Single crystals of Ca9Y(VO4)7 (1), Ca9Y(VO4)7:Li+ (2) and Ca9Y(VO4)7:Mg2+ (3) were grown by the Czochralski method. Their chemical composition was analyzed by ICP spectroscopy and their crystal structure was examined by single crystal X-ray analysis. The crystals are characterized by trigonal symmetry, space group R3c. Hexagonal unit-cell parameters are as follows: a=10.8552(1) Å, c=38.0373(2) Å, V=3881.65(1) Å3 for 1; a=10.8570(1) Å, c=38.0161(3) Å, V=3880.77(4) Å3 for 2; a=10.8465(1) Å, c=38.0366(2) Å, V=3875.36(3) Å3 for 3. All crystals are characterized by β-Ca3(PO4)2-type structure with statistical distribution of Ca2+ and Y3+ over M1, M2 and M5 sites in different ratios and with completely empty M4-cationsite. The impurity of Mg2+cations in structure 2 has been detected in octahedral M5 site. Ferroelectric phase transitions are evidenced by DSC and SHG. At about 1220 and 1300 K, they demonstrate phase transitions. Upon heating the symmetry of the crystal structure changes according to the scheme R3c→R3̅c→R3̅m and is restored during consequent cooling. The first of them is of ferroelectric and the second of non-ferroelectric nature. Even a small amount of impurities in Ca9Y(VO4)7 structure is accompanied by a noticeable decrease in the temperature of the ferroelectric-paraelectric phase transition.
Abstract. The phosphates Ca 9-x Pb x Dy(PO 4 ) 7 were obtained by standard solid state reaction found to be isotypic with whitlockite-type β-Ca 3 (PO 4 ) 2 (polar space group R3c). The introduction of one or several doped cations preserves the structure and creates multi-colored and tunable phosphors. The luminescence properties are discussed. The Ca 9-x Pb x Dy(PO 4 ) 7 phosphates shows green result emission and can be used as a potential materials for LED devises.
The system of phosphates Ca9−xPbxHo(PO4)7 were obtained by solid-state reaction and were found to be isotypic with whitlockite-type β-Ca3(PO4)2 (polar space group R3c). The crystal structure encloses five crystallographic sites M1–M5 different in size and oxygen coordination. The unit-cell parameters were determinate using Le Bail decomposition. Rietveld method structural refining showed that Ho3+ ions are located statistically with calcium in M1 and M2 sites, while Pb2+-ions are located in the M3 site. Examination of optical second-harmonic generation evidences non-linear optical activity and confirms polar space group R3c. Structural mechanisms and dielectric features of phase transitions are strongly influenced by the exact distributions of atoms over the crystallographic sites.
The conformational properties of a dansyl amide molecule (CH 3 ) 2 N-C 10 Н 6 -SO 2 NH 2 , which is widely used for fluorescent labeling of compounds, e.g., amino acids or nucleotides and compounds able to form liquid crystals, are studied. It is found that the molecule has six conformers with relative energies 0/0.13, 1.13/1.38, 0.06/0.06, 0.31/0, 2.95/2.70, 0.31/0 (B3LYP/cc-pVTZ and МР2/cc-pVTZ methods respectively). In all conformers the naphthalene core becomes non-planar under the effect of two substituents. The effect of the nature of substituents on the geometric parameters of the conformers is considered. In terms of the NBO analysis a substantial deviation of the S-N bond in the -SO 2 NH 2 substituent from orthogonality and also the asymmetrical position of the -N(СH 3 ) 2 group with respect to the naphthalene core are explained. The paths and barriers of conformational transformations of the dansyl amideа molecule are determined. The structure of the (CH 3 ) 2 N-C 10 H 6 -SO 2 NН-X moiety in the crystals is analyzed and the relationship between the relative energy of the conformers of a free molecule and the probability of the occurrence of their structures in the crystals is determined.
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