New cation-disordered quaternary selenides Tl2Ga2TtSe6 (Tt=Ge, Sn)

Two new quaternary selenides of the alpha-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) angstrom, with a refined composition of Tl1-xGa1-ySny Se-2 (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) angstrom, refined composition Tl1-xGa1-ySny Se-2, x=0343(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.