A universal equation of state for solids

Thermal Equation of State of Cubic Silicon Carbide at High Pressures

We have performed in situ X-ray diffraction measurements of cubic silicon carbide (SiC) with a zinc-blende crystal structure (B3) at high pressures and temperatures using multi-anvil apparatus. The ambient volume inferred from the compression curves is smaller than that of the starting material. Using the 3rd-order Birch-Murnaghan equation of state and the Mie-Grüneisen-Debye model, we have determined the thermoelastic parameters of the B3-SiC to be K0=228±3 GPa, K0',=4.4±0.4, q=0.27±0.37, where K0, K0' and q are the isothermal bulk modulus, its pressure derivative and logarithmic volume dependence of the Grüneisen parameter, respectively. Using the 3rd-order Birch-Murnaghan EOS with the thermal expansion coefficient, the thermoelastic parameters have been found as K0=221±3 GPa, K0',=5.2±0.4, α0=0.90±0.02 ⋅ 10-5 ⋅ K-1, where α0 is the thermal expansion coefficient at room pressure and temperature. We have determined that paired B3-SiC - MgO calibrants can be used to estimate pressure and temperature simultaneously in ultrahigh-pressure experiments up to 60 GPa.

High-pressure structure and reactivity of crystalline bibenzyl: Insights and prospects for the synthesis of functional double-core carbon nanothreads

The high-pressure synthesis of double-core nanothreads derived from pseudo-stilbene crystals represents a captivating approach to isolate within the thread chromophores or functional groups without altering its mechanical properties. These entities can be effectively utilized to finely tune optical properties or as preferential sites for functionalization. Bibenzyl, being isostructural with other members of this class, represents the ideal system for building co-crystals from which we can synthesize double-core nanothreads wherein bridging chromophores, such as the azo or ethylene moieties, are embedded in the desired concentration within a fully saturated environment. To achieve this, a critical step is the preliminary characterization of the high-pressure behavior of crystalline bibenzyl. We report here an accurate investigation performed through state-of-the-art spectroscopic techniques, Raman and Fourier transform infrared spectroscopy, and x-ray diffraction up to 40 GPa. Our findings reveal a strongly anisotropic compression of the crystal, which determines, at pressures between 1 and 2 GPa, consistent modifications of the vibrational spectrum, possibly related to a torsional distortion of the molecules. A phase transition is detected between 9 and 10 GPa, leading to a high pressure phase where, above 24 GPa, the nanothread formation is observed. However, the observed reaction was limited in extent and required significantly higher pressures in comparison to other members of the pseudo-stilbene family. This comprehensive study is imperative in laying the foundation for future endeavors, aiming to synthesize double-core nanothreads from pseudo-stilbene crystals, and provides crucial insights into the high-pressure behavior and phase transitions of crystalline bibenzyl.

Anisotropic Interlayer Force Field for Group-VI Transition Metal Dichalcogenides

An anisotropic interlayer force field that describes the interlayer interactions in homogeneous and heterogeneous interfaces of group-VI transition metal dichalcogenides (MX2, where M = Mo, W, and X = S, Se) is presented. The force field is benchmarked against density functional theory calculations for bilayer systems within the Heyd-Scuseria-Ernzerhof hybrid density functional approximation, augmented by a nonlocal many-body dispersion treatment of long-range correlation. The parametrization yields good agreement with the reference calculations of binding energy curves and sliding potential energy surfaces. It is found to be transferable to transition metal dichalcogenide (TMD) junctions outside of the training set that contain the same atom types. Calculated bulk moduli agree with most previous dispersion-corrected density functional theory predictions, which underestimate the available experimental values. Calculated phonon spectra of the various junctions under consideration demonstrate the importance of appropriately treating the anisotropic nature of the layered interfaces. Considering our previous parametrization for MoS2, the anisotropic interlayer potential enables accurate and efficient large-scale simulations of the dynamical, tribological, and thermal transport properties of a large set of homogeneous and heterogeneous TMD interfaces.

Pressure-induced structural phase transitions of zirconium: anab initiostudy based on statistical ensemble theory

Recently, we put forward a direct integral approach to solve the partition function with ultrahigh efficiency and precision, which enables the rigorous ensemble theory to investigate phase behaviors of realistic condensed matters and has been successfully applied to the phase transition of vanadium metal (Ninget al2022J. Phys.: Condens. Matter34425404). In this work, the approach is applied to the structural phase transitions of zirconium metal under compressions up to 160 GPa and ultrahigh calculation precision is achieved. For the obtained equation of state with pressure over 40 GPa, the deviations from latest experiments are within0.7%and the computed transition pressure ofα→ωis 6.93 GPa, which is about five times larger than previous theoretical predictions and in excellent agreement with the measured range of 5-15 GPa. Our results support the argument that there is no existence of the isostructural phase transition of Zr metal that was asserted by recent experimental observations.

Ab initio calculation of mechanical, electronic and optical characteristics of chalcogenide perovskite BaZrS3 at high pressures

A theoretical examination of the structural, elastic, electronic and optical properties of the chalcogenide perovskite BaZrS₃ under pressures of 0 and 20 GPa was performed using density functional theory ab initio calculations. The lattice constants of the BaZrS₃ structure are well reproduced from our first-principles calculations and are in excellent agreement with experimental measurements. Moreover, the values of mechanical parameters, such as the elastic constant, increased under applied pressure. The electronic parameters indicate that the chalcogenide perovskite BaZrS₃ has a direct band gap of 1.75 eV. To understand the optical response, the real and imaginary parts of the dielectric function of BaZrS₃ have been studied, as well as the absorption coefficient, reflectivity and extinction coefficient. The induced pressure is found to enhance the optical parameters in the different energy regions. Our calculations predict that the studied chalcogenide perovskite BaZrS₃ could be a candidate in photovoltaic, optoelectronic and mechanical applications.

Equation of state of Iridium: from insight of ensemble theory

The equations of state (EOS) of Iridium are, for the first time, obtained by solving the high-dimension integral of partition function based on a recently developed approach of ultrahigh efficiency and precision without any artificial parameter, and the deviation of 0.25% and 1.52% from the experiments was achieved respectively for the isobaric EOS in a temperature range of 300 K-2500 K and the isothermal EOS at 300 K up to 300 GPa. Specific comparisons show that the deviation of EOS based on harmonic approximation even including anharmonic effect, manifests worse than ours by several times or even one order of magnitude, indicating that ensemble theory is the very approach to understand the thermodynamic properties of condensed matter.

Unexpected low thermal expansion coefficients of pentadiamond

A new carbon allotrope, pentadiamond, was recently reported in the literature. Herein, we investigate its thermal expansion and thermoelastic properties by first principles. It is observed that the bulk modulus and hardness of pentadiamond are far less than those of diamond, but the thermal expansion of pentadiamond is lower than that of diamond in the range of 0 K to 2000 K, and even negative in the temperature range of 0-190 K. The negative thermal expansion at low temperature originates from the transverse vibrations of the edge-shared atoms in the coplanar double-pentagon. The low thermal expansion at high temperature is contributed by the strong bonds in pentadiamond. Benefiting from the low thermal expansion, the elastic constants of pentadiamond decrease very slowly with respect to temperature compared with those of diamond. The low sensitivity of thermodynamic and thermoelastic properties to temperature makes pentadiamond a promising material for high anti-thermal-shock and accurate electronic device applications.

The Crystal Structure of Carbonic Acid

Ubiquitous carbonic acid, H2CO3, a key molecule in biochemistry, geochemistry, and also extraterrestrial chemistry, is known from a plethora of physicochemical studies. Its crystal structure has now been determined from neutron-diffraction data on a deuterated sample in a specially built hybrid clamped cell. At 1.85 GPa, D2CO3 crystallizes in the monoclinic space group P21/c with a = 5.392(2), b = 6.661(4), c = 5.690(1) Å, β = 92.66(3)°, Z = 4, with one symmetry-inequivalent anti-anti shaped D2CO3 molecule forming dimers, as previously predicted. Quantum chemistry evidences π bonding within the CO3 molecular core, very strong hydrogen bonding between the molecules, and a massive influence of the crystal field on all bonds; phonon calculations emphasize the locality of the vibrations, being rather insensitive to the extended structure.

Delving into guest-free and He-filled sI and sII clathrate hydrates: a first-principles computational study

The dynamics of the formation of a specific clathrate hydrate as well as its thermodynamic transitions depend on the interactions between the trapped molecules and the host water lattice. The molecular-level understanding of the different underlying processes benefits not only the description of the properties of the system, but also allows the development of multiple technological applications such as gas storage, gas separation, energy transport, etc. In this work we investigate the stability of periodic crystalline structures, such as He@sI and He@sII clathrate hydrates by first-principles computations. We consider such host water networks interacting with a guest He atom using selected density functional theory approaches, in order to explore the effects on the encapsulation of a light atom in the sI/sII crystals, by deriving all energy components (guest-water, water-water, guest-guest). Structural properties and energies were first computed by structural relaxations of the He-filled and empty sI/sII unit cells, yielding lattice and compressibility parameters comparable to experimental and theoretical values available for those hydrates. According to the results obtained, the He enclathration in the sI/sII unit cells is a stabilizing process, and both He@sI and He@sII clathrates, considering single cage occupancy, are predicted to be stable whatever the XDM or D4 dispersion correction applied. Our results further reveal that despite the weak underlying interactions the He encapsulation has a rather notable effect on both lattice parameters and energetics, with the He@sII being the most energetically favorable in accord with recent experimental observations.

Improved volume variable cluster model method for crystal-lattice optimization: effect on isotope fractionation factor

The isotopic fractionation factor and element partition coefficient can be calculated only after the geometric optimization of the molecular clusters is completed. Optimization directly affects the accuracy of some parameters, such as the average bond length, molecular volume, harmonic vibrational frequency, and other thermodynamic parameters. Here, we used the improved volume variable cluster model (VVCM) method to optimize the molecular clusters of a typical oxide, quartz. We documented the average bond length and relative volume change. Finally, we extracted the harmonic vibrational frequencies and calculated the equilibrium fractionation factor of the silicon and oxygen isotopes. Given its performance in geometrical optimization and isotope fractionation factor calculation, we further applied the improved VVCM method to calculate isotope equilibrium fractionation factors of Cd and Zn between the hydroxide (Zn-Al layered double hydroxide), carbonate (cadmium-containing calcite) and their aqueous solutions under superficial conditions. We summarized a detailed procedure and used it to re-evaluate published theoretical results for cadmium-containing hydroxyapatite, emphasizing the relative volume change for all clusters and confirming the optimal point charge arrangement (PCA). The results showed that the average bond length and isotope fractionation factor are consistent with those published in previous studies, and the relative volume changes are considerably lower than the results calculated using the periodic boundary method. Specifically, the average Si-O bond length of quartz was 1.63 Å, and the relative volume change of quartz centered on silicon atoms was  - 0.39%. The average Zn-O bond length in the Zn-Al-layered double hydroxide was 2.10 Å, with a relative volume change of 1.96%. Cadmium-containing calcite had an average Cd-O bond length of 2.28 Å, with a relative volume change of 0.45%. At 298 K, the equilibrium fractionation factors between quartz, Zn-Al-layered double hydroxide, cadmium-containing calcite, and their corresponding aqueous solutions were [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] respectively. These results strongly support the reliability of the improved VVCM method for geometric optimization of molecular clusters.

Structural complexity in ramp-compressed sodium to 480 GPa

The properties of all materials at one atmosphere of pressure are controlled by the configurations of their valence electrons. At extreme pressures, neighboring atoms approach so close that core-electron orbitals overlap, and theory predicts the emergence of unusual quantum behavior. We ramp-compress monovalent elemental sodium, a prototypical metal at ambient conditions, to nearly 500 GPa (5 million atmospheres). The 7-fold increase of density brings the interatomic distance to 1.74 Å well within the initial 2.03 Å of the Na+ ionic diameter, and squeezes the valence electrons into the interstitial voids suggesting the formation of an electride phase. The laser-driven compression results in pressure-driven melting and recrystallization in a billionth of a second. In situ x-ray diffraction reveals a series of unexpected phase transitions upon recrystallization, and optical reflectivity measurements show a precipitous decrease throughout the liquid and solid phases, where the liquid is predicted to have electronic localization. These data reveal the presence of a rich, temperature-driven polymorphism where core electron overlap is thought to stabilize the formation of peculiar electride states.

Pressure-Induced Structural Phase Transition in EuNi2P2

EuNi2P2 was studied with a diamond anvil cell (DAC) and X-ray diffraction (XRD). The structural evolution of powder crystal EuNi2P2 under high pressure up to 137 GPa and its single-crystal structure up to 9 GPa were reported. The unique structural phase transition of this 122-type crystal occurred above 70 GPa in powder crystal EuNi2P2. The diffraction data from single-crystal EuNi2P2 revealed the coordinate change of the P atom, and the stability of the crystal at 9 GPa was confirmed. The crystal EuNi2P2 remained stable with a tetragonal phase without obvious symmetry changes during compression to 137 GPa.

Evidence and Stability Field of fcc Superionic Water Ice Using Static Compression

Structural transformation of hot dense water ice is investigated by combining synchrotron x-ray diffraction and a laser-heating diamond anvil cell above 25 GPa. A transition from the body-centered-cubic (bcc) to face-centered-cubic (fcc) oxygen atoms sublattices is observed from 57 GPa and 1500 K to 166 GPa and 2500 K. That is the structural signature of the transition to fcc superionic (fcc SI) ice. The sign of the density discontinuity at the transition is obtained and a phase diagram is disclosed, showing an extended fcc SI stability field. Present data also constrain the stability field of the bcc superionic (bcc SI) ice up to 100 GPa at least. The current understanding of warm dense water ice based on ab initio simulations is discussed in the light of present data.

Origin of observed narrow bandgap of mica nanosheets

Mica nanosheets possess peculiar feature of narrowed bandgap with the decrease of thickness but a conclusive theoretical understanding of the narrowing mechanisms is still under development. In this report, first-principles calculations were carried out to investigate the electronic band structure of mica nanosheets with the deposition of K₂CO₃. Bulk mica shows an indirect bandgap of 4.90 eV. Mica nanosheets show similar electronic structures to bulk mica with a gradually increased bandgap of 4.44 eV, 4.52 eV and 4.67 eV for 1-layer, 2-layers and 3-layers nanosheets, respectively, which is attributed to the lattice relaxation. K₂CO₃ is found to have strong affinity towards mica nanosheets. The K₂CO₃ deposited mica nanosheets showed an increased bandgap with the increase of thickness, consistent with experimental observations. The calculated bandgap of K₂CO₃ deposited mica for 2-layers and 3-layers nanosheets are 2.60 eV and 2.75 eV, respectively, which are comparable with the corresponding experimental values of 2.5 eV and 3.0 eV. Our theoretical findings support the experimental evidence of surface contamination of mica by K₂CO₃, and provide new insight into the structure and properties of 2D mica.

On the Stiffness of Gold at the Nanoscale

The density and compressibility of nanoscale gold (both nanospheres and nanorods) and microscale gold (bulk) were simultaneously studied by X-ray diffraction with synchrotron radiation up to 30 GPa. Colloidal stability (aggregation state and nanoparticle shape and size) in both hydrostatic and nonhydrostatic regions was monitored by small-angle X-ray scattering. We demonstrate that nonhydrostatic effects due to solvent solidification had a negligible influence on the stability of the nanoparticles. Conversely, nonhydrostatic effects produced axial stresses on the nanoparticle up to a factor 10× higher than those on the bulk metal. Working under hydrostatic conditions (liquid solution), we determined the equation of state of individual nanoparticles. From the values of the lattice parameter and bulk modulus, we found that gold nanoparticles are slightly denser (0.3%) and stiffer (2%) than bulk gold: V0 = 67.65(3) Å3, K0 = 170(3)GPa, at zero pressure.

Anisotropic Interlayer Force Field for Transition Metal Dichalcogenides: The Case of Molybdenum Disulfide

An anisotropic interlayer force field that describes the interlayer interactions in molybdenum disulfide (MoS2) is presented. The force field is benchmarked against density functional theory calculations for both bilayer and bulk systems within the Heyd-Scuseria-Ernzerhof hybrid density functional approximation, augmented by a nonlocal many-body dispersion treatment of long-range correlation. The parametrization yields good agreement with the reference calculations of binding energy curves and sliding potential energy surfaces for both bilayer and bulk configurations. Benchmark calculations for the phonon spectra of bulk MoS2 provide good agreement with experimental data, and the calculated bulk modulus falls in the lower part of experimentally measured values. This indicates the accuracy of the interlayer force field near equilibrium. Under external pressures up to 20 GPa, the developed force field provides a good description of compression curves. At higher pressures, deviations from experimental data grow, signifying the validity range of the developed force field.

Materials informatics platform with three dimensional structures, workflow and thermoelectric applications

Since the proposal of the "Materials Genome Initiative", several material databases have emerged and advanced many materials fields. In this work, we present the Materials Informatics Platform with Three-Dimensional Structures (MIP-3d). More than 80,000 structural entries, mainly from the inorganic crystal structural database, are included in MIP-3d. Density functional theory calculations are carried out for over 30,000 entries in the database, which contain the relaxed crystal structures, density of states, and band structures. The calculation of the equations of state and sound velocities is performed for over 12,000 entries. Notably, for entries with band gap values larger than 0.3 eV, the band degeneracies for the valence band maxima and the conduction band minima are analysed. The electrical transport properties for approximately 4,400 entries are also calculated and presented in MIP-3d under the constant electron-phonon coupling approximation. The calculations of the band degeneracies and electrical transport properties make MIP-3d a database specifically designed for thermoelectric applications.

Crystal Structure and Non-Hydrostatic Stress-Induced Phase Transition of Urotropine Under High Pressure

High-pressure behavior of hexamethylenetetramine (urotropine) was studied in situ using angle-dispersive single-crystal synchrotron X-ray diffraction (XRD) and Fourier-transform infrared absorption (FTIR) spectroscopy. Experiments were conducted in various pressure-transmitting media to study the effect of deviatoric stress on phase transformations. Up to 4 GPa significant damping of molecular librations and atomic thermal motion was observed. A first-order phase transition to a tetragonal structure was observed with an onset at approximately 12.5 GPa and characterized by sluggish kinetics and considerable hysteresis upon decompression. However, it occurs only in non-hydrostatic conditions, induced by deviatoric or uniaxial stress in the sample. This behavior finds analogies in similar cubic crystals built of highly symmetric cage-like molecules and may be considered a common feature of such systems. DFT computations were performed to model urotropine equation of state and pressure dependence of vibrational modes. The first successful Hirshfeld atom refinements carried out for high-pressure diffraction data are reported. The refinements yielded more realistic C-H bond lengths than the independent atom model even though the high-pressure diffraction data are incomplete.

High-pressure and low-temperature structural study of claudetite I, a monoclinic layered As2O3 polymorph

High-pressure structural studies of claudetite I reveal that it exhibits the lowest As coordination numbers despite being the densest As₂O₃ polymorph.

Semi-empirical and ab initio calculations for crystals under pressure at fixed temperatures: the case of guanidinium perchlorate

A simple semi-empirical approach is proposed to calculate structure and properties of crystals under pressure at fixed temperatures. The computed semi-empirical pressure dependencies for guanidinium perchlorate are in good agreement with available experimental data. Ab initio results within quasi-harmonic approximation for guanidinium perchlorate are also presented.

Melting Curve and Isostructural Solid Transition in Superionic Ice

The phase diagram and melting curve of water ice is investigated up to 45 GPa and 1600 K by synchrotron x-ray diffraction in the resistively and laser heated diamond anvil cell. Our melting data evidence a triple point at 14.6 GPa, 850 K. The latter is shown to be related to a first-order solid transition from the dynamically disordered form of ice VII, denoted ice VII^{'}, toward a high-temperature phase with the same bcc oxygen lattice but larger volume and higher entropy. Our experiments are compared to ab initio molecular dynamics simulations, enabling us to identify the high-temperature bcc phase with the predicted superionic ice VII^{''} phase [J.-A. Hernandez and R. Caracas, Phys. Rev. Lett. 117, 135503 (2016).PRLTAO0031-900710.1103/PhysRevLett.117.135503].

Pressure-Induced Polymerization of Polycyclic Arene-Perfluoroarene Cocrystals: Single Crystal X-ray Diffraction Studies, Reaction Kinetics, and Design of Columnar Hydrofluorocarbons

Pressure-induced polymerization of aromatic compounds leads to novel materials containing sp³ carbon-bonded networks. The choice of the molecular species and the control of their arrangement in the crystal structures via intermolecular interactions, such as the arene-perfluoroarene interaction, can enable the design of target polymers. We have investigated the crystal structure compression and pressure-induced polymerization reaction kinetics of two polycyclic 1:1 arene-perfluoroarene cocrystals, naphthalene/octafluoronaphthalene (NOFN) and anthracene/octafluoronaphthalene (AOFN), up to 25 and 30 GPa, respectively, using single-crystal synchrotron X-ray diffraction, infrared spectroscopy, and theoretical computations based on density-functional theory. Our study shows the remarkable pressure stability of the parallel arene-perfluoroarene π-stacking arrangement and a reduction of the interplanar π-stacking separations by ca. 19-22% before the critical reaction distance is reached. A further strong, discontinuous, and irreversible reduction along the stacking direction at 20 GPa in NOFN (18.8%) and 25 GPa in AOFN (8.7%) indicates the pressure-induced breakdown of π-stacking by formation of σ-bonded polymers. The association of the structural distortion with the occurrence of a chemical reaction is confirmed by a high-pressure kinetic study using infrared spectroscopy, indicating one-dimensional polymer growth. Structural predictions for the fully polymerized high-pressure phases consisting of highly ordered rods of hydrofluorocarbons are presented based on theoretical computations, which are in excellent agreement with the experimentally determined unit-cell parameters. We show that the polymerization takes place along the arene-perfluoroarene π-stacking direction and that the lateral extension of the columns depends on the extension of the arene and perfluoroarene molecules.

Thermomechanical properties of zero thermal expansion materials from theory and experiments

Origin and composition dependence of the anisotropic thermomechanical properties are elucidated for Ba_1-xSr_xZn₂Si₂O₇ (BZS) solid solutions. The high-temperature phase of BZS shows negative thermal expansion (NTE) along one crystallographic axis and highly anisotropic elastic properties characterized by X-ray diffraction experiments and simulations at the density functional theory level. Ab initio molecular dynamics simulations provide accurate predictions of the anisotropic thermal expansion in excellent agreement with experimental observations. The NTE considerably decreases with increasing Sr content x. This is connected with the composition dependence of the vibrational density of states (VDOS) and the anisotropic Grüneisen parameters. The VDOS shifts to higher frequencies between 0-5 THz due to substitution of Ba with Sr. In the same frequency range, vibrational modes contributing most to the NTE are found. In addition, phonon calculations using the quasi-harmonic approximation revealed that the NTE is mainly connected with deformation of four-membered rings formed by SiO₄ and ZnO₄ tetrahedra. The thermomechanical and vibrational properties obtained in this work provide the basis for future studies facilitating the targeted design of BZS solid solutions as zero or negative thermal expansion material.

Equations of State of Simple Solids (Including Pb, NaCl and LiF) Compressed in Helium or Neon in the Mbar Range

The equations of state measured under ambient temperature in the Mbar range are reviewed, focusing on experiments using diamond anvils cells with a quasi-hydrostatic pressure transmitting medium (helium or neon) and coupled with X-ray diffraction. Equations of state (EoS) parameters are listed with an unified pressure metrology for all data. This metrology is based on the efforts made in the 2000s to update the ruby luminescence pressure scale, after the collection of original data. To complete this database, unpublished P-V data for lead (Pb), sodium chloride (NaCl) and lithium fluoride (LiF) are also provided with the same metrology. Systematic effects of the pressure metrology on the EoS parameters are discussed.

Ground-State and Thermodynamical Properties of Uranium Mononitride from Anharmonic First-Principles Theory

We report on an advanced density-functional theory (DFT) approach for investigating the ground-state and thermodynamical properties of uranium mononitride (UN). The electronic structure for UN at zero temperature is obtained from DFT that utilizes the generalized gradient approximation (GGA) for the electron exchange and correlation functional and includes spin-orbit interaction and an extension with orbital polarization. Thermodynamical properties are computed within the quasi-harmonic approximation in the Debye–Grüneisen model while anharmonicity is captured in the self-consistent ab initio lattice dynamics (SCAILD) scheme. Anharmonic phonons have heretofore never been modeled from first-principles for UN but they turn out to be important. The computed free energy compares well with that of a CALPHAD (CALculation of PHAse Diagrams) assessment of available experimental data.

Pressure-Induced Polymerization and Electrical Conductivity of a Polyiodide

We report the high-pressure structural characterization of an organic polyiodide salt in which a progressive addition of iodine to triiodide groups occurs. Compression leads to the initial formation of discrete heptaiodide units, followed by polymerization to a 3D anionic network. Although the structural changes appear to be continuous, the insulating salt becomes a semiconducting polymer above 10 GPa. The features of the pre-reactive state and the polymerized state are revealed by analysis of the computed electron and energy densities. The unusually high electrical conductivity can be explained with the formation of new bonds.

Evidence for Crystalline Structure in Dynamically-Compressed Polyethylene up to 200 GPa

We investigated the high-pressure behavior of polyethylene (CH₂) by probing dynamically-compressed samples with X-ray diffraction. At pressures up to 200 GPa, comparable to those present inside icy giant planets (Uranus, Neptune), shock-compressed polyethylene retains a polymer crystal structure, from which we infer the presence of significant covalent bonding. The A2/m structure which we observe has previously been seen at significantly lower pressures, and the equation of state measured agrees with our findings. This result appears to contrast with recent data from shock-compressed polystyrene (CH) at higher temperatures, which demonstrated demixing and recrystallization into a diamond lattice, implying the breaking of the original chemical bonds. As such chemical processes have significant implications for the structure and energy transfer within ice giants, our results highlight the need for a deeper understanding of the chemistry of high pressure hydrocarbons, and the importance of better constraining planetary temperature profiles.

First-principles study of thermal expansion and thermomechanics of group-V monolayers: blue phosphorene, arsenene, and antimonene

Using first-principles calculations based on quasiharmonic approximation (QHA), we systematically investigate the vibrational properties, thermal expansion and thermomechanics properties of three monoatomic group-V monolayers with honeycomb structure: blue phosphorene, arsenene and antimonene. It is found antimonene shows negative thermal expansion in the temperature range studied, while blue phosphorene and arsenene display small positive thermal expansion at high temperature. The physical mechanisms of their different thermal properties are explored. The results of Grüneisen theory are also exhibited for comparison, and the similarities and differences are studied. It is concluded the discrepancies arise from the high order anharmonicity of phonons.

Construction of a hybrid clamped cell for high-pressure neutron-diffraction experiments with a large diamond window

A hybrid pressure cell was fabricated from commercially available copper-beryllium and custom-made Ni-Cr-Al Russian alloy, tailored for usage as a reaction vessel supplying a volume of about 400 mm³. In order to directly (in situ) monitor pressure and chemical reactions within the chamber, a large diamond window suitable for spectroscopic sample analysis was implemented. The performance of the hybrid cell was validated from high-pressure neutron-diffraction measurements on carbon dioxide.

Pressure-induced isosymmetric phase transition in biurea

An isosymmetric phase transition of biurea has been found in which the molecules appear to “snap” from one conformer to another upon compression.

Stability of Hydrogen Hydrates from Second-Order Møller-Plesset Perturbation Theory

The formation of gas hydrates and clathrates critically depends on the interaction between the host water network and the guest gas species. Density functional calculations can struggle to quantitatively capture these dispersion-type interactions. Here, we report wave function-based calculations on hydrogen hydrates that combine periodic Hartree-Fock with a localized treatment of electronic correlation. We show that local second-order Møller-Plesset perturbation theory (LMP2) reproduces the stability of the different filled-ice-like hydrates in excellent agreement with experimental data. In contrast to various dispersion-corrected density functional theory implementations, LMP2 correctly identifies the pressures needed to stabilize the C₀, C₁, and C₂ hydrates and does not find a spurious region of stability for an ice-I_h-based dihydrate. Our results suggest that LMP2 or similar approaches can provide quantitative insights into the mechanisms of formation and eventual decomposition of molecular host-guest compounds.

Effects of pressure on the structure and lattice dynamics of ammonium perchlorate: A combined experimental and theoretical study

Ammonium perchlorate NH₄ClO₄ (AP) was studied using synchrotron angle-dispersive X-ray powder diffraction (XRPD) and Raman spectroscopy. A diamond-anvil cell was used to compress AP up to 50 GPa at room temperature (RT). Density functional theory (DFT) calculations were performed to provide further insight and comparison to the experimental data. A high-pressure barite-type structure (Phase II) forms at ≈4 GPa and appears stable up to 40 GPa. Refined atomic coordinates for Phase II are provided, and details for the Phase I → II transition mechanics are outlined. Pressure-dependent enthalpies computed for DFT-optimized crystal structures confirm the Phase I → II transition sequence, and the interpolated transition pressure is in excellent agreement with the experiment. Evidence for additional (underlying) structural modifications include a marked decrease in the Phase II b'-axis compressibility starting at 15 GPa and an unambiguous stress relaxation in the normalized stress-strain response at 36 GPa. Above 47 GPa, XRD Bragg peaks begin to decrease in amplitude and broaden. The apparent loss of crystalline long-range order likely signals the onset of amorphization. Three isostructural modifications were discovered within Phase II via Raman spectroscopy. A revised RT isothermal phase diagram is discussed based on the findings of this study.

Examining the chemical and structural properties that influence the sensitivity of energetic nitrate esters

The sensitivity of explosives is controlled by factors that span from intrinsic chemical reactivity and chemical intramolecular effects to mesoscale structure and defects, and has been a topic of extensive study for over 50 years. Due to these complex competing chemical and physical elements, a unifying relationship between molecular framework, crystal structure, and sensitivity has yet to be developed. In order to move towards this goal, ideally experimental studies should be performed on systems with small, systematic structural modifications, with modeling utilized to interpret experimental results. Pentaerythritol tetranitrate (PETN) is a common nitrate ester explosive that has been widely studied due to its use in military and commercial explosives. We have synthesized PETN derivatives with modified sensitivity characteristics by substituting one -CCH2ONO2 moiety with other substituents, including -CH, -CNH2, -CNH3X, -CCH3, and -PO. We relate the handling sensitivity properties of each PETN derivative to its structural properties, and discuss the potential roles of thermodynamic properties such as heat capacity and heat of formation, thermal stability, crystal structure, compressibility, and inter- and intramolecular hydrogen bonding on impact sensitivity. Reactive molecular dynamics (MD) simulations of the C/H/N/O-based PETN-derivatives have been performed under cook-off conditions that mimic those accessed in impact tests. These simulations infer how changes in chemistry affect the subsequent decomposition pathways.

How and Why Does Helium Permeate Nonporous Arsenolite Under High Pressure?

Investigations into the helium permeation of arsenolite, the cubic, molecular arsenic(III) oxide polymorph As₄ O₆ , were carried out to understand how and why arsenolite helium clathrate As₄ O₆ ⋅2 He is formed. High-pressure synchrotron X-ray diffraction experiments on arsenolite single crystals revealed that the permeation of helium into nonporous arsenolite depends on the time for which the crystal is subjected to high pressure and on the crystal history. The single crystal was totally transformed into As₄ O₆ ⋅2 He within 45 h under 5 GPa. After release of the pressure, arsenolite was recovered and a repeated increase in pressure up to 3 GPa led to practically instant As₄ O₆ ⋅2 He formation. However, when a pristine arsenolite single crystal was quickly subjected to a pressure of 13 GPa, no helium permeation was observed at all. No neon permeation was observed in analogous experiments. Quantum mechanical computations indicate that there are no specific attractive interactions between He atoms and As₄ O₆ molecules at the distances observed in the As₄ O₆ ⋅2 He crystal structure. Detailed analysis of As₄ O₆ molecular structure changes has shown that the introduction of He into the arsenolite crystal lattice significantly reduces molecular deformations by decreasing the anisotropy of stress exerted on the As₄ O₆ molecules. This effect and the pΔV term, rather than any specific As⋅⋅⋅He binding, are the driving forces for the formation As₄ O₆ ⋅2 He.

First principles study of the Mn-doping effect on the physical and chemical properties of mullite-family Al2SiO5

Transition metal (TM) modification is a common strategy for converting an earth-abundant mineral into a cost-effective catalyst for industrial applications. Among a variety of minerals, Al₂SiO₅, which has three phases, andalusite, sillimanite and kyanite, is emerging as a promising candidate for new catalyst development. In this paper, we use Mn to demonstrate the rationale of 3d TM doping at the Al sites in each of these three phases through first-principles calculations and the cluster expansion method. The results of cluster expansion show that Mn has a strong site preference for the six-coordinated Al octahedral chains in the andalusite and sillimanite phases, while distributing randomly in the kyanite phase. Moreover, Mn can only replace Al in sillimanite and kyanite in low concentrations; however, higher concentrations of Mn can replace Al in andalusite. We found that the concentration sensitivity is due to the Jahn-Teller distortion and 3d orbital splitting. This finding can also explain the low doping concentrations of other 3d TMs (Fe, Cr and V) in Al₂SiO₅ compounds. Based on the calculated Helmholtz free energy, we constructed a (Mn_xAl_1-x)AlSiO₅ temperature-composite phase diagram, which explains the physical mechanisms behind the results for 3d transition metal doping and phase transitions in Al₂SiO₅. This work could shed light on the related physics, chemistry, and geoscience of (Mn_xAl_1-x)AlSiO₅, and more importantly, a design rationale for the engineering of cheap catalysts.

The elastic solid solution model for minerals at high pressures and temperatures

Non-ideality in mineral solid solutions affects their elastic and thermodynamic properties, their thermobaric stability, and the equilibrium phase relations in multiphase assemblages. At a given composition and state of order, non-ideality in minerals is typically modelled via excesses in Gibbs free energy which are either constant or linear with respect to pressure and temperature. This approach has been extremely successful when modelling near-ideal solutions. However, when the lattice parameters of the solution endmembers differ significantly, extrapolations of thermodynamic properties to high pressures using these models may result in significant errors. In this paper, I investigate the effect of parameterising solution models in terms of the Helmholtz free energy, treating volume (or lattice parameters) rather than pressure as an independent variable. This approach has been previously applied to models of order-disorder, but the implications for the thermodynamics and elasticity of solid solutions have not been fully explored. Solid solution models based on the Helmholtz free energy are intuitive at a microscopic level, as they automatically include the energetic contribution from elastic deformation of the endmember lattices. A chemical contribution must also be included in such models, which arises from atomic exchange within the solution. Derivations are provided for the thermodynamic properties of n-endmember solutions. Examples of the use of the elastic model are presented for the alkali halides, pyroxene, garnet, and bridgmanite solid solutions. Elastic theory provides insights into the microscopic origins of non-ideality in a range of solutions, and can make accurate predictions of excess enthalpies, entropies, and volumes as a function of volume and temperature. In solutions where experimental data are sparse or contradictory, the Helmholtz free energy approach can be used to assess the magnitude of excess properties and their variation as a function of pressure and temperature. The formulation is expected to be useful for geochemical and geophysical studies of the Earth and other planetary bodies.