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The tellurides Tl(10-x)La(x)Te(6) were synthesized from the elements in stoichiometric ratios at 873 K, followed by slow cooling. These materials are substitution variants of Tl(5)Te(3), crystallizing in space group I4/mcm, with lattice dimensions of a = 8.9220(4) Å, c = 13.156(1) Å, V = 1047.2(1) Å(3), for x = 1 (Z = 2). Increasing the La content occurs with an increase in the unit cell volume and the c axis, but a decrease of the a axis. Tl(5)Te(3) is a metallic compound, while Tl(9)LaTe(6) was calculated to be semiconducting. Correspondingly, the Seebeck coefficient increases with increasing x, while the electrical and thermal conductivity both decrease. The highest thermoelectric figure-of-merit determined thus far is 0.21 at 581 K for cold-pressed Tl(9)LaTe(6).

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The ternary thallium chalcogenides of the general formula Tl(4)MQ(4) (M = Zr or Hf; Q = S or Se) were obtained from high-temperature reactions without air. These sulfides and selenides are isostructural, crystallizing in the triclinic system with space group P1 and Z = 5, in contrast to Tl(4)MTe(4) compounds that adopt space group R3. The unit cell parameters for Tl(4)ZrS(4) are as follows: a = 9.0370(5) Å, b = 9.0375(5) Å, c = 15.4946(9) Å, α = 103.871(1)°, β = 105.028(1)°, γ = 90.138(1)°, and V = 1183.7(1) Å(3). In contrast to the corresponding tellurides, the sulfides and selenides exhibit edge-shared MQ(6) octahedra, propagating along the c axis in a zigzag manner. All elements occur in the most common oxidation states, according to the formulation (Tl(+))(4)M(4+)(Q(2-))(4). Electronic structure calculations predict energy band gaps of 1.7 eV for Tl(4)ZrS(4) and 1.3 eV for Tl(4)ZrSe(4), which are in accordance with the large resistivity values observed experimentally.

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