Proton ordering in (NH4)3H(SO4)2 at low-temperature phase transitions.

Single-crystal neutron diffraction was used to investigate the H-atom disorder in triammonium hydrogen disulfate (TAHS), (NH4)3H(SO4)2, below room temperature. Crystal structure analysis of the monoclinic phase III shows an increase of proton ordering with decreasing temperature in the (SO4)H(SO4) dimer. Moreover, the NH4(+) groups on a general position begin ordering in this phase. The monoclinic unit cell of TAHS-IV doubles in the b direction and a slight distortion of SO4(2-) and NH4(+) tetrahedra is observed. The order parameter introduced by Landau was determined for the second-order II/III and III/IV phase transitions from the intensities of the superstructure reflections. TAHS-V has a triclinic space group and the crystal structure seems to be completely ordered according to a structure analysis by single-crystal X-ray diffraction measurements. In addition, the decisive role of the dynamical disorder of different ammonium groups on successive phase transitions is discussed. Additional peaks were observed by X-ray powder diffraction measurements at ∼ 70 K on cooling, which refers to the V/VII phase transition. These additional peaks remained up to ∼ 85 K on heating. They were described with a doubling of the unit cell along all three principal crystallographic directions.

Disorder of (NH4)3H(SO4)2 in the high-temperature phase I.

The highly disordered crystal structure of triammonium hydrogen disulfate, (NH(4))(3)H(SO(4))(2), in the high-temperature phase I was studied using single-crystal neutron diffraction. It is known that the O atom involved in hydrogen bonding between neighbouring SO(4) tetrahedra is disordered and takes a split-atom position, building a two-dimensional hydrogen-bond network in the (001) plane. The H atoms in these SO(4)-H-SO(4) hydrogen bonds are disordered and hence refined with a split-atom model. Moreover, from the much larger anisotropic mean-square displacements of ammonium protons the NH(4)(+) groups were refined with a reasonable split-atom model, and their motional behaviour was also analysed by rigid-body treatment. Finally, careful consideration was given to show possible supplementary proton migration between the ammonium protons and those of the hydrogen bonds in this high-temperature phase.

Ba3Li2V2O7Cl4, a new vanadate with a channel structure.

The tribarium dilithium divanadate tetrachloride Ba(3)Li(2)V(2)O(7)Cl(4) is a new vanadate with a channel structure and the first known vanadate containing both Ba and Li atoms. The structure contains four non-equivalent Ba(2+) sites (two with m and two with 2/m site symmetry), two Li(+) sites, two nonmagnetic V(5+) sites, five O(2-) sites (three with m site symmetry) and four Cl(-) sites (m site symmetry). One type of Li atom lies in LiO(4) tetrahedra (m site symmetry) and shares corners with VO(4) tetrahedra to form eight-tetrahedron Li(3)V(5)O(24) rings and six-tetrahedron Li(2)V(4)O(18) rings; these rings are linked within porous layers parallel to the ab plane and contain Ba(2+) and Cl(-) ions. The other Li atoms are located on inversion centres and form isolated chains of face-sharing LiCl(6) octahedra.

The diphyllosilicate Rb2(VO)2[Si8O19].

Dirubidium divanadyl phyllooctasilicate, Rb(2)(VO)(2)[Si(8)O(19)], is the first known anhydrous diphyllosilicate containing V(IV). The structure consists of silicate double layers which are separated by [V(2)O(8)](8-) dimers and is related to that of the compounds A(2)Cu(2)[Si(8)O(19)] (A = Rb or Cs), although the title compound crystallizes in a noncentrosymmetric orthorhombic space group. The silicate double layers contain four tetrahedrally coordinated Si sites in general positions and 12 O sites, nine in general positions and the other three on mirror planes. The vanadyl dimers have two square-pyramidally coordinated V sites (site symmetry m). There are two different 10- and 12-fold coordinated Rb sites with site symmetry m, one of which is a split position located between the dimers in the interlayer space, while the other is in a channel within the silicate layer.

Temperature dependence of the AV3O7 (A = Ca, Sr) structure.

The V(4+) (spin (1/2)) oxovanadates AV(3)O(7) (A = Ca, Sr) were synthesized and studied by means of single-crystal X-ray diffraction. The room-temperature structures of both compounds are orthorhombic and their respective space groups are Pnma and Pmmn. The previously assumed structure of SrV(3)O(7) has been revised and the temperature dependence of both crystal structures in the temperature ranges 297-100 K and 315-100 K, respectively, is discussed for the first time.

Synthesis and structural characterization of BaV4O9.

The new spin (1/2) V(4+) barium oxovanadate BaV(4)O(9) was synthesized and studied by means of single-crystal X-ray diffraction. Its room-temperature structure is monoclinic, space group P2/c. We discuss the temperature evolution of the crystal structure and thermal expansion tensor of the material between 293 and 100 K.

Modulated corrugations in the crystal structure of the superconductor CaAlSi.

We report the crystal structure analyses of CaAlSi from single-crystal and powder X-ray diffraction and the existence of two commensurately modulated phases, a sixfold and a fivefold modulated structure. This polymorphism seems to be correlated to the thermal history of the sample. We describe both modulated structures using a three-dimensional and a (3 + 1)-dimensional formalism.

Reinvestigation of the structure of BaCuSi(2)O(6) -- evidence for a phase transition at high temperature.

Superstructure reflections have been observed in the room-temperature X-ray diffraction pattern of BaCuSi(2)O(6), barium copper disilicate. The tetragonal structure has a fourfold unit-cell volume compared with the original structure determined by Finger et al. At T(s) = 610 K BaCuSi(2)O(6) undergoes a structural phase transition upon which the superstructure reflections disappear. The description of the structure in the larger cell removes the crystal-chemical inconsistencies observed for the original structure.