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

Abstract

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.