Volume 222, Issue 8

Intensive researches on specific treatments of radioactive cesium, occurring in high level nuclear wastes, are in progress in France. Nowadays, a cesium immobilization in host matrices with a high chemical durability seems to be the favourite option. In that perspective, titanium based upon hollandite is a good candidate as a host matrix because of its high cesium incorporation ability, its excellent chemical stability, and its capacity to support charge compensation during the Cs + into Ba 2+ transmutation process. In this study we report the preparation and crystal growth in a BaF 2 —B 2 O 3 flux melt of the Ba x Cs y M z Ti 8– z O 16 hollandite phase and its characterization by single crystal X-ray diffraction. An attention was focused on materials with M = Al or Al + Fe and with barium only since single crystals including cesium could not be obtained from the flux method. An incommensurate modulation of the Ba cation site occupancy, with I 4/ m (00 γ )00 as a superspace group, was observed for all the single crystals under study.

The crystal structure of Li 4 [(UO 2 ) 2 (W 2 O 10 )] ( 1 ) has been solved by direct methods and refined to R 1 0.036. The compound is orthorhombic, Cmc 2 1 , a = 7.3823(3), b = 14.0498(5), c = 10.1887(3) Å, V = 1056.77(7) Å 3 . The structure is based upon uranyl tungstate sheets with the composition [(UO 2 ) 2 (W 2 O 10 )] 4– . The sheets are strongly corrugated and are parallel to the (001) plane. The sheet can be described as consisting of two types of chains. UO 7 pentagonal bipyramids share edges to form UO 5 chains, whereas WO 6 octahedra share corners to form WO 5 chains. Both chains extend along the a axis. The Li + cations reside in between the sheets providing their linkage into a three-dimensional structure. The structure can be considered as a derivative of complex oxides observed in the U—W—O and U—Mo—W—O systems. However, the topology of polyhedral linkage is different as a result of the presence of Li + cations in the structure. The crystal chemistry of Li uranyl tungstates is discussed.

We have determined all eight components of the piezoelectric tensor of a monoclinic crystal, BiB 3 O 6 , by means of synchrotron X-ray diffraction technique. The experiments have been performed on a four circle goniometer using the ω -scan method introduced by Graafsma [1] which is universal in the choice of appropriate reflections. The determined values of the piezoelectric tensor are in good agreement with those obtained earlier for BiB 3 O 6 with the use of an optical Michelson interferometer.

The fractions of aluminium, [Al]/[Al + Si], and oxygen, [O]/[O + N], in crystallographically distinct sites of BaYb[Si 4– x Al x O x N 7– x ] oxonitridoaluminosilicate (space group P 6 3 mc , No. 186) were refined based on the results of neutron powder diffraction for a synthetic sample with the composition of x = 2.2(2) and simulated as functions of temperature for the compositions x = 2 and x = 2.3 using a combination of static lattice energy calculations (SLEC) and Monte Carlo simulations. The SLEC calcu lations have been performed on a set of 800 structures differing in the distribution of Al/Si and O/N within the 2 × 2 × 2 supercell containing 36 formula units of BaYb[Si 4– x Al x O x N 7– x ]. The SLEC were based on a transferable set of empirical interatomic potentials developed within the present study. The static lattice energies of these structures have been expanded in the basis set of pair-wise ordering energies and on-site chemical potentials. The ordering energies and the chemical potentials have been used to calculate the configuration energies of the oxonitridoaluminosilicates (so-called sialons) using a Monte Carlo algorithm. The simulations suggest that Al and O are distributed unevenly over two non-equivalent T(Si/Al) and three L(N/O) sites, respectively, and the distribution shows strong dependence both on the temperature and the composition. Both simulated samples exhibit order/disorder transitions in the temperature range 500–1000 K to phases with partial long-range order below these temperatures. Above the transition temperatures the Si/Al and N/O distributions are affected by short-range ordering. The predicted site occupancies are in a qualitative agreement with the neutron diffraction results.

High resolution neutron powder diffraction data recorded at 4.5 K confirm the orthorhombic structure of methyl iodide reported previously by Kawaguchi et al. (1973) from an X-ray single-crystal study at 193 K. The present study, using neutron powder diffraction, provides more precise structural parameters and reliable atomic co-ordinates for the hydrogen atoms enabling the methyl group libration to be analysed as a function of temperature up to 193 K.

The polymorphism of Ag 3 VO 4 was investigated with temperature dependent synchrotron powder diffraction, and differential scanning calorimetry in the temperature range 295 K ≤ T ≤ 893 K. Three crystal structures of Ag 3 VO 4 were identified and determined from powder diffraction data: α -Ag 3 VO 4 ( T = 295 K, C 2/ c , a = 10.2672(2) Å, b = 4.9814(1) Å, c = 10.2240(2) Å, β = 116.0(1)°, V = 470.0(2) Å 3 ), β -Ag 3 VO 4 ( T = 423 K, I -42 m , a = 4.9968(1) Å, c = 9.6911(3) Å, V = 241.97(1) Å 3 ), and γ -Ag 3 VO 4 ( T = 703 K, Fm -3 m , a = 7.8386(1) Å, V = 481.62(1) Å 3 ). The crystal structures of all three phases of Ag 3 VO 4 can be described as cubic closed packed arrangements of [VO 4 ] 3– anions with the silver cations occupying all octahedral and tetrahedral voids.

( 1 ) C 6 H 2 N 3 O 7 – · C 5 H 12 NO 2 + , Mr = 346.26, P 2 1 / c , a = 7.2356(6), b = 10.5765(9), c = 19.593(2) Å, β = 95.101(6)°, V = 1493.5(2) Å 3 , Z = 4, R 1 = 0.0414; ( 2 ) C 6 H 2 N 3 O 7 – · C 6 H 8 NO + , Mr = 338.24, P 2 1 / n , a = 7.8713(5), b = 6.1979(7), c = 28.697(3) Å, β = 90.028(7)°, V = 1400.0(2) Å 3 , Z = 4, R 1 = 0.0416. The packing units in both compounds consist of hydrogen bonded cation-anion pairs. The (hyper)polarizabilities have been calculated for the crystallographic and optimized molecules, by AM1 and at the DFT/B3LYP(6-31G**) level.

The complex [Cu(C 4 H 12 N 2 ) 2 (H 2 O) 2 ] 2+ [(C 4 H 4 NO 4 S) – ] 2 , which is the first structurally characterized complex with acesulfamate as the counter anion, has been synthesized and characterized by elemental analyses, magnetic and conductivity measurements, UV/vis, FT-IR and mass spectra. The thermal behaviour of the complexes was studied by simultaneous TG, DTG and DTA methods in a static air atmosphere. In the crystal structure, the copper cation, lying on a crystallographic centre of symmetry, adopts an elongated octahedral trans -[CuN 4 O 2 ] coordination geometry, with long axial Cu–O(aqua) bond length and short equatorial Cu–N bonds, of 2.479(2), 2.035(2) and 2.051(2) Å, respectively. The coordinated water molecules together with the amine part of the dimethylethylene diamine molecule link the acesulfamate ions to the metal complex via intermolecular hydrogen bonds.

4- o -(4-methoxybenzoyl)-2-methyl- N -(2-pyridal)-2H-1,2-benzothiazine-3-carboxamide-1,1-dioxide, C 23 H 19 N 3 O 6 S (I), a piroxicam derivative has been synthesized and characterized by X-ray powder structure analysis. The molecules of (I) are linked through pairs of intermolecular N–H···O and C–H···O hydrogen bonds forming centrosymmetric dimeric R 2 2 (14)(M) and R 2 2 (22)(N) rings which combine to form infinite one-dimensional MNMN… chain along the [2¯11] direction. The C–H···π (arene) hydrogen bonds interlink the parallel chains into two-dimensional sheets lying in the (102) plane. These sheets are further connected through intermolecular C–H···O and C–H···N bonds, so generating a three-dimensional framework structure in (I).