A series of thallium ternary chalcogenides with the composition Tl 2x In 2(1Àx) Se 2 , x = 0.2, 0.3,...0.9, have been studied by X-ray powder and, for some of them, singlecrystal 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 re®ned 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.

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