Denis Andrault

Thermoelastic properties and crystal structure of MgSiO3 perovskite at lower mantle pressure and temperature conditions

A new synchrotron X-ray diffraction study of MgSiO3 perovskite at high-pressure and high-temperature has been carried out in a laser-heated diamond-anvil cell to 94 GPa and temperatures above 2500 K. MgSiO3 perovskite is shown to be stable in this P–T range and adopts an orthorhombic structure (space group Pbnm), thus ruling out any phase transition or decomposition to an assemblage of denser oxides. Structural refinements show an increase of the orthorhombic distortion with increasing pressure, counterbalanced by a decrease of this distortion at high temperature, as evidenced by the Si-O-Si angle evolution. In addition, thermoelastic parameters identified from this new pressure-volume-temperature data set indicate that a significant amount of magnesiowustite is required to match PREM bulk modulus profile, thus making a pure perovskite lower mantle unlikely.

Thermal expansion of forsterite up to the melting point

As determined from powder X-ray diffraction experiments with synchrotron radiation, the thermal expansion coefficient of forsterite increases smoothly from 2.8 to 4.5 K−1 from 400 K to 2160 K. No anomalous increases of the cell parameters are observed near the melting point. The consistency between the observed and calculated value of the initial slope of the melting curve of forsterite suggests that defects do not make a large contribution to thermal expansion near the melting point. Along with previous results, the new data confirm the influence of anharmonicity on the high-temperature heat capacity of forsterite and indicate that both the Gruneisen parameter and αKT (α = thermal expansion coefficient, KT = bulk modulus) have nearly constant values at high temperatures.

Cation sites in Al-rich MgSiO 3 perovskites

Abstract Local structure analysis of Al-containing magnesium silicate perovskite has been carried out with X-ray absorption spectra recorded at the Mg, Al, and Si K-edges using the SA32 beam-line of SuperAco (Orsay, France). The Al-XAFS spectrum of (MgSi)0.85Al0.3O3 perovskite (synthesized in a multi-anvil apparatus) cannot be explained by assuming that Al3+ occurs in octahedral or dodecahedral sites only. This conclusion is based on comparison between Al-spectrum and those recorded at the Mg and Si K-edges for the same structure, general trends found for Al-spectra in various atomic sites, and ab-initio calculations using the FEFF-6 code. Thus, Al appears to be partitioned between both octahedral and dodecahedral perovskite sites. However, the structural accommodations needed to stabilize Al3+ cation in such different sites are not straightforward. Also, Mg K-edge spectra in enstatite and perovskite were compared with those previously reported at the Fe K-edge for the same structures, confirming that these two elements are located in the same polyhedra in both structures, and thus that Fe2+ enters the dodecahedra of the silicate perovskite.

Thermal expansion of natural orthoenstatite to 1473 K

The volume thermal expansion of powdered natural orthoenstatite [(Mg_(0.994)Fe_(0.002)Al_(0.004))2(Si_(0.996)Al_(0.004))_2O_6] has been measured to 1473 K using energy dispersive synchrotron X-ray diffraction. Over the temperature range examined, the data are consistent with a volume thermal expansion, α, that linearly increases with temperature as given by the expression α(T) = 29.7(16) × 10^(−6)K^(−1) + 5.7(11) × 10^(−9)K^(−2)T. An analysis in terms of the often-used constant expansion coefficient yields α_0 = 34.5(17) × 10^(−6) K^(−1), which is in good agreement with several previous experimental results on orthoenstatite (Mg_2Si_2O_6) over a similar temperature range. Our results do not support the extreme upper and lower bounds reported in earlier studies for the thermal expansivity of Fe-rich orthoenstatite, but rather suggest that the thermal expansion of this phase is approximately midway between those extreme values.

Oxygen K-edge fine structures of water by x-ray Raman scattering spectroscopy under pressure conditions

Fine structure of the oxygen K edge was investigated for water at ambient pressure, 0.16, 0.21, 0.27, 0.47, and 0.60 GPa using x-ray Raman scattering spectroscopy (XRS). Similarity in near-edge structures at 0.16 and 0.60 GPa suggests little difference in the electronic state of oxygen in the low-pressure and high-pressure forms of water. Yet, we observed significant variation of preedge structure of the XRS spectra with compression. The intensity of the preedge peak at 535.7 eV has a minimal value at around 0.3 GPa, indicating that the number of hydrogen bonding increases first and then decreases as a function of pressure.

Study of partial melting at high-pressure using in situ X-ray diffraction

The high-pressure melting behavior of different iron alloys was investigated using the classical synchrotron-based in situ X-ray diffraction techniques. As they offer specific advantages and disadvantages, both energy-dispersive (EDX) and angle-dispersive (ADX) X-ray diffraction methods were performed at the BL04B1 beamline of SPring8 (Japan) and at the ID27-30 beamline of the ESRF (France), respectively. High-pressure vessels and pressure ranges investigated include the Paris–Edinburgh press from 2 to 17 GPa, the SPEED-1500 multi-anvil press from 10 to 27 GPa, and the laser-heated diamond anvil cell from 15 to 60 GPa. The onset of melting (at the solidus or eutectic temperature) can be easily detected using EDX because the grains start to rotate relative to the X-ray beam, which provokes rapid and drastic changes with time of the peak growth rate. Then, the degree of melting can be determined, using both EDX and ADX, from the intensity of diffuse X-ray scattering characteristic of the liquid phase. This diffuse contribution can be easily differentiated from the Compton diffusion of the pressure medium because they have different shapes in the diffraction patterns. Information about the composition and/or about the structure of the liquid phase can then be extracted from the shape of the diffuse X-ray scattering.

CARBON NITRIDES : A PROMISING CLASS OF MATERIALS

(Received 14 July 2001; In final form 28 September 2001)Using a thermal decomposition process of an organic precursor (thiosemicarbazide) under an argon flow, a brownorange solid was prepared. Chemical analysis, X-ray diffraction and infrared spectroscopy characterizations revealthe formation of a well-crystallized bidimensional carbon nitride compound with a composition close to C

The high pressure synthesis of the graphitic form of C3N4

Abstract Basing on “ab-initio” calculations, C3N4 was claimed to be an ultra-hard material with a bulk-modulus close to that of diamond. Five different structural varieties were announced: the graphitic form, the zinc blende structure, the α and β forms of Si3N4 and another form, isostructural with the high pressure variety of Zn2Si04. Using the same strategy as that developed for diamond or c-BN synthesis, it appears that the graphitic form could be an appropriate precursor for preparing the 3D varieties. Two main problems characterize the C3N4 synthesis: (-) the temperature should be reduced in order to prevent nitrogen loss, (-) the reactivity of the precursors should be improved. Consequently, we have developed a new process using the solvothermal decomposition of organic precursors containing carbon and nitrogen in the presence of a nitriding solvent. The resulting material, with a composition close to C3N4, has been characterized by different physico-chemical techniques.