtered CuK,. ring of ABs, were involved for the phases shown in this radiation. Lattice constants were taken form the back work. These two types of typical unit layers are shown in reflection peaks.Sharp peaks and good doublet resolu- Figure 4 and are respectively defined by ABsSh(9) and tion in the high angle region (2e > 120°) constituted the criterio n of adequate stress relief. The phase boundaries 100 were determined by extrapolation of the second phase amount from the X-ray charts obtained from the speci- mens stress-relieved at elevated temperatures. The metallographic examinationsofthe specimens ofTaRh3- TaNis and Talrs - TaPta systems were used as a cross- reference. The specimens are etched in conc. HCI solu- tion for 3 to 10 minutes, depending on the compositions. The specímens of composition grater than 5Qatomic per- 14.&0° "'1. cent TaN~ oí the former system wereetched electrolyti- 14.00 1••• cally with 50% HCI from a few seconds to two minutes, 1::1.00 ii 13.00 ~ o • > decreasing etching time as the TaNis content increased. 12.50 ~ (lJ¡,l~i:m 12.00---1 RESULT AND DISCUSSION 1.740 I.no~--<>-----o----o-__ --Ó- Of the six pseudobinary systems studied, the follo- I 1.660 wingthree systems, YPts - MoPts, ZrPts -MoPts and SiNig - NbNis, do not form intermediate phases. The extent of . !I:l (CAIl ¡~~~F-='::=::"::=-=~=-:::':--=--::::;-~-=:;, solubility limits of all phases including the terminal pha- , , ses, their lattice parameters, average atomic volumes, 1.610 (CllIlr' ,.,...- and close-packing characteristics of the rest of the sys- e l1.600 tems are presented in Figures 1, 2 and 3. The detailed crystallographic data ofthe phases are shown in Table 2, 3 and 4., All encountered intermediate phases, A(Bt-xB'x), between the terminal phases, ABgand AB's, (0\1%10 100 are characterized by the close-packed ordered AI3g- type (AIIN132 35 structures [10,23]. The structures of all the phases were TaRh3 TaNI3 composed by the combination of ordered close-packed layers and stacking schemes of these layers. Among the Fig. 1. TaRha - TaNia pseudobinary phase diagram and Plots of many possible types oí layers for ABs as described by reduced axial ratios, average atomic volumes and % of h-líke packing densities of Ta(Rh1-xNixls alloy phases vs. composi- Beattie [24],two different types of ordered close-packed tion and AGN.

TABLE 1

SCHEDULE OF TYPICAL HEAT TREATMENTS

'1) TaIrs - TaPts System, 1600 °C - 1day } 1200 °C _ 2 days followedf;by quench and Stress relieve, both 15 mino

;2) TaRhs - TaNis System, 1200 °C - 1.5 day } 1000 0C_ 2 days followed by quench and Stress relieve, 1200 °C (15 min), 100°C (20 mín).

3) YPda - YAga System. 700°C - 3 days (in Vycor tu be) } Added extra: Stress relieve 700°C (30 min).· 6% Ag to compensate 2% y the melt loss

4) SiNia - NbNis System. 950°C - 2 days and Stress relieve 15 - 20 mino 5) ZrP1:.g- MoPts: as cast only

6) YPts - MoPts: as cast only Revista Latinoamericana de Metalurgia y Materiales. Vol. 7. Nos. 1 & 2 (1987) 29

\ 100 \ \ ~ \ r 150

ol------L...-

1.90 '·'5 t 1.10 ~ ~ 1.75 ! 1.70'----- ·11.11

(%1,"74~!r.150 _,..--. .<)00 ...0-o- _

B J 1.7Z0 '& lE ;¡. 1.170-- ~ (101'1.110 - i J..IIIO- ~ 1.140· : 1.150

TOIOI (AI%) o 14o-DS)tllCt,0.. n YPd, o 10 o 'o 40 110 lO 70 100 !2 52.1 ".! ".1 11 Fig. 3. YPds-YA~ pseudobinary phase diagram and plots of redu- Talr3 TOP'3 ced axial ratios, average atomicvolumes and % of h-like pack- ing densities of Y(Pd1-xAg,.).¡ alloy phases vs. composition ~ - TaRt¡¡ pseudobinary phase diagram and plots of redu- and number of total (4d-5s) electrons. eed a;ria] ratios, average atomic volumes and % of h-líke pac-

o densities of Ta(Irl-xPt.>s alloy phases vs. composition AGN. represent the composition, the number of close-packed layers per unit cell, and the shift density in the unitlayer, .&II:iplIIllUJ2) type layers, The Sh(1f2) represents the shift respectively. Structure factors employed in indexing the • ~;ruISpl)Sition of [100] rows of atoms by half of lattice phases were calculated through FunltceU= FlayerXF.tacking. _a;mt from the hexagonal Al3gSh(O) close-packed Generally, structures with shift density zero are hexago- nal, rhombohedral, or cubic (for l = 3), structures with over-all structure can now be represented by a Sh ;a!Ó O) are orthorhombic, monocliníc, or tetragonal .:r.Ilftnre symbol, ~Bn(l)8h(Z); where ~Bn' l, and Z (for l = 3). .

TABLE2A CRYSTALLOGRAPHIC DATA FOR Ta(Rhl_xN~>S Cry8tal SY8tem No. of Atoms Percent of and Type of per Layer Structure h-coordinate Composition Space Group • Structure Unit Cell Symbol Atoms Limits (x)% (S} Cubic L~-AuCU8 4 ABs (3)Sh O O 0'"'-'22±3 ~-Pm3m .•ff. is (6) Hexagonal PuAI (VCO ) 24 ABg (6)Sh O 331 34±2'"'-'38±2 s s 3 D~h-P6m2 Hexagonal D024-TiNia 16 ABg(4) Sh O 50 57 ± 2'"'-' 58 ± 2 D:h-P6g/mmo Rhombohedral f3-Ta (Pd, Rh)g 36 ABg (9) ShO 6~ 66 ± 2'"'-' 67 ± 2 3 D~-Rsm Ordered Sm Hexagonal D019-MgCds 8 ABg (2) ShO 100 79 ± 2'"'-' 83 ± 2 D~h-P6g/mme 1 (28) Orthorhombic no.-nc», 8 Al3g (2) Sh~ 86± 2'"'-' 94± 2 014-Pmnm 1.2S) Monoclinic f3-NbPta 48 AB3 (12) 8h~ 97 ± 2'"'-' 100 ~-P2um 30 LatinAmerican Journai o/ Metall~rgy and Mtüerials, Vol. 7, Nos. 1 & 2 (1987)

TABLE2B

LATTICE PARAMETERS, REDUCED AXIAL RATIOS AND AVERAGE ATOMIC VOLUMES OF THE ORDERED CLOSE-PACKED INTERMEDIATE

Ta(Rh1-xNix)a PHASES

Lattíce Parameters Á Atornic Vol At. Peto Phases 30 h:> Co (::) (V~~1 (::) (~ TaN\¡ a-TaRha (3) 3.8443 1.632g 1.732¡ 14.202,¡ 10.00 1.628g Ta (Rh, Ni)s (6) 5.3786 13.1381 1.732 13.715 35.00 Hi24¡¡ Ta (Rh, NOs (4) 5.32~ 8.6454 1.732 13.2491 57.50 1.625 1.732¡ Ta (Rh, Ni)a (9) 5.2886 1~.341s 3 13.0134 67.50 Ta (Rh, Ni)s (2) 5.240g 4.292g 1.638g 1.732 12.76~ 80.00

Ta (Rh, Ni)g (2S) 5.164¡ 4.5658 4.253¡ 1.6472 1.61~ 1.768 12.538s 90.00 * f3 - TaNig (128) 5.11 4.54 25.50 a = 90° 38'

• from reference 7.

TABLE3A

CRYSTALLOGRAPHIC DATA FOR Ta (Ir1-xPt,.)a

Grystal System No. of Atoms Percent of and Type of per Layer Structure h-coordinated. Gomposition Phnsee Space Group , Siructure Unit Gell Symbol Atoms Limits (x)% a-Talr3 (3) Cubic L~ -. AuCu3 4 ABa (3) ShO O 0'Ü17±3 ~-Pm:im Ta (Ir, Pt)3 (6) PuAl (VCO ) 1 34±2'V36±2 Hexagonal a s 24 AB3 (6) 8hO 333 D~h-P6m2 'fa (Ir, Pt)3 (9) 66~ 56±2'V58±2 Rhornbohedral f3-Ta (Pd, Rhh 36 ABs (9) 8hO 3 D~-~m Ordered Sm ,i ~: 1 Tá'(Ir, Pt)3 (28);: Orthorhornbic .DOa - 'l)C\1s 8' ABa (2) Sh"2 78 ± 2'V 89 ± 2 .'. i•• D~~-Pmnm- f3-TaPt.J (128) Monoclinic ~ f3-NbPt.J 48 ABg (12) Sh~ 94 ± 2'V 100 Cih-P21/m .. ~.. ;.

l'ABLE 3B

LATTICE PARAMETERS, REDUCED AXIAL RATIOS AND AVERAGE ATOMIC VOLUMES OF THEORDERED CLOSE-PACKED INTERMEDIATE Ta (Irl~xPt:)3 PHA8ES

Lattiee Param'eterdA¡ Atomjc Vol. At. Peto Phases 30 h:> Co (~~~) TaPt,¡ (::) (::) (~ 'a-Talr, (3) 3.893g 1.633 .v • : 1.732¡ 14.758g 10.80 Ta(Ir, Pt)3 (6) 5.533g 1.732¡ , 13.5111 1.6275 14.9298 38.00 Ta(Ir, Pt)3 (9) 1.624 5.5607 20.3264 4 1.732 15.1201 55.00 Ta(Ir, Pt)s (28) 5.560 4.84.5 " 4.53~ 1.63~ 1.62~ 1.742¡¡ 15.274g 80.00 Ta(Ir, Pt)3 (28) 1.742g 5.556 4.8414 4.5518 1.63~ 1.6284 15.3045 88.00 * f3-TaP~ (12S) 5.537 4.869 .23.33 . -. a=90~32.4'

J ~.;. • From reference'6, Revista Latinoamericana de Metalurgia y Materiales, Vol. 7, Nos. 1 & 2 (1987) 31

TABLE 4A I CRYSTALLOGRAPHIC DATA FOR Y(Pd1-xAgx)g

/ Grystal System No. of Atoms Pereeni of cmd Type of per Lauer Structure h-coordinated Gomposition Space Group Structure Unit Gell Sumbo! Atoms Limits (x)%

YPdg (3) Cubic LIz-AuCua 4 ARa (3) ShO O O"V 16'± 3 ~-PmSm y¡ d, Ag)8 (4) Hexagonal D02CTiNis 16 ABa (4) ShO 50 36±2"V38±2 D:h-P6s/mme (pd, Ag)s (2) Hexagonal D019-MgCds 8 ABs~) ShO 100 64 ± 2"V 77± 4 D~h-P63/mmc

TABLE 4B

LAl"rICE PARAMETER, REDUCED AXIAL RATIOS AND AVERAGE ATOMIC VOLUMES OF THE ORDERED CLOSE-PACKED INTERMEDIATE Y(Pd1-xAgx)3 PHASES

o Lattice Parameters (A) Atorrljc Vol. At. Pct Phases ao ~ Co ~} (V~~1 (~} (AJ YAgs YPd3 (3) 4.074 1.633 1.732 16.905 0.00

Y(Pd, Ag), (4) 5.7995 9.568 1.6498 1.732 17.418 37.00 Y(pd, Ag)3 (2) 5.937 4.936 1.663 1.732 18.833 70.00

k = 3n and k rt= 3n, respectively, for orthorhombic and O • monoclinic structures. The indexed patterns were com- • Q.A3. pared with theexistíng data for assurance. The typical O b O O X-ray patterns are presented in 'I'ables 5 and 6.

• O\~· In indexing the orthorhombic 2-layer Sh(l/2) pha- O c;JbO ~ ses, Ta (RhlONi90)s and Tá (Ir16Pts4)s whose basal plane A2,..1 I (stacking layer)ofthese structures has rectangularcrys- • (j- tollagraphic symmetry, the patterns were first tentati- vely indexed on the disordered hexagonal close-packed (A): AB3Sh(O) basal plane (8): AB3Sh( ~) basal plane cell with the exception that the basal plane refections were splít into a doublet or triplet, In order to account for g. 4. (A): Relation between ordered hexagonal cell and orderea the weak diffraction lines.as superlattice lines an orde- orthohexagonal cell of the close-packed ABs-type triangular red atomic arrangéments is assumed. 'This arises from basal planeo (B): Relation between imaginary ordered hexa- the fact that if one neglects a small orthorhombic distor- gonal cell and ordered rectangular (orthorhombic) cell of the close-packed ABa-type rectangular basal planeo Atoms in tion, this structure can be indexed on the basis oí a two- [100] rows are shifted by A1/2. layer hexagonal cell indentical to the MgCd3 structure. The tentative hexagonal indices were thus transformed into orthorhombic indices according to the transforma- Structure factors for the .ordered close-packed tion matrix (Fig. 5). yers can be given: For layer ABsSh(O),by h, k = 2n; Fbyer= (fA + 3fB) and h, k rt= 2n; F'ayer= (fA - fB), for yer ABsSh(1/2), by (h/2 + k) = 2n; Flayer= (fA + SfB) and (h/2 + k) rt= 2n; F¡ayer= (fA - fB). F.tackingfor hexago- nal structures is given for the two cases: (h- k)hexago. ,.! = 3n and (h - k)hexagona¡rt= 3n. This corresponds to 32 LatinA71UJrieanJournai of Metallurgy and Materials, Vol. 7, NOB.1 & fJ (1987)

TABLA5 TABLA6

X-RAY DIFFRACTION PATTERN OF X-RAY DIFFRACTION PATTERN OF Ta(Rh, Ni)s (9) Sh(O) Ta (Rh, Ni)s (2S) Sh (1/2) AT 67.5 ATOMIC PERCENT TaN~, AT 90 ATOMIC PERCENT TaNis, CuK,. RADIATION CJlK,.RADIAT!ON o ao = 5.164r A, bo = 4.565s A, Co = ~.253¡ A lob. 2 (hk'l) Sin~(}ob•. Sin (}calc. arbitrary 100. 10.4 .0541 .0525 4 (hkl) Sin'9ob1 Sin'9coú. arbitrary .0609 6 10.5 .0682 .0681 7 010 .0284 .0285 5 U.O} { .0850 110 .0509 .0508 3 10.6 .0854 .0855 7 101 .0553 .0551 11 10.7 .1062 011 .0615 .0613 11 20.1 .1148 .1149 11 111 .0831 .0836 4 20.2 .1195 .1197 7 200 .0886 .0891 9 00.9} {'1287 020 .1142 .1140 13 10.8 .1289 .1300 66 210 .1173 .1176 18 20.4 .1392 .1375 31 022 .131a .1314 27 .1469 14 120 .1363 20.5 .1534 .1530 '30 021 .1466 .1468 38 10.10 .1872 .1872 4 211 .1501 .1504 70 -20.7 .1915 .1912 6 012 .1602 .1599 3 21.1 .1997 .1999 3 121 .1691 21.2 .2047 112 .1822 11.9} .2146 {'2137 220 .2036 .2031 2 20.8 .2150 13 202 .2203 .2205 6 10.11 .2206 31O} {'2290 21.4 .2227 .2225 3 301 .2328 .2333 6 21.5 .2385 .2380 3 221 .2365 .2359 9 30.0 .2547 .2550 2 022} {'2454 20.10 .2727 .2722 9 212 .2490 .2490 21 21.7 .2762 030 .2566 10.13 .2968 .2968 2 311 .2618 21.8 .2987 .3000 2 122 .2677 20.11 .3070 .3055 3 130 .2787 031 .317 .2894 103 .3174 .3179 5

aorth The plots of average atornic volumes per atom for o each of the phases behave differently in the three pseu- Di,ordered HexoQonol Axea dobinaries (Figs, 1,2 and 3). The TaRhg-TaNig system Al, A2, A" e exhibits almost a straight line, praetieally equivalent to o Vergard's Law. All the intermediate phases of Talrg- Ordered Orthorhombie Alte, TaPl{¡ show a slight positive deviation from the Ver- bOrth 0, b, e gard's. The last system YPd3-YAgg, however, seems tú deviate negatively. In orderto compare the close-packed Flg. ó. Axial transformatíon scheme from disordered hexagonal eell struetures, all the structure cells are reduced to the same (Al. A2• As. e) to ordered orthorhombic cell (110. 4:.. ~) in the type ofunit pseudo-orthorhombie eell through the simple ordered close-packed ABa-type rectangular basal planeo geometrical normalization procedure (Figs. 4A, B). In all three systems the reduced axial ratios (b/a), of the ABgSh(O) phases are al! near the ideal value, that is (b/a), = 1.732.As the basal plane ehanges from Sh(O) to Sh (1/2), the reduced axial ratios (b/a), are increases. By doing this alllines could be indexed. The exten- Howe ver, the redueed (e/a), deviate negatively through sive diseussion on this matter ean be found in ref. [23]. the 9-layer Sh(O) in both TaRhs-TaNi3 and Talrs-TaPl{¡ (b/a)reducedratios for Ta (Rh, Ni)s (2) Sh (1/2) and Ta (Ir, systems. In passing over into the Sh (1/2) close-packed Pt)8 (2) Sh (1/2) are respeetively 1.768 and 1.742. This plane, the curvesstart to climb again as does (b/a); To indicates an expansion in the b direction compared to the see the deviation of the cel! more clearly, the (e/a)r were ideal orthorhombic cell; (b/a)reduced= bo/(ao/2) = V3= calculated by inserting an ideal axial ratio in the basal .= 1.732 plane as; Revista Latinoamericana de Metalurgia y Materiales, Vol. 7. Nos. 1 & 2 (1987) 33

(b/ a), = V3, that is king sequence has been observed. The stacking sequence is defined by the sequence of the occupation of the three available tetrahedral sites in the hexagonal unit cell, with a repeat distance of llayers in the e direction. A des- .cription of close- paeked stacking sequences can be facili- Carnr.aril' y, they deviate negatively from the ideal values tated by applying the coordination polyhedra in sub- 1 and 2). It seems that they are compensating for sequent layers. Eaeh A-atom is surrounded by a com- a r expallsion. This indicates an expansion in the b plete shell of twelve nearest-neighbor B-atoms. All stac- ion in theABa (l) 8h (1/2) structures. In the lastsys- king variants of both layer types (c-like and h-like) pro- YPdg-YAga, however, the ratio, (e/a)¿ inereases duce a coordination number (CN) 12 polyhedron around YiDearily instead of decreasing as those of the first two the A-atom which is comprised entirely of b-atoms, for- :!!1stems have shown. At above about 77 ± 4 atomic per- ming the two different coordínatíon polyhedra. If the oí YAgg unknown phases were encountered. X-ray atom has an fcc environment (three layers), the polyhe- pa;tterns at 90 and 95 atomie percent were almost alike. dron is the c-type; if it has an hcp environment (two difficulty was expected in the region, sinee an layers), the polyhedron is the h-type. Since only c- and h- e:l:]pec:ted stoichiometric YAga compound was not obtai- types are possible, the stacking sequence can be expres- [28]. Compositionofthis phasewasfound to be about sed as a sequence of coordination polyhedra. In the - atomic percent les s than the expected value and it ABs-type close-paeked phases the different ways of stac- assigned as YAgx in the Y-Ag binary phase dia- king these two, e and h, result in different structures, [28]. gíving a eharacteristic fractional or percentage value of h-type stacking to each structure. This proportion of h- Equilibrium ABa ordered phases with l = 1,2,3,4,5, coordinated atoms could play an important role in ex- 7.8,9, lO, 12 and 15 stacking sequences are presently plaining the sequence of occurrence of ABa-type phases. wn. Table 7 lists the stacking sequence and the per- The phase sequences and the percentages of h-like pac- tage of hexagonal stacking character of some ABs king of the structures based on the hexagonallayers are es. Exceptfor the six-layer structure, a single stae- thus as follows:

raRhg (3) .(6) .(4) .(9) ,(2) --'(28) ---+{J-TaNis (128) 0% 33 1/3% ,50% ,66 2/3% ,100% h-like f3-TaPtg (128) TaIra (3) (6) ,(9) ,(28) 0% ,33 1/3% ,66 2/3% h-like a-YPda (3) .(4) ,(2) ,YAgs (X) 0% ,50% ,100% h-like

TABLE 7

THE 8TACKING 8EQUENCE AND THE PERCENTAGE OF HEXAGONAL CHARACTER OF 80ME ORDERED ABs CLOSE-PACKED PHASES

% 01 h-like N° of Close-Packed Description of some possible stacking modes characler of crystal structure Layers per Unit Cell (Notqtion. with ABC) (Notation with h and e) the strueture lattice type

21ayer AB hh 100% Hexagonal MgCds 31ayer ABC cee 0% Cubic AuCus 41ayer ABAC chch 5~ Hexagonal TiNis 6 layer ABCACB hcchec 33~% Hexagonal VCOs (6) layer ABABAC chhhch 66~% Hexagonal 9 layer ABCBCACAB hchhchhch 663 Rhombohedral {J-Ta (Pd, Rh)g 10 layer ABCBABACAB hchchhchch 60% Hexagonal roTa (Pd, Rh)s 121ayer ABCACABCBCAB hcchhcchhcch 50% Monoclinic {J-NbPtg 15 layer ACABCBABCACBCAB chcchehcchcheeh 40% Rhombohedral HoAls 34 LatinAmencan Journal of Metallurgy and Materials, Vol. 7, Nos. 1 & 2 (1987)

From the above it is noticed that, the sequences of the an homologous series Pb3+2nSbg~5+2n(n== 0,1,2,3) [30); equilibrium phases with the structures with Sh(O) basal Füloppite Pb3Sb8~Ó (n = O) [31], Plagioníte Pb5Sbg~7 plane have an increasing proportion of h-like packing in (n = 1) [32], Heteromorphite Pb7Sbg~9 (n = 2) [33) and the stacking sequence, ranging from 0% to the saturated Semseyite P~Sb8S..!1 (n = 3) [33). These monoclinic sul- values of 100% h-like. In all systems shown above, one or fosalts are composite structures built of domains with more phases are absent, but the characteristic sequence structure closely related to Galena, PbS, like rocksalt based on the h-like packing density is maintained. The structure type [34]. Succesive rnembers differ only in the transformation: of the close-packed basal plane from width of the rocksalt unit. Slabs of rocksalt structure of type Sh(O) to Sh(l/2) is, however, not directly related to two atoms in thickness are oriented parallel to {H2l clea- the saturation of h-like packing. N o layer structure with vage plane in the structures. Addition of Pb and S atoms transition types between these two extremes [24, 29] changes the composition in accord with the homologous exists in these systems. Crystal structures which are series and serves to increase the length of the chain-like based on two typical different basal planes, A(B1-xB'x)3 asymmetric unit and hence the width of the rocksalt-like (9) Sh(O) and A(B1-xB'x)3 (12) Sh (1/2) are presented res- rib bono This process extends one of the axes of the struc- pectively in Figures 6 and 7. ture with n [32, 34).

I I I I I III I I

h

e

Q.) h (.) c:: Q.) ::3 h o- Q) (J) e 01

..la::·S h (.) e CJ h (J)-

e

h a ' \ \ \ \ ...... ---'·b' \ \ . \ • A atom O 8 or 8' ato m _ A atom o O 8 or 8'atum .OinX=O,a .Oinx=+~· Oinx=+~

Fig. 7. (100) projection oí A(~'-xB'x}¡¡ (12) Sh (1/2) Structure. ex). -O in X=l,O •0 inX=+t 00 in X=±t TiCIl3,Ta(Rh, Ni)s (2S) and Ta (Ir, Ptls (2S); atomic sizes are not drawn to scale. Fig, 6. (11.0) Projection of A(B¡-xB'x}g(9) Sh(O)strueture, The struc- ture is characterized by 66 2/3% h-like paeking with stacking sequence hehhehheh --- All equilibrium diagrams (Figs. 1, 2 and 3) are drawn with the number of dos electrons (or average ex). Ta(Rh, Nih(9), Ta(Ir, Pt)s(9) and Ta(Pd, Rh)g(9); atomic sizes are not drawn to seale. group number, AGN, in the transition metal pseudobi- nary systems; TaRhs-TaNis and TaIrs-TaP~) increasing from left to right. In comparison between the systems, It should be of interesting for readers to observe the TaRha-TaNig and TaIrg-TaPtg, the solubility limits ofthe similar structural characteristics that two lattice cons- corresponding phases do not occur at about the same tants remain almost invarient while the third changes composition even though these two systems employ the with composition in some sulfosalt minerals in the PbS- identical arrangements of AGN. The AGN is not able to

Sb2Sa system. A series of four Pb-Sb sulfosalt mineral s of directly predict the stability ranges of the particular

Plagionite Group, xPbS-4Sb2Sa(x = 3,5,7,9), eonstitute A(B1-xB'x)s phases unless certain variations of AGN are lh:.::lri!Ues. Y01. 7, _-os. 1 el: 2 (1987) 35

C!!{IT'¡:eJa¡;io' TI be- on the formation of ABs(3)S phase from other ABs(l)S and iÍH~A.(i.·':fnrllÍle'phases, the phases are referred to Ref. [35]. re of phases based OD e close-packed layer type . such that the equilíbrium phases have an increa- REFERENCES roportion oí h-Iike packing in the stacking se- re, ranging from 0% to 100% h-like, with increasing 1. P. Greenfield and P. A. Beck: Trans. TMS-AIME, 206 (1956) 265. GN oí the phases. In the system, TaIra- TaPl:g, the O) and (2)Sh(O) phases are absent comparad to 2. A. E. Dwight and P. A. Beck: Trans. TMS·AIME, 21.5 (1959) 976. oí the system, TaRh3-TaNia, the characteristic ence of the phases is still maintained. This result is 3. W. H. Ferguson Jr., B. C. Giessen and N. J. Grant: Trans. TMS- AIME, 227 (1963) 1401. ting to compare with the Hume-Rothery's corre- .on [9] between the occurrenee of different phases and 4. B. C.Giessen, Harma Iback and N. J. Grant: Traná. TMS·AIME . 230 (1964) 113. e AGN in the transition metal binary systems. But in 5. D. L. Ritter, B.,e. Giessen and N. J. Grant: Trans. TMS·AIME, - last system, YPds Y.Ags, silver is not a transition 230 (1964) 125u, 1259. and palladiurn has a closed 4d-electron shell. 6. B. C. Giessen and N. J. Grant: Acta Cryst. 17 (1964) 615. _-evertheless, the number of total electrons in d-s shells 7. B. C. Giessen and N. J. Grant: Acta Cryst, 18 (1965) 1080. the phases increase from left to right on the diagram 8. B. C. Giessen, R. H. Kane and N. J. Grant, Trans. TMS-AIME, 233 d tbe sequence of the A(Bl-xB'x)a (l) Sh(O) phases is, (1965) 855. . maintained as those of the rest of the two systems stated 9. C. W. Harwoth and W. Hurne-Rothery, Phil. Mag. 3 (1958) ahove. From the results obtained, although it is tenta- 1013. . e, it may be concluded that the sequence of occurrence 10. M. V. Nevitt, in Electronic Structure and Alloy Chernistry oí the close-packed ordered intermediate phases of the Transition Elernents. Ed. P. A. Beck, Interscience Pub., New York types, A(Bl-xB'x)s (l) 8h(O), is such that with increasing (1963) . .AGN or number oí total electrons in d-s shells in alloys 11. J. H. Westbrook (Editor), Interrnetallic Compounds, John-Wiley e equilibrium phases have an increasing proportion of & Sons, New York (1967). -like packing in the stacking sequence, ranging from 0%. 12: B. C. Giessen (Editor), Developrnents in the Structural Chernistry to 100% h-like. oí Alloy Phases, The MS oí AIME Proceedings, Plenurn Press,. New York (1969). Effect oí cold work and annealing treatments on 13. A. Raman, J. Less Cornrnon Metals, 26 (1972) 199. structural modifications from the ABa(l = 2 and 12)8 14. K. H. J. Buchow, J. Less Cornrnon Metals, 26 (1972) 329. phases to the TiAIg - type ABs(3)S phases have further been studied recently [35], where S stand s for the Sh(l/2) 15. C. A. Polly and K. N. R. Taylor, J. Less Cornrnon Metals, 27 (1972) 95. (ayer. The cold work (cw) changed the structures as 16. J. H. N. van Vuchtand K. H.J. Buschow, Philips Res. Reports,29 follows (1971) 49. . 17. E. Veleckis, R. V. Schabaske, 1.Johnson and H. M. Feder, Trans. TaNig(2) S TaNi3(3)S TMS-AIME, .239 (1967) 58. Ta(Rho.l N io.9M2)S Ta(Rhol Nio.9M3)S 18. Gonde Kiessler, Erich Gebhardt and Siegfried Steeb, J. Less Com- rnon Metals, 26 (1972) 293. and 19. Tilo Godeeke and Werner Kóster, Z. M;etallkde., 63 (1972) 422. TaNi (3)S 20. Yasuaki Nakagawa and Torniei Hori: Trans, Jap. Inst. Metals, TaNi3(12)S s 13 (1972) 167. TaPl:g(12)S TaPl:g(3)S 21. B. Rawal and K. P. Gupta: J. Less Cornrnon Metals, 27 (1972) 65. while the annealing changed as such 23. S. Saíto and P. A. Beck, Trans. TMS-AIME, 215 (1959) 933. TaNis(l2)S TaNis(2)S 24. Harry J. Beattie Jr.: Intermstallic Compounds, J. H. West;:. brook, ed., JohnWiley & Sons, New York (1967). TaPl:g(12) S TaPl:g(2)S 25. F. Laves and H. J. Wallbaum, Z. Krist, (A) 101 (1939) 78. 26. G. Kurdjumov, V. Miretskü and T. Stelletskaya, J. Phys. USSR. More detailed study on TaNia phase revealed .the 3(1940) 297. following phase relations with treatments [35]: 27: Saíto Shozo: Acta Cryst., 12 (1959) 500. ,28. E. GebhardtM. V. Erdberg and U. Lüty, in Nuclear Metallurgy,J. P. Waber et al ed., Vol. io, AIME, IMD Rept. N° 13 (1964). 29. K. Schubert, Kristallstrukturen Zweikornponentiger Phasen, Springer-Verlag, Berlin (1964) .. TaNi3(2)S ...-__. TaNis(3)S 30. S. ·A. Cho and B. J. Wuensch, Nature, 225 (1970) 444. 31. E. W. Nuffield, Prograrn Abstr. Surnrner Meet. Am, Crystallogr. anneal cw cw Assoc., University Park, Pennsylvania (1974) 270. 32. S. -A. Cho and B. J. Wuensch, Zeit. Kristallogr. 139.(1974) cw anneal cw 351. . 33. J. J. Kohatsu and B. J. Wuensch: Acta Crystallogr. B 30 The stabilization oí the phase TaNia(2)S on annealing (1974) 2935. was attributed to the surface contamination by oxygen, 34. B. J. Wuensch, Sulfide Mineralogy, vol. 1, P.H. Ribbe, editor, which acts as electron acceptors, The three phases, Mineralogical Society oí América, Washington, D.C. (1974) W· TaNis(12)S, TaNia(3)S and TaNia(2)S, are thus designa- 41. ted as TaNia, TaNis-cw and TaNigOx' Detailed conditions 35. B. C. Giessen and N. J. Grant: Acta Metall. 15 (1967) 871.