Structures and properties of the ternary thallium chalcogenides Tl2MQ3 (M = Zr, Hf; Q = S, Se)

We have synthesized new compounds of the formula Tl(2)MQ(3), with M = Zr and Hf and Q = S and Se, and studied their crystallographic features, electronic structures and electrical conductivity. These isostructural compounds crystallize in the monoclinic space group P2(1)/m (Z = 2), with unit cell parameters for the representative Tl(2)ZrS(3) of a = 7.9159(10) Å, b = 3.7651(5) Å, c = 10.275(2) Å, and β = 97.476(2)°. The Zr atoms of Tl(2)ZrS(3) are (distorted) octahedrally coordinated by the S atoms, with two such octahedra sharing edges along the c axis and forming infinite double chains running parallel to the b axis. Tl atoms separate these chains from one another along the a and c axes. The Tl atoms are also surrounded by S atoms in a distorted octahedral coordination. The structure may be viewed as alternating layers of Zr/Tl atoms and S atoms, and is therefore a distorted, ordered variant of the α-NaFeO(2) structure type. All atoms are in their standard oxidation states: Tl(+), Zr(4+), S(2-). The sulphide Tl(2)ZrS(3) has a calculated band gap of 1.15 eV, and the selenide Tl(2)HfSe(3) a gap of 0.57 eV. The electrical conductivity values of Tl(2)ZrS(3) and Tl(2)HfSe(3) at room temperature are 7.1 × 10(-6)Ω(-1) cm(-1) and 3.9 × 10(-3)Ω(-1) cm(-1), respectively.