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The crystal structure of BiOCl has been redetermined with 3570 observed reflections of which 174 were unique measured on a computer controlled Philips PW 1100 single crystal diffractometer. The structure belongs to the tetragonal space groupThe positional and thermal parameters, with anisotropic temperature factors, were refined by full-matrix least-squares calculations to a finalEach Bi atom is eight-coordinated by 4 O and 4 Cl atoms at distances of 2.316 Å and 3.059 Å respectively thus forming a decahedron. The faces of the decahedron are 2 rectangles (O–O–O–O and Cl–Cl–Cl–Cl) with sides 3.487 Å and 8 isosceles triangles (four O–Cl–O and four Cl–O–Cl) with sides O–Cl 3.249 Å and O–O or Cl–Cl 3.487 Å.The decahedra are linked to each other by a common O–Cl edge along the

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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.