Guillaume Fiquet

Synthesis of superhard cubic BC2N

Cubic BC2N was synthesized from graphite-like BC2N at pressures above 18 GPa and temperatures higher than 2200 K. The lattice parameter of c-BC2N at ambient conditions is 3.642(2) A, which is larger by 1.48% than would be expected based on ideal mixing between diamond and cubic boron nitride. The bulk modulus of c-BC2N is 282 GPa which is one of the highest bulk moduli known for any solid, and is exceeded only by the bulk moduli of diamond and c-BN. The hardness of c-BC2N is higher than that of c-BN single crystals which indicates that the synthesized phase is only slightly less hard than diamond.

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.

Melting of Peridotite to 140 Gigapascals

Under Pressure In order to understand the behavior of materials in the solid deep Earth, it is important to be able to estimate how a material melts at high pressure. To this end, Fiquet et al. (p. 1516) performed experiments using a laser-heated diamond anvil cell coupled to in situ synchrotron measurements of peridotite rock—a mixture of minerals thought to represent Earths upper mantle—across a wide pressure range. The results suggest that liquid phases may exist at very high pressure values, such that seismically anomalous zones near the boundary between the core and the mantle may result from isolated pockets of melt. Along similar lines, the base of primitive Earths mantle may have acquired its trace element signature from partial melting of certain mineral phases higher up in the mantle. High-temperature and -pressure experiments reveal details about how and where the mantle melts. Interrogating physical processes that occur within the lowermost mantle is a key to understanding Earth’s evolution and present-day inner composition. Among such processes, partial melting has been proposed to explain mantle regions with ultralow seismic velocities near the core-mantle boundary, but experimental validation at the appropriate temperature and pressure regimes remains challenging. Using laser-heated diamond anvil cells, we constructed the solidus curve of a natural fertile peridotite between 36 and 140 gigapascals. Melting at core-mantle boundary pressures occurs at 4180 ± 150 kelvin, which is a value that matches estimated mantle geotherms. Molten regions may therefore exist at the base of the present-day mantle. Melting phase relations and element partitioning data also show that these liquids could host many incompatible elements at the base of the mantle.

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.

Density measurements of liquid Fe‐S alloys at high‐pressure

We present the first static measurements of the density of metallic liquids in the Fe-S system in the pressure and temperature range 1.5 GPa-6.2 GPa and 1500 K - 1780 K. Density is inferred from X-ray absorption experiments carried out with a large volume press at the European Synchrotron Radiation Facility. It is shown that increasing the amount of sulfur in liquid iron decreases the bulk incompressibility by -2.5 GPa per 1 weight% of S. These data are important for constraining the presence and amount of sulfur in the cores of small planetary bodies.

High-Temperature Thermodynamic Properties of Forsterite

The high-temperature thermodynamic properties of forsterite were reviewed in the light of a new determination of the isobaric heat capacity (C(p)), up to 1850 K, and Raman spectroscopic measurements, up to 1150 K and 10 GPa. The C(p) measurements and available data on thermal expansion (alpha) and bulk modulus (K) show that the isochoric specific heat (C-nu) exceeds the harmonic limit of Dulong and Petit above 1300 K. This intrinsic anharmonic behavior of C-nu) can be modeled by introducing anharmonic parameters a(i) = (partial-lnv(i)/partial-T)V which are calculated from the measured pressure and temperature shifts of the vibrational frequencies. These parameters are all negative, with absolute values lower for the stretching modes of the SiO4 tetrahedra (a(i) almost-equal-to - 1 x 10(-5) K-1) than for the lattice modes (a(i) almost-equal-to 2 x 10(-5) K-1). Through the relation C(p) = C-nu) + alpha-2K(T)VT, the calculated anharmonic C-nu) and the measured C(p) are then used to determine the temperature dependences of the thermal expansion and bulk modulus of forsterite, up to 2000 K, in agreement with recent experimental results. Finally, all these data point to an inconsistency for the Gruneisen parameter of forsterite, whereby the macroscopic parameter gamma = alpha-VK(T)/C-nu) cannot be evaluated simply at high temperature by summation of the individual isothermal mode Gruneisen parameters gamma-iT = K(T) (partial-lnv(i)/partial-P).

High‐temperature heat capacity and premelting of minerals in the system MgO‐CaO‐Al2O3‐SiO2

Relative enthalpy measurements have been made from 800 to 1700 or 1800 K for periclase, lime, spinel, diopside, pseudowollastonite, and anorthite. Significant premelting effects have been observed for diopside. pseudowollastonite, and anorthite, showing that only part of the structural changes that take place near the melting point can be quenched to room temperature. With the heat capacities derived from these measurements and other data for SiO2 polymorphs, corundum, forsterite and pyrope, the validity of proposed Cp equations has been tested, particularly with respect to extrapolations at higher and lower temperatures. For high-temperature extrapolations, the best results have been obtained with an equation of the form Cp = k0 + kln ln T + k1/T + k2/T2 + k3/T3. Comparisons of experimental heat capacities with values calculated from polyhedral modeling show that significant deviations from additivity can be found above 1000 K, likely as a result of anharmonic factors. This raises the possibility that enthalpies and entropies of formation are temperature dependent under mantle conditions. Finally, the difficulties of extrapolating differential scanning calorimetry data for mantle minerals are emphasized.

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.

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.

Spin crossover in ferropericlase at high pressure: a seismologically transparent transition?

An iron spin transition has no effect on the seismologic properties of lower-mantle minerals. Seismic discontinuities in Earth typically arise from structural, chemical, or temperature variations with increasing depth. The pressure-induced iron spin state transition in the lower mantle may influence seismic wave velocities by changing the elasticity of iron-bearing minerals, but no seismological evidence of an anomaly exists. Inelastic x-ray scattering measurements on (Mg0.83Fe0.17)O-ferropericlase at pressures across the spin transition show effects limited to the only shear moduli of the elastic tensor. This explains the absence of deviation in the aggregate seismic velocities and, thus, the lack of a one-dimensional seismic signature of the spin crossover. The spin state transition does, however, influence shear anisotropy of ferropericlase and should contribute to the seismic shear wave anisotropy of the lower mantle.