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Two new quaternary selenides of the α-TlSe structure type have been synthesized and characterized. Single crystal X-ray diffraction analysis has revealed that Tl2Ga2SnSe6crystallizes with space group I4/mmc, a= 8.095(1), c= 6.402(1) Å, with a refined composition of Tl1−xGa1−ySnySe2(x= y= 0.345(5)), Z= 4, R1 = 0.028; wR2 = 0.066. The crystal structure of the isostructural compound Tl2Ga2GeSe6has been determined by means of powder X-ray diffraction: space group I4/mmc, Z = 4, a = 8.0770(4), c = 6.2572(5) Å, refined composition Tl1−xGa1−yGeySe2, x = 0.343(5), y = 0.35(2), (RB(I)= 0.084; RP= 0.041; RPw= 0.058). According to their optical absorption spectra all compounds are semiconductors with relatively narrow direct band gaps of 2.15(3) and 2.05(5) eV for the Ge and Sn phase, respectively.

Tl2HgGeSe4crystal was successfully, for the first time, synthesized by the Bridgman–Stockbarger technology, and its electronic structure and peculiarities of optical constants were investigated using both experimental and theoretical techniques. The present X-ray photoelectron spectroscopy measurements show that the Tl2HgGeSe4crystal reveals small moisture sensitivity at ambient conditions and that the essential covalent constituent of the chemical bonding characterizes it. The latter suggestion was supported theoretically by ab initio calculations. The present experiments feature that the Tl2HgGeSe4crystal is a high-resistance semiconductor with a specific electrical conductivity of σ ∼ 10–8Ω–1cm–1(at 300 K). The crystal is characterized by p-type electroconductivity with an indirect energy band gap of 1.28 eV at room temperature. It was established that a good agreement with the experiments could be obtained when performing first-principles calculations using the modified Becke–Johnson functional as refined by Tran–Blaha with additional involvement in the calculating procedure of the Hubbard amendment parameter U and the impact of spin–orbit coupling (TB-mBJ + U+ SO model). Under such a theoretical model, we have determined that the energy band gap of the Tl2HgGeSe4crystal is equal to 1.114 eV, and this band gap is indirect in nature. The optical constants of Tl2HgGeSe4are calculated based on the TB-mBJ + U+ SO model.