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IR, Raman, X-ray, electron absorption and luminescence studies have been performed for novel laser Nd 3 BWO 9 and Eu 3 BWO 9 borotungstates exhibiting non-centrosymmetric crystal structures. The assignment of observed vibrational modes to respective symmetry and vibrations of atoms has been proposed. These studies have shown that vibrational and electronic properties of these crystals can be better explained when P 6 3 symmetry is assumed, instead of previously proposed P 3 one. The crystal structure refinement has also confirmed that symmetry of the Eu 3 BWO 9 borotungstates is P 6 3 , not P 3.

Dielectric and ultrasound measurements of berthierite FeSb 2 S 4 reported in the literature revealed a phase transition at T c =348 K. In the present study the phase transition was confirmed by Mossbauer spectra exhibiting anomalous behavior close to 350 K. Precise lattice parameters measured in the temperature region 305–380 K show a change of slope at about 350 K. The crystal structure of FeSb 2 S 4 has been determined using single-crystal X-ray diffraction data at both room temperature and 370 K above the phase transition. The crystal structure is orthorhombic, Z =4. At 295 K a =11.412(2) A, b =3.763(1) A, c =14.161(3) A, and R =0.0280, and at 370 K a =11.421(2) A, b =3.771(1) A, c =14.173(3) A, and R =0.0274. No symmetry change has been observed. At both temperatures the space group of FeSb 2 S 4 is Pnma with small changes in atomic parameters and interatomic distances. The transition observed at 348 K is, therefore, to be classified as a weak phase transition going on without essential changes in the crystal structure.

The aim of the paper was a search for polymorphism in LiNH4SeO4 (LASe) single crystals and the study of the structure of the only known α-modification of LASe. We found the α-LASe to be isomorphous with α-LiNH4SO4 (the space group Pca21). Two kinds of polytypes, the A with cA = c and the B with cB = 2c, were observed. The two polytypes were found to exist simultaneously in each single crystalline sample with the B-polytype prevailing. This superstructure is found to affect the α rightarrow β polymorphous transformation.

X-ray investigations of Eu2+-rich precipitates in NaCl single crystals have revealed the presence of two phases. Phase A of chemical formula EuCl2 with a CaF2-type structure and lattice parameter a = 6.969 (2) A preserves strictly the orientation of the matrix lattice. Phase B with the lattice parameter a = 7.47 (1) A and tentative formula EuCl2.2NaCl, containing Eu2+ ions and charge-compensating cation vacancies, is localized at subgrain boundaries and is thermally less stable than phase A.