High-pressure study of X-ray diffuse scattering in ferroelectric perovskites

We present a high-pressure x-ray diffuse scattering study of the ABO$_3$ ferroelectric perovskites BaTiO_3 and KNbO_3. The well-known diffuse lines are observed in all the phases studied. In KNbO_3, we show that the lines are present up to 21.8 GPa, with constant width and a slightly decreasing intensity. At variance, the intensity of the diffuse lines observed in the cubic phase of BaTiO_3 linearly decreases to zero at $\sim 11$ GPa. These results are discussed with respect to x-ray absorption measurements, which leads to the conclusion that the diffuse lines are only observed when the B atom is off the center of the oxygen tetrahedron. The role of such disorder on the ferroelectric instability of perovskites is discussed.Comment: 4 pages, Accepted in PR

A combined XAS and XRD Study of the High-Pressure Behaviour of GaAsO4 Berlinite

Combined X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) experiments have been carried out on GaAsO4 (berlinite structure) at high pressure and room temperature. XAS measurements indicate four-fold to six-fold coordination changes for both cations. The two local coordination transformations occur at different rates but appear to be coupled. A reversible transition to a high pressure crystalline form occurs around 8 GPa. At a pressure of about 12 GPa, the system mainly consists of octahedral gallium atoms and a mixture of arsenic in four-fold and six-fold coordinations. A second transition to a highly disordered material with both cations in six-fold coordination occurs at higher pressures and is irreversible.Comment: 8 pages, 5 figures, LaTeX2

Ab initio study of the beta$-tin->Imma->sh phase transitions in silicon and germanium

We have investigated the structural sequence of the high-pressure phases of silicon and germanium. We have focussed on the cd->beta-tin->Imma->sh phase transitions. We have used the plane-wave pseudopotential approach to the density-functional theory implemented within the Vienna ab-initio simulation package (VASP). We have determined the equilibrium properties of each structure and the values of the critical parameters including a hysteresis effect at the phase transitions. The order of the phase transitions has been obtained alternatively from the pressure dependence of the enthalpy and of the internal structure parameters. The commonly used tangent construction is shown to be very unreliable. Our calculations identify a first-order phase transition from the cd to the beta-tin and from the Imma to the sh phase, and they indicate the possibility of a second-order phase-transition from the beta-tin to the Imma phase. Finally, we have derived the enthalpy barriers between the phases.Comment: 12 pages, 16 figure

Superhard Phases of Simple Substances and Binary Compounds of the B-C-N-O System: from Diamond to the Latest Results (a Review)

The basic known and hypothetic one- and two-element phases of the B-C-N-O system (both superhard phases having diamond and boron structures and precursors to synthesize them) are described. The attention has been given to the structure, basic mechanical properties, and methods to identify and characterize the materials. For some phases that have been recently described in the literature the synthesis conditions at high pressures and temperatures are indicated.Comment: Review on superhard B-C-N-O phase

Investigation of cracks in GaN films grown by combined hydride and metal organic vapor-phase epitaxial method

Cracks appeared in GaN epitaxial layers which were grown by a novel method combining metal organic vapor-phase epitaxy (MOCVD) and hydride vapor-phase epitaxy (HVPE) in one chamber. The origin of cracks in a 22-μm thick GaN film was fully investigated by high-resolution X-ray diffraction (XRD), micro-Raman spectra, and scanning electron microscopy (SEM). Many cracks under the surface were first observed by SEM after etching for 10 min. By investigating the cross section of the sample with high-resolution micro-Raman spectra, the distribution of the stress along the depth was determined. From the interface of the film/substrate to the top surface of the film, several turnings were found. A large compressive stress existed at the interface. The stress went down as the detecting area was moved up from the interface to the overlayer, and it was maintained at a large value for a long depth area. Then it went down again, and it finally increased near the top surface. The cross-section of the film was observed after cleaving and etching for 2 min. It was found that the crystal quality of the healed part was nearly the same as the uncracked region. This indicated that cracking occurred in the growth, when the tensile stress accumulated and reached the critical value. Moreover, the cracks would heal because of high lateral growth rate

Détermination de périodicités dans des phénomènes aléatoires historiques stationnaires

Nous nous intéressons ici à la détermination pratique de périodicités dans des phénomènes « historiques », c'est-à-dire aléatoires, non reproductibles, dont les mesures sont limitées dans le temps, et qui en outre comportent des manques dus à diverses causes fortuites. Nous présentons ici, avec critiques, plusieurs méthodes d'étude : périodogramme, spectre de puissance, fonction d'autocorrélation, et enfin une méthode mixte (autocorrélation multipliée par spectre de puissance) qui nous a donné de bons résultats

Détermination de périodicités dans des phénomènes aléatoires historiques stationnaires

We are concerned here with the practical study of periodicities in phenomena which can be qualified as « historical », i. e. stochastic, non-reproducible, whose measurements extend over a limited time, and contain vacancies due to incidental conditions. Several methods are discussed here : periodogram, power spectrum, autocorrelation function, and a mixed method (autocorrelation multiplied by power spectrum) which yielded some good results.Nous nous intéressons ici à la détermination pratique de périodicités dans des phénomènes « historiques », c'est-à-dire aléatoires, non reproductibles, dont les mesures sont limitées dans le temps, et qui en outre comportent des manques dus à diverses causes fortuites. Nous présentons ici, avec critiques, plusieurs méthodes d'étude : périodogramme, spectre de puissance, fonction d'autocorrélation, et enfin une méthode mixte (autocorrélation multipliée par spectre de puissance) qui nous a donné de bons résultats

Emission and enrichments of radon daughters from Etna volcano magma

International audienceA program of measurements in and outside the plume of Etna volcano has shown that radon daughters (as well as gaseous radon) are directly emitted from the magma. Enrichment factors have been observed: 4 for bismuth vs lead and 6 for polonium vs lead. The 210pb total output measured for the Etna plume is an insignificant source for this nuclide. Procedures During June 1976 aerosol was collected on filters at various locations near the Etna volalso ß Universit• de Picardie, also ß Terres Australes et Antarctiques Fran•aises

Volcanic Output of Long-Lived Radon Daughters

International audienceThe concentrations of long-lived radon daughters have been measured in the exhausts of 11 volcanoes 11,104 LAMBERT ET AL.' RVEAC MAGMATIC PHASE 238 226 U Ra 92 88 Short-lived daughters VOL&TILE PHASE 222 86 21 0pb The undetectable activity is revealed through the apparent 20 years half-life in the activities •3 of Z10Bi and a of 210po ' 210 82 Ph (RaD) 210 Zi0 An excess of Bi/ Pb is revealed by a S d-decay of the •3 activity An excess of 210po/Z10pb is revealed by a 138 d-decay of the a activity