The localization of hydrogen atoms is an essential part of crystal structure analysis, but it is difficult because of their small scattering power. We report the direct localization of hydrogen atoms in nanocrystalline materials, achieved using the recently developed approach of dynamical refinement of precession electron diffraction tomography data. We used this method to locate hydrogen atoms in both an organic (paracetamol) and an inorganic (framework cobalt aluminophosphate) material. The results demonstrate that the technique can reliably reveal fine structural details, including the positions of hydrogen atoms in single crystals with micro- to nanosized dimensions.

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Phosphate tungsten bronzes have been shown to be conductors of low dimensionality. A review of the crystallographic and structural properties of this huge series of compounds is given here, corresponding to the present knowledge of the different X-ray studies and electron microscopy investigations. Three main families are described, monophosphate tungsten bronzes, A x (PO 2 ) 4 (WO 3 ) 2m , either with pentagonal tunnels (MPTBp) or with hexagonal tunnels (MPTBh), and diphosphate tungsten bronzes, A x (P 2 O 4 ) 2 (WO 3 ) 2m , mainly with hexagonal tunnels (DPTBh). The general aspect of these crystal structures may be described as a building of polyhedra sharing oxygen corners made of regular stacking of WO 3 -type slabs with a thickness function of m, joined by slices of tetrahedral PO 4 phosphate or P 2 O 7 diphosphate groups. The relations of the different slabs with respect to the basic perovskite structure are mentioned. The structural description is focused on the tilt phenomenon of the WO 6 octahedra inside a slab of WO 3 -type. In this respect, a comparison with the different phases of the WO 3 crystal structures is established. The various modes of tilting and the different possible connections between two adjacent WO 3 -type slabs involve a great variety of structures with different symmetries, as well as the existence of numerous twins in MPTBp's. Several phase transitions, with the appearance of diffuse scattering and modulation phenomena, were analysed by Xray scattering measurements and through the temperature dependence of various physical properties for the MPTBp's. The role of the W displacements within the WO 3 -type slabs, in two modulated structures (m = 4 and m = 10), already solved, is discussed. Finally, the complexity of the structural aspects of DPTBh's is explained on the basis of the average structures which are the only ones solved.

Large crystals of the monoclinic a-CoV 2 O 6 cobaltite have been grown. A clear dielectric anomaly is found at 15 K which coincides with the magnetic ordering temperature. For large enough magnetic field values, this upturn in the 3 0 (T) curve is suppressed. This is related to the different magnetic states (antiferro (AF), ferri (FI) and ferromagnetic (FO)) revealed by the isothermal magnetization curves. At 5 K, the magnetodielectric curves are found to mimic the M(H) T ¼ 5 K curves: two abrupt changes are observed at H C1 ¼ 1.65 T and H C2 ¼ 3.3 T which correspond to the AF to FI and FI to FO transitions, respectively, and also hysteretic behavior for T < 5 K. These features demonstrate the strong coupling between magnetic and dielectric order parameters.6436 | J. Mater. Chem.,

Single crystals of the stoichiometric iron calcium oxysulfide CaOFeS have been grown by a solid-state reaction. Structural analysis of CaOFeS at room temperature by combining single-crystal X-ray diffraction data and transmission electron microscopy leads to a stoichiometric hexagonal noncentrosymmetric P63mc layered structure isostructural to CaOZnS. It is built from alternating layers made of FeOS3 tetrahedra sharing sulfur apexes and stacked with Ca(2+) planes. All Fe-O bonds are parallel to the stacking axis; this breaks the centrosymmetry, leading to a polar structure. The dielectric measurements reveal the existence of a magnetodielectric effect near 33 K in good agreement with the Neel temperature, as evidenced near 35 K by specific heat measurements reported by a different group.

A new ternary layered compound In2Ge2Te6, belonging to the hexatellurogermanate family has been synthesized from the reaction of appropriate amounts of the pure elements at high temperature in sealed silica tubes.