Raymond F. Smith

Magnesium oxide (MgO) is a prototypical strongly bonded ionic solid and an expected major constituent of rocky planet mantles and gas giant cores, yet its high-pressure phase diagram and shock response remain incompletely understood. Here we combine laser-driven decaying-shock and steady-shock compression with nanosecond x-ray diffraction, velocimetry, and optical pyrometry to investigate phase transitions in single-crystal MgO compressed to 600 GPa and 13 000 K. We observe a pronounced crystallographic orientation dependence in the B 1 → B 2 transformation pathway and subsequent melting behavior. For compression along [111], the shock-temperature evolution exhibits distinct plateaus consistent with equilibrium B 1 → B 2 → liquid transitions. In contrast, compression along [110] produces a single temperature discontinuity and diffraction signatures indicative of B2 nucleation from a transient disordered layer at the shock front. The combined thermodynamic and structural data indicate that near-instantaneous disordering can mediate stress relaxation under rapid uniaxial compression. Above 572 ± 24 GPa , all orientations converge to complete melting at 13 200 ± 600 K , constraining the B2-liquid boundary at multimegabar pressures and identifying MgO as the highest-temperature solid measured at planetary interior conditions. These results demonstrate that crystallographic orientation governs phase-transition pathways in strongly compressed ionic solids and provide a benchmark for modeling matter under extreme planetary interior conditions.

We determine the crystal structure and temperature of tantalum ramp and shock ramp compressed to hundreds of gigapascals at the National Ignition Facility. With x-ray diffraction, we find that tantalum remains in the body-centered-cubic structure with no evidence for the proposed phase up to ramp-compression stresses of 531 . 7 − 18.7 + 21.3 GPa . In separate experiments measuring single-shot extended x-ray absorption fine structure (EXAFS) at the L 3 edge, we find that tantalum's temperature agrees well with radiation-hydrodynamics calculations incorporating strength models for ramp and shock-ramp compression to ∼ 200 GPa , but not ∼ 400 GPa . The discrepancy at ∼ 400 GPa can plausibly be explained by underestimated ablation-plasma x-ray heating, with the magnitude of this heating increasing with laser irradiance. These results constitute the highest‐stress structural determination for Ta, the first L -edge EXAFS thermometry in ramp and shock‐ramp experiments, and a stringent benchmark of hydrodynamic models. They validate L ‐edge EXAFS as a robust platform for thermometry in dynamic compression and underscore the need for careful ablator design in future high‐pressure studies.

The phase stability, crystal structure, and melting of Nb have been examined under high pressure shock compression to 365 GPa using ultrafast x-ray diffraction measurements on two x-ray free electron laser facilities. On compression, Nb remains stable in the bcc phase up to 220 GPa, with coexistence of bcc-Nb and liquid from 249 to 298 GPa, and complete melting at 301 GPa, with melt identified by diffuse liquid diffraction. Melting initiates at higher pressure than expected based on theoretical predictions, and the data consistently excludes the presence of other suggested phases of Nb at these pressures, including and hcp, thereby resolving the long-standing structural discrepancy. Singh-type strength analysis provides experimental evidence for a change in sign of the elastic anisotropy parameter at 121 GPa, consistent with the previously reported heat-induced hardening to heat-induced softening transition in Nb.

We directly measure melting of shock compressed Fe3C to 300 GPa using in situ x-ray diffraction. We find melting begins at a shock stress state near 150 GPa and is fully liquid by 190(10) GPa. We find no evidence for crystalline Fe7C3, which is the incongruent melting phase observed in static compression experiments. These observations indicate that the equilibrium melting reaction of Fe3C to liquid and Fe7C3 is kinetically inhibited on nanosecond timescales.

The generation of defects during dynamic compression is well established; however, directly imaging their formation has not been spatially resolved routinely. To address this, we developed an x-ray topography diagnostic to work toward the observation of spatially resolved strain gradients associated with deformation in ambient and shocked materials at the Matter at Extreme Conditions beamline of the Linac Coherent Light Source (LCLS). X-ray topography is a near field technique that reveals defects in single-crystal samples as spatially varying intensity contrast within the diffracted x-ray beam. Owing to its high-quality single-crystal structure and exceptional strength, diamond is an outstanding material for investigating defect generation at both ambient and extreme conditions. Using single x-ray pulses from LCLS, we obtained high-quality topographic images from 〈100〉, 〈110〉, and 〈111〉 oriented diamond crystals under ambient conditions. We then drove laser-generated shocks over a range of stresses to explore whether x-ray topography can visualize the real-time creation of defects governing the onset of plasticity.

This paper presents an enhanced analysis method for investigating material properties at high strain rates, extending the capability of established experimental techniques to gain more information. The ramp-release method has been applied to many experiments reported at high (≈105 − 106 s−1) strain-rates. More recent data gathered at the National Ignition Facility (NIF) has enabled higher (≈108 s−1) strain-rates to be studied. Here, we present an initial application of ramp-release analysis to NIF ramp-compression data, illustrating both the opportunities and the practical challenges of extending these methods to laser-driven platforms. The higher strain-rates accessed at the NIF mean that there is more strain-rate enhancement to strength, and the experimental configuration means that this enhancement is more readily seen in the data. This is enabled by the capability of avoiding peak-compression attenuation through the sample thickness with a designed hold period made possible by the pulse-shaping capability of NIF. We propose that this combination of experimental conditions and an enhanced analysis method enables the strain-rate enhancement to strength to be studied, and potentially for this to inform physics models at smaller scales than the continuum.

In situ x-ray diffraction at both the National Ignition Facility and Omega-EP Laser Facility has been utilized to determine the crystallographic state of ramp and shock-ramp compressed gold up to 1.2 TPa (1 TPa=10 Mbar=10×10^{6} atmospheres). In this Letter, we describe a series of experiments that explore a variety of pressure-temperature states in Au, accessed using tailored laser pulses. We find that the ambient pressure face-centered-cubic phase is stable under ramp compression to pressures of at least 1 TPa where the body-centered-cubic phase is observed simultaneously.