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Lead and Bismuth Chalcogenide Systems Hurr.Lnc Lru, Luxn L. Y. CH

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Lead and Bismuth Chalcogenide Systems Hurr.Lnc Lru, Luxn L. Y. CH American Mineralogist, Volume 79, pages 1159-l 166, 1994 Lead and bismuth chalcogenide systems Hurr.lNc Lru, Luxn L. Y. CH,c.Nc Department of Geology, University of Maryland, College Park, Maryland2O742,U-5.4. Ansrnlcr Phaserelations in the systemsof lead and bismuth chalcogenideswere examined in the temperature range of 500-900 "C. In the system PbS-PbSe-BirSr-BirSer,a complete series of solid solutions exists between PbnBioS,,(heyrovskyite) and PbnBioSe,r,whereas along the join from Pb.BiuS,,(lillanite) to PbrBiuSe,,there are two terminal solid solutions. The join PbBirSo-PbBirSeois characterizedby an extensivesolid solution basedon weibullite. Six extensive ranges of solid solution exist in the system PbSe-PbTe-BirSer-BirTer,and their compositionsfall along the joins in Pb(Se,Te):Bir(Se,Te),ratios of l:0, 2.7:1, 2:I, l:1, l:2, and0: I . New phasesand solid solutions in the systemsof lead and bismuth sulfide and telluride and selenide and telluride are all structurally related, and their structures are based on stacked layers of PbTe (two layers) and BirTe, (five layers). The total number of layers (N) in the unit cell of a phase, mPbTe'nBirTer, is expressedby the equation N : Z(2m * 5n), where Z is the number of formula units in the unit cell and can only be derived from experimental data. For the four phasesalong the galena-tetradymitejoin in the PbS- PbTe-BirSr-BirTe, system, PbrBirSrTer, PbrBioSrTeo,PbBirSrTer, -and PbBinS'Teohave N, Z, a, and c-respectively:9, l, q.2l x, and ft.j1 A;32,2,4.23 A, and 60.00A;21,3, 4.8 A. and 39.83A; and 12,1,4.24 A, and 23.12A- Ixrnouugrron a small range of solid solution, representedby PbrBirSe' at 500 lrad and bismuth chalcogenidesare ofinterest because "C. Phaserelations along the join PbTe-BirTe, have been of the large number of minerals reported in the group. the subject of several studies. Elagina and Abrikosov The atomic structures of Pb and Bi differ by a single 6p (1959) and Golovanova and Zlomanov (1983) reported electron,and their crystallochemicalbehaviors show many a single intermediate phase, PbBioTe', stable along the similarities, which are well illustrated in their sulfides. join. Hirai et al. (1967)also reporteda singleintermedi- The BiSu polyhedron has bond lengths and bond angles phase,but with a compositionof Pb.BioTer.Zhukova very similar to those of the PbSu octahedron. Conse- ate Zaslavskli(1976) and Skoropanovet al. (1985) ob- quently, various galenalike structural units can be formed and the formation of PbrBirTer, PbBirTeo, and Pb- by combining Pb and Bi polyhedra(Makovicky, l98l). served Bi.Te,. Chami et al. (1983)and Dumas et al. (1985)pro- However, no significant solid solutions were found be- poseda join containing two intermediatephases, PbBirTen tween their respectivechalcogenides. The purpose ofthis and PbBi"Te,. study is to examine the distinctions and similarities in the Pb- and Bi-containing systems sulfide and selenide, selenideand telluride, and sulfide and telluride. ExpnnnvrnxrAr, PRoCEDURES Phase relations among lead sulfbismuthinides were Reagent-gradePb, Bi, S, Se, and Te, all with 99.990/o studiedby Van Hook (1960)and Craig (1967).There are purity, were used in the preparation of starting compo- four lead sulfbismuthinides stable along the join PbS- sitions. Heat treatment was performed in electric furnaces BirS.: PbnBios,r,PbsBi6S,7, PbBizSo, and PbBinSr.The in which the temperaturewas controlled to within +2"C, first three phaseshave been shown to be analogous to and the conventional technique of sealed,evacuated sil- heyrovskyite, lillianite, and galenobismutite, respective- ica glasscapsules (Kullerud and Yoder, 1959)was used. ly. PbBioS, is a high-temperature phase stable between Generally, the duration of the treatment ranged from 60 680 and 725 "C. d at 500 "C to I d at 900 "C. At the end of the heat Along the join PbSe-Bi,-Se.,Elagina (1961) reported treatment, the sampleswere quenched to room temper- three intermediate phases, PbrBioSen, PbBirSeo, and ature by compressedair to preserve phase assemblages PbBioSer,and a wide range of PbSe-typesolid solution synthesizedat high temperatures.Test experimentsdem- extendingto 20 molo/oBirSe, at 720'C. Godovikov et al. onstratedthat the quenchingprocedure is capableoflow- .C (1967)reported two phases,PbBirSeo and PbBioSer,and ering the temperature from the 500-900 range to 100 0003-o04x/94llI l2-1 159$02.00 I 159 I 160 LIU AND CHANG: LEAD AND BISMUTH CHALCOGENIDES ToC "C in 2-3 s. Visual examination of reaction betweensam- ple and capsuleglass was made, and any experiment with r 100 evidence of reaction was discarded. As a test of equilib- rium, some samples were heated to complete melting, quenched,ground under acetone,and annealedat the de- for length 1000 sired temperatures the same of time as their counterparts. If the final assemblagesshowed no difer- encescaused by changingthe procedure,equilibrium was assumedto have been attained. 900 X-ray powder diffraction, reflected-light microscopy, and electron microprobe analysis were used for phase characteization. No high-temperature X-ray diffraction 800 was performed to examine phase assemblagesat experi- mental temperatures.Cell dimensions were computed us- ing a least-squaresrefinement program (Benoit, 1987). 700 For the determination of melting, the quenched samples were made into polished sections and examined micro- scopically for the presence of liquid textures. Phase 600 boundaries in the subsolidus region were constructed by both parametric and phase-disappearancemethods. 500 ExpBnrurr.rrAl. REsuLTs The binary joins Phaserelations along the join PbSe-BirSe,established 400 in the present study are shown in Figure la. Three inter- mediate phaseswere found: PbrBioSe,, (phase U), 40 60 80 D ! . ^ PbrBiuSe,,(phase V), and a PbBirSeo-basedsolid solution PbSe t'12583 mol% 6hasi w). trtr first two are Se analoguesto heyrovskyite and lillianite, respectively.The PbBirSeo-basedsolid so- ToC lution (weibullite) has a range of 47-57 molo/oBirSe. at 500 "C and 47-53 molo/oBi,Se. at 400 "C. All lead bis- muth selenidesmelted congruently,and the melting points oC 1000 determinedare 560 + 3 "C for PbrBioSe,s,575 + 3 for PbrBiuSe,r,and 655 + 3 "C for PbBirSeo.Among the reported phases, only PbBirSe4was confirmed in this study. 900 Along the join PbTe-BirTer, results obtained in the present study are shown in Figure lb. There are two lead bismuth tellurides: PbBi,Te, (phase A) and PbBioTe, 800 (phaseM). PbBi.Teomelted incongruently at 585 + 3 'C, and PbBioTe, melted congruently at 570 + 3 "C. 700 The lead and bismuth chalcogenides Phaserelations in the system PbS-PbSe-BirSr-BirSe,at 500 "C are shown in Figure 2a. Heyrovskyite and the 600 seleniananalogue of heyrovskyite form a complete series of solid solution, whereas,between lillianite and the se- c o Qoo ]o a o lenian analogueof lillianite, two terminal solid solutions exist. Cell dimensions determined are listed in Table l. o o o o o o s00 I io o The cell dimensions of hevrovskvite obtained are com- 400 (- Fig. l. Phase relations along the joins (a) PbSe-BirSe,and 40 60 80 (b) PbTe-BirTer. Open circles, solid circles,and half-solid circles PbTe Bi2Te3 representliquid, solid, and two-phaseassemblages, respectively. mol% Irtters denote single phases. LIU AND CHANG: LEAD AND BISMUTH CHALCOGENIDES 1l6l r60 t4.o 54.4 .3 ro.o a o< I .E cg 80 -cl / ./. ./D /+-.++-.H-+-{4 o to 40 50 60 70 too 40 60 Bi2s3 Bi2Se3 PbBi2s4 DolU PbBi2Se4 PbBl2Se4 molo/" Fig.3. Variationof celldimensions with compositionfor the (b) PbBi,(S,Se)"(phase W) solidsolution synthesized at 500'C. PbSe PbTe parable with those reported by Klominsky et al. (1971), although the naturally occurring heyrovskyite from its type locality contains Ag. The most distinct feature in the systemis the extensive region of solid solution based on weibullite [Pb,*,Bir*"(S,Se)o*.,*,sr], which rangesfrom PbBirSeoto 90 molo/oPbBirSo at n and y : 0. Along the PbS-BirSe, join, the seriesextends to the compositions that include the Pb-Bi ratio of wittite (Johansson,1924). Powder X-ray diffraction data of synthetic weibullite match those from the weibullite obtained by Mumme (1980), and varia- tions of cell dimensions with compositions along the PbBir(S,Se)"join are shown in Figure 3. The changesre- flect the size differencebetween S and Se.The miscibility 20 40 60 join as- Bi2se3 gap along the BirSr-BirSe, forms a three-phase mol"/" semblagewith the weibullite solid solution. Phase relations in the PbSe-PbTe-BirSer-BirTe, syst€m (c) at 500 oC are shown in Figure 2b. Becauseofthe binary Pbs PbTe phasesalong the PbTe-BirTe, join, phaseA and phaseM form two extensive ranges of solid solutions by substitu- tion of Se for Te. Phase A extends from PbBirTeo to PbBir(SeorrTeorr)oand phase M from PbBioTe' to Pb- Bio(SeorrTeor)r.A 2:l type solid solution is slable in the system between PbrBir(Seo*Teo.ro),irnd PbrBir(SeorrTeo rr)r. Despite the small range of solid solution of Se analogueof lillianite and the absenceofa 8:3 phase(heyrovskyite{ype) (- Fig.2. Phaserelations at 500 .C in the systems(a) PbS-PbSe- Bi,S,-BirSer, (b) PbSe-PbTe-Bi,Ser-Bi,Ter,and (c) PbS-PbTe- BirSr-BirTer. Solid circles, half-solid circles, and triangles rep- resentone-, two-, and three-phaseassemblages. L€tters denote a 40 60 Bi253 Bi2Te3 single phase. Shaded area in a representsweibullite solid solu- molo/o tion. tt62 LIU AND CHANG: LEAD AND BISMUTH CHALCOGENIDES TABLE1. cell dimensionsof the phasessynthesized in the system pbs-pbse-Birs.-Birse. Chemicalcomposition (molo/o) Celldimension
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