Measurement of the B1-B2 transition pressure in NaCl at high temperatures

Pressure and temperature stability boundaries of cubic SiC polymorphs: a first-principles investigation

A better understanding of the effects of temperature and pressure on the wide gap SiC semiconductor is necessary for both (i) an improvement of the performance of this compound in a variety of technological applications and (ii) a clarification of controversial issues related to the stability of its cubic polymorphs at high pressure and high temperature. Bearing in mind this double demand, we perform first-principles calculations of the phonon band structures, vibrational density of states, and thermal and mode Grüneisen parameters of the zinc blende (B3) and rock-salt (B1) cubic polymorphs of 3C-SiC covering pressures and temperatures up to 120 GPa and 3000 K, respectively. Under a martensitic description of the B3-B1 transformation, we found that the large hysteresis pressure range observed at room temperature (35-100 GPa) disappears at around 1100 K. The calculated Clapeyron slope of this transformation is slightly negative, with average values of -2.9 MPa K^-1 in the 0-3000 K interval and -3.7 MPa K^-1 at 2000 K. We also study the decomposition reaction of the two cubic polymorphs into their elemental constituents (C and Si), finding a decreasing (increasing) decomposition temperature for the B3 (B1) phase as the pressure is applied. All these results are sustained by good agreement with other recently reported experimental and theoretical thermodynamic data that have also been evaluated under our quasi-harmonic approximation framework.

High-pressure thermal conductivity and compressional velocity of NaCl in B1 and B2 phase

Sodium chloride (NaCl) is an important, commonly used pressure medium and pressure calibrant in diamond-anvil cell (DAC) experiments. Its thermal conductivity at high pressure–temperature (P–T) conditions is a critical parameter to model heat conduction and temperature distribution within an NaCl-loaded DAC. Here we couple ultrafast optical pump-probe methods with the DAC to study thermal conductivity and compressional velocity of NaCl in B1 and B2 phase to 66 GPa at room temperature. Using an externally-heated DAC, we further show that thermal conductivity of NaCl-B1 phase follows a typical T⁻¹ dependence. The high P–T thermal conductivity of NaCl enables us to confirm the validity of Leibfried-Schlömann equation, a commonly used model for the P–T dependence of thermal conductivity, over a large compression range (~ 35% volume compression in NaCl-B1 phase, followed by ~ 20% compression in the polymorphic B2 phase). The compressional velocities of NaCl-B1 and B2 phase both scale approximately linearly with density, indicating the applicability of Birch’s law to NaCl within the density range we study. Our findings offer critical insights into the dominant physical mechanism of phonon transport in NaCl, as well as important data that significantly enhance the accuracy of modeling the spatiotemporal evolution of temperature within an NaCl-loaded DAC.

Nucleating a Different Coordination in a Crystal under Pressure: A Study of the B1-B2 Transition in NaCl by Metadynamics

Here we propose an NPT metadynamics simulation scheme for pressure-induced structural phase transitions, using coordination number and volume as collective variables, and apply it to the reconstructive structural transformation B1-B2 in NaCl. By studying systems with size up to 64 000 atoms we reach a regime beyond collective mechanism and observe transformations proceeding via nucleation and growth. We also reveal the crossover of the transition mechanism from Buerger-like for smaller systems to Watanabe-Tolédano for larger ones. The scheme is likely to be applicable to a broader class of pressure-induced structural transitions, allowing study of complex nucleation effects and bringing simulations closer to realistic conditions.

Novel Unexpected Reconstructions of (100) and (111) Surfaces of NaCl: Theoretical Prediction

We have predicted stable reconstructions of the (100) and (111) surfaces of NaCl using the global optimization algorithm USPEX. Several new reconstructions, together with the previously reported ones, are found. For the cleaved bare (100) surface, pure Na and pure Cl are the only stable surface phases. Our study of the (111) surface shows that a newly predicted Na₃Cl-(1 × 1) reconstruction is thermodynamically stable in a wide range of chlorine chemical potentials. It has a sawtooth-like profile where each facet reproduces the (100) surface of rock-salt NaCl, hinting on the preferred growth of the (100) surface. We used Bader charge analysis to explain the preferable formation of this sawtooth-like Na₃Cl-(1 × 1) reconstruction of the (111) surface of NaCl. We find that at a very high chemical potential of Na, the polar (and normally absent) (111) surface becomes part of the equilibrium crystal morphology. At both very high and very low chemical potentials of Cl, we predict a large decrease of surface energy and fracture toughness (the Rehbinder effect).

Combination of pulsed light heating thermoreflectance and laser-heated diamond anvil cell for in-situ high pressure-temperature thermal diffusivity measurements

By combining thermoreflectance measurements and laser heated diamond anvil cell (LHDAC) techniques, an instrument for the measurement of in situ high pressure-temperature thermal diffusivity of materials was developed. In an LHDAC system, high-power continuous-wave laser beams irradiate both faces of a disk-shaped metal sample loaded into diamond anvil cells (DACs), to maintain a stable high-temperature condition. During the operation of the LHDAC system, temperature of the sample is determined from the thermal radiation spectrum between 640 and 740 nm to fit Planck's law. Subsequently, a pulsed laser beam irradiates the metal disk to induce a temperature gradient inside the sample, and the transient temperature, caused by heat diffusion, is measured by a continuous wave probe laser based on the thermoreflectance phenomenon. We determined the thermal conductivities of Pt and Fe up to approximately 60 GPa and 2000 K using the measured thermal diffusivities and obtained values consistent with previous works. The uncertainties in the pressure and the temperature are estimated to be approximately 10%, and that in the thermal conductivity is estimated to approximately 15%. The system developed in this study enables us to determine thermal transport properties of materials under pressure-temperature conditions of the deep Earth.

Pressure-induced metallization in layered ReSe2

The evolution of the crystal structure and electrical transport properties of distorted layered transition metal dichalcogenide ReSe₂ was studied under high pressure up to ~90 GPa by Raman spectroscopy and electrical resistivity measurements accompanied by ab initio electronic band structure calculations. Raman spectroscopy studies indicate an isostructural phase transition due to layer sliding at ~7 GPa, to the distorted 1T-phase which remains stable up to the highest pressures employed in these experiments. From a direct band gap semiconductor at ambient pressure, ReSe₂ undergoes pressure-induced metallization at pressures ~35 GPa, in agreement with the ab initio calculations. Resistivity measurements performed with different loading conditions reveal the possible emergence of superconductivity, which is most likely not an intrinsic property of ReSe₂, but is rather conditioned by internal stresses upon compression.

Alchemical screening of ionic crystals

We introduce alchemical perturbations as a rapid and accurate tool to estimate fundamental structural and energetic properties in pure and mixed ionic crystals.

Theoretical predictions of novel potassium chloride phases under pressure

Above 350 GPa KCl assumes an hcp lattice that is reminiscent of the isoelectronic noble gas Ar.

Graphitic Phase of NaCl. Bulk Properties and Nanoscale Stability

We applied the ab initio approach to evaluate the stability and physical properties of the nanometer-thickness NaCl layered films and found that the rock salt films with a (111) surface become unstable with thickness below 1 nm and spontaneously split to graphitic-like films for reducing the electrostatic energy penalty. The observed sodium chloride graphitic phase displays an uncommon atomic arrangement and exists only as nanometer-thin quasi-two-dimensional films. The graphitic bulk counterpart is unstable and transforms to another hexagonal wurtzite NaCl phase that locates in the negative-pressure region of the phase diagram. It was found that the layers in the graphitic NaCl film are weakly bounded with each other with a binding energy order of 0.1 eV per stoichiometry unit. The electronic band gap of the graphitic NaCl displays an unusual nonmonotonic quantum confinement response.

Activation energy of the low-load NaCl transition from nanoindentation loading curves

Access to activation energies E(a) of phase transitions is opened by unprecedented analyses of temperature dependent nanoindentation loading curves. It is based on kinks in linearized loading curves, with additional support by coincidence of kink and electrical conductivity of silicon loading curves. Physical properties of B1, B2, NaCl and further phases are discussed. The normalized low-load transition energy of NaCl (Wtrans/µN) increases with temperature and slightly decreases with load. Its semi-logarithmic plot versus T obtains activation energy E(a)/µN for calculation of the transition work for all interesting temperatures and pressures. Arrhenius-type activation energy (kJ/mol) is unavailable for indentation phase transitions. The E(a) per load normalization proves insensitive to creep-on-load, which excludes normalization to depth or volume for large temperature ranges. Such phase transition E(a)/µN is unprecedented material's property and will be of practical importance for the compatibility of composite materials under impact and further shearing interactions at elevated temperatures.

An introduction to “Computational Crystallography”

The currently available methods for the computation of structures and their properties are reviewed. After a brief introduction into some common technical aspects, the capabilities and limitations of the most commonly used approaches are discussed. Examples are given to show the state of the art in Computational “Crystallography”, and possible future developments are outlined

Role of temperature in the numerical analysis of CaO under high pressure

In this paper we focus on the elastic and thermodynamic properties of the B1 phase of CaO by using the modified TBP model, including the role of temperature. We have successfully obtained the phase transition pressure and volume change at different temperatures. In addition elastic constants and bulk modulus of B1 phase of CaO at different temperatures are discussed. Our results are comparable with the previous ones at high temperatures and pressures. The thermodynamical properties of the B1 phase of CaO are also predicted.

Nucleation and growth in pressure-induced phase transitions from molecular dynamics simulations: mechanism of the reconstructive transformation of NaCl to the CsCl-type structure

We perform path sampling molecular dynamics on the pressure-induced reconstructive phase transition from NaCl to CsCl type structure. Unlike the molecular dynamics simulations prior to this work our approach does not drive the process by applying elevated pressure. As a consequence, we are able to observe nucleation events that initiate the successive transformation of the crystal. The competing phases are separated by an interface exhibiting a well-defined structure that propagates through the crystal during phase transition.

Activation volumes for solid-solid transformations in nanocrystals

The transition between four- and six-coordinate structures in CdSe nanocrystals displays simple transition kinetics as compared with the extended solid, and we determined activation volumes from the pressure dependence of the relaxation times. Our measurements indicate that the transformation takes place by a nucleation mechanism and place strong constraints on the type of microscopic motions that lead to the transformation. The type of analysis presented here is difficult for extended solids, which transform by complicated kinetics and involve ill-defined domain volumes. Solids patterned on the nanoscale may prove to be powerful models for the general study of structural transitions in small systems, as well as in extended solids.

Pressure-Induced Coordination Changes in Alkali-Germanate Melts: An in Situ Spectroscopic Investigation

The structure of liquid Na(2)Ge(2)O(5).H(2)O, a silicate melt analog, has been studied with Raman spectroscopy to pressures of 2.2 gigapascals. Upon compression, a peak near approximately 240 wavenumbers associated with octahedral GeO(6) groups grows relative to a peak near approximately 500 wavenumbers associated with tetrahedral GeO(4) groups. This change corresponds to an increase in octahedral germanium in the liquid from near 0% at ambient pressures to >50% at a pressure of 2.2 gigapascals. Silicate liquids plausibly undergo similar coordination changes at depth in the Earth. Such structural changes may generate decreases in the fusion slopes of silicates at high pressures as well as neutrally buoyant magmas within the transition zone of the Earth's mantle.