Session B26: Materials at High Pressure: Phase Transitions

Pre-compression for Dynamic Gas Gun Loading

Equation of state properties for materials off the principal Hugoniot and isentrope are currently poorly constrained. The ability to directly probe regions of phase space between the Hugoniot and isentrope under dynamic loading will greatly improve our ability to constrain equation of state properties under a variety of conditions and study otherwise inaccessible phase transitions. Large diameter ($>$4 mm) samples are typically required for dynamic loading on gas guns. We are developing the ability to pre-compress large diameter samples to 1+ GPa. Compressible materials (such as liquids and gases) possess a significantly stiffer pre-compressed Hugoniot, allowing access to lower temperature states on shock compression. Challenges and initial static and dynamic testing results of the pre-compression hardware will be discussed.   

High-pressure phases of alumina

Alumina (Al2O3) has been widely used as a pressure standard in static diamond anvil cell experiments and is a major chemical component of the Earth. So a detailed knowledge of its high-pressure stability is of great importance in both materials science and deep Earth science. A phase transition is known to occur at roughly 80-100 GPa between corundum and the Rh2O3 (II) structure. A second phase transition to the CaIrO3 structure occurs at even higher pressures. Here we present a computational structure search to reveal three additional structures which are competitive at these pressures but hitherto unknown to be stable in high-pressure alumina.   

Design and Fabrication of RF/CRF Aerogel Flier-Plates with Graded Density For Laser-Driven Quasi-Isentropic Compression Experiments

Resorcinol Formaldehyde (RF)/Carbonized Resorcinol Formaldehyde (CRF) aerogel flier-plates with graded density were designed and fabricated via simple and effective approaches, for increasing the peak pressure and shaping the compression profile in Laser-driven quasi-isentropic compression (ICE) experiments. Sol-gel technique and flexible micro-mould were involved in launching density gradients into aerogel. Resorcinol (R)-formaldehyde (F)-water system catalyzed by sodium carbonate (C) was employed to provide organic RF aerogel, various of sol with different recipes were cast into the mould layer by layer; A carbon dioxide (CO$_{2})$ supercritical fluid drying (SCFD) process and a four-step pyrolysis process were applied to convert RF hydrogel into RF aerogel, RF aerogel into CRF aerogel, respectively. After a four-step pyrolysis process, RF aerogel was converted to CRF aerogel. The strategies were demonstrated to be simple and effective in launching density gradients into RF/CRF aerogel flyer-plates. It was found in the Laser-Driven Quasi-Isentropic compression experiments of Al that the rise time of the ramp compression wave was about 50{\%} longer for the graded density RF aerogel case.   

Laser Driven, Extreme Compression Science

Extreme-compression science is blessed by a number of new techniques and facilities that are shattering previous experimental limitations: static pressures above 600 GPa, equation of state (EOS) experiments on pulsed-power machines, picosecond-resolved x-ray diffraction on free-electron lasers, and many new experiments on high-energy lasers. Our goals, using high-energy lasers, have been to push the limits of high pressure accessible to measurement and to bridge the gap between static- and dynamic-compression experiments by exploring off-Hugoniot states. I will review laser techniques for both shock- and ramp-compression experiments, and discuss a variety of diagnostics. I will present recent results including: impedance-matching Hugoniot experiments, absolute-Hugoniot implosive-shock radiography, coupled radiometry and velocimetry, ramp-compression EOS, and in-situ x-ray diffraction and absorption spectroscopy into the TPa regime. As the National Ignition Facility (NIF) transitions to a laser user facility for basic and applied science, we are transferring many of these techniques. The unprecedented quality and variety of diagnostics available, coupled with exquisite pulse-shaping predictability and control make the NIF a premier facility for extreme-compression experiments.   

Quantum Monte Carlo simulations of a single iron impurity in MgO

Ferropericlase [(Mg,Fe)O] is the second most abundant mineral in Earth's lower mantle. A high-spin to low-spin transition in Fe2+ that occurs in the middle of the lower mantle has been observed in diamond anvil experiments and confirmed within density functional theory (DFT). The spin transition has significant influence on the physical properties and behavior of the lower mantle. However, details on the mechanism of spin transition are still being understood in both experiment and DFT [1]. Here, we aim to benchmark the high-spin to low-spin transition of a single iron atom impurity in MgO using quantum Monte Carlo (QMC). High-spin and low-spin equations of state are initially computed using density functional theory within the LDA+U approximation, which provide trial Slater-Jastrow wave functions for QMC. Equations of state are then computed with variational and diffusion Monte Carlo in 8- and 64-atom cells using the QMCPACK code.[2]. QMC results are in general agreement with experiment and DFT studies.\\[4pt] [1] J.-F. Lin, S. Speziale, Z. Mao, and H. Marquardt, Rev. Geophys., 51, 244 (2013).\\[0pt] [2] http://qmcpack.cmscc.org/   

Quantum Monte Carlo Calculation for the Equation of State of MgSiO$_{3}$ perovskite at high pressures

Magnesium silicate (MgSiO$_{3})$ is among the most abundant minerals in the Earth's mantle. Its phase behavior under high pressure has important implications for the physical properties of deep Earth and the core-mantle boundary. A number of experiments and density functional theory calculations have studied perovskite and its transition to the post-perovskite phase. Here, we present our initial work on the equation of state of perovskite at pressures up to 200 GPa using quantum Monte Carlo (QMC), a benchmark ab initio method. Our QMC calculations optimize electron correlation by using a Slater-Jastrow type wave function~with a single determinant comprised of single-particle orbitals extracted from fully converged DFT calculations.~The equation of state obtained from QMC calculations agrees with experimental data.   

Thermally induced velocity variations of ferropericlase in Earth's lower mantle

Understanding the origin of lateral velocity heterogeneities in the mantle is crucial to understand the constitution of and internal processes at work in the Earth. The spin crossover in iron in ferropericlase (Fp) and the unusual and well documented elastic anomalies introduce unfamiliar effects on seismic velocities. In this work we investigate by first principles calculations potential velocity anomalies caused by lateral temperature variations in the presence of a spin crossover in Fp under mantle conditions. Anti-correlation between shear velocity (V$_{\mathrm{S}})$ and bulk sound velocity (V$_{\mathrm{\phi }})$ in the mantle has long been viewed as an indicator of compositional or mineralogical heterogeneity. This view is not entirely justified in the presence of spin crossover in ferropericlase. We also identify new effects that exist in the presence of a spin crossover. Signatures of these effects appear to exist in the lower mantle.   

Design principles and high-pressure syntheses of novel superhard materials

The development of novel high-performance superhard materials, guided by reliable design theories, is highly anticipated for continuous progresses in processing techniques. In the past decade, we have established the hardness model for polar covalent single crystals, and revealed an extra hardening mechanism for polycrystalline materials, which shows a hardness$-$microstructural size correlation and provides further hardening at deep nanoscale due to quantum confinement effect. Therefore, nanostructuring diamond and cBN is still an effective way to enhance hardness. Based on our model, we estimate the hardness upper limits for diamond and cBN with nanograined and nanotwinned microstructures, respectively. Transformed from graphite-like carbon and BN precursors at high pressure and high temperature (HPHT), nanograined diamond and cBN with the smallest grain size of $\sim$15 nm can be synthesized, showing enhanced hardness but reduced thermal stability. Starting from onion-like BN and carbon, we successfully synthesized nanotwinned cBN and diamond with average twin thickness of 5 nm or below at HPHT. The simultaneous enhancements in hardness, fracture toughness, and thermal stability were confirmed in our nanotwinned cBN and diamond. Our approach offers a general pathway to nano-structure superhard materials for excellent stability and ultrahardness, as well as exceptional tradeoff between hardness and toughness, through the creation of nanotwinned microstructure.   

Structures of xenon oxides at high pressures

For many years, it was believed that noble gases such as xenon were entirely inert. It was only in 1962 that Bartlett first synthesized a compound of xenon. Since then, a number of other xenon compounds, including oxides, have been synthesized. Xenon oxides are unstable under ambient conditions but have been predicted to stabilize under high pressure. Here we present the results of a combined theoretical and experimental study of xenon oxides at pressures of 80-100 GPa. We have synthesized new xenon oxides at these pressures and they have been characterized with X-ray diffraction and Raman spectroscopy. Calculations were performed with a density-functional theory framework. We have used the \textit{ab-initio} random structure searching (AIRSS) method together with a data-mining technique to determine the stable compounds in the xenon-oxygen system in this pressure range. We have calculated structural and optical properties of these phases, and a good match between theoretical and experimental results has been obtained.   

The $\varepsilon $-$\eta $' transition in solid oxygen at high pressure

Despite extensive theoretical and experimental studies, the stability of solid oxygen at high pressure remains an open question. The exact structure of $\varepsilon $-O was only recently defined experimentally as consisting of O$_{8}$ units. More recent measurements have indicated that a new phase, $\eta $'-O, may be stable above $\varepsilon $-O in the pressure range from 44 to 90 GPa and at temperatures near 1000 K. However, the supporting experimental evidence is not conclusive. In this work, we study the phase diagram of solid oxygen up to 120 GPa and 1200 K from first principles. Full free energy calculations with hybrid exchange functionals have been performed to establish the mechanical and thermodynamic stability of $\eta $'-O. This structure has unusual dynamical properties, which will be discussed. Analysis of the low-temperature stability of $\varepsilon $-O to resolves inconstancies between previous experimental and theoretical results will be presented as well.   

Pressure-induced structural transformations in nanomaterials: a linear-scaling DFT investigation

Semiconductor nanomaterials display a number of peculiar and tunable properties that distinguish them from their bulk counterparts. Of particular interest is their response to applied pressure, as they transform from one crystalline or amorphous structure to another. Accurate simulations are important for understanding finite size effects in the atomistic mechanisms of phase transformations (difficult to observe clearly in macroscopic experiments), for the opportunity to uncover novel metastable phases stabilized in finite systems, and for potentially innovative applications of nanomaterials. First-principles methods are essential to accurately describe the bond breaking/making in phase transformations and the realistic description of surfaces (often covered by complex surfactants). However the computational cost limits both the length- and time-scales attainable. We have combined an O(N) density functional theory code for large systems and an electronic-enthalpy method[1] to apply pressure to finite systems to model with quantum mechanical precision processes induced by pressure in nanomaterials under realistic conditions. The focus is on Si, Ge and CdS nanocrystals that are currently favoured for technological applications. [1] Corsini et al, J. Chem. Phys. 139,084117 (2013)