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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The crystal structures of four quaternary R 3 Mg 0.5 DSe 7 compounds (structure type La 3 Mn 0.5 SiS 7 , space group P6 3 , Pearson code hP23, a = 10.669(1) Å, c = 6.0611(8) Å, R1 = 0.0252 for Ce 3 Mg 0.5 SiSe 7 ; a = 10.531(1) Å, c = 6.0420(1) Å, R1 = 0.0275 for Pr 3 Mg 0.5 SiSe 7 ; a = 10.6579(7) Å, c = 6.0430(7) Å, R1 = 0.0249 for Ce 3 Mg 0.5 GeSe 7 ; a = 10.615(1) Å, c = 6.0329(7) Å, R1 = 0.0406 for Pr 3 Mg 0.5 GeSe 7 ) were determined by means of X-ray single-crystal diffraction. The R atoms occupy trigonal prisms capped by two additional atoms. The Mg atoms are located in octahedra and the D atoms in tetrahedra.Rare-earth compounds / Chalcogenides / Crystal structure / X-ray single-crystal diffraction

The new title compounds are prepared from the elements (1150°C, 3 h) and characterized by single crystal XRD and magnetic measurements. The isotypic compounds crystallize in the hexagonal space group P63 with Z = 2 (La3CuSiS7-type structure). The structure is a three-dimensional network of edge-sharing SmX7 (X: S, Se) distorted trigonal prisms which are connected with GeX 4 tetrahedra and CuX 3 triangles by sharing edges. Both compounds are paramagnetic down to 1.72 K.

La3Ge1.48Se7, Ce3Ge1.47Se7, Pr3Ge1.49Se7, Sm3Ge1.48Se7, Gd3Ge1.45Se7, and Tb3Ge1.43Se7 crystallize with the Dy3Ge1.25S7 type structure in the space group P63 with Z = 2 (single crystal XRD).