New cation-disordered quaternary selenides Tl₂Ga₂ TtSe₆ (Tt=Ge, Sn)

Abstract Two new quaternary selenides of the α-TlSe structure type have been synthesized and characterized. Single crystal X-ray diffraction analysis has revealed that Tl2Ga2SnSe6 crystallizes with space group I4/mmc, a = 8.095(1), c = 6.402(1) Å, with a refined composition of Tl1− x Ga1− y Sn y Se2 (x = y = 0.345(5)), Z = 4, R1 = 0.028; wR2 = 0.066. The crystal structure of the isostructural compound Tl2Ga2GeSe6 has 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− x Ga1− y Ge y Se2, x = 0.343(5), y = 0.35(2), (R B(I) = 0.084; R P = 0.041; R Pw = 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.