X-ray Diffraction Study of the Crystal Structure of the Tl Ternary Chalcogenides Tl_2xIn_2(1−x)Se₂, x = 0.2, 0.3,...0.9

A series of thallium ternary chalcogenides with the composition Tl_2xIn_2(1−x)Se₂, x = 0.2, 0.3,...0.9, have been studied by X-ray powder and, for some of them, single-crystal diffraction. They are tetragonal, space group I4/mcm, Z = 4, and isostructural with the binary semiconductor TlSe. Their crystal structures have been solved by direct methods and refined by the Rietveld method to a precision which is satisfactorily comparable to single-crystal results. As x is changed from x = 0.2 to x = 0.9 the unit-cell parameters and volume decrease or increase following Kurnakov's law, which is valid for solid solutions. Refined positional parameters of Se, In—Se and Tl—Se bond lengths vary with x also according to the same law. The distribution of In and Tl cations in 4(a) and 4(b) sites depends on the stoichiometry x and the crystals are composed of [In³⁺Se₂] chains along the c axis in which InSe₄ tetrahedra share edges; the chains are interconnected with Tl⁺(In⁺) ions.

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