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

We have synthesized new compounds of the formula Tl2MQ3, 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 P21/m (Z = 2), with unit cell parameters for the representative Tl2ZrS3 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 Tl2ZrS3 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-NaFeO2 structure type. All atoms are in their standard oxidation states: Tl+, Zr4+, S2−. The sulphide Tl2ZrS3 has a calculated band gap of 1.15 eV, and the selenide Tl2HfSe3 a gap of 0.57 eV. The electrical conductivity values of Tl2ZrS3 and Tl2HfSe3 at room temperature are 7.1 × 10−6 Ω−1 cm−1 and 3.9 × 10−3 Ω−1 cm−1, respectively.