F. Marumo

Structure refinement of CaO·6Al2O3

Abstract The crystal structure of CaO·6Al2O3 at 294 K was investigated by X-ray diffraction and refined to an R value of 0.025 for 2735 reflections (1492 independent reflections). It belongs to the space group P6 3 mmc with a = 5.5587(1), c = 21.8929(3) A, V = 585.83(1)A3, and Z = 2, Dx = 3.79 g/cm3, and μ(MoKα) = 15.86 cm−1. Three different models are refined for the bipyramidal Al3+ ions: (1) the central atom model, (2) the central atom model with anharmonic vibration, and (3) the split atom model. The results indicate that the split atom model where the Al3+ ion occupies statistically one of two equivalent sites is the real structure at room temperature.

β-barium borate single crystal grown by a direct Czochralski method

Abstract High purity single-crystal of β-BaB2O4 has been successfully grown from a BaB2O4 melt with the direct Czochralski method. The obtained crystal was confirmed to be that of the low-temperature β-phase by powder X-ray diffraction and the second harmonics generation. The growth rate was 0.6 mm/h, which is more than ten times as targe as that in the flux method. The growth direction was [001]. The present method requires to satisfy several important factors, such as choice of the starting material and temperature gradient at the growing point. The reason why a crystal of the low-temperature form is grown directly from a melt of the compound can be explained by the Ostwald step rule.

Study of the bipyramidal site in magnetoplumbite-like compounds, SrM12O19 (M = Al, Fe, Ga)

Abstract Structures of magnetoplumbite-type crystals Sr M 12 O 19 ( M = Al, Fe, Ga) have been refined by the single-crystal X-ray diffraction method. The space group is P 6 3 / mmc with Z = 2. The cell dimensions are a = 5.5666(2) and c = 22.0018(8) A for SrAl 12 O 19 , a = 5.8836(1) and c = 23.0376(9)A for SrFe 12 O 19 , and a = 5.7929(1) and c = 22.8123(7) A for SrGa 12 O 19 at 22°C. Four structural models were tested with respect to the states of M atoms at trigonal bipyramidal sites: central atom and split atom models with and without anharmonic thermal vibrations. The simple split atom model gave lower R w values than the central atom model with anharmonic thermal vibrations for SrAl 12 O 19 and SrGa 12 O 19 , whereas it gave an R w value approximately equal to that given by the central atom model with anharmonic vibration for SrFe 12 O 19 . The present study suggested that the potential around the M atom with trigonal bipyramidal coordination has double minima in all the magnetoplumbite-type crystals at lower temperatures.

β-BaB2O4 single crystal growth by Czochralski method. II

Abstract The development of a Czochralski technique for the growth of high quality single crystals β-BaB 2 O 4 is reported. The β phase crystal could be directly grown from a BaB 2 O 4 composition melt without flux elements, although the β phase is metastable. The size of the grown crystal was extended to 15 mm in diameter and 40 mm in length. Neither cracks nor inclusions were observed. The effects of the preparation method of the starting material and the crystal growth conditions for obtaining the β phase single crystal were investigated. The relation between the β phase nucleation and the interfacial energy was discussed.

The crystal structure of quinolinium bis(7,7,8,8-tetracyanoquinodimethanide), Q+(TCNQ)2−

Crystals of Q+(TCNQ)~are monoclinic, with space group C2/c and a = 28.468, b = 3.838, c = 25.704/~ and ,fl = 113.6 °. There are four formula units in the unit cell. The structure was deduced from Patterson syntheses and refined by the block-diagonal least-squares method to an R value of 0.120 for 1274 observed reflexions. The structure consists of columns of monadic units of TCNQ parallel to the b axis. The TCNQ molecules are stacked in a fashion characteristic of the structures of TCNQ salts. The average spacing between the TCNQ molecules in the column is 3.22/~. The shape and size of the TCNQ units seem to be intermediate between those of TCNQ ° and TCNQ-. Quinolinium cations are arranged in the channels formed by the TCNQ columns. The structure is disordered, with quinolinium cations adopting either of the alternative orientations with equal probability. They also form a stack parallel to the b axis with the much wider spacing of 3.50 A,. The estimation of molecular interaction energy for the system (TCNQ)~has revealed that the geometrical relationship of the two TCNQ units is determined primarily by the charge-transfer interaction and in part by interactions between the nonbonded atoms.

Structure analysis of MgSiO3 glass

Abstract The structure of MgSiO3 glass was investigated by a combination of radial distribution function (RDF) analysis with an intensity comparison method on the basis of X-ray diffraction data. There appeared five peaks at 1.63, 2.08, 2.58, 3.25 and 4.2 A in the RDF curve. The short range structure was found to be similar to that observed in pyroxenes: single chains composed of SiO4 tetrahedra are linked by Mg2+ ions laterally. The Mg2+ ion is surrounded by six oxygen atoms on average, four of which form MgO pairs with an average distance of 2.08 A and the remainder Mg O pairs of 2.5 A which contribute to the formation of the third peak at 2.58 A in the RDF curve together with the OO pairs (2.66 A) in SiO4 tetrahedra. The calculated intensity curve based on this model was in good agreement with the observed one.

Structure of barium chlorapatite

BasCl(PO4) 3, hexagonal, P63/m, a = 10.284 (2), c = 7.651 (3) A, Z = 2. The crystals were grown by hydrothermal reaction of BaCI2.2H20 and K2HPO 4 at 473 K. Refinements were carried out by the full-matrix least-squares method to R = 0.034 and R w = 0.039 with 810 independent reflexion data. Comparisons with the structures of other chlorapatites are presented. Introduction. Chlorapatites, MsCI(PO4) 3 (M = Ca, Ba, Sr, Mn, Cd, etc.), belong to the apatite group of compounds. The crystal structures have been investigated for CasCI(PO4) 3 (Mackie, EUiott & Young, 1972), CdsCI(PO4) 3 (Sudarsanan, Young & Donnay, 1973), SrsCI(PO4) 3 (Sudarsanan & Young, 1974) and Mn5Cl0.9OH0.1(PO4) 3 (Engel, Pretzsch, Gramlich & Baur, 1975) by X-ray diffraction. All these apatites have the space group P63/m, except CasCI(PO4) 3 which crystallizes in P2~/b. The locations of the CIions are not the same among these crystals. The anion occupies the special position 2(b) (0,0,0) in Sr5CI(PO4) 3, and 2(a) (0,0,1⁄4) in Cd5CI(PO4) 3 and MnsCl0.9OH0.t(PO4) 3. In CasCI(PO4)3, it lies at 4(e) (0,0,z) with z = 0.06, giving a similar atomic configuration to that of SrsCI(PO4) 3. As part of a study of the apatite group compounds, single crystals of BasCI(PO4) 3 were synthesized by a hydrothermal reaction and the crystal structure was refined. The structure is in principle the same as that of SrsCI(PO4)3. Single crystals of barium chlorapatite were prepared by hydrothermal reaction of BaC12.2H20 and K2HPO 4. 0.05 mol of BaC12.2H20 and 0.03 mol of K2HPO 4 in 50 ml of distilled water were heated at 473 K for 2 weeks. Single cv./stals of barium chlorapatites grew at pH 5-6 with BaHPO 4. The crystals obtained were colourless, transparent hexagonal prisms 0 .1-0 .3 mm in length and 0 .03-0.1 mm in diameter. The infrared spectrum of BasCI(PO4) 3 was recorded in the range 4000 to 400 cm -~, but the OH stretching band was not observed. This indicates that C1 is not replaced by OH in the crystal. 0567-7408/79/102382-03501.00 Weissenberg photographs showed Laue symmetry 6/m and the systematic absences 001 for l odd. The possible space groups are, therefore, P63/m (centrosymmetric) and P63 (noncentrosymmetric). P6a/m was employed in the structure refinement, since all the apatite group compounds known to date belong to this space group. Intensities were measured on an automated fourcircle diffractometer (Philips PW 1100/20), with graphite-monochromated Mo Kt~ radiation, by the 20-o3 scan technique with a scan speed of 4 ° min -~ in 09, using a crystal 0.15 mm in length and 0.06 mm in diameter. 810 independent reflexion data IF o > 3a(Fo)] were obtained within the range 20 _< 80 °. Intensities were corrected for Lorentz and polarization factors. Absorption corrections (p = 14.868 mm -1 for Mo KCt) were made with the program A CACA (Wuensch & Prewitt, 1965). The refinement was started from the parameters of cadmium hydroxyapatite, CdsOH(PO4) 3 (Hata, Okada, Iwai, Akao & Aoki, 1978), and the position of CI was determined in the difference Fourier syntheses. The calculation with the full-matrix least-squares program LINUS (Coppens & Hamilton, 1970) converged to the residual R = ~ [IFol IFcl~/~ IFol = 0-034 and the weighted residual R,. = ~ w(IFol -IFcl)2/~ WlFo 12 = 0.039 with anisotropic temperature factors. The weighting scheme employed was that of Hughes, with w = 1 if F , < Fomax, and w = Fomax/F o if Table 1. Final positional parameters (x 104) and isotropic thermalparameters (x 104) for BasCI(PO4) 3

Structural analysis of CaSiO3 glass by X-ray diffraction and Raman spectroscopy☆

Abstract The short-range structure of CaSiO3 glass was studied by a combination of radial distribution analysis with an intensity comparison method on the basis of X-ray diffraction data. Three main peaks appeared at 1.64, 2.43 and 3.64 A in the radial distribution function curve. The Raman bands were observed at 367, 642, 878 and 972 cm−1 and the Raman spectrum was found to be similar to that observed in pyroxenes: single chains consisting of SiO4 tetrahedra are linked by calcium ions laterally. The calcium ion is surrounded by six oxygen atoms at an average distance of 2.34 A and the remainder CaO pairs at a distance of 2.54 A. The calculated intensity curve based on this model was in good agreement with the observed one.

Structural analysis of 12CaO.7Al2O3glass

Abstract The short-range structure of 12CaO.7Al 2 O 3 glass was investigated by the radial distribution function method together with the intensity comparison method. The coordination numbers of Al and Ca atoms were estimated to be 4.2 and 5.6, with average metal-oxygen distances of 1.77 and 2.37 A, respectively. Other peaks in the radial distribution function D(r) curve were observed at 3.20, 3.55 and 6.2 A. Three structure models were examined to interpret the D(r) curve: crystalline 12CaO.7Al 2 O 3 consisting of three dimensional framework, crystalline 5CaO.3Al 2 O 3 consisting of sheets and crystalline (Mg,Fe,Ca)O.SiO 2 consisting of chains. Among these models, the 5CaO.3Al 2 O 3 model gave good agreement with the observed and calculated S.i(S) curves.

A high resolution exafs and near edge study of GeO2 glass

High resolution Ge K-EXAFS and XANES spectra were measured at 80 K and 300 K for trigonal and tetragonal GeO2 crystals, and GeO2 glass by using synchrotron radiation. By comparing the χ(k) and ¦F(r)¦ curves of these samples, it was found that the averaged short-range structure of GeO2 glass resembles that of trigonal GeO2 crystals. By the parameter fitting method, the Ge-O and Ge-Ge distances of the GeO2 glass were determined to be 1.74 and 3.15 A respectively, which are close to those of trigonal GeO2 crystals. However, an apparent Debye-Waller type factor for Ge-Ge pairs of GeO2 glass is larger than that of trigonal GeO2 crystals. The fine structures observed in high resolution near-edge spectra indicate that the linkage of GeO4 tetrahedra in the GeO2 glass is quite similar to that in trigonal GeO2 crystals.