Sebastian Prinz

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.

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.

Influence of different grinding wheel and dressing roller specifications on grinding wheel wear

A targeted adjustment of the dressing results and the methodological influence of the dressing process on the non-stationary wear of a grinding wheel after dressing increases the productivity and the reproducibility of grinding processes. Despite the great economic importance of grinding processes with vitrified corundum grinding wheels and the great relevance of the dressing process for the application behavior of these grinding wheels, quantitative models are missing for the purposeful design of the dressing process. In previous studies, a dressing model was successfully developed which predicts the dressing force in the dressing process as well as the workpiece roughness and the grinding wheel wear behavior in a grinding process for a specific grinding wheel and form roller specification. However, a transferability of this model to other grinding wheel and form roller specifications is not possible because the influence of the grain size and the hardness of the grinding wheel as well as the dressing tool topography on the grinding wheel wear and thus on parameters of the dressing model are not known. The objective of this work was to extend the model to additional grinding wheel and form roller specifications to ensure a broad applicability of the model.

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.