A previously unknown modification of dicopper(I) triselenostannate(IV), Cu(2)Se(3)Sn, has been obtained from the Cu(2)Se-SnSe(2) quasi-binary system and investigated using X-ray single-crystal diffraction. The Se atoms are stacked in a closest-packed arrangement with the layers in the sequence ABC. The Cu atoms occupy one-third of the tetrahedral interstices, whereas the Sn atoms are located in one-sixth of the tetrahedral interstices. All the atoms occupy general positions. The structure possesses pseudo-inversion symmetry. The Cu(2)Se(3)Sn structure investigated in this paper (96 atoms per unit cell, ordered distribution of Cu and Sn over 12 cation positions) is a superstructure of the reported cubic (eight atoms per unit cell, random distribution of Cu and Sn over one cation position) and monoclinic (24 atoms per unit cell, ordered distribution of Cu and Sn over three cation positions) modifications.

The phase diagram of the quasi-binary Ag 2 SeAs 2 Se 3 system was constructed using differential-thermal and X-ray phase analyses. The crystal structure of the ternary AgAs 3 Se 5 compound (space group R   m, a = 0.38195(1) nm, c = 5.0082(2) nm, R I = 0.0868) was investigated using X-ray powder diffraction. The Se atoms form a close-packed arrangement with the layers stacked in the sequence ABC. The atoms of a statistical mixture M (0.5Ag+0.5As) and the remaining As atoms occupy 4/5 of the octahedral interstices.

The isothermal section of the quasi-ternary Ag 2 Se-SnSe 2-As 2 Se 3 system at 520 K was constructed using differential-thermal and X-ray phase analyses. The existence of a quaternary compound, Ag 2 SnAs 6 Se 12 (space group R 3 m, a = 0.38118(1) nm, c = 3.9724(3) nm) with incongruent type of melting at 690 K, was established. The Se atoms in the structure of the Ag 2 SnAs 6 Se 12 compound are stacked in a close-packed arrangement with the layers in the sequence ABC. A statistical mixture of cations (0.333Ag+0.167Sn+0.500As) occupy one half of the octahedral interstices. The remaining As atoms are distributed over the other octahedral interstices.

We have performed the investigation of the nonlinear optical properties namely the third harmonic generation (THG) of the glass-formation region in the Cu2Se–GeSe2–As2Se3 system. The samples were synthesized by direct single-temperature method from high-purity elementary substances. We have found that the value of disorder parameter Δ depends on the composition of the glassy alloys. The measurements show that increasing the Cu2Se concentration leads to increased slope of the absorption edge, which may be explained by the decrease of the height of random potential relief for the electrons in the tails of the state density which border the band edges. A very sharp increase in the THG at low temperature was observed. Significant enhancement in THG was obtained with decreasing the energy gap, which agreed well with the nonlinear optical susceptibilities obtained from other glasses.

Monoclinic Cu 2 Se 3 Sn. -Single crystals of a new monoclinic modification of Cu2Se3Sn are obtained from the elements at 1380 K (evacuated silica tube, 2 h). The structure of the present polymorph (space group Cc, Z = 16) is a superstructure of the cubic and monoclinic modifications reported earlier. The eight independent Cu atoms and four independent Sn atoms occupy one-third and one-sixth of the tetrahedral holes, resp., in a pseudo-close-packed Se lattice. The cation positions are fully ordered. -(GULAY, L. D.; DASZKIEWICZ*, M.; OSTAPYUK, T. A.; KLYMOVYCH, O. S.; ZMIY, O. F.; Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 66 (2010) 5, i58-i60, DOI:10.1107/S0108270110014034; Trzebiatowski Inst. Low Temp. Struct. Res., Pol. Acad. Sci., PL-50-950 Wroclaw, Pol.; Eng.) -Schramke 32-007