Y. Le Godec

Equation of state of cubic boron nitride at high pressures and temperatures

We report accurate measurements of the equation of state (EOS) of cubic boron nitride by x-ray diffraction up to 160 GPa at 295 K and 80 GPa in the range 500-900 K. Experiments were performed on single crystals embedded in a quasihydrostatic pressure medium (helium or neon). Comparison between the present EOS data at 295 K and literature allows us to critically review the recent calibrations of the ruby standard. The full P-V-T data set can be represented by a Mie-Grueneisen model, which enables us to extract all relevant thermodynamic parameters: bulk modulus and its first pressure derivative, thermal expansion coefficient, and thermal Grueneisen parameter and its volume dependence. This equation of state is used to determine the isothermal Grueneisen mode parameter of the Raman TO band. A formulation of the pressure scale based on this Raman mode, using physically constrained parameters, is deduced.

Thermoelastic behaviour of hexagonal graphite-like boron nitride

Abstract A synchrotron X-ray diffraction study on hexagonal graphite-like boron nitride (h-BN) was performed under high pressures and temperatures. From the measured P-V-Trelation for h-BN (with a three-dimensional ordering parameter P3 = 0.9) in the temperature range from 298 to 1273 K and up to 6.7 GPa, the thermoelastic parameters are derived by fitting a modified high temperature Birch-Murnaghan equation of state. The results are: bulk modulus B0[GPa] = 27.6-0.0081(T[K]-298) and its pressure derivative B1 = 10.5 + 0.0016(T [K] - 298). These values are for samples with P3 = 0.9 and are quite different for samples with different values of the order parameter. This parameter is shown to have a leading role in the determination of the thermoelastic properties of h-BN, which explains and reconciles the differences between previous results.

The α−γ−ε triple point of iron investigated by high pressure–high temperature neutron scattering

We present high pressure–high temperature diffraction data in the 0–10 GPa and 300–1000 K range of all three main iron phases. The refinements of the diffraction patterns give molar volumes to high precision and show that the α−γ−e triple point is located at 8.2±0.1u2002GPa and 678 K. These values are significantly lower than currently admitted (10.4 GPa/740 K), but are consistent with reported in situ x-ray diffraction data obtained in multianvil presses. These measurements have been made possible by the use of a high P/T cell which uses metallic toroidal gaskets and which suffers from less absorption, gives clean diffraction patterns, and shows better pressure performance than other setups.

Temperature- and pressure-driven spin-state transitions in LaCoO{sub 3}

Crystal structure, spin state, and semiconductor-metal transitions of LaCoO{sub 3} in the 10-905 K temperature range were studied by neutron diffraction at high pressures up to 4.5 GPa and x-ray diffraction and Raman spectroscopy at high pressures up to 17.5 GPa. The susceptibility and thermal-expansion anomalies in the studied pressure range can be described successfully by the thermal excitation of two orbitally nondegenerate magnetic states with different electronic configuration, intermediate spin (IS), and high spin (HS) from the low-spin (LS) nonmagnetic ground state. The pressure-induced compression of Co-O bonds leads to a substantial increase of LS-IS and LS-HS energy splittings E{sub 1} and E{sub 2} with d ln E{sub 1}/dP=0.37 and d ln E{sub 2}/dP=0.23 GPa{sup -1}. The onset of the semiconductor-metal (S-M) transition in LaCoO{sub 3} correlates with the vanishing of several Raman modes forbidden for rhombohedral R3c symmetry and originating from local distortions due to short-range e{sub g}-orbital order of IS Co{sup 3+} ions. The S-M transition temperature increases rapidly under pressure with dT{sub S-M}/dP{approx_equal}50 K/GPa.

Compact multianvil device for in situ studies at high pressures and temperatures

We describe a recently developed device for in situ studies at pressures up to 25 GPa and temperatures up to 2300 K. The system consists of a 450 ton V7 Paris-Edinburgh press combined with a Stony Brook ‘T-cup’ multianvil stage. Such a compact large-volume set-up has a total mass of 100 kg only and can be readily used on most synchrotron radiation facilities. The optimization of the set-up by off-line tests is detailed, and we present some X-ray diffraction results which demonstrate the potential of the technique.

Equations of state of MgO at high pressure and temperature

Abstract A synchrotron X-ray diffraction study on MgO has been done at simultaneous high pressure and temperature. The lattice parameter of MgO has been measured up to a static pressure of 6 GPa and a temperature of 1273 K, using a large volume pressure cell and energy-dispersive synchrotron X-ray powder diffraction. The compression was made following six high-temperature isotherms. A Vinet equation of state was used to fit the experimental P-V-T data. The Vinets model compares very well with the experimental data above the Debye temperature (760 K) and allows the use of MgO as an alternative internal pressure calibrant for experiments at high temperature.

Polyamorphism in cerium based bulk metallic glasses: Electronic and structural properties under pressure and temperature by x-ray absorption techniques

High pressure and high temperature x-ray absorption near edge spectroscopy experiments have been carried out on Ce60Al20Cu20 bulk metallic glass showing an electronic delocalization of the 4f-electron of cerium under pressure. In parallel, high pressure extended x-ray absorption fine structure spectroscopy reveals large structural modifications of the cerium local environment. This study provides experimental evidence that an electronic driven structural transformation occurs in cerium based bulk metallic glasses (Ce-BMGs). The effect of temperature on the hysteresis of this amorphous-amorphous phase transition is also discussed, suggesting the existence of a critical point in the phase diagram of Ce-BMGs. This work will encourage further investigations on Ce-based metallic glasses phase diagrams in order to support, or refute, the actual theoretical understanding of polyamorphism.

X‐Ray Diffraction Measurements in GaSb under High Pressure and Temperature

The P–T phase diagram of GaSb has been studied by energy dispersive X-ray diffraction using synchrotron radiation at pressures up to 7 GPa and temperatures up to 973 K. A detailed determination of the melting curve of the low (GaSbI) and high (GaSbII) pressure phases has been made. A slope change in the GaSbI melting curve is clearly observed, confirming the existence of two different liquids (LI and LII) in the melt, as reported recently by volume and electrical resistance anomalies. The GaSbI–LI–LII and GaSbI–GaSbII–LII triple point coordinates in the P–T plane are found to be (3.8 ± 0.1 GPa, 765 ± 10 K) and (4.9 ± 0.1 GPa, 657 ± 10 K), respectively. The GaSbI–GaSbII phase transition has also been studied. The transition pressure has been measured from room temperature to 573 K in both upstroke and downstroke. A strong hysteresis (>4 GPa) is obtained at room temperature. The phase transition boundaries determined in the upstroke and downstroke converge both to the GaSbI–GaSbII-LII triple point position, at which the hysteresis is reduced to zero.

Techniques for structural studies of liquids and amorphous materials by neutron diffraction at high pressures and temperatures

This paper describes new techniques which permit the study of liquids and amorphous materials using neutron diffraction at high pressures and high temperatures up to 6.5u200aGPa and 1000u200aK using the Paris–Edinburgh cell. The set-up is based on an internal heating system, and combines a novel encapsulation method and highly transparent sintered diamond anvils to maximise the signal from a sample of 60u200amm3 volume. We describe in detail the experimental set-up, the operation conditions, and the procedure for absorption correction and background subtraction. The method is illustrated by recent data collected on D2O water at 6.5u200aGPa and 672u200aK.

BC8 Silicon (Si-III) is a Narrow-Gap Semiconductor

Large-volume, phase-pure synthesis of BC8 silicon (Ia3[over ¯], cI16) has enabled bulk measurements of optical, electronic, and thermal properties. Unlike previous reports that conclude BC8-Si is semimetallic, we demonstrate that this phase is a direct band gap semiconductor with a very small energy gap and moderate carrier concentration and mobility at room temperature, based on far- and midinfrared optical spectroscopy, temperature-dependent electrical conductivity, Seebeck and heat capacity measurements. Samples exhibit a plasma wavelength near 11u2009u2009μm, indicating potential for infrared plasmonic applications. Thermal conductivity is reduced by 1-2 orders of magnitude depending on temperature as compared with the diamond cubic (DC-Si) phase. The electronic structure and dielectric properties can be reproduced by first-principles calculations with hybrid functionals after adjusting the level of exact Hartree-Fock (HF) exchange mixing. These results clarify existing limited and controversial experimental data sets and abxa0initio calculations.