Journal of Research of the National Bureau of Standards Vol. 62, No.1, Janua ry 1959 Research Paper 2925

Phase Equilibrium Relations in the Binary System Lead Oxide-Niobium Pentoxide Robert S. Roth

The phase equilibrium diagram for the binary system lead oxide-niobium pentoxide has been construct.ed from observations of fusion characteri s tics and X-ray diffraction data. The system contains six binary compounds with PbO : Nb20 , ratios of 3: 1, 5: 2, 2:1,3:2, 1:1, and 1: 2. The compound PbO·NbzO, was found to melt congruently at 1,343° C and have a stable, reversible phase t ransformation temperature from t he low­ temperature rhombohedral form to the high-temperature tetragonal form at 1,150° C. , The 5: 2, 2: 1, and 1: 2 compounds melt congruently at 1,220°, 1,233°, and 1, 337° C, respec­ t ively; the 3: 1 and 3: 2 compounds melt incongruently at 985° and 1,233° C, respectivcly.

1. Introduction Jb20 5- High-purity grade niobium pentoxide, over 99.7 percent. Speetroscopic analyses showed A study of phase relationships in the binary 0.01 to 0.1 percent Zr, 0.001 to 0.01 percent Fe and system PbO-NbzOs has been conducted as part of a Si, and 0.0001 to 0.001 percent Ca and Mg. program of fundamental phase equilibria studies of Caleulations of weight composition were made to ceramic materials. The compounds PbO·Nb20 s and ± 0.01 percent, no correction being made for the 2PbO·Nb20 s have bee n previously reported by percentage purity of the raw materials, except for a Goodman [1] / and a phase transformation in the faetor due to weight lost on ignition. The starting PbO·Nb20 s compound has been reported by Fran­ materials, in sufficient quantities to give a 5.0-g combe [2J . Cook and Jaffe [3, 4] have studied sample, "" ere weighed to the nearest 0.1 mg. They 2PbO·Nb20 s and composiLions deficient in PbO and were then mixed by hand, tumbled for 1 hI', ground report a cubic phase to occ ur at a composition of for 1 hI' in an agate mortar, and formed into %-in.­ 3PbO·2Nb20 s. However, no systematic attempt diam disks at a pressure of 10,000 Ib/in.2. The has been made to stud y the phase-equilibrium pressed disks were fired for 1 hI' at 750 0 C on platinum relations in the entire binary system. foil in an ail' atmosphere, using an electrically­ X -ray diffraction data, together with the deter­ heated furnace wound with 80 percent Pt- 20 percent mination of the melting points of the compounds Rh wire. and of the solidus and liquidus temperatures at Following the preliminary heat treatment, th e yarious composi tions . across the system, have sup­ di sks were ground, and for those specimens selected plied data from which an equilibrium diagram has fo l' t he X-ray study new disks about 7~ in. high were been constructed. formed at 15,000 Ib /in. 2 in a ~~ -in . -diam mold and Because of the volatility of PbO, many of Lhe ex­ refired for 7~ to 1 hI' at various appropriate tempera­ periments were conducted in sealed Pt tubes. As tures between 800 0 C and 1,400 0 C. In order to PbO tends to vaporize markedly at Lhe temperature reduce PbO loss, a system of inverted Pt crucibles studied, all the data may not be exactly equivalent similar to that r eported by Jaffe, Roth, and Marzullo to Lhe true values at atmospheric pressure. How­ [5J was utilized. For those specimens which still eyer, as these experiments could not be conducted indicated appreciable weight loss, a sealed Pt tube, in air and the pressure inside the tube was relatively co ntaining a small amount of the ground, calcined small, it is felt that the data show the best available mixture, was quenched from the appropriate tem­ approach to the equilibrium conditions prevailing perature. Equilibrium conditions, except in the under atmospheric pressure. immediate vicinity of a phase transformation, could usually be obtained in the sealed tube in 10 to 15 2. Sample Preparation and Test Methods min. X-ray diffraction powder patterns were made, using a high-angle recording Geiger-counter Spec­ The following starting materials were ll sed in the trometer and Ni filtered Cu radiation, with th e preparation of samples: Geiger counter traversing th e specimen at XO /min PbO- R eagent grade yellow lead oxide. This and th ,-radiation being recorded on the chart at material was found by X-rays to contain only the 1° 20/in. ) high-temperature modification of PbO, massicotite, Specimens for solidus and liquidus determinations and none of the tetragonal r.ed lead o:A'ide, lithargite. were mixed with a few drops of a 20-percent solution Spectroscopic analyses showed 0.01 to 0.1 percent of poly1ethy1ene glycol in water, pressed into disks Co, 0.001 to 0.01 percent Bi and Tl, 0.0001 to in the same manner as for the X-ray specimens, and 0.001 percent Ag, AI, Ca, Cu, and Si and less than ground in the form of small four-sided pyramids 0.0001 percent :Mg. grooved on each side as desoribed by Geller et al. [6J.

1 Figures in brackets indicate the literature references at the end of this paper. Solidus and liquidi1s deteTminations were made in 27 the electrically heated furnace using a Pt versus forms, with an irreversible phase transformation, 'Y to 9.0 Pt- l0 Rh thermocouple and an indicating poten­ a (low to high) at 8300 C. They gave the parameters I tIOmeter for temperature measurements. The of the high-temperature form as, monoclinic, a= 21.34 specimens were placed on a Pt disk which was on A, b= 3.816 A, c= 19.47 A, and {J = 1200 20'. Magneli a support of mullite. An oxidizing atmosphere and Lagergren [9] have used the parameters a= 21.50 existed in the furnace at all times. The specimens A, b= 3.825 A, c= 20.60 A and {3 = 121 ° 45'. Holz­ were placed around the circumference of a circle, berg et al. have indexed the diffraction peak at 1 in. away from the thermocouple, maintained at 1.915Aas (712). The index of this peak should prob­ temperature for J~ hr, and allowed to cool by shutting ably be (020). Holzberg [10] has agreed that the b off the furnace. The solidus and liquidus tempera­ value of 3.825 A is probably more correct. Table 1 tures were arrived at by visual inspection of the shows the indexed X -ray pattern of high-temperature cooled specimen. The solidus temperature was Nb20 5, as found in the present study, compared with recorded as the first sign of liquid formation, as the indexed values shown by Holzberg et al. [8] and indicated by rounding of the corners of the test Magneli and Lagergren [9]. The parameters found pyramid. The temperature of complete melting, as for the material used in the present study are a = indicated by the formation of a flat button, was 21.08 A, b= 3.823 A, c= 19 .33 A, and {J = 119° 48'. recorded as the liquidus temperature. In all cases many tests had to be made to determine these 3.2. PbO two values. In general, this pyramid method was only usable PbO occurs in two polymorphic forms. The low­ for those specimens containing less than 50 percent temperature form, stable at room temperature, is of PbO. Specimens containing more than 50 percent lithargite, red PbO, tetragonal with a= 3.976 A, of PbO tended to change composition rapidly when c= 5.023 A [11]. It transforms to massicotite, heated in air and it was necessary to seal these com­ yellow lead oxide, at 4890 C [12] . Yellow PbO is positions in small Pt tubes. For specimens which orthorhombic, and the material used for the present had been heated in the sealed tubes, visual observa­ study has the parameters a= 5.48 A, b= 5.88 A, tion of partial melting of the quenched specimens c= 4.75 A. The high-temperature polymorph is could usually locate the solidus temperature. In found to exist, metastably , at room temperature when some cases, the appearance of a new phase in the quenched from any temperature in the sealed Pt X-ray pattern with different heat treatments was the tube. Even 65 hr at 4700 C, just below the trans­ best means of locating the solidus. The liquidus tem­ formation temperature, showed no sign of red PbO. peratures could probably be found only by detailed Petersen [12] has shown that it takes two to three optical microscope examination. No attempt has weeks at 4200 to 4300 C to convert the yellow lead been made to locate accurately the liquidus tempera­ oxide into the red form. tures in that portion of the system containing greater than 50 percent of PbO. 3.3. Compound PbO·Nb20 . The methods of determination of temperature lim­ its of a particular phase are subject to a number of The compound PbO·Nb20 5 has been reported to sources of error. Among these are the possible intro­ occur in four different symmetry modifications.2 duction of impurities into the specimens in the form­ The compound was first described by G. Goodman ing and grinding operations, the possibility of reduc­ [1] as ferroelectric, orthorhombic. Francombe [2] first reported a low-temperature rhombohedral phase, tion of Nb20 5, oxidatioll and/or differential loss of PbO, difference of temperature between one or more and Roth [14] showed that a tetragonal modification specimens and the thermocouple, and the inherent probably also existed. Cook [15] has indicated that a pyrochlore-type phase is present if the material is difficulty in the visual determination of the solidus 0 and liquidus temperatures. It is believed that the not heated above 700 C. temperatures as recorded are accurate to 50 C. ± a . Low-Temperature Rh ombohedral Modification Francombe [2] reported that the compound 3. Compounds Involved in the PbO- Nb20 S PbO·Nb20 5 was polymorphic, having a phase trans­ System formation at about 1,2000 to 1,2500 C. The low temperature form is not ferroelectric and was re­ 3.1. Nb20 S ported to be rhombohedral with the "simplest­ structure-cell" possessing the dimensions aR = 6.206 A Brauer [7] has found Nb20 5 to occur in three poly­ morphic forms. The H-form or high-temperature and aR = 58° 18'. However, the X-ray powder form was reported as stable at temperatures above pattern given by Francombe [2] was indexed on a. 1,2000 to 1,2500 C. In this study wherever free larger "pseudo-cubic-cell" with dimension a'R= 8.664 A and a' R= 88° 30'. Nb20 5 was found, the high-temperature form was the one observed in all heated specimens, although the 2 A paper in Russian by Efimov et al. [13] d iscusses tbe reactions involved in low-temperature form was used as the starting ma­ producing PbO·Nb,Os and reports a Cace centered cubic lattice of 5.277 A fo r tbis compound. As only an abstract oftbis paper was av a il abl ~ in English, this result terial. Holzberg, Reisman, Berry, and Berkenblit cannot be critically analyzed. However, it seems likely tbat the X-ray pattern [8] concluded that Nbz0 5 has only two crystalline giving rise to this data was of tbe pyrochlore-type structure occurring at a ratio of 3PbO to 2Nb20 s, whicb bas a param eter of approximately twice that given in polymorphs, the high- and the low-temperature the Russian work. 28 TABLE 1. X -ray powder diffraction data for hi gh temperature NbzOs

M agnoli and Lagergren [9] Holzberg et a!. [8] Present wo rk CUK C% l radia tion

d obl! 1/10 hkl- dohs I/Io hklb I /Io hkl d l /d ~alo c ----1------17.0 30 10T/~f 16.9 10 001 16.66 6 0.0036 0.0036 001 10.6 40 10.6 20 201 10. 517 5 . 0090 . 0090 20T 10, 063 2 .C099 .0099 10 1 9.7 10 102 9.7 10 102 9.615 2 . 01 08 . 0107 102 9. 2 20 200 9. 2 < 10 200 9. 148 5 . 0120 . 01 20 200 8.5 20 002 8.4 < 10 002 8.354 4 .C143 . 014 2 002 6.942 2 . 0208 . 0207 301 6. 486 3 . 0238 . 0237 102 6.35 30 103 6.32 10 -----i03 6.285 11 .0253 . 0253 103 5. 60 10 003 5.60 < 10 003 5.590 4 . 0320 . 0320 003 5.30 10 402 5.29 < 10 402 5. 273 6 . 0360 . 0361 402 5. 15 60 401 5. 13 20 401 5. 116 48 .0382 . 0384 401 4.74 < 10 103 4.734 4 . 0446 . 0447 103 4.63 50 104 4. f,3 20 104 4.616 36 . 0469 . 0469 104 3.852 6 . 0674 . 0672 300 205 3.83 20 f 3.83 < 10 010 3.821 .0685 { .0684 010 1. 010 . 0684 20" 3, 75 . 07 14 11 0 3. 75 110 70 40" { 100 3.74 50 110 3. 737 100 . 0716 . 0717 liT . 0719 400 3.65 100 105 3. 65 100 105 3.636 100 .0756 .0757 105 3.59 10 111 3.56 < 10 111 3.577 4 . 0782 . 0782 111 3.56 20 112 o 11 2 3.553 4 . 0792 . 0791 11 2 3.515 . C809 . 0804 210 .0812 603 . 08 16 212 603 } 3. 49 100 602 3. 49 50 212 3.483 100 . 0824 . 0826 505 012 . 0826 012 . 0829 602

3. 41 10 604 3. 41 < 10 c,Qi ~. 4C6 4 . 0862 . 0867 604 o 402 3.383 4 . 0874 . 0879 402 3.36 60 005/31I 3. 36 10 005 3.351 28 .C89 1 .0889 005 . 0891 31I 3.32 20 601 3. 32 < 10 211 3.316 .0909 . 0904 211 . 0917 601 3.28 < 10 213 . 0929 213 3.27 20 113 11 3 3.264 4 .0939 . 0937 113 3.247 4 .0949 . 0948 204 3. 17 40 013 3. 17 20 3. 153 11 . 1006 . 1004 013 . 1045 412 3. 09 20 506 3. 09 412 3.078 . l C55 . 1055 506 106 3.00 30 } 3.01 106 2. 994 . 111 6 . 111 3 214 { 214 . 1116 106 2.95 10 Jl4 . 1153 114 512 2. 84 90 014 } 2.84 10 512 2.832 36 . 1247 . 1249 512 2.82 10 513 2.83 10 701 2.826 38 . 1252 . 1253 0 14 . 1265 51 . 1273 70T 2.77 80 5JI 2. 78 20 5JI 2. 771 31 . 1303 . 1305 511 . 1306 601 2.73 10 514 . 1352 514 2. 70 80 215 2. 71 20 -----215 2.701 34 . 1371 . 1369 215 2. 70 10 706 . 1383 706 415 2.67 10 41 5 2.677 2.668 . 1405 { . 1403 415 207 . 1408 207 2. 65 20 510 2.64<1 . 1431 . 1431 510 . 1441 110 2.63 20 803 2. 635 115 2. 628 . 1448 { . 1444 804 . 1454 803 515 ' . 1510 2.58 10 ------{ 515 612 } . 1513 612 802 . 153(; 802 2.54 70 614 } 2.549 ]0 107 2.543 36 . 1546 { . 1515 107 503 . 1501 614 2.53 10 412 2.537 < 10 503 . 1553 503 o 404 . 1564 412 o 412 . 1565 313 .1565 404 2.53 30 01 5 2.529 < 10 313 2.523 . 1571 . 1573 015 2.49 50 707 2. 496 10 806 2.491 26 . 1612 . 1619 707 2.48 30 214 2. 483 < 10 511 2.478 8 . 1629 . 1629 511 . 1632 214 . 1637 806 2. 45 20 416 2. 457 610 2. 452 . 1664 . 1665 416 . 1688 801 S ee footnotes at end of table.

29 -_._---

TABLE 1. X-ray powder diffraction data for high temperature Nb2O;-Continued

Magneli and Lagergren [9] Holzberg et al. [8] Present work CuK«, radiation

doba 1/10 hkl" dob!! 1/10 hklb d 1/1. , hkld l/d;b. l / d~ ,dc ------.------0. 1837 71 2 . 1840 807 2. 339 20 712 2.340 < 10 903 . 1851 905 616 . 1865 903 2. 315 80 413 2.319 10 208 2. 314 29 0.1868 . 1871 413 { 208 } . 1873 616 . 1877 208 2.282 10 708 . 1925 708 2.210 20 702 .2060 702 2. 175 10 907 2. 181 814 . 2121 907 < 10 . 2128 814 2. 162 20 617 813 . 2139 813 0 . 2141 617 513 2. 111 20 414 2. 112 { 4 0.2244 . 2237 513 < 1~ 414 2. III . 2249 414 . 2316 207 2.075 80 10,0,3 2. 079 207 . 2321 . 2321 816 10 2.076 38 . 2324 10,0, 6 . 2328 315 . 2336 10, 0,3 2. 041 20 10,0, L 0 809 .2410 2.036 80 20!) 2. 037 10 604 2. 037 34 . 24 11 711 .2411 518 . 2416 209 . 2422 604 .2459 809 . 2462 10,0,L 1. 985 10 817 . 2524 817 117 1. 931 10 6, 0,m 1. 934 117 .2683 < 10 .2687 6, 0, m 118 .2730 118 020 1. 912 100 1. 915 712 1. 912 29 . 2736 020 { 901 20 . 2736 . 2738 712 11, 0, 6 } .2744 901 . 2747 11 , 0 ,~ . 2827 221 11, O. 7 1. 872 30 I. 876 221 . 2853 3,0, m { 220 } < 10 1. 873 .2852 . 2856 220 . 2862 11, 0,7 022 1.865 10 . 2879 022 { 009 } ---_ . . { . 2880 009 10, 1.4 .2890 619 1. 855 20 208 { < I ~ 208 } 1. 856 3 . 2903 .2891 811 1. 857 { . 2914 208 . 2945 10, 1. 4 1. 840 10 -·--:iiii" . 29,\8 3H"l 223 1.830 10 } ----- . 2981 223 { 123 . 2989 123 .3005 320 2, O. . 3020 10, 1,1 1.820 GO Ii'i I. 822 320 20 .3020 { 11,0, S } 10 I. 819 { .3027 2, 0, 10 . 3039 11, O. .3120 421 1.793 10 902 1. 789 16 . 3124 . 3127 407 { . 3128 222 . 3145 002 1. 762 <10 10, 0,1i] 1. 7115 .3211 . 3214 420 l. 744 10 119 1. 712 23 . 3295 . 3204 11, 0. 1 . 3302 119 . 330:1 10, 0, m .3355 4, 0, i1 1. i27 25 . 3353 . 3355 10, 1.8 { . 3357 521 .3421 1. 709 . 3424 225 10 { .3422 4, 1, j] . 3483 520 1. 694 10 520 1. 692 20 . 349" . 3493 125 10 . 3529 . 3522 1i06 1. 6~5 3, I,m 1. 683 51 { .35:17 3,1.10 1. 627 8 . 377fi . 3778 10, 0, 2 1. 590 <10 522 1. ':;92 27 .3945 . 3946 1, 1, j] .3919 522 1. 584 10 10, 1,1 l. 582 20 . 2994 . 3997 11, O. II 1. 579 18 . 4012 . 4009 721 { . 4016 026 .4031 10, 1,1

1. 557 10 904 1. 556 22 . 4130 . 4138 13, 0. 9 { . 4140 12,1, l! .4155 904

• The parameters upon which these hkl values are based are· a=21.50 A, b=3.825 A, c=20.60A, /l =121° 45'. b 'rhe parameters upon which these hkl values are based are a=21.34 A, b=3 . ~lGA , c=19.47 A, /l=120o 20'. , Only those values are listed which were either found as peaks in the present stndy or roported as peaks by previOUS workers. d The parameters upon which these hkl values are based are a=21.0S A, b=3.823 A, c=19.33 A,/l=1l9° 48' . 30 It can easily be scen that the larger "pseudo-cell" TABLE 2. X -ray diffraction powder data .rOT low temperatllT( , is r elated to the smaller "simplest-cell" by a doubling Thombohedral PbNb20 6 (CuKa [ Radiation) of the hexagonal a axis. For the "simplest-cell" aH = 6.04 A, Cll = 15 .3 94 A and for the "pseudo-cell" d l i To 2 . Rhombo- TIexagonal l /d o~ , 1/dc a. l c hed ral au = 12.093 A, cH = 15.394 A. The pa,ttern was ---- indexed by Francombe using hexagonal hkl values hkl hkl for the larger cell. However, a simple examination 7. 184 5 0.0194 0.0196 010 011 5.270 13 . 0360 . 0363 oII 110 of this indexing r eveals that many of the hkl values 4.889 3 . 0418 .0420 lIO 102 4.231 7 . 0559 . 0558 III 2Dl used are not allowed by a rhombohedral cell, where 3. 109 100 . 1035 . 1036 120 113 h- lc + l= 3n. Furthermore, the observed versus 3.034 100 . 1086 . 1088 11 2/T3I 300 calculated d spacings do not fit for many of the lines. 2.957 10 . 1144 . 11 46 121 212 The same observa tions are true for the "simplest­ 2.626 11 . 1450 . 1451 022 220 2.383 7 . 1760 . 176 1 03~Jf1 033 cell. " 2. 170 69 .2123 . 2124 223 Cook [15] has given the rhombohedral unit cell of 2. 11 6 6 .2233 _ 22~4 222 402 2.061 5 . 2353 .2354 131 025 the low-temperature form of PbO·Nbz0 5 as aR = 2.054 10 .2371 .2372 032 23 1 7.168 A and £xR = 93 ° 52'. These parameters corre- 1.9840 48 . 2541 .2539 123032 410 pond to a hexazonal unit cell of a= 10.473 A and 1. 9270 9 .2693 . 2693 222 006 1. 90 12 5 . 2767 .2769 231 314 c= 11.549 A. This cell is related to Francombe's 1. 8093 10 .3055 .3056 231 11 6 "simples t-cell" by a multiplication and rotation of 1. 7904 8 . 3099 .3097 223 501 1. 7640 100 .3214 . 32 12 041/232 143 th e unit-cell parameters of th e hexagonal cell: 1.7502 58 .3265 . 3264 033 330 1. 6477 4 .3683 .3685 0-12 242 au (Cook) all (Francombe), 1. 6263 40 .3781 . 3781 141/330 036 =-13 240 .4146 . 4144 226 Cll (Cook) = 3/4 CJ{ (Francombe). 1. 5530 7 { .4149 331 207 1. .5 156 22 .4353 .4353 224/242 600 1. 3826 10 . 523 1 .5232 150/341 146 An X-l'fl,y powder pattern of the rhombohedral form 1. 3620 18 . 539 1 . 5389 052/243 253 of PbO·Nb20 5, indexed on the basis of the cell sug­ 1. 3483 20 .5501 .5498 152 162 1. 3360 5 .5603 .5600 151 047 gested by Cook, is shown in table 2. The unit-cell 1.3336 5 .5623 .5618 342 515 parameters of this material are found to be aR = 1. 3126 13 .5804 .5803 044 440 7.184 A, £xR = 93 ° 55'; all= 10 .5 10 A, clI = 11.562 A. 1. 2955 22 .5958 . 5957 251 336 1. 2542 6 .6357 . 6359 251 138 1. 2481 5 . r,420 . 6422 342 lI9 b. Metastable Orthorhombic Ferroelectric Modification 1. 2425 4 . 6478 .6477 153 443 1. 1912 11 .7047 . 7046 060/442 066 The orthorhombic modification was the first form 1.1613 7 .74 l5 .7414 160 157 1.1536 6 .7514 . 75 10 35 1 229 of PbO·Nb20 5 to be reported. It was discovered 1. 1493 100 .7571 .7565 344/1.92 713 Lo be ferroelectric by G_ Goodman [1] who reported 1.1421 8 . 7666 .7674 154 542 . 7812 . 7810 161 058 a n orthorhombic unit cell having the approximate 1. 1314 -1 { . 78 13 335 801 dimensions a = 25 A, b= 25 A, c= 7.0 A. Fran combe 1. 1067 6 .8165 .8157 353 625 1. 0958 5 .8328 .8327 352 1,2, 10 [2] found that the ferroelectric form only occurred 1. 0 46 12 . 850 1 .8496 262 446 when the specimen had b een heated above the phase 1.0782 28 .8602 . 8598 261/450 149 transformation temperat ure of 1,200° to 1,250° C . • 'rhese values are based on the unit-cell parameters aH = IO.501 A, CJI= 11.562 H e reported this form to be orthorhombic with A, an= 7.184 A, an= 93° 55'. a = 17.51 A, b= 17 .81 A , c= 7.7 3 A. In a previous publication arising from the present work [14] the orthorhombic strlletm e in a form apparently stable X-ray powder diffraction pattern of the ferroelectric at room temperature regardless of further heat modification of PbO·Nb20 5 was indexed on the basis treatments in laboratory time. A solid solution, of an orthorhombic unit cell with a = 17.63 A, of PbO·Nb20 5 plus 2 weight percent of Zr02·TiOz, h= 17 .93 A, c= 3.868 A. Long film exposures using was examined by high-temperature X-rays at single-crystal techniques have shown that the true 600 ° C, above the ferroelectric Curie point. The unit cell actually has a C axis value of t wice that resulting pattern could be indexed on the basis of a given [15]. This larger value does not show up in t etragonal unit cell with a= 12.56 A and c= 3.925 A a Geiger counter tracing of the powder diffraction [14]. It is still not known whether the c axis should pattern. It has been shown [14] that the ferro­ be doubled for the true tetragonal cell. electric modification is only metastable, reverting High-termperature X-ray patterns made in this t o the rhombohedral form if heated several times laboratory, indicate that pure PbO·Nbz0 5 has from room temperature to 700° C. It, therefore, tetragonal symmetry, as a stable modification, has no fi eld of stability in the binary system from the temperature of the high-low phase trans­ PbO- Nb20 5• formation to the melting point. If the high­ temperature modification is cooled quickly below c. High-Temperature Tetragonal Modification t he transformation point, it will maintain the tetragonal structure in a metastable condition_ Goodman [1] added small amounts of Zr02 and/or When it reaches the Curie point, reported as 590° C Ti02 to his materials in order to obtain dense by Goodman [1], it transforms metastably and ceramics . Roth [14] has shown that the addition reversibly to the orthorhombic ferroelectric modi­ of such impurities has t he effect of producing the fication. 31 d. Pyrochlore-Type Modification TABLE 3. X -ray diffraction powder data for rhombohedral 2PbO.Nb20; (CuK"" radiation) Cook [15] has claimed that another low-tempera­ 0 ture form of PbO·Nb20 5 exists below 700 C and d I /fo Rbombo· Hexagonal that it " appears to be a pyrochlore structure pre­ lf1 !b. l/d!alc& bedral viously unreported for lead niobate of this formula." - hkl hkl Examination of the X-ray pattern of PbO·Nb20 5 7.10 3 0. 0199 ------(b) heated at 550 0 C for 16 hI' as well as that of a speci­ 6. 29 7 . 02 .~3 0.0252 111 003 3.543 4 . 0796 . 0791 030/221 303 men supplied by Cook, showed that the pyrochlore­ 3.393 3 . 0869 . 0867 130 124 3.142 65 . 1013 . 1009 222 type phase reported actually was 3PbO·2Nb20 5, and 006 that rhombohedral PbO·Nb20 5 and free Nbz0 5 3.056 100 . 1071 . 1069 222 402 also were present. It can therefore be concluded 2.667 100 . 1406 . 1405 040 044 2.438 .1657 331 405 3 . 1683 { . 1703 133 421 that PbO·Nb20 5 does not form a pyrochlore-type 2.410 7 . 1722 .1726 240 226 structure under any conditions. 2.390 5 .1751 . 1787 04.2 242 2.015 8 .2464 . 2449 342 119 3.4. Compound 2PbO·Nb20 5 l. 9064 39 .2751 .2750 440 408 1. 8664 45 .2871 . 2871 044 440 1. 6305 27 .3762 . 371\9 262 0,4,10 The compound 2PbO·Nb20 5 has been studied by 1.6053 33 . 3881 .3879 262 446 Cook and Jaffe [3,4], Shirane and Pepinsky [17], 1. 5935 18 .3938 .3940 262 082 Hulm [18], and Jona, Shirane, and Pepinsky [19]. 1. 5744 7 . 4034 . 4034 444 0,0,12 1. 5297 19 . 4274 . 4276 441 804 Cook and Jaffe [3] list the compound 2PbO.Nbz0 5 as 1. 3333 9 . 5625 . 5621 080 088 rhombohedral with a= 5.285 A and a = 89° 15'. 1. 2450 4 .6452 . 6448 662 4,0, 14 They later [4] imply that the a axis should be 1. 2282 7 . 6629 . 6629 662 8,0, 10 1. 2122 14 .6805 .6810 266 842 doubled, thus suggesting a rhombohedral distortion 1. 2028 9 . 6912 . 6905 480 4,4,12 of the pyrochlore-type structure. Jona, Shirane, 1. 1832 11 . 7143 . 7147 084 484 1.1092 3 .8128 . 8129 484 0,4,16 and Pepinsky [19] give values of a= 10.674 A, 1. 0857 6 .8484 . 8491 484 848 a = 88° 50'. The values obtained in the present 1. 0781 4 .8604 .8612 448/484 12,0,0 study are a= 1O .676 A, a= 88° 44', and the indexed X-ray diffraction pattern is given in table 3. • Tbese values are based on tbe unit·cell parameters

Shirane and Pepinsky [17] and Hulm [18] have shown aH = 14.931 A, cH= 18.893 A, an=IO.676 A, «=880 44'. a dielectric anomaly in 2PbO·Nbz0 5 at about 14 0 Ie, b This peak does not fit t be proposed rhombohedral structure, but it is repro· indicating a phase transformation, but nothing is ducible and apparently indicates a lower symmetry for the true structure of 2PbO·Nb,0,. Such a peak also occurs jn tbe patterns of the compound known of the structure of this phase. 5PbO·2Nb, O, and tbe solid solutions between tbe two.

3 .5. Compound 3PbO·2Nb20 5 2PbO·Nb20 5. The parameters observed for the 5:2 Cook and Jaffe [4] reported the observation of compound are aR= 10.709 Ai aR=88° 14'. a cubic pyrochlore-type phase at approximately " the The composition of the 5:2 compound can be composition Pb1.5Nb20 6.5" but did not definitely arrived at by analogy to the pyrochlore-type struc­ call it a compound. They observed a unit-cell ture. The theoretical cubic pyrochlore-type struc­ parameter for this composition of 10.561 A. In the ture contains 16 A ions, 16 B ions, and 56 negative present study this phase has been identified as a ions. Of the negative ions, 48 occupy one set of compound with little or no solid solution, of the equivalent positions and 8 ions occupy another. composition 3PbO·2Nb20 5 . It has been found to In the PbO·Nb20 5 system a unique compound is melt incongruently at 1,233° C to liquid plus formed at a ratio of IX Pb+2 ions to 2Nb+5 ions or PbO·Nb20 S' Pb1.5Nb20 6.5 (Pb12Nb1604s04)' On the high-PbO side of the pyrochlore-type formula a likely compound 3 .6. Compound 5PbO·2Nb20 5 is one in which 8 oxygen ions are missing, those be­ longing to the 8-fold equivalent position. This type This compound has not been previously reported. of compound would be Pb2Nb1.606 (Pb16Nb12 .S0 4S) It was discovered in the presen t work by observing which has a PbO:Nb20 5 ratio of 5:2. This com­ that compositions in the area containing a single-phase position is observed to be the high-PbO end of the rhombohedral pyrochlore-type structure did not solid solution series. begin melting at temperatures to be expected from a simple solid-solution region. It was obvious that 3.7. Compound 3PbO·Nb20 S a new compound of approximately the ratio 5:2 had to be introduced in order to explain the melting­ The compound 3PbO·Nb20 s has also not been point data. This composition containing 71.43 mole previously reported. It has a distorted pyrochlore­ percent of PbO is the high PbO end member of this type structure which resembles 2PbO.Nb20 5, except solid-solution series. The composition containing that the distortion is of a pseudo-tetragonal rather 72 mole percent of PbO shows some of the compound than a rhombohedral type. The face-centered 3PbO·Nb20 5 • The compound 5PbO·2Nb20 5 ap­ tetragonal parameters for this phase are a= 10.658 A. parently has the same symmetry as the 2:1 com­ c= 10.824 A. As can be seen in the X-ray pattern pound, probably rhombohedral. The 5:2 compound of table 4, there are some diffraction peaks which shows a larger distortion from the ideal cubic pyro­ cannot be indexed on the basis of this face-centered chlore-type structure than does the compound cell, but do fit a primitive cell of the same dimen- 32 'fA BLE 4. X-ray di:f!raclion powde?' data Jor the comp01md T A BLE 5. X -my di,O'mclion powdel' data fo ?' the compound 3PbO·Nb; Os (CuKa ! radiation) PbO·2Nb20 , (CuK",! mdiation)

d l / fo l/d~b. l ld ~a l c a hlil d 1/10 likl ' d l i To hlila --- 6. 78 7 0. 0217 ------8. 84 7 110 2. 441 041 6. 18 16 . 0262 0.0262 111 6.24 15 020 2. 3J3 250 5.886 3 . 0289 ------5. 576 15 120 2. 139 5. 611 5 .0318 ------3.948 32 001 2. 126 350 3.419 7 . 0856 . 0856 b 103 3. 85 7 6 2 073 060 3.218 12 .0966 .0966 311 3. 601 5 1. 996 5 260 3.093 100 . 1045 . 1046 222 3.528 8 1. 972 J5 002 2. 943 14 . 1151 . 1144 b320 3. 455 14 230 1. 858 7 2. 706 69 . 1366 . 1366 004 3. 333 JG 021 1. 850 .1 2.662 100 . 1411 . 1408 400 3. 2J8 55 121 1. 757 4 'J51 2. 461 13 . 165 1 . 1649 313 3. 109 5 040 I. 739 4 2. 444 8 . 1674 . 1670 331 3.067 8 1. 71 1 'I 2. 404 4 . 1731 ------3.020 100 140 1. 681 8 2. 354 5 . 1805 . 1806 b214 2. 94 2 32 330/221 1. 649 16 142 2.077 4 . 2319 . 2310 115 2.861 5 1. 637 (i 2. 051 8 . 2378 . 2374 511 2. 786 60 240/131 1. 604 15 242 1. 8989 100 . 2773 . 2774 404 2. 627 4 1. 508 4 1. 8834 54 . 2819 . 2817 440 2. 596 23 231 1. 4,10 1. 6268 58 . 3779 . :;777 226 2.520 4 1. 312 1. 6091 100 . 3862 . 381)3 622 1. 5461 41 . 41 3 . 4183 444 • The ""I "alues are given by analogy to tbO tetragonal structure of 1. 3530 8 .5463 .5463 008 hi gh temperaLure PbO·Nb,O,. This is only a pseudo·eell, as there arc 1. 3323 15 .5634 .5634 800 many peaks wbich ea rulOt be indexed on tbis basis. The pseudo· 1. 2317 37 . 6592 . 6594 626 tetragonal cell has unit·cell parameters of a= 12.44 JI , c= 3.94 A. 1. 2247 . 6667 13 .6679 662 1.2224 J3 . 6692 } 1. 2066 17 . 6869 .6871 408 congruently melting compounds 5PbO·2Nb20 5, 1.1953 17 . 6999 . 699<.J 804 1.1923 18 . 7034 . 7042 840 2PbO·Nb20 S, PbO·Nb20 S, and PbO·2Nb20 5 ; and 1. 0991 11 .8278 .8230 448 two incongruently melting compounds 3PbO·Nb20 5 1. 0912 16 .8398 .8408 844 and 3PbO·2Nb20 5 • 1. 0406 7 . 9235 . 9240 2, 2, 10 1. 0313 7 . 9,102 . 94 11 666 PbO has been accepLed as melting at 883 ± 5° C 1. 0262 16 . 9496 . 9496 10, 2, 2 [20]. There is an eutectic between PbO and 3PbO· Nb 20 S at approximately 94 mole percent of PbO and aThese values arc based on a tetrag onal unit cell wiLll a= 1O .658 A, c= 10.824 A. 835 0 C. The eutectic tempera,ture was located by b These peaks arc not allowed by a Iaee-eentered cell. X-ray difl'ra ction analyses O! quenched specim en.s from sealed Pt tubes (see sectlOll 5). ' ''Then a sp eCl­ sions. However, there are still other cl-spaeings men in the Lwo-phase region was quenched from which cannot be indexed at all on this tetragonal below the solidus, yellow PbO and 3PbO·NbzOs celL Although the patt.ern is very strongly pseudo­ were found to coexi t. Changes in d-spacings of the tetragonal, it must be coneluded that the true sym­ X-ray patLel'l1 s indicate that a sm all amount of s

488871- 59--3 33 1400 I- -

1300 I- -

1200 I- -

\ - .. - PN 2 SS \ I \ I \ - I \ I \ I \ - I \ I \ I \ I \ I \ I \ I \ \ -

FIGUR E 1. Phase equiLib1'imn diagmm f o1' the system PbO- N b20 5.

P,-yellow PbO, orthorhombic; P 3N- 3P bO·;\!b,0 ,; P,N,-5PbO·2:\b,O,; P ,;\!"-2P bO·:\b,05; P ,K,-3PbO·2Kb,0 5; T-P.'<- tetragonal PbO. :-Jb,O,; H-P'\- rllOm­ boheciral PbO·Nb,0 5; P N,- P bO·21\Tl),0 5; N - ;\!b,O,; S8- solid sol ut ion; O-no melting; e - partial melting; . - co mpletely melted .

No phase other than that of 2PbO·Kb20 5 was ob­ When these specimens 'were quenched in scaled ser ved in the X-ray pattern of a quencbed specimen Pt tubes a clear picture of a two -phase region with of the melted material, thus stron gl~T indicating co n­ little or no solid solution was observed for tempera­ gruent melting. tures of 1,220° C or below. At 1,230° C the cubic Compositions b etween 2PbO·Nb20 5 andPbO·Nb20 5 3PbO·2Nb20 5 phase greatly predominated through­ give conHieting data when examined as quenched out all the specimens from 65 to 61 mole perce nt of specimens from scaled tubes and as cer amic pellets PbO. The rhombohedral 2PbO·Nb20 5 phase was heated in air. Cook and J aff e [4] obser ved a single­ found to coat the walls of the P t tube and was diffi­ phase cubic pyrochlore-type structure at a composi­ cult to recover, thus indicating that this phase had tion corres ponding to "Pbl'5Nbz0 6'5" or 60 mole per­ been in the vapor state and that these specimens had cen t of PbO. This observation was confirmed in been above the eu tectic temperature. The eu tectic the present study for both air and sealed-tube tech­ composition could not be located exactly bu t is niques. However, Cook [21] has stated that in "a probably close to 66 percent of PbO. The eu tectic closely spaced series of compositions between Pbz- temperature was thus located at 1,225 ± 5 ° C. In Nb20 7 and PbI 5Nb20 6'5 . . . extra lines of the the scaled tube, the composition containing 60 mole rhombohedral spiitting do not draw closer to the percen t of PbO was found to melt incongruen tly at main lines of the pattern, as you would expect. 1,233° C, forming some PbO·Nb20 5. Compositions They just fade out ." A similar phenomena was of 59 and 58 mole percent of PbO contained both observed in the present study for sp ecimens pre­ cubic 3PbO.2Nb20 5 and PbO·N bz0 5 when quenched pared at I-mole-percent intervals between 65 and in the sealed tube from below the solidus. There­ 60 mole percent of PbO, when heated in covered P t fore, a compound of the ratio 3PbO·2Nbz0 5 has crucibles. been fully established. 34 TABLE 6. B xpe!'imenial data fa!' composil'ions in the binal'y system PbO- Nb20 5

Composition Experimental H eat treatment Res ults procedure

Sealed T emper· PbO Nb,O, Air Pt tu be, atnrc Time PhysiCa.1 obscrvaUon X - Ray d ilfraction analyses d Qu enched ------1------1------",ol % ",o l % ° C hr 470 30 Brigh t red _. ______...... ______.. . __ .. . Pb30 ,. X 471 65 Yellow ____ .. _____ . __ .... ______.. _. Orth.- PbO. 100 0 :: ~ :: X 487 lfo Ye ll ow_. _. ______. _.. ______Do. 1------X 798 1. 5 Yell ow_. ______. ____ . ______Do. X 900 0. 16 Yell ow, melted ______. ___ Do. 786 1 Orth· PbO+3P bO· :\h,O,. X 471 65 Orange· yellow _.. .. ____ . _. ______. __ .. _. . _ D o. 95 X 487 16 Orange-yellow _... . ____ . ______. ___ ... Do. { :: ~ :: X 798 1. 5 Do. X 832 0. 11 Yellow wi th red spots, partially meILed . _ Orth.P bO+tet.P hO". X 800 .05 Dark reddish brown, melted ______'l'et·P bO .. + trace of 3 p b O·I\I>,0 5 and orth·P bO, 94 6 { :::::: e X 824 16.00 Black-metall ic Ju ster, grinds to yellow- OrL h.P bO+3 pbO,l\b,O, (no teL . P hO ..). ish brown. 93 7 X 860 0. 03 Reddish brown , melted _.. . ______.. _. T et· pbO,,+ 3PbO· Nh,O,. 92 8 X 860 .03 Reddish brown , melted . __ . ______Do. 90 10 750 I Orth. PhO+ 3PbO·l\b,O,. { .- ~ -. X il60 .05 M ostly melted, vesic ular. ______. ______Tet. P bO.. + 3PbO· :\b,O,. 786 I 3PbO·Nb,O,+orLh .P bO, .'{ at melteeL. ___ ..... ______. . __ .. 85 15 ··x·· 830 . 08 Do . {:: ~ :: X 840 .08 Some melti ng ___. ______.. _ T et·Pbo" +3P bO·Nb, O,, X 978 . 08 Mostly melted , vesieular ..... _. ______Do . 80 20 X 78H I 3PI>0·Nb,O,+orLh. P bO. X 753 I 31' bO·l\b,O" X 870 I Do, X 900 I D o. X 920 l Lost weigh I,. _ __ ••• _••••• _._ • ______• __ • 3PbO·Nb, O,+5P bO·2:\b,O" X 950 J :Lo, t weighL. . __.. . ___ __... ______. ___ D o. i5 25 X 1, 008 l Lo~t weighL. ______. ______. _. ___ 5phO·2Nb,0 ,. X 951 0.25 No mc!U ng______31'bO·Nb,O• . X 980 . 16 )Jo melti ng______. ______Do. X 900 . 16 M elting (7) • • ______•• 3PbO·:-Jb,O.+5PbO·2Nb,O.+tet. PbO ... X 1. 000 . 16 Some meILin g ____ .____ _. ______5PbO·2:-Jb,O,+tel. PbO". X 1,053 . 16 Some molting______. ______Do. -_ ... - X 800 1. 5 5P bO·2Nb,O.+3P bO· :\1>,0 ,. 72 28 {------X 1.000 0.33 So me melting (7) ______._ ••••• ____ • • • ••• 5PbO·2N b,0 .+ tel. PbO". X 1, 225 . 08 Some melLi ng (7} -.------71. 4285 28.5715 :::::: X J,2OO .08 So m eI Li llg _. __ . . ___ . ___ ._. ______.• 5PbO·2Nb,O,. (.,:2) {------X J, 220 . 08 MelLed ___ . . . ______. ______. ______5PbO·2Nl),0 .+2P bO·l\b,05.. + tet. P bO ... X 1. 230 .02 Completely melted ______.... _. _____ . __ X 753 1 31'bO·Kb,O,+5pbO·2:-fb,0 .-2p bO·Nb,O... . X 1, 100 I. Lost weight- ______5PbO·2Nb,O,-2P bO·Nb,O, ... X 747 M D o. 70 30 X 856 0.5 Do. X 1, 100 . 08 Xo melting ______.______Do. X 1. 180 . 16 No mel ting______. ______Do. X 1. 230 .08 :0:0 m elt. ing ______, __ . __ _ X 1, 250 . 08 ]\[elted ______.______5PbO·2Xh,O,-2PbO·.'{i),O, ..+ tct. p bO" . 68 32 { --- _.. X 860 l. 5 5PbO·2Nb,O,-2PbO·Nb,O,,,. X 1, 225 0. 02 Consid erable mel Li fig ______• ______51' bO·2Nb,O,-2PbO· Nb,O, ..+ tet. P b O" . X 1, 075 l 2P bO·Nb,O,. X 1,225 0.25 r;OL meILed ___ .. ____ . ____ . ______. ____ _ Do. ___ . . do ______.... ______. .. _._ 66.67 33.33 X 1, 230 . 16 Do . X 1,235 . 25 J\-Ielted ______...... __ . __ .. . Do. X 1, 247 . 16 _. __ _d o .... ______. __ .. . ______Do. X 1,260 16 ____ _d o . . . __ ._ .... ______. ______Do. X 1, 100 1 3PbO·2Xb,O,+2PbO·Nb,O,. 65 35 { X 1, 100 1 , 2PbO·Nb,O,+3P bO·2Nb,0 •. X 1,230 0 5 Somc melting 2P bO·Nb,O,+3P bO·2Nb,O,. 64 36 { X l.100 I 3PbO·2Nb,O,+2P bO.Nb,0 •. X 1, 230 0. 25 So me melting_ Do. 1, 100 1 Do. N o melting . . . ____ . ______. .. 6a 37 X 1, 220 1. 5 2P bO oN b,O,+3P b 0· 2 N b,O,. {:: ~ :: X 1,230 0.16 M el ling ('!) • _____ • _• • _____ • ____ •• •• __ • __ Do. X 1, 240 . 08 Some melting_ . __ .. _._. _____ . _._ . _._ ... 3PbO·2. b,O,+2P bO ·Xb,0 ., 62 38 1, 100 1 Do. { --: .. X J, 230 0.5 Some melting_ Do. 61 39 1, 100 1 Do. X 1, 230 0.16 Some melting (?) _•• • ______••• ______D o. { --::-- 1, 100 1 3P bO·2Nb,O,. No melting ______60 40 X 1,225 0. 25 D o. {: : ~ :: X 1, 235 . 25 So me melLin g_ .. ____ . _. .. ______. . _ 3P bO·2Nb,O,+orth. PbO·Kb,O, . X I. 2~5 0.08 1\1ost ly melted . __ . ______.. _.. . _...... __ _ Do. 59 41 { X 1. 100 1 3PbO·2Nh,O,. X 1, 2~0 0.11 No melting ______3 PbO·2Nb,O,+ PbO· Nb, O,. ,.g 42 { X 1, 100 1 3PbO·2Nb,O,. X 1. 2:1O 0. 11 No melting ______3PbO·2NI>,O,+orth. PbO·Nb,O" 56 ..5 43 . .5 X 1. 259 I 3PbO·2 :'< b, O,"' rhom· P bO·Kb,O, . 55 45 X 1, 100 1 Do. ,,3.5 46.5 X 1,259 1 Rhom· pbO·N b,O,+3PbO·2Nb,O,. ,!)2 48 X 1, 299 1 Ortll. PbO ·Nb , O ,+ ~ PbO·2Nb,O,.

488871- 59--4 35 TABLE 6. Experimental data for compositions in the binm'y system PbO- Nb20 S-Continued Experimental Composition procedure Heat treatm ent Results Sealed PbO Air P t tube, Tempol'­ Time Physical observation X - R ay diD'ractjon analyses d quenched ature 1-~m~o'I~%,--I-~m~o~l~o/t~o---I-----'------o-C-- --Th~T -- I ------; -----·------1 X 550 16 Not completely reacted______3PbO·2Nb, O,+rllom. PbO·Nb, O,+mon.Nb, O,. X 800 0.5 Oompletely reacted ______Rhom. PbO· Nb, O,. X 1,200 . 5 ______D o. X 1,300 . 5 ______Rllom. PbO·Nb, O,+crth. PbO·Nb,O,. X 1,350 .5 M elted ______Ortil. PbO·Nb,O,. X 1, 150 2 ______Rhom. PbO·Nb, O,. X 1, 150 24 ______Rhom. PbO·Nb, O,+orth. PbO·Nb, O,. X 1. 160 2 ______Do. 50 50 X 1, 200 0.5 ______Do. X 1,243 . 25 ______Orth, PbO.Nb, O,+rhom. PbO·Kb, O,. X 1,329 .5 No melting ______Orth. PbO· Nb, O,. X 1, 340 . 25 No melting______Do. :x 1,345 0 Melted______Do. aX 1, 000 0 . ______Rllom. PbO·Nb, O,. aX 1,236 a ______Rllom. PbO·Nb, O,+tet. PbO·Nb,O,. 'X ' X 1: ~~ ~ g :::: :~:: ::: ~:::: :: :::::::: ::: : :: :::::: :::: ;f ~t ~~g :~ ~ : g::rhom . PbO.~b , O ,. X 1, 280 1 ______Orth. PbO·Nb, O, ..+PbO·2Nb,O,,,. bX 1,296 O. ,5 No melting ______bX 1, 314 . ,J Some melting ______bX I. 330 .5 Oonsiderable melting ______48 52 bX 1,338 .5 Completely m elted ______.. :x 1, 170 .25 Rhom. PbO·Nb, O,,,+PbO·2Nb, O,,,. X 1,180 1 Rllom. PbO·Nb, O, ..+orth. PbO·Nb, O,.. + PbO·2Nb, O, ... X 1,200 0. 25 Orth. PbO·Nb, O, ..+PbO.2Nb,O, ,,. X 1, 280 1 Do. bX 1,296 0. 5 No melting ______46 54 bX I. 314 .5 Some mclting ______bX 1,330 . 5 Almost completely melted ______1bX 1,338 . 5 Completely melted ______X 1,200 . 16 Rllom. PbO.Nb,O, ..+ PbO·2Nb,O,,,. X 1,265 0 Orth. PbO·Nb, O, .. + PbO·2Nb, O,,,. 44 56 bX 1,296 0.5 No melting ______bX 1.314 . 5 ~Oon side r able melting ______1bX 1,330 . 5 Oompletely melted ______._ X 1.200 1 ______Rllom PbO.Nb,O,.. + PbO·2Nb, O,,,. X 1, 265 0 ______Orth. PbO·Nb, O, ..+ PbO·2Nb,O,,,. No melting ______42 bX 1,296 0.5 58 bX 1,314 . fJ Almost completely m elted ______1b X 1,330 . 5 Completel y melted ______X 1. 265 0 ______PbO·2Nb, O, ..+ orth. Pb O·N b, 05 ... bX 1.296 0.5 No melting ______bX Almost completely meHed ______40 60 1.314 .5 bX 1. 330 . 5 Completely melted ______X 1.310 . 25 P artly melted ______X 1,330 .25 Completely m elted ______38 62 X 1,200 1 PbO·2Nb, O, .. + rllom. PbO·Nb,0 5... 36 64 1, 200 1 PbO·2Nb, O,,,. {~ 1,299 1 Do. 34 66 X 1, 304 1 Do. X 1, 257 0 PbO·2Nb, O,. X 1,30l 48 Do. X 1,350 0.5 Melted ______Do. bX 1,296 .5 No melting ______33.33 66.67 bX 1. 314 . 5 No melting ______bX 1. 330 . 5 ]\TO melting ______bX 1,338 .5 Completely m elted ______X 1,336 .25 No melting ______PbO·2N b, O,. X 1,338 . 16 Completely m elted ______. Do. 32 68 ---x-- 1, 304 1 PbO·2Nb, O,+mon.Nb, O,,,. 1, 304 I Do. 30 70 1,340 0. 5 Completely m elted ______X 1,338 . 16 Completely m elted ______{~b - X~ -- Almost completely melted ______27 73 1,340 . 5 bX 1,360 .5 Completely melted ______X 1, 265 0 PbO·2Nb205+mon.Nbz05!Js, bX 1. 296 0.5 Ko melting ______bX 1,314 .5 Ko melting ______25 75 bX 1,330 . 5 N o melting ______bX 1, 338 . 5 Considerable melting ______bX 1, 340 . 5 Considerable melting ______bX 1,360 . 5 Almost completely melted ______bX 1, 380 . 5 Completely melted ______X 1, 300 65 MOll.Nb, O, ..+ PbO·2Nb,O,. Some melting ______20 bX 1, 340 0.5 80 bX 1, 360 . 5 Moderate melting ______{ bX 1,380 . 5 Considerable melting ______X 1, 300 65 ______Mon.Nb,0 5.. + P bO·2Nb,05. bX No melting ______15 85 1,340 O. 5 bX 1, 360 .5 Some melting ______{ bX 1, 380 .5 Moderate melting ______X 1, 257 0 ______Mon.Nb,O,.. + PbO·2Nb,O,. bX K 0 melting ______10 90 1,340 0.5 bX 1, 360 . 5 Ko melting ______{b X 1,380 .5 Ko melting ______X 770 I. 0 ______Man. Nb2 0 08S. X 1, 300 65 Do. 5 95 bX 1,340 0.5 No melting ______bX 1,360 .5 No melting ______1b X 1. 380 . 5 N o melting ______a r:l"'hlS expcnment was conducted III a hIgh-temperature X-ray furn a(;c. d rrhe phases Identified are glven in the order of the amount present at room b The specimens had been gronnd into the sbape of four-sided -grooved pyra- temperature. The phases are not necessarily those presen t at t he temperature mids. to wh ich the specimen was heated : Mon.- monoclinic; ortb.- orthorllombie; tet.-- o The melted specim en was sealed in another Pt tube and reheated to deter- tetragonal; rhom.- rhombohedral; ss-solid solution. mine if equilibrium could be obtained from the nonequilibrium tetragonal PbO • Tilis X-ray pattern was made on t he same specimen as preceding after grind- solid solution. ing the surface of the specimen. 36 L_ The compound PbO·Nb20 5 was found to melt transformation from the low to the high form of congruently fit 1,343 ± 2° 0 by both the sealed-Pt­ Nb20 5 • The specimen containing 5 mole percent of Lube method find the grooved-pyramid method. PbO heated to 770° 0 for 1 hr contained only the Hov/ever, the temperature of the rhombohedral­ high-temperat ure form of Nb2 0 s. This form was LeLra.go nal phase transformation could not be arrived also found in the nonequilibrium assemblage of the at by heating in air. It appears that this phfise 50 : 50 specimen at 550° 0 for 16 hr. No attempt has Lransformation. is quite sluggish. High-temperature been made to locate accurately the temperature of X-ray patterns in air show that several hours at this phase transformation in tbe presence of PbO. 1,287° 0 are required to convert all of the low-tem­ The low temperature form of Nb20 ; has not been perature rhombohedral form to the high-temperature indicated on tbe phase diagram of figure 1 as it tetragonal form. A speeimen heated at 1,200° 0 seems likely that this polymorph is only metastable for 1 hr in air and examined at room temperature and has no true equilibrium position in the binary showed only the low-temperature form. However, system. The melting point of Nb20 s was accepted Lhe same material heated in a sealed Pt tube for 30 as 1,500 ± 10° 0 as found by Roth and Ooughanour min at 1,2 00 ° 0 was conver ted in part to the bigh­ [22 ]. temperature form. A 2-hr h eat treatment as low as 1,160° 0 gave some of the high-temperature form 5. Occurrence of Q Nonequilibrium Red while the same time at 1,150° 0 gave only low-tem­ PbO Solid Solution perature PbO·Nb20 5 • However, holding the 1: 1 composition at 1,150° 0 for 24 hr did conver t a small \V I1011 specimens of eomposiLion between pure n,mount of the material to the high-tempern, ture PbO and the 2 : 1 compound arc quenched from Jorm. It seems that fl maximum temperature of a bove the solidus, a phase similar Lo the tetragonal 1,150° 0 is as elose Lo the LUllsfonnaLion tempera­ red PbO is observed in the X-ray patLern. This ture as can be obtained in reasonable laboratory time. phase has an a parameter about the same as pure PbO.Nb20 S will accept a small amount of Nb20 5 teLragonal PbO but a c par31neLer which is much inLo solid solution as shoWIl by significant chn,n ges larger. This c parameter is always approximately in d-spacings of the X-ray patterns. However, the tbe same, regardless of I he lemperature or composi­ composition co ntaining 48 mole percent of PbO Lion of the quenched specimell. It has a value of has two phases, indicati ng tllat solid solution is less abouL 5. 08 A as compared Lo the value of 5.0 23 A for than 2 mole percent. This small amount of solid pure PbO. Specimens conLaining 95 percent of PbO solution is euough to raise t l10 temp era Lure of the co nsisted olll.Y of yellow PbO and thi s red PbO solid phase transformation from the rhombohedral to the solution, wlten quenehed from above t be solidus. tetragonal form by about 25° 0, as shown by quench­ "Vhell Lhe compositio n is on the hi gh-Nb20 ; side ing experiments in scaled Pt t ub es. of th e eutec Li c, the teLragonal solid solution occurs A eutectic exists betwee n PbO·Nb 20 ; and PbO· wiLh the compound 3PbO·Nb20 s. When specimens 2NbzOs at about 41 mole percent of PbO Rlld1,31 0° O. of 3PbO·Nb20 5 arc heated above the incongruent This eutectic was found by the grooved-pyramid melt in g point, abou t 985° 0 , this tetragonal phase technique and checked with experiments us in g is observed to occ ur with the 5PbO·2Nb20 5 com­ sealed Pt tubes. The compound PbO·2Nb20 5 was pound. Speeimens of the 5: 2 compound and found to melt, probably congruently, at 1,337° ± 2° O. compositions betwee n 5 : 2 and 2 : 1 con tain, upon PbO·2NbzOs takes more than 2 but less thn,n 4 mole quenchin g from above the solidus, red PbO solid percent of PbO into solid solution at the solidus solution plus a rhombohedral pyrochlore-Lype solid temperature. This solid solution is reLained on solution. relatively slow cooling, as shown by X-ray patterns A specimell of the euLecL ic composition, 94 mole of specimens co ntaining 36 and 38 mole percent perce nt of PbO, showed almost en t ircl~r the red of PbO. PbO solid solution when quenched from above the Although PbO·2Nb20 S appears to melt sharply, melting point. This spec im en was put back in a no euteetic has been definitely located on the high sealed P t tube and heJcl overnight just below the Nb20 b side. However, some melting-point data of melting point. The res ul ti ll g protluct was an equilib­ the compositions containing 32 and 30 mole percen t rium mixture of yellow PbO and the compound of PbO indicate a eutectic in this region. Both of 3PbO·Nb20 .\ . Thus t he red PbO solid solution has these compositions appear completely melted, by been proven to be metastable. the pyramid met,hod, at 1,344° O. If the 1 : 2 com­ There are several possible explanations of this pound melted incongruently, these specimens would nonequilibrium phenomenon. The most obvious be only partially melted at 1,344° O. A eutectic ean, conclusion would be that oxidation of PbO and t herefore, be placed at about 30 mole percent of reduction of Nb20 .\ occur when they are heated PbO and 1,334 ± 4° O. together in a closed tube. However, this oxygen N b20 s is found to accept a little over 15 mole solid solution would then hn,ve parameters depending perce n t of PbO into solid solution at the eutectic upon the amount of Nb20 5 present, which is not temperature. As shown by X-ray diffraction this observed to be the case. Another possibility is that amount decreases to a little less than 10 mole percent the tetragonal PbO solid solution represents the at room temperature. The addition of PbO to composition of the liqnid in equilibrium at the Nb20 5 seems Lo lower the temperat ure of the phase temperature from which the material was quenched. 37 That is, the liquid wh en quenched devitrifies into The rhombohedral-tetragon al inversion of a single-phase crystalline solid of the same composi­ PbO·Nb20 s was found to take place very sluggishly tion. However, this explanation also would req uire at approximately 1,150° C in a sealed Pt tube. different parameters of the solid solution for different The temperature of this phase transition is raised temperatures and compositions. An alternative by the Nb20 s solid solution to about 1,175° C. hypothesis is that the phase does not represent the equilibrium liquid but that the liquid follows its A recent paper by M. H. Francombe and B. Lewis regular course of crystallization until it reaches the [24] suggests a s olid-solution region from eutectic composition. This composition then crystal­ PbO : 1.5Nb20 5 to PbO: 3Nb20 5• This covers a lizes out as a single-phase nonequilibrium solid range in which two phases have bee n found, for the solution, having a structure similar to the low­ most part, in the present work. No explanation temperature form of PbO. can be found for the observa.tion of such a large It does not seem likely that this solid solu tion is amount of solid solution in the PbO-Nb20 5 system . of the substitutional type. The radius of Pb+2 is The assistance of Charles R . Drew in preparing 1.20 A and that of Nb+s is 0.69 A [23]. The ratio and heating most of the specimens is gratefully between the two is well beyond the limit at which acknowledged. John M. Cm'ceo assisted in t he even partial substitution might be expected. There­ measurements of some of the X-ray data. ::-'Iany fo re, it must be concluded t hat this nonequilibrium helpful suggestions were received from Carl R . solid solution is of an interstitial type. Robbins and Ernest M. Levin. There is also the possibility that this apparent red PbO solid solution is a true compound in the' 7. References system, having a minimum thermal stability at the [I] G. Goodman, J . Am. Ceram. Soc. 36, 368 (1953). temperature indicated here for the eutectic tempera­ [2] M. H . Francombe, Acta Cryst. 9, 683 (1956). [3] W. R. Cook, Jr., and H . J affe, Phys. R ev. 88, 1426 ture, 835° C, and a composition equal to the hypo­ (1952) . thesized eutectic composition. That such a com­ [4] 'vV. R. Cook, Jr., and H. Jaffe, Phys. R ev. 89, 12l;!7 pound should have the sam e structure as red PbO (1953) . seems highly unlil(ely and is not co nsidered to be [5] B. Jaffe, R. S. Roth, pnd S. Marzullo, J. Research ). BS the true explanation of the observed phenomena. 55, 239 (1955) RP2626. [6] R. F. Geller, P. J . Yavorsky, B. L. Steierman, and A. S. Creamer, J. Research NBS 36, 277 (J 946) RP1703. 6 . Summary [7] G. Brauer, Z. anorg. u. aUgem. C hem. U8, 1 (19-11 ). [8] F. Holzberg, A. Reisman, M. Berry, and M. Berkenblit , J. Am. Chem . Soc. 79, 2039 (1957). Tlte system PbO-NbzOs was studied by means of [9] A. Mllgneli and S. Lagergren, AST M I ndex of X-ray solid-state reactions, fusion characteristics, and D iffr2ctiOll Data, Card No. 5- 0379. X-ray diffraction data. The existence of six com­ [101 F. Holzberg, Private communication. pounds in this system was shown. They are [1l] H . E. Swanson and R. K Fuyat, NBS Circ. 539, ' "01. II (1953). 3PbO·Nbz0 5 which melts incongruently at about [12] M. P etersen, J. Am. Chcm. Soc. 63, 2617 (1941 ). 985° C, 5PbO·2NbzOs which melts congruently at [13] A. F. Efimov, V. A. Pchelkin, Yu. P. Simano\', E. P . 1,220° C, 2PbO·Nb2 0 S which melts co ngruently at A rtamr nova, and A. V. Lapits kii , Vestnik l\Ioskov. about 1,233° C, 3PbO·2NbzOs which melts incon­ Univ. 9, No.6, Sel'. Fiz. Mat. i. Estestven Nauk No. 4, 77 (1954). gruently at about 1,233° C, PbO·Nbz0 5 which melts [14] R. S. Rot h, Acta Cryst. 10, 437 (1957). co ngruently at about 1,343° C, and PbO·2Nbz0 5 [15] W. R. Cook, Bull. Am. Ceram. Soc. 37 , No.4, Program, which melts co ngruently at about 1,337° C. The 33 (1958). four eutectic poillts are located as follows : about 94 [Ie] W. R. Cook, P "i vate communication. [1 7] G. Shirane and R. P ep insky, P hys. Rev. 92, 50-1 (1953). mole percent of PbO and 835° C, about 66 mole [18] J. Ie. Hulm, Phys. R ev. 92, 504 (1953). percent of PbO and 1,225° C, 41 mole percent of [19J F. Jonq, G. Shirane, and R. P epinsky, Phys. R e\". 98, PbO and 1,3 10° C, and about 30 mole percent of 903 (1955) . PbO and 1,334° C. [20] R. F. Geller, A. S. Creamer, fin d E. K. Bunting, J. Rese9rch NBS 13, 237 (1934) np705. Complete solid solution was no ted between [21] W. R. Cook, Private comm.unication. 2PbO·Nb20 S and 5PbO·2Nb zOs' Partial solid solu­ [22] R. S. Roth p nd L. ViTo Coughanour, J. R esearch ).'BS tion occurred between the two compounds PbO·Nb20 S 55, 209 (1955) RP2621. [23] L. H . Ahrens, Geochim. Cosmochim. Acta 2, 155 (1952). and PbO·2Nb 0 5• ~ ' b 20 S accepts about 15 mole z [24] M. H . Francombe and B . Lewis, Acta Cry st. 11, 69G percent PbO in solid solution at the eutectic tcm­ (1958). perature. No solid wlution occurs on either sid e of the compound 3PbO·2Nb20 5 • W ASHING'l'OK, July 11 ,1958.

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