Structure and Properties of the Semiconductors Tl 2 SnAs 2 Q 6 (Q = S, Se)
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Structure and Properties of the Semiconductors Tl2SnAs2Q6 (Q = S, Se) Ratnasabapathy G. Iyer1, Daniel Bilc2, S. D. Mahanti2 and Mercouri G. Kanatzidis1 1
Department of Chemistry, 2Department of Physics and Astronomy Michigan State University, East Lansing, MI 48824 ABSTRACT We describe the Tl2SnAs2Q6, (Q= S, Se) compounds which consist of [SnAs2S6]2- layers with the Tl+ cations lying in between. Tl2SnAs2S6 and Tl2SnAs2Se6 crystallize in the space group P-3 with a = 6.706(4) Å, c = 7.187(6) Å and a = 6.996(3) Å, c = 7.232(4) Å respectively. These compounds are semiconductors with band gaps of 1.68 eV for the sulfide and 1.08 eV for selenide corresponding to their dark red and black colors respectively. Band structure calculations suggest indirect band gaps in these materials. INTRODUCTION A major goal in solid state chemistry is to develop methodologies that can predict and prepare materials with pre-designed structures and properties. In the case of multinary chalcogenides, this purpose is served by the hugely successful alkali metal polychalcogenide and polychalcophosphate flux methods. 1 These fluxes allow reactions to be done at lower temperatures and often reduce thermodynamic control over the reaction pathway. Under such conditions molecular building blocks can form and become available for bonding. These blocks show a variety of binding modes resulting in different structural assemblies. Recently we began investigating the reactivity of main group and transition metals in alkali polythioarsenate fluxes.2 A main motivation for these investigations is the dissimilar redox behavior of the As3+/As5+ couple in a sulfur environment compared to the P4+/P5+ couple. The structures of thioarsenate compounds are expected to be different from those observed in chalcophosphate chemistry. We chose Sn because of our success with this metal in polychalcogenide and polychalcophosphate fluxes. The reactions of Sn in AAsxSy (A = K, Rb, Cs) fluxes gave the onedimensional compounds NaSnAsS4 and ASnAsS5 (A = K, Rb), the layered compounds A2SnAs2S6 (A = K, Rb) and the molecular Cs2SnAs2S9, which features discrete molecules. 3 The layered phase, A2SnAs2S6, crystallizes in the trigonal space group P-3. The band gap of K2SnAs2S6 is 1.89 eV. Because of the highly symmetrical crystal structure, we became interested in analogs of this phase as possible semiconductors. If Tl could be substituted for the alkali metal, there could be a lowering of the band gap because of the tendency of Tl to act in a more covalent fashion compared to the alkali metals. Additional reduction in the band gap can be achieved by substituting Se for S. Compounds with low band gaps and high symmetry are desirable for possible applications in solar energy and thermal energy conversion.5 Another useful class of thallium thioarsenate materials which has gained considerable interest is the glassy system Tl2S-As2S3, where Tl is used as a modifier to modify the glassy As2S3 network and hence change its properties. 4 Thin films of Tl-As-S glasses have shown pot
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