On the identification of oxygen and fluorine atoms in disordered inorganic oxyfluoride compounds

It is considered practically impossible to differentiate between oxygen and fluorine atoms by X-ray diffraction in disordered structures of oxyfluoride compounds due to the similarity of their ionic radii and diffusion factors. Indeed, many transition metal oxyfluoride compounds containing polar pseudo-octahedral MO_xF_6–x (x = 1-3) anions form crystal structures without any fluorine-oxygen (F/O) ordering owing to a large number of local anion configurations. Because of this static disorder, it is impossible to determine the positions of O and F atoms and find the real geometry of the polyhedron. However, this becomes possible in the case of dynamic disorder of oxyfluoride anions when the central atom is displaced from the center of the octahedron toward a vertex, edge, or face (depending on the number of oxygen atoms in the polyhedron), which enables the identification of O and F atoms owing to inherent differences between M–O and M–F bonding. On cooling, such compounds undergo phase transitions of the order–disorder type with substantial changes in the entropy. The examples of static and dynamic orientational disorder in oxyfluoride compounds of d⁰ transition metals are given.     

Crystal structure and NMR study of 4-amino-1,2,4-triazolium hexafluoridoniobate(V) and hexafluoridotantalate(V)

4-Amino-1,2,4-triazolium hexafluoridoniobate(V) and hexafluoridotantalate(V) (C₂H₅N₄)MF₆ (M = Nb, Ta) crystallizing in the monoclinic system (space group P2₁/n) are synthesized for the first time and their crystal structures and spectroscopic features are studied by single crystal X-ray diffraction and ¹H and ¹⁹F NMR spectroscopy. The crystal structures of isostructural (C₂H₅N₄)MF₆ compounds are formed of octahedral complex [MF₆]^– anions (M = Nb, Ta) and monoprotonated heterocyclic 4-amino-1,2,4-triazolium cations (C₂H₅N₄)⁺ organized in a three-dimensional structure via N–H···F and N–H···N hydrogen bonds. The character and types of ion motions in the fluoride sublattice of (C₂H₅N₄)MF₆ are determined in a wide temperature range.     

Structural investigation of cooperite (PtS) crystals

The single-crystal structure of cooperite, a natural platinum sulfide PtS, is studied by X-ray diffraction supported by high-resolution scanning transmission electron microscopy and X-ray spectrum microanalysis. It is found that, in addition to the main reflections corresponding to the known tetragonal cell (a = 3.47 and c = 6.11 Å; space group P4₂/mmc), many weak reflections with intensities I ≤ 60σ(I) are clearly observed. These reflections fit the tetragonal cell (space group I4/mmm) with doubled parameters. In structures with small (P4₂/mmc) and large (I4/mmm) cells, the S atoms occupy statistically two special positions. It is shown that the chemical composition of the cooperite crystals deviates from the stoichiometric composition: sulfur-deficient specimens predominate.     

Crystal Structures of Ammonium and Cesium Pentadecafluorotetraantimonates(III)

Antimony(III) fluoride complexes with compositions (NH₄)₃Sb₄F₁₅(I) and Cs₃Sb₄F₁₅(II) are structurally characterized. Crystals Iare triclinic: a= 8.317(3) Å, b= 10.419(6) Å, c= 10.826(3) Å, = 63.71(4)°, = 73.24(3)°, = 77.42(5)°, Z= 2, _calcd= 3.42 g/cm³, _exp= 3.45 g/cm³, space group P , R= 0.051; crystals IIare monoclinic: a= 8.079(2) Å, b= 29.116(8) Å, c= 8.162(2) Å, = 117.08(2)°, Z= 4, _calcd= 4.549 g/cm³, _exp= 4.50 g/cm³, space group P2₁/c, R= 0.036. Structure Iis composed of SbF₄E and SbF₅E polyhedra combined into tetranuclear chains; crystals IIconsist of SbF₄E polyhedra, two of which form a dimer Sb₂F₇E₂, while the other two are isolated (E is the antimony lone electron pair).     

The Technique of Studying X-Ray Scattering over Wide Temperature Range in an Electric Field

The technique of studying the synchrotron radiation scattering on single crystals over wide temperature range using an original miniature device for applying an electric field to a sample is described. A specific feature of the device is the possibility of its application with various heating and cooling systems. The technique was used at BM01 beamline at the European Synchrotron Radiation Facility (ESRF) for the study of the influence of electric field on the processes of structural transformation in lead zirconate–titanate single crystal with a low titanium concentration. The efficiency of the technique proposed in this work is demonstrated and the results of the preliminary analysis of the results are presented.

     

Crystal structure of acrylamide

The crystal structure of acrylamide is re-determined by single crystal X-ray diffraction (133(1) K, BRUKER SMART 1000 CCD, a = 8.228(1) Å, b = 5.759(1) Å, c = 9.760(1) Å, β = 120.04(1)°, V = 400.3(1) Å,³, space group P2₁/c, Z = 4, R = 0.0543 for 867 reflections). In the structure strong hydrogen bonds N-H...O join the molecules of C₃H₅NO into bi-molecular layers that make C...C molecular contacts. It is demonstrated that the process of solid phase polymerization of acrylamide should proceed through the cleavage of double bonds C(1)=C(2) in the monomers and formation of bonds C(1)-C(1) and C(2)-C(2) between the closest carbon atoms of different layers.