Matthew L. Whitaker

Transition metal carbides find widespread use throughout industry due to their high strength and resilience under extreme conditions. However, they remain largely limited to compounds formed from the early d‐block elements, since the mid‐to‐late transition metals do not form thermodynamically stable carbides. We report here the high‐pressure bulk synthesis of large single crystals of a novel metastable manganese carbide compound, MnC_x (P6₃/mmc), which adopts the anti‐NiAs‐type structure with significant substoichiometry at the carbon sites. We demonstrate how synthesis pressure modulates the carbon loading, with ~40 % occupancy being achieved at 9.9 GPa.

Seismic observations show the Earth's inner core has significant and unexplained variation in seismic attenuation with position, depth and direction. Interpreting these observations is difficult without knowledge of the visco‐ or anelastic dissipation processes active in iron under inner core conditions. Here, a previously unconsidered attenuation mechanism is observed in zinc, a low pressure analog of hcp‐iron, during small strain sinusoidal deformation experiments. The experiments were performed in a deformation‐DIA combined with X‐radiography, at seismic frequencies (∼0.003–0.1 Hz), high pressure and temperatures up to ∼80% of melting temperature. Significant dissipation (0.077 ≤ Q⁻¹(ω) ≤ 0.488) is observed along with frequency dependent softening of zinc's Young's modulus and an extremely small activation energy for creep (⩽7 kJ mol⁻¹). In addition, during sinusoidal deformation the original microstructure is replaced by one with a reduced dislocation density and small, uniform, grain size. This combination of behavior collectively reflects a mode of deformation called “internal stress superplasticity”; this deformation mechanism is unique to anisotropic materials and activated by cyclic loading generating large internal stresses. Here we observe a new form of internal stress superplasticity, which we name as “elastic strain mismatch superplasticity.” In it the large stresses are caused by the compressional anisotropy. If this mechanism is also active in hcp‐iron and the Earth's inner‐core it will be a contributor to inner‐core observed seismic attenuation and constrain the maximum inner‐core grain‐size to ≲10 km.

It is well known that elasticity is a key physical property in the determination of the structure and composition of the Earth and provides critical information for the interpretation of seismic data. This study investigates the stress‐induced variation in elastic wave velocities, known as the acoustoelastic effect, in San Carlos olivine. A recently developed experimental ultrasonic acoustic system, the Directly Integrated Acoustic System Combined with Pressure Experiments (DIASCoPE), was used with the D‐DIA multi‐anvil apparatus to transmit ultrasonic sound waves and collect the reflections. We use the DIASCoPE to obtain longitudinal (P) and shear (S) elastic wave velocities from San Carlos olivine at pressures ranging from 3.2–10.5 GPa and temperatures from 450–950°C which we compare to the stress state in the D‐DIA derived from synchrotron X‐ray diffraction. We use elastic‐plastic self‐consistent (EPSC) numerical modeling to forward model X‐ray diffraction data collected in D‐DIA experiments to obtain the macroscopic stress on our sample. We can observe the relationship between the relative elastic wave velocity change (ΔV/V) and macroscopic stress to determine the acoustoelastic constants, and interpret our observations using the linearized first‐order equation based on the model proposed by Hughes and Kelly (1953), https://doi.org/10.1103/physrev.92.1145. This work supports the presence of the acoustoelastic effect in San Carlos olivine, which can be measured as a function of pressure and temperature. This study will aid in our understanding of the acoustoelastic effect and provide a new experimental technique to measure the stress state in elastically deformed geologic materials at high pressure conditions.

Seismic observations of the Earth’s inner-core testify to it being both a complex and dynamic part of the Earth. It exhibits significant variation in seismic attenuation and velocity with position, depth and direction. Interpretation of which is difficult without knowledge of the anelastic processes active in the inner-core is difficult. To address this, we used zinc, a low-pressure analogue of the hexagonal close pack ( hcp ) structured iron that forms the inner-core, to provide first-order constraints on the anelasticity of hcp- metals at high pressure, seismic frequencies (∼0.003-0.1Hz), homologous temperatures (T/T _m ) up to ∼0.8. Measurements were made in a deformation-DIA combined with X-radiography. The data was analysed using an improved image processing method that reduces systematic errors and improves strain measurement precision by up to 3 orders of magnitude. Using this algorithm significant dissipation and softening of zinc’s Young’s modulus is observed. The softening occurs in the absence of significant impurities or a fluid phase and is caused by grain boundary sliding coupled with dynamic recrystallisation.The recrystallisation results in a steady-state grain-size and low dislocation density. A softened Young’s modulus predicts a reduction in shear wave speed 2-3 times greater than that for compressional waves, which is consistent with anelasticity playing a significant role in the seismic velocity of the inner-core. Comparison of elastic wave speeds from experimental or computed material properties with anelastically-retarded inner-core seismic velocities will tend to over-estimate the light element budget of the inner-core. Therefore anelastic effects in hcp -iron must be considered in the interpretation of the inner-core.

We measured the elastic velocities of a synthetic polycrystalline β-Mg₂SiO₄ containing 0.73 wt.% H₂O to 10 GPa and 600 K using ultrasonic interferometry combined with synchrotron X-radiation. Third-order Eulerian finite strain analysis of the high P and T data set yielded K_so = 161.5(2) GPa, G_o = 101.6(1) GPa, and (∂K_s/∂P)_T = 4.84(4), (∂G/∂P)_T = 1.68(2) indistinguishable from K_so = 161.1(3) GPa, G_o = 101.4(1) GPa, and (∂K_s/∂P)_T = 4.93(4), (∂G/∂P)_T = 1.73(2) from the linear fit. The hydration of the wadsleyite by 0.73 wt.% decreases K_s and G moduli by 5.3% and 8.6%, respectively, but no measurable effect was noted for (∂K_s/∂P)_T and (∂G/∂P)_T. The temperature derivatives of the K_s and G moduli from the finite strain analysis (∂K_S/∂T)_P = −0.013(2) GPaK⁻¹, (∂G/∂T)_P = −0.015(0.4) GPaK⁻¹, and the linear fit (∂K_S/∂T)_P = −0.015(1) GPaK⁻¹, (∂G/∂T)_P = −0.016(1) GPaK⁻¹ are in agreement, and both data sets indicating the |(∂G/∂T)_P| to be greater than |(∂K_S/∂T)_P|. Calculations yield ∆V_p(α-β) = 9.88% and ∆V_S(α-β) = 8.70% for the hydrous β-Mg₂SiO₄ and hydrous α-Mg₂SiO₄, implying 46–52% olivine volume content in the Earth’s mantle to satisfy the seismic velocity contrast ∆V_s = ∆V_P = 4.6% at the 410 km depth.

We present the results of a high-pressure semi-hydrostatic study of two plagioclase minerals, andesine and albite, using diamond anvil cells (DACs) to characterize in situ variations in Raman spectra under different static pressures. In this work, we also examined the kinetic effects on deformation at both long and short timescales through non-traditional experiments in which the DAC was either dropped or struck with a mallet. We examined the effects of strain rate, quench rate, and pressure duration on the Raman spectra of plagioclase. We observed that amorphization occurred in all the plagioclase samples we analyzed, and that amorphization onset and permanence differ depending on the composition, kinetics, energy input, and peak pressure. In andesine, samples pressurized above a peak pressure of 18 GPa, amorphization is permanent. Below this critical pressure, the phase has ‘memory’, and crystalline andesine reforms on decompression. Our findings suggest the presence of a thermodynamic energy well in andesine around 18 GPa, and we show that any additional energy input while close to 18 GPa results in amorphization becoming permanent. The effect of the energy well may be relevant for longer duration impacts. For such impacts, equilibrium state studies of deformation and phase formation were previously considered to be applicable. These experiments illuminating the presence of memory effects in plagioclase have implications for comparing static compression, in which samples are measured while under compression, with shock compression, for which samples are measured after decompression.

In this study, quartz, coesite, and stishovite were deformed concurrently with an olivine reference sample at high pressure and 850 ± 50 °C. Olivine deformed with an effective stress exponent (n) of , which we interpret to indicate that the Peierls creep deformation mechanism was active in the olivine. Quartz and coesite had very similar strengths and deformed by a mechanism with n = and , respectively, which are consistent with previous measurements of power law creep in these phases. Stishovite deformed with n = and was stronger than both olivine and the other silica polymorphs. The high stress exponent of stishovite is greater than that typically observed for power law creep, indicating it is probably (but not certainly) deforming by Peierls creep. The rheology of SiO₂ minerals appears therefore to be strongly affected by the change in silicon coordination and density from fourfold in quartz and coesite to sixfold in stishovite. If the effect of Si coordination can be generalized, the increase in Si coordination (and density) associated with bridgmanite formation may explain the tenfold to 100‐fold viscosity increase around 660 km depth in the Earth.