Chantel M. Aracne

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.

Determination of the high pressure elasticity of cobalt from measured interfacial acoustic wave velocities

We have used impulsive stimulated light scattering to measure the velocity of an acoustic wave propagating along the interface formed by a cobalt single crystal in contact with liquid helium to a pressure of 10GPa. We have combined the measured velocities with x-ray diffraction data of cobalt under compression to obtain the elastic tensor elements c44 and c66, and with lower precision c11, c12, and c13. We further show that using published inelastic x-ray scattering results for c33 the associated uncertainties of c11, c12, and c13 are substantially reduced.

Phonons of the anomalous element cerium

Many physical and chemical properties of the light rare-earths and actinides are governed by the active role of f electrons, and despite intensive efforts the details of the mechanisms of phase stability and transformation are not fully understood. A prominent example which has attracted a lot of interest, both experimentally and theoretically over the years is the isostructural γ - α transition in cerium. We have determined by inelastic X-ray scattering, the complete phonon dispersion scheme of elemental cerium across the γ → α transition, and compared it with theoretical results using ab initio lattice dynamics. Several phonon branches show strong changes in the dispersion shape, indicating large modifications in the interactions between phonons and conduction electrons. This is reflected as well by the lattice Grüneisen parameters, particularly around the X point. We derive a vibrational entropy change , illustrating the importance of the lattice contribution to the transition. Additionally, we compare first principles calculations with the experiments to shed light on the mechanism underlying the isostructural volume collapse in cerium under pressure.

Preparation and characterization of single crystal samples for high-pressure experiments

To date, most research utilizing the diamond anvil cell (DAC) at very high pressure has been conducted with polycrystalline samples, thus the results are limited to addressing average bulk properties. However, experiments on single crystals can yield data on a range of orientation-dependent properties such as thermal and electrical conductivity, magnetic susceptibility, elasticity and plasticity. Here, we report new procedures to produce extremely high-quality metallic single crystal samples of size compatible with DAC experiments in the Mbar range. So far, we have produced samples of zinc, Al2O3, cobalt, molybdenum and cerium, and have evaluated the quality of the finished samples with white-light interferometry, synchrotron X-ray diffraction and inelastic X-ray scattering.

Using Simultaneous Time‐Resolved SHG and XRD Diagnostics to Examine Phase Transitions of HMX and TATB

Simultaneous SHG (second harmonic generation) and XRD (x‐ray diffraction) diagnostics have been applied to examine the phase behavior of energetic materials, HMX (octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine) and TATB (1,3,5‐triamino‐2,4,6 trinitrobenzene). This unique capability provides information about both volume and surface effects that occur during the solid‐solid transformation process. This paper reports XRD results for HMX and TATB at elevated temperatures and on simultaneous SHG and XRD experiments on HMX at fixed temperature. Our results do not indicate that a solid‐solid phase transformation occurs for TATB even at temperatures up to 340°C. XRD results on HMX held at 165°C and 1 bar, indicate that the β to δ transformation is incomplete after a period of 4.5 hours which do not temporally correlate with SHG. Overall information indicates that the observed SHG intensities from surface effects can, in some cases, dominate over volume generated SHG contributions. Finally, we have run in situ ...

First Results of Reaction Propagation Rates in HMX at High Pressure

We have measured the reaction propagation rate (RPR) in octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine (HMX) powder in a diamond anvil cell over the pressure range 0.7–35 GPa. In order to have a cross‐comparison of our experiments, we conducted RPR experiments on nitromethane (NM) up to 15 GPa. Our results on NM are indistinguishable from previous measurements of Rice and Foltz. In comparison to high‐pressure NM, the burn rates in solid HMX are 5–10 times faster at pressures above 10 GPa. Numerical simulations of the burn rate of pressurized HMX were also performed for comparison to the results obtained. The simulated burn rates closely approximate the observed rates at pressures up to 3 GPa. However, further refinement to the computational model is required for the calculated burn rates to approach those observed at higher pressures.

Phonon triggered rhombohedral lattice distortion in vanadium at high pressure

In spite of the simple body-centered-cubic crystal structure, the elements of group V, vanadium, niobium and tantalum, show strong interactions between the electronic properties and lattice dynamics. Further, these interactions can be tuned by external parameters, such as pressure and temperature. We used inelastic x-ray scattering to probe the phonon dispersion of single-crystalline vanadium as a function of pressure to 45 GPa. Our measurements show an anomalous high-pressure behavior of the transverse acoustic mode along the (100) direction and a softening of the elastic modulus C44 that triggers a rhombohedral lattice distortion occurring between 34 and 39 GPa. Our results provide the missing experimental confirmation of the theoretically predicted shear instability arising from the progressive intra-band nesting of the Fermi surface with increasing pressure, a scenario common to all transition metals of group V.

Kinetics of the isostructural γ to α transition in cerium investigated by ultrasonics

We performed ultrasonic pulse echo measurements on high-purity polycrystalline cerium across the isostructural γ to α-fcc-to-fcc transition, up to 13 kbar and then back down to 0.3 kbar. Our results confirm the anomalous decrease of the longitudinal sound velocity in the γ-phase approaching the transition, a significant hysteresis loop, and definitively highlight the strong difference in the long wavelength limit of the lattice dynamics of the γ and α phases. We have also studied the kinetics of the transition, which is well described by an Arrhenius behavior, with an activation volume of 10 cm3/mol.