Pseudo-merohedrally twinned praseodymium hexacyanoferrate(III) tetrahydrate

Crystals of the title compound, diaquahexa-mu-cyano-ferrate(III)praseodymium(III) dihydrate, Pr[Fe(CN)6]\ub74H2O or [PrFe(CN)6(H2O)2]\ub72H2O, are twinned with three components. The Pr atom is coordinated by eight atoms, viz. six N and two symmetry-related water O atoms. The Pr polyhedron (Pr has site symmetry m2m, Wyckoff position 4c) is linked to an FeC6 octahedron (Fe located on a site with imposed 2/m symmetry, Wyckoff position 4b) through N atoms, forming an infinite array. The second (symmetry independent) water molecule lies on a mirror plane, is not included in coordination and is weakly hydrogen bonded to N atoms

Pseudo-merohedrally twinned praseodymium hexacyanoferrate(III) tetrahydrate

Crystals of the title compound, diaquahexa-mu-cyano-ferrate(III)praseodymium(III) dihydrate, Pr[Fe(CN)6]\ub74H2O or [PrFe(CN)6(H2O)2]\ub72H2O, are twinned with three components. The Pr atom is coordinated by eight atoms, viz. six N and two symmetry-related water O atoms. The Pr polyhedron (Pr has site symmetry m2m, Wyckoff position 4c) is linked to an FeC6 octahedron (Fe located on a site with imposed 2/m symmetry, Wyckoff position 4b) through N atoms, forming an infinite array. The second (symmetry independent) water molecule lies on a mirror plane, is not included in coordination and is weakly hydrogen bonded to N atoms

γ-Alumina: a single-crystal X-ray diffraction study

The structure of γ-alumina (Al21+1/3◊2+2/3O32) crystals obtained as a product of a corrosion reaction between β-sialon and steel was refined in the space group Fd-3m. The oxygen sublattice is fully occupied. The refined occupancy parameters are 0.83(3), 0.818(13), 0.066(14) and 0.044(18) for Al ions in 8a, 16d, 16c and 48f positions, respectively. The Al ions are distributed over octahedral and tetrahedral sites ina 63:37 ratio, with 6% of all Al ions occupying non-spinelpositions

Dipotassium heptafluorotantalate(V), &#946-K2TaF7, at 509 K

Upon heating to 509 K, monoclinic &#945-K2TaF7 (P21/c) transforms to orthorhombic &#946-K2TaF7 (Pnma) which is composed of discrete K+ cations and TaF72- anions. In the anion, the Ta atom is sevenfold coordinated by F atoms, with the Ta-F distance varying from 1.88(2) to 1.965(19) \uc5. The two independent K+ cations are surrounded by F atoms positioned within 2.740(17)-2.979(10) and 2.989(17)-3.15(5) \uc5. Ta, both K and one of the four F atoms are located on a mirror plane, Wyckoff position 4c

Layered strontium phenylphosphonate: synthesis, thermal properties and crystal structure from X-ray powder diffraction data

Strontium phenylphosphonate, Sr(PhPO3H)2, was synthesized by the reaction of strontium carbonate and phenylphosphonic acid. The compound was structurally characterized by Rietveld refinement on X-ray powder diffraction data. It consists of a layered structure with inorganic framework of SrO8 polyhedra from which phenyl groups are pointing out. The compound is isomorphous with the already known calcium and barium phenylphosphonates. Infrared spectroscopy analysis shows the presence of (P-)O-H...O hydrogen bonds in the solid material. The thermal behavior of the compound in the range 30650\ub0C, studied by thermogravimetry (TG), differential thermal analysis (DTA) and X-ray powder thermodiffractometry, revealed that strontium phenylphosphonate is stable up to 330\ub0C. The compound undergoes consequent thermal decomposition and phase transitions above 330\ub0C temperatures until it converts to crystalline Sr(PO3)2 above 570\ub0C

Redetermination of Na3TaF8

The crystal structure of trisodium octafluoridotantalate,Na3TaF8, has been redetermined using diffractometer data collected at 153 K, resulting in more accurate bond distances and angles than obtained from a previous structure determination based on film data. The structure is built from layers running along [101], which are formed by distorted [TaF8] antiprisms and [NaF6] rectangular bipyramids sharing edgesand corners. The individual layers are separated by eightcoordinated Na ions. Two atoms in the asymmetric unit are in special positions: the Ta atom is on a twofold axis in Wyckoff position 4e and one of the Na ions lies on an inversion centre in Wyckoff site 4d

Static and dynamic structure of deca-dodecasil 3R

The structure of calcined deca-dodecasil 3R (DD3R), with formula Si120O240, a very well-suited material for the synthesis of inorganic/organic composites structured on a nanometer level, was investigated in details. So far, a highly complicated twinning has hampered its structure description on a desirable level of precision and accuracy. This twinning has been resolved and a new structure determination was completed. Structure refinement done in the R-3 space group revealed a rather large Ueq of a bridging oxygen (O), shorter than expected bridging Si - O bonds and the straight Si-O-Si bond angle dictated by the atom positions at a threefold axis. A structure model based on statically displaced bridging O-atom on one hand improved accuracy of Ueq s and of the Si-O bonds of interest, but provided unacceptable O-O contacts. To explain this dilemma ab initio molecular dynamics calculations were done in order to inspect possible configurations. The finite-temperature simulations of the dynamical properties were done at 300 K and a canonical ensemble with a Nos\ue9 thermostat procedure was used. The Verlet velocity algorithm with (i) a time step ∆t = 1fs (fast sampling) and simulation time of 4 ps and (ii) ∆t = 4fs (slow sampling) and simulation time of 20 ps were used. The x,y coordinates of Si-O-Si atoms corresponded to the time-averaged structure with all three atoms are on a threefold axis. It was thus evident, that structure analysis itself cannot provide a completely true picture of DD-3R structure. To reveal possible dynamic disorder in the structure and to analyze the spatial distribution of the atoms during a longer time interval, the 20ps MD simulation was used. To un-reveal basic features hidden in such a time series wavelet denoising and non-decimating wavelet transform were applied. Synchronous changes of both Si-O distances pointed to synchronous movement of the whole building units (tetrahedra). To calculate the frequency of such a Si-O-Si bending vibration total and partial vibrational density of states were calculated from atom velocities. The interpretation of calculated spectra could be based on INS spectra of various silica polymorphs collected by several authors. It can be concluded that a good agreement of the low frequency features in the INS spectra with our results from MD indicate, that the nature of disorder in DD-3R is dynamic rather than static

Second-degree twinning and dynamic disorder in the crystal structure of deca-dodecasil 3R

The structure of deca-dodecasil 3R (DD-3R), Si120O240, a very well suited material for the synthesis of inorganic/organic composites structured on a nanometer level, has been investigated in detail. So far, a highly complicated twinning has hampered its structure description at a desirable level of accuracy. This twinning has now been resolved and a new structure determination is presented. Structure refinement in the R-3 space group revealed a large, unusually shaped atomic displacement ellipsoid for oxygen-bridging units (tetrahedra), bridging Si-O bonds shorter than expected and the linear Si-O-Si\u27 bond angle dictated by special positions at a threefold axis. A structure model based on a statistically disordered bridging O atom improved the accuracy of the Si-O bonds of interest, but provided unacceptable O-O contacts. To solve this dilemma, ab initio NVT molecular dynamics calculations were performed to study the possible configurations. Wavelet analysis of the time variations of selected Si-O distances pointed to a synchronous shift of the whole building units (tetrahedra). Low-frequency features of the calculated phonon density of states agree well with the published INS (inelastic neutron scattering) spectra of several silica polymorphs, indicating that the nature of the disorder in DD-3R is dynamic rather than static