Daniel Häusermann

P-V-T equation of state of MgSiO3 perovskite

A pressure-volume-temperature data set has been obtained for MgSiO3 perovskite, using synchrotron X-ray diffraction with a laser-heated diamond-anvil cell. The unit cell parameters of the silicate perovskite were measured by angle dispersive X-ray diffraction using imaging plates up to pressures of 57 GPa and temperature in excess of 2500 K. These measurements are in good agreement with the previously reported data of Funamori et al. [Funamori, N., Yagi, T., Utsumi, W., Kondo, T., Uchida, T., Funamori, M., 1996. J. Geophys. Res. 101 (B4), 8257–8269] at lower pressure and yield (∂K∂T)P = −0.027 GPa K−1 for a fixed value K′0 of 4 and a zero-pressure thermal expansion parameter α = 1.55 × 10−5 K−1 at 300 K. Assuming that the thermoelastic parameters of MgSiO3 perovskite are applicable to perovskites with moderate iron content, the comparison of the density and KT profiles calculated for a mixture of perovskite and magnesiowustite and for PREM model indicates that a pure perovskite lower mantle is very unlikely. On the other hand, we obtain a very good match with PREM density and KT profiles for a mixture of 83 vol % (Mg0.93Fe0.07)SiO3 perovskite and 17 vol % (Mg0.79Fe0.21)O magnesiowustite, compatible with a pyrolytic lower mantle.

Equation of state of magnetite and its high-pressure modification: Thermodynamics of the Fe-O system at high pressure

Abstract Fe3O4 has been studied by high-pressure diffraction to 43 GPa. No major changes in the spinel-type structure of magnetite is observed below 21.8 GPa. At higher pressure a sluggish transition to a highpressure modification, h-Fe3O4, is observed. The X-ray diffraction pattern of the high-pressure modification is consistent with the orthorhombic unit cell (CaMn2O4-type structure, space group Pbcm) recently proposed for h-Fe3O4 by Fei et al. (1999), however, it is also consistent with a more symmetric CaTi2O4- type structure (space group Bbmm). Bulk modulus values for magnetite, KT0 = 217 (2) GPa, and h-Fe3O4, KT0 = 202 (7) GPa, are calculated from the pressure-volume data using a third-order Birch-Murnaghan equation of state. A thermodynamic analysis of the Fe-O system at high pressure is presented. The proposed equation of state of h-Fe3O4 gives an increased stability of wüstite relatively to a two-phase mixture of iron and h-Fe3O4 compared to earlier equations of state and removes an inconsistency in the thermodynamic description of the Fe-O system at high pressure.

P‐V‐T equation of state of periclase from synchrotron radiation measurements

The volume of periclase (MgO) has been measured by monochromatic X-ray diffraction in a laser-heated diamond anvil cell up to a pressure of 53 GPa and a temperature of 2500 K. The X-ray source was synchrotron radiation at the European Synchrotron Radiation Facility (Grenoble, France). In addition to laser heating, the use of argon as a pressure transmitting medium provided quasi-hydrostatic conditions in the cell assembly. In order to take thermal pressure effect into account, pressure was measured using an internal pressure calibrant (platinum). By analysis of the experimental P-V-T data set the following parameters were obtained: at ambient temperature, K′0 = 3.94 ± 0.2 when K0 is fixed to 161 GPa (with a Birch-Murnaghan equation of state); under high temperature, α(P = 0,T) = (3.0 + 0.0012T) × 10−5 K−1; (∂KT/∂T)P = −0.022(3) GPa K−1. The quasi-harmonic Debye model appears to describe correctly the temperature dependence of the volume at high pressure within experimental errors, with the following parameters: θD0 = 800 K, γ0 = 1.45 (Gruneisen parameter under ambient conditions), and q = 0.8 ± 0.5.

Melting of tantalum at high pressure determined by angle dispersive x-ray diffraction in a double-sided laser-heated diamond-anvil cell

The high-pressure and high-temperature phase diagram of Ta has been studied in a laser-heated diamond-anvil cell (DAC) using x-ray diffraction measurements up to 52 GPa and 3800 K. The melting was observed at nine different pressures, the melting temperature being in good agreement with previous laser-heated DAC experiments, but in contradiction with several theoretical calculations and previous piston–cylinder apparatus experiments. A small slope for the melting curve of Ta is estimated ( at 1 bar) and a possible explanation for this behaviour is given. Finally, a P–V –T equation of states is obtained, the temperature dependence of the thermal expansion coefficient and the bulk modulus being estimated.

Paris-Edinburgh large-volume cell coupled with a fast imaging-plate system for structural investigation at high pressure and high temperature

A new set-up for collecting high-quality data suitable for structural refinement at high pressure and high temperature has been developed on beamline ID30 at the ESRF. The possibility of using high X-ray energies, high brilliance of third-generation sources and a new fast imaging-plate detector interfaced to the Paris–Edinburgh large-volume press has lead to a significant reduction of acquisition time and improvement of the quality of the data. The feasibility of angle-dispersive X-ray diffraction experiments at high pressure and high temperature, even on light elements, has been demonstrated.

Equation of state of wurtzitic boron nitride to 66 GPa

The compressibility of wurtzitic boron nitride (wBN) taken in mixture with cubic BN has been measured at room temperature up to 66 GPa, using a diamond anvil cell and powder diffraction of synchrotron radiation. From the obtained pressure-volume relation for wBN the isothermal bulk modulus of B0=375±9 GPa and its first pressure derivative of dB0/dp=4.9±0.7 have been calculated indicating that this phase has nearly the same compressibility as cBN (B0=377±4 GPa and dB0/dp=4.1±0.2). Thermodynamic calculations using our findings on wBN equation of state have shown that wurtzitic boron nitride is metastable over the whole ranges of pressures and temperatures.

Density measurements of liquid under high pressure and high temperature

A new method for density measurements by means of X-ray absorption under high pressure and high temperature using synchrotron radiation has been developed. The method has been modified for a large-volume Paris-Edinburgh press and combined with intense high-energy X-rays at the ESRF. In order to overcome effects of deformation of sample shape under pressure, a ruby cylinder was used as a sample container. The density was determined from the intensity profile of transmitted X-rays. The densities of crystalline and liquid Bi were successfully measured up to 750 K at 1 GPa.

Lack of the critical pressure for weakening of size-induced stiffness in 3C–SiC nanocrystals under hydrostatic compression

Using in situ high-pressure x-ray diffraction methods, the compressibility of 30nm 3C–SiC nanocrystals was studied under hydrostatic conditions while helium was used as pressure transmitting medium, as well as under nonhydrostatic conditions without pressure medium. No threshold pressure phenomenon was observed for the compressibility of the nanocrystals during compression in hydrostatic conditions, while the critical pressure around 10.5GPa was observed during nonhydrostatic compression. These indicate that the threshold pressure phenomena, recently reported that the nanocrystals initially exhibited much higher bulk modulus below the threshold pressure during compression [Appl. Phys. Lett. 83, 3174 (2003); J. Phys. Chem. 107, 14151 (2003)], were mainly caused by the nonhydrostatic effect instead of a specific feature of nanocrystals upon compression. The bulk modulus of 3C–SiC nanocrystals is estimated as 220.6±0.6GPa based on the hydrostatic compression data.

Analysis and interpretation of the first monochromatic X-ray tomography data collected at the Australian Synchrotron Imaging and Medical beamline.

The first monochromatic X-ray tomography experiments conducted at the Imaging and Medical beamline of the Australian Synchrotron are reported. The sample was a phantom comprising nylon line, Al wire and finer Cu wire twisted together. Data sets were collected at four different X-ray energies. In order to quantitatively account for the experimental values obtained for the Hounsfield (or CT) number, it was necessary to consider various issues including the point-spread function for the X-ray imaging system and harmonic contamination of the X-ray beam. The analysis and interpretation of the data includes detailed considerations of the resolution and efficiency of the CCD detector, calculations of the X-ray spectrum prior to monochromatization, allowance for the response of the double-crystal Si monochromator used (via X-ray dynamical theory), as well as a thorough assessment of the role of X-ray phase-contrast effects. Computer simulations relating to the tomography experiments also provide valuable insights into these important issues. It was found that a significant discrepancy between theory and experiment for the Cu wire could be largely resolved in terms of the effect of the point-spread function. The findings of this study are important in respect of any attempts to extract quantitative information from X-ray tomography data, across a wide range of disciplines, including materials and life sciences.

The Effect of Micro-Strain and Pressure Medium on the High-Pressure Phase of GaSb

Angle dispersive X-ray diffraction experiments have been performed in a diamond anvil cell using different pressure media and a large volume cell (Paris-Edinburgh cell) in order to determine the effect of pressure–temperature conditions on the high-pressure phase of the binary compound gallium antimonide GaSb. In particular, fine analysis of the diffraction patterns obtained at high pressure and/or temperature have clearly evidenced the importance of the pressure medium and of micro-strains which considerably modified the structure at high pressure.