The crystal structures of the low-temperature (space group C2/c, Pearson symbol mS144, a = 7.1379(4) Å, b = 12.3823(7) Å, с = 27.3904(9) Å, β = 94.308º, R I = 0.0765) and high-temperature (space group Fm-3m, Pearson symbol cF12, a = 5.859(1) Å, R1 = 0.0391) modifications of Cu 2 Se were determined by means of X-ray powder and single crystal diffraction. The basic features of the two modifications are similar. The Se atoms are stacked in a close-packed arrangement with the layers in the sequence ABC. The Cu atoms are distributed over all tetrahedral interstices. The positions of Cu are completely ordered in the lowtemperature modification, whereas they are disordered in the high-temperature modification.

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crystals of the title compound are obtained from a melt of the elements (evacuated silica tube, 1420 K, 3 h). La3Cu 4.88Se7 crystallizes in the orthorhombic space group Pnma with Z = 4. The structure contains edge-sharing [LaSe6] and [LaSe7] polyhedra. Whereas the positions of the La and Se atoms are ordered and lie on mirror planes, all positions for the Cu atoms are partially occupied. The five Cu atom positions determine an ionic diffusion path in the structure. -(GULAY, L. D.; DASZKIEWICZ*, M.; STROK, O. M.; PIETRASZKO, A.; Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 67 (2011) 3, i24-i26, http://dx.doi.org/10.1107/s0108270111005889 ; Trzebiatowski Inst. Low Temp. Struct. Res., Pol. Acad. Sci., PL-50-950 Wroclaw, Pol.; Eng.) -W. Pewestorf 25-004

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