John M. Newsam

Structural studies of the high-temperature modifications of sodium and silver orthophosphates, II-Na3PO4 and II-Ag3PO4, and of the low-temperature form I-Ag3PO4

Abstract The crystal structures of the high-temperature modifications of sodium and silver orthophosphates have been determined using powder neutron diffraction (PND) data. II-Na3PO4 adopts the space group Fm3m with a=7.512(3) A at 400°C. The PO3−4 group is centered around the origin, but it shows high orientational disorder. The sodium ions occupy the 1 4 1 4 1 4 and 1 2 1 2 1 2 sites. II-Ag3PO4, at 650°C, is similar with a=7.722(5) A . The structure of I-Ag3PO4 at room temperature ( P 4 3 n , a=6.0095 (6) A ) has been re-examined by single-crystal X-ray diffraction. The derived model, with R=0.019 for 116 independent reflections, is in agreement with the latest work reported in the literature. The structure of I-Ag3PO4 at 375°C, as determined by PND, has a=6.061 (1) A , and displays no gross modifications from that observed at 25°C, although the anisotropic nature of the silver sites is markedly more pronounced at this higher temperature. The cation mobility is discussed in relation to the high-temperatures structures.

Non-stoichiometry in beta aluminas

Abstract The nature of non-stoichiometry in beta alumina is discussed and mechanisms of non-stoichiometric compensation reviewed. Particular attention is given to the modification of non-stoichiometry, notably demonstrated in investigation of hydrogen beta aluminas where more-nearly-stoichiometric materials have been obtained. We survey the properties of both non-stoichiometric and more-nearly-stoichiometric beta aluminas and provide interpretations of the dependence on stoichiometry of conducting plane composition and structure, lattice constants and ionic conductivity.

Slow Neutron Polarization by Longitudinally Polarized Proton Filter

The neutron beam with the energy between 10-2 eV and 10 eV could be polarized by the longitudinally polarized proton filter. In this experiment, we found that the polarization cross section for the longitudinally polarized filter is approximately the same value as that for the transversely polarized one.

Molecular Dynamics Simulations of Metal Clusters and Metal Deposition on Metal Surfaces

The geometries and energies of Pdn clusters (n=2-15) and the deposition of Pd atoms on the Cu(110) surface at several temperatures have been studied by molecular dynamics using embedded-atom type potentials. Compact structures were obtained for the small Pdn clusters; these geometries are compared with results for Many-Body Alloy (MBA) and Lennard-Jones potentials. For Pd atoms on the Cu(110) surface, the simulations indicate no substantial Pd penetration into the bulk, but unconstrained thin slab models do exhibit surface-driven melting and Pd penetration. The motion Pd adatoms on the surface is influenced by the surface dynamics, which manifests anharmonicity and roughening.

The solid solutions Na3(1−x)Alx▭2xPO4: room temperature powder neutron diffraction studies of the two compositions x=0.025 and x=0.2

Abstract The system Na 3(1−x) Al x ▭ 2x PO 4 , where ▭ represents a cation vacancy, has been examined under ambient conditions at the two compositions x =0.025 and x =0.2 by powder neutron diffraction. The presence of Al 3+ ions stabilises the high-temperature face-centred cubic phase of sodium orthophosphate, II-Na 3 PO 4 , with a 0 =7.424(2) A (x=0.025) and a 0 =7.421(2) A (x=0.2) at 25°C. At the lower dopant concentration, the structure corresponds closely to that of II-Na 3 PO 4 . The PO 3- 4 groups are orientationally disordered about the origin with sodium ions occupying the 1 4 1 4 1 4 and 1 2 1 2 1 2 sites in space group Fm3m. For x =0.2, vacancies are found solely at the 1 2 1 2 1 2 position. This is consistent with the interpretation of these phases in terms of a cation-excess anti-fluorite structure. The influence of the structural evolution on the sodium-ion conductivity is discussed and a basis for the formation of low melting-point glasses for x > 0.5 is proposed.

Some comments on “relation entre structure et conductivite ionique basse temperature Ag3PO4”

Abstract The structural and conductivity data on silver orthophospate described recently by Deschizeaux-cheuy et al. are considered in the context of earlier work on the same subject. The results of powder neutron (PND) and single crystal X-ray diffraction (SXD) analyses of Ag 3 P0 4 provide a consistent picuture of how the I-Ag 3 P0 4 structure evolves with temperature. A comparison of independent analyses of II-Na 3 P0 4 by PND and SXD demonstrates that solution and refinement of the structures of even highly disordered superionic materials can, in certain cases, be accomplished using powder diffraction data.

Triply occupied mirror planes in beta-aluminas

Abstract A simple analysis of the number of required vacancies in the mirror planes of normal beta-aluminas leads to their formulation as M1+xAl11−xBx2+O17, 0⩽x⩽0.67 or M1+xAl11O17+x/2, 0⩽x⩽0.57. For the former materials, the upper limit of the composition range is shown to require the occurrence of a 3a × 3a superstructure with plane group symmetry P31m. For the latter, however, no analogous ordering scheme is required. The mechanisms by which super-cation-rich materials can be formed, with cation excesses beyond these limiting compositions, are considered.

Thermal evolution of the structure of ammonium β-alumina

Abstract Single crystal neutron diffraction measurements at room temperature have revealed the structures of two derivatives of ammonium β-alumina which are produced by sequential loss of ammonia on heating. These crystallographic analyses afford insight into the behaviour of the parent compound and the manner in which the ammonium ions are affected as the temperature is raised above ambient. The mechanism by which stoichiometric materials are formed is considered and the results also shed light on the effective ionic size of the ammonium ion in the host structure.

Ammonium beta alumina

Abstract The preparation and crystal structure of ammonium beta alumina is reviewed. The thermal decomposition behaviour and the evolution of the conducting plane composition with temperature is described. The progressions from ammonium beta alumina to hydrogen beta alumina, and thence ultimately to α-alumina, are multi-step processes and a combination of infra-red, DTA, TGA and neutron diffraction investigations have led to a broad understanding of the changes occuring.

Novel β-lactams derived from the photoisomer of N-benzyloxycarbonylmethylene-2-pyridone

Transformations of the cyclobutene ring in the photoisomer of N-benzyloxycarbonylmethylene-2-pyridone gives some novel fused β-lactams.