Synthesis and structure of the new semiconductor compounds Tl2BIIDIVX4 (BII-Cd, Hg; DIV- Si, Ge; X-Se, Te) and isothermal sections of the Tl2Se-CdSe-Ge(Sn)Se2 systems at 570 K

Isothermal sections of the Tl2Se-CdSe-Ge(Sn)Se-2 systems at 570 K were investigated by X-ray powder diffraction (XRPD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The existence of the Tl2CdSnSe4 compound was confirmed, and the formation of the Tl2CdGe3Se8, Tl2CdGeSe4, Tl2CdSiTe4, Tl2HgSiTe4 compounds was established. The four equimolar isostructural compounds crystallize in the tetragonal SG I-42m (Pearson symbol tI16) which is a superstructure to the Tl+1 [Tl+3Se2](-1) structural type (SG I4/mcm). The lattice parameters of the quaternary compounds are a = 0.80145(9), c = 0.67234(9) nm (Tl2CdGeSe4); a = 0.84121(6), c = 0.70289(9) nm (Tl2CdSiTe4); a = 0.83929(4), c = 0.70396(5) nm (Tl2HgSiTe4). The optical bandgap is 1.71 and 1.39 eV for Tl2CdGeSe4 and Tl2CdSnSe4, respectively.