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Investigation of sulfur- and selenium-containing quasi-ternary systems Tl2X–MIIX–MIVX2 (MII = Cd, Hg; MIV = Si, Ge, Sn) by DTA and XRD methods at 520 K found the formation of twelve quaternary compounds, Tl2HgSi(Ge,Sn)S4, Tl2PbSiS4, Tl2CdSi(Ge,Sn)Se4, Tl2HgSi(Ge,Sn)Se4, Tl2PbSi(Ge)Se4, at the vertical sections Tl2MIVX3–MIIX, and seven compounds, Tl2CdGe3Se8, Tl2HgSi(Ge)3S8, Tl2PbSi(Ge)3S8, Tl2HgSi(Ge)3Se8, at the Tl2MIIMIVX4–MIVX2 sections. Additionally, series of analogous compounds Tl2Cd(Hg)SiTe4 of the 2-1-1-4 type and Tl2Mg(Mn,Zn)Ge3Se8 of the 2-1-3-8 type, were obtained. The compounds of both 2-1-1-4 and 2-1-3-8 groups that form in the AI2Х–ВIIХ–DIVX2 system possess non-centrosymmetric crystal structure making them potential candidates for optical and other properties valuable for materials science. Similar quasi-ternary systems AI2Х–ВIIХ–DIVX2 feature quaternary phases, individual known representatives of which such as Li2CdGeSe4, Li2CdSnSe4, Cu2CdGeSe4, Cu2CdSnSe4, belong to diamond-like semiconductors and have already found use in non-linear optics and other fields of semiconductor applications. The structure of all quaternary compounds Tl2MIIMIVX4, except Pb-containing ones, was determined in isotropic approximation with the Tl2HgGeTe4 structure as model (tetragonal symmetry, space group I-42m), as well as six other tellurides Tl2Mn(Cd,Hg)Ge(Sn)Te4. Tl2PbSiS4 and Tl2PbGeS4 are isostructural and crystallize in the monoclinic symmetry, space group Р21/а. A family of compounds of the general formula Tl2MIIMIV3X8 (MII = Mg, Zn, Cd, Hg, Mn; MIV = Ge; X = Se, Te) are analogous to the quaternary phases with alkaline metals α-K2ZnSn3S8 and Cs2MIIMIV3Se8 which crystallize in the orthorhombic symmetry, space group P212121. |
