High-energy-density laser facilities and advances in dynamic compression techniques have expanded access to material states in the Terapascal regime relevant to inertial confinement fusion, planetary science, and geophysics. However, experimentally determining the material temperature in these extreme conditions has remained a difficult challenge. Extended X-ray Absorption Fine Structure (EXAFS), referring to the modulations in x-ray absorption above an absorption edge from photoelectrons’ interactions with neighboring atoms, has proven to be a versatile and robust technique for probing material temperature and density for mid-to-high Z elements under dynamic compression. The current platform at the National Ignition Facility has developed six configurations for EXAFS measurements between 7 and 18 keV for different absorption edges (Fe K, Co K, Cu K, Ta L3, Pb L3, and Zr K) using a curved-crystal spectrometer and a bright, continuum foil x-ray source. In this work, we describe the platform geometry, x-ray source performance, spectrometer resolution and throughput, design considerations, and data in ambient and dynamic-compression conditions.

The structural and electronic transformations of liquid silane as a function of pressure are studied using firstprinciples molecular dynamics simulations. We present results for two isotherms, 1000 K and 2000 K, in the pressure range from ambient up to 265 GPa. The phases of dense liquid silane bear some similarities with its crystalline phases but there are significant differences as well. Notably, we find the emergence of a mixed polymeric-molecular H2 phase with strong temperature dependence and metallization in the liquid occurring at significantly lower pressure than in the solid. We report dc conductivity, which reaches higher values, and has a different pressure dependence and physical origin than recent analysis of experimental measurements has suggested.

The structure of solid oxygen has been studied at pressures from 50 to 140 GPa using static structure search methods and molecular dynamics simulations with density functional theory and a hybrid exchange functional. Several crystalline structures with space group symmetries Pnma, , P 21/m, 1 / m , Pm, and P 6 3 / mmc have been identified as candidates for the phase of oxygen at 0 K. Within the hybrid exchange functional framework and at 300 K temperature, Pm is shown to be energetically most favorable above 111 GPa. A comparison with experimental x-ray diffraction, spectroscopic, and superconductivity measurements is provided for all competing structures.

We report measurements of the compressibility of ramp compressed tantalum to a final stress of 2.3 TPa corresponding to threefold volumetric compression. Using these data, we extended the experimental constraint on the Ta cold compression curve by an order of magnitude in pressure. By combining the resulting data with previous measurements of shock compression and ambient pressure heating, we construct an experimentally bounded and thermodynamically consistent equation of state model for Ta which has 2% uncertainty in pressure at 1 TPa. We therefore propose Ta as an in situ pressure scale for laser-heated static compression experiments which were recently able to reach terapascal pressures and thousands of degrees Kelvin. Our new equation of state of Ta is experimentally constrained at extreme pressures and temperatures relevant to a wide range of planetary interiors and will allow for more accurate comparison between experimental measurements and theory at extreme conditions.

Large laser facilities have recently enabled material characterization at the pressures of Earth and Super-Earth cores. However, the temperature of the compressed materials has been largely unknown, or solely relied on models and simulations, due to lack of diagnostics under these challenging conditions. Here, we report on temperature, density, pressure, and local structure of copper determined from extended x-ray absorption fine structure and velocimetry up to 1 Terapascal. These results nearly double the highest pressure at which extended x-ray absorption fine structure has been reported in any material. In this work, the copper temperature is unexpectedly found to be much higher than predicted when adjacent to diamond layer(s), demonstrating the important influence of the sample environment on the thermal state of materials; this effect may introduce additional temperature uncertainties in some previous experiments using diamond and provides new guidance for future experimental design.

Investigating how solid matter behaves at enormous pressures, such as those found in the deep interiors of giant planets, is a great experimental challenge. Over the past decade, computational predictions have revealed that compression to terapascal pressures may bring about counter-intuitive changes in the structure and bonding of solids as quantum mechanical forces grow in influence 1 – 6 . Although this behaviour has been observed at modest pressures in the highly compressible light alkali metals 7 , 8 , it has not been established whether it is commonplace among high-pressure solids more broadly. We used shaped laser pulses at the National Ignition Facility to compress elemental Mg up to 1.3 TPa, which is approximately four times the pressure at the Earth’s core. By directly probing the crystal structure using nanosecond-duration X-ray diffraction, we found that Mg changes its crystal structure several times with non-close-packed phases emerging at the highest pressures. Our results demonstrate that phase transformations of extremely condensed matter, previously only accessible through theoretical calculations, can now be experimentally explored.

The transformation of carbon monoxide (CO) from a molecular liquid to a polymeric solid under isothermal compression at room temperature is investigated using first principles theory. We report structural and thermodynamic properties from ambient density up to 2.45 g/cc obtained using density functional theory molecular dynamics simulations, including hybrid exchange corrections. The theoretical results are compared with newly obtained polymeric CO samples, synthesized in a large volume press. The explosive performance of polymeric CO is predicted and discussed. Under most favorable assumptions, it is found to be comparable to trinitrotoluene.

Sodium is a simple metal at ambient conditions, while it transits to an electride phase at pressures above \ensuremath{\sim}160 GPa along the room-temperature isotherm. We explore the thermal effects on the electronic properties of the $\mathit{hP}4$ phase of sodium along the $\ensuremath{\rho}=5.872\phantom{\rule{0.16em}{0ex}}\mathrm{g}/\mathrm{c}{\mathrm{m}}^{3}$ isochore. We quantitatively classify this phase as an insulator based on the criterion of nearsightedness of the one-particle density matrix. Ab initio calculations suggest that the band gap of this insulator decreases with increasing temperature along the isochore, primarily because of ionic distortions, culminating in an insulator-to-metal transition upon melting at ${T}_{m}\ensuremath{\approx}2100\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. This transition is accompanied by residual electronic localization (in $d$ orbitals) in the form of dynamic electron bubbles and a change in hybridization from p-d to s-p upon melting. This transition is explored by tracking the electronic and electro-optical properties along the isochore under consideration.

Sodium is the most abundant alkali-metal element and has one of the simplest electronic structures of any metal. At ambient conditions, sodium forms a body-centered-cubic lattice. However, during cooling, it undergoes a partial martensitic phase transition to a complex mixture of rhombohedral polytypes commencing from 36 K. Although the Fermi surface (FS) of bcc sodium has been extensively studied, not much attention has been given to the FS of the martensite structure. Here we report results for the Fermi surface and quantum oscillation (QO) frequencies of several energetically favorable crystal structures of Na at low temperature from first-principles calculations. Interestingly we find that despite drastic differences in the crystal structures of the candidate low-temperature phases of sodium, for all these phases the strongest quantum oscillation peak is centered at 28 kT. Our theoretical results are accompanied by experimental data of QO on a multigrain sodium sample at T = 0.3 K and B-max = 18 T exhibiting a sharp peak at 28 kT, independent of the sample orientation. The persistence of this peak even in the presence of the structural transitions has an implication for using the quantum oscillations of polycrystalline sodium for high magnetic field calibration.

Topological semimetals generally contain heavy elements. Using density-functional theoretic calculations, we predict that three dense lithium polymorphs in the pressure range 200-360 GPa display nontrivial semimetallic electronic structure. Specifically, these high-pressure phases exhibit Fermi pockets which are degenerate over a loop in k space, around which an encircling k-space path is threaded by +/-pi Berry phase. Accordingly, these dense lithium phases are topological nodal loop semimetals involving a single light element.

Metal hydrides represent promising solutions for low-pressure vehicular hydrogen storage in emerging fuel cell electric vehicles. Charge transfer processes within these materials often accompany surface hydrogen dissociation, as well as diffusion across interfaces between hydrided and dehydrided phases. Using first- principles computations on the model Mg/MgH2 metal hydride system, we investigate the energetics and charge distribution upon introduction/extraction of H atoms to/from the system. Our study indicates that the Mg-H bonds share properties associated with ionic and metallic interactions, and that the relative contribution of these two types of interactions can be correlated with the local coordination of Mg atoms. The ionicity of Mg-H bonds is found to change significantly only upon addition/removal of a hydrogen pair, consistent with the tendency of Mg to form MgH2 stoichiometry. Mg/MgH2 interfaces feature properties that are intermediate between the parent phases, with semi-localized electronic states appearing at under- or overcoordinated atoms in the interfacial region.

First-principles molecular dynamics (FPMD) simulations are performed on 6 and 12% Na in dense liquid N. A detailed description of structural and electronic properties leading to an understanding of the effect of Na doping on the polymerization phase transition of N is presented. Compression of the mixtures from 5 to 90 GPa shows three distinct regions of characteristic local order separated by pressures near 30 and 65 GPa. Computation of Gibbs free energies of mixing shows that these mixtures are thermodynamically stable beyond 20 and 15 GPa for 6 and 12% Na, respectively.

The role of quantum ion dynamics in the low melting temperatures of Li is investigated from first principles theory. Free energies of solid and liquid phases are obtained at the classical and quantum ion levels. The results are used to determine the Li melting curve in the 40--60 and 110--150 GPa pressure ranges and are in excellent agreement with experimental data around 50 GPa. They predict the resumption of a positive melting slope at higher pressure. Quantum corrections to individual energy terms are far more significant than their net effect on the melting temperatures near 50 GPa, even though lithium behaves as a quantum solid at this pressure. The scales of these corrections increase with compression. A case is made for the possibility for anomalous melting at much higher pressures, where quantum ion dynamics are expected to play a prominent role.

The high temperature phase boundaries of CO2 in the proximity of the Earth's adiabat are determined using first-principles molecular dynamics simulations based on density functional theory. The melting curve, predicted here up to 71GPa, and the molecular to polymeric solid phase transition are computed through a phase coexistence approach from free energy calculations. The resulting CO2 phase IV–phase V-liquid triple point is at 31.8GPa and 1636K, in excellent agreement with the available experimental data. The Earth's geotherm crosses into the non-molecular phase V near 40GPa and 2160K, indicating that free deposits of carbon dioxide in the lower mantle would exist as a polymeric solid. We have also examined the thermodynamic stability of phase V and find no indication of transformations into a dissociated diamond and oxygen phase at mantle conditions.

The thermodynamic stability of the Pnma structure of CaTiO3 has been studied using hybrid density functional theory and Gibbs free energy calculations, including anharmonic effects. The use of the screened Heyd-Scuseria-Ernzerhof (HSE06) functional shifts the room temperature transition pressure from perovskite to post-perovskite structure by around 18 GPa, or 37%, compared with GGA-PBE. Both the lattice dynamics and choice of exchange functional play a significant role in stabilizing the Pnma structure at finite temperature. The Pnma structure is found to be stable up to 65 GPa at 300 K, consistent with the recent experimental observation of stable Pnma up to 60 GPa.

We report low-frequency high-resolution Raman spectroscopy and ab-initio calculations on dense lithium from 40 to 200 GPa at low temperatures. Our experimental results reveal rich first-order Raman activity in the metallic and semiconducting phases of lithium. The computed Raman frequencies are in excellent agreement with the measurements. Free energy calculations provide a quantitative description and physical explanation of the experimental phase diagram only when vibrational effect are correctly treated. The study underlines the importance of zero-point energy in determining the phase stability of compressed lithium.

We report on the use of first-principles molecular dynamics calculations to examine properties of liquid carbon dioxide in the pressure-temperature range of 0-1 TPa and 200-100 000 K. The computed equations of state points are used to predict a series of shock Hugoniots with initial starting conditions that are relevant to existing and ongoing shock-wave experiments. A comparison with published measurements up to 70 GPa shows excellent agreement. We find that the liquid undergoes a gradual phase transition along the Hugoniot and have characterized this transition based on changes in bonding and structural properties as well as the conductivity and reflectivity of the fluid.

The phase diagram of Ca is examined using a combination of density-functional theory (DFT) and diffusion quantum Monte Carlo (DMC) calculations. Gibbs free energies of several competing structures are computed at pressures near 50 GPa. Existing disagreements for the stability of Ca both at low and room temperature are resolved with input from DMC. Furthermore, DMC calculations are performed on 0 K crystalline structures up to 150 GPa and it is demonstrated that the widely used generalized gradient approximation of DFT is insufficient to accurately account for the relative stability of the high-pressure phases of Ca. The results indicate that the theoretical phase diagram of Ca needs a revision.

The phase diagram of Ca up to 100 GPa and 3500 K is studied using first-principles density-functional theory. We propose two solid phases with orthorhombic $Cmcm$ and $Pnma$ structures and determine their finite-temperature phase boundaries. In molten Ca, we present predictions for liquid transitions under compression. Our results describe significant electronic and structural changes that are qualitatively different from those found in dense alkali liquids. The predicted liquid and solid transitions provide a consistent description of the Ca phase diagram and insight into the expected properties of other alkaline-earth metals.

Hydrogen-helium mixtures at conditions of Jupiter's interior are studied with first-principles computer simulations. The resulting equation of state (EOS) implies that Jupiter possesses a central core of 14-18 Earth masses of heavier elements, a result that supports core accretion as the standard model for the formation of hydrogen-rich giant planets. Our nominal model has about 4 Earth masses of planetary ices in the H-He-rich mantle, a result that is, within a modeling uncertainty of 6 Earth masses, consistent with abundances measured by the 1995 Galileo entry probe mission, suggesting that the composition found by the probe may be representative of the entire planet. Interior models derived from this first-principles EOS do not give a match to Jupiter's gravity moment J(4) unless one invokes interior differential rotation, implying that Jovian interior dynamics has an observable effect on the high-order gravity field.