[en] The allocution of Mr Pierre Bouvier, chairman of the French butane and propane Committee, in the opening session of the 113. gas conference is reported here. Mr Bouvier has particularly recalled the important part of butane and propane in the national development. (O.M.)

[en] Recent work concerning a scale covariant theory of gravitation and corresponding astrophysical tests, has called for a revival of the goemetrical considerations put forward by Weyl half a century ago, together with a better understanding of the mathematical foundation of Weyl's geometry. The present paper is to be regarded as a contribution in that direction. (Auth.)

[en] The pressure-temperature phase diagrams of different zirconia samples prepared by oxidation of Zircaloy-4 and Zr-1%Nb-0.12O alloys were monitored by Raman spectrometry from 0.1 MPa to 12 GPa and from 300 to 640 K. These new diagrams show that the monoclinic-tetragonal equilibrium line is strongly downshifted in temperature compared to literature measurements performed on usual polycrystalline zirconia. In addition, the monoclinic-orthorhombic equilibrium line is slightly shifted to higher pressure (i.e. 6 GPa). The crystallite sizes smaller than 30 nm, are thought to be responsible for these equilibrium line displacements. The tetragonal phase obtained in temperature under high pressure can be quenched at room temperature, if the pressure is maintained, and it is destabilised and transforms completely into monoclinic phase if the pressure is released. These results confirm that coupled effects of stress, temperature and nanosized grain are responsible for the formation of the tetragonal phase near the metal/oxide interface during the oxidation of zirconium-based alloys

[en] We report on a study of pressure-dependent structural properties of LaAlO₃, which is considered to be a model material of the class of 'tilted' perovskites. Our high-pressure investigation by means of Raman spectroscopy and synchrotron radiation shows that LaAlO₃ presents structural instabilities under pressure and undergoes a soft-mode-driven rhombohedral-to-cubic phase transition in the vicinity of 14 GPa, which can be analysed in the framework of a Landau model. The structural changes are described in terms of a collective restoring of the initially tilted AlO₆ octahedra towards the ideal cubic perovskite structure without tilting. Up to 40 GPa, the pressure dependence of the volume of the pseudo-cubic phase is described by a third-order Birch-Murnaghan equation of state with parameters V₀=54.57(4) A³, K_T=190(5) GPa and K'=7.2(4). (author)

[en] The structure of nanocrystallized zirconia has been investigated in the 1-40-GPa pressure range at ambient temperature. A continuous transformation towards the ideal fluorite cubic structure is observed from 8 GPa. The complete transformation seems to be achieved at 30 GPa. The compressibility of the tetragonal and cubic forms are derived and discussed

[en] High pressure measurements of the ferroelastic phase transition of SrTiO₃ (Guennou et al 2010 Phys. Rev. B 81 054115) showed a linear pressure dependence of the transition temperature between the cubic and tetragonal phase. Furthermore, the pressure induced transition becomes second order while the temperature dependent transition is near a tricritical point. The phase transition mechanism is characterized by the elongation and tilt of the TiO₆ octahedra in the tetragonal phase, which leads to strongly nonlinear couplings between the structural order parameter, the volume strain and the applied pressure. The phase diagram is derived from the Clausius-Clapeyron relationship and is directly related to a pressure dependent Landau potential. The nonlinearities of the pressure dependent strains lead to an increase of the fourth order Landau coefficient with increasing pressure and, hence, to a tricritical-second order crossover. This behaviour is reminiscent of the doping related crossover in isostructural KMnF₃.

[en] The phase relations and equation of state of ZrO₂ were investigated up to 100 GPa by means of in situ observation using laser heating in a diamond anvil cell and synchrotron radiation. A cotunnite (PbCl₂)-type phase, which appears above 12.5 GPa, is stable to a pressure of 100 GPa and a temperature of 2500 K. No post-cotunnite phase was observed under the present experimental conditions. The unit-cell parameters and the volumes of the cotunnite-type ZrO₂ were determined as a function of pressure at room temperature using a laser-annealing technique. The cotunnite-type ZrO₂ shows rather isotropic compression. The bulk modulus calculated using the Birch-Murnaghan equations of state is 278 GPa, which suggests that high-density ZrO₂ is a candidate for potentially very hard materials. In situ high-temperature experiments performed below 12.5 GPa revealed that a tetragonal fluorite (CaF₂)-type phase is stable up to 3000 K, although a cubic fluorite-type phase has been assumed to exist in this high-temperature regime. The result suggests the possibility that stoichiometric ZrO₂ does not show the cubic structure up to the melting temperature

[en] The oxidation of chromium and iron, present as impurities in zirconium or as alloy elements in zircaloy-4 has been studied by macro and micro-photo-electrochemical techniques. The chemical nature, the size and lateral distribution of phases containing iron and chromium have been characterized in thin layers formed by oxidation at 470 C in oxygen on pure zirconium and zircaloy-4. In the case of zirconium, the iron and chromium impurities lead to the formation of oxides distributed in a homogeneous way in the zirconia layer, when in the case of zircaloy-4, these elements, present under the form of intermetallic particles in the substrate, lead to the localized formation of hematite Fe₂O₃, of solid solution (Fe_xCr_1-x)₂O₃ and of chromine Cr₂O₃. The local distribution of these phases has been studied with a resolution of about the μm, owing to their different forbidden band widths. (O.M.)

[en] We present a detailed study on the memory effect results in Na_0.5 paragraph 5CoO₂ single crystals. We analyze the temperature dependence of the nonvolatile current-pulse-induced resistance memory state. These results allow us to have more insight in the mobility of Na⁺ ions induced by current and their effect on the memory effect. We also developed X-ray diffraction studies under pressure at ambient temperature in the N_a0.5CoO₂ powder compound. An orthorhombic to hexagonal phase transition was observed at 9GPa. This transition can be explained taking into account the Na ions displacement between two allowed positions. These structural results allow us to confirm that the non-volatile resistive commutation can be interpreted by the displacement of the Na ions induced by the current pulses.

[en] A high-pressure Raman study of microcrystalline tungsten oxide was performed in the 0.1 MPa-30 GPa pressure range under hydrostatic and non-hydrostatic conditions. Two phase transitions are evidenced; they take place below 0.1 GPa and at about 22 GPa and are of first order. Two spectral anomalies are observed at about 3 and 10 GPa; they may be related to diffuse weak structural transitions. The number of observed Raman bands remains practically unchanged in the 0.1-30 GPa range and thus the symmetry changes are likely to be small. Surprisingly, the non-hydrostatic conditions do not induce inhomogeneous band broadening and do not modify the transition sequence observed in hydrostatic conditions. The compressibilities of the different observed phases are estimated from spectral data and discussed within Hazen's polyhedral approach. (author)