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Boron phosphide (BP) is a (super)hard semiconductor constituted of light elements, which is promising for high demand applications at extreme conditions. The behavior of BP at high temperatures and pressures is of special interest but is also poorly understood because both experimental and conventional ab initio methods are restricted to studying refractory covalent materials. The use of machine learning interatomic potentials is a revolutionary trend that gives a unique opportunity for high-temperature study of materials with ab initio accuracy. We develop a deep machine learning potential (DP) for accurate atomistic simulations of the solid and liquid phases of BP as well as their transformations near the melting line. Our DP provides quantitative agreement with experimental and ab initio molecular dynamics data for structural and dynamic properties. DP-based simulations reveal that at ambient pressure, a tetrahedrally bonded cubic BP crystal melts into an open structure consisting of two interpenetrating sub-networks of boron and phosphorous with different structures. Structure transformations of BP melt under compressing are reflected by the evolution of low-pressure tetrahedral coordination to high-pressure octahedral coordination. The main contributions to structural changes at low pressures are made by the evolution of medium-range order in the B-subnetwork and, at high pressures, by the change of short-range order in the P-subnetwork. Such transformations exhibit an anomalous behavior of structural characteristics in the range of 12–15 GPa. DP-based simulations reveal that the Tm(P) curve develops a maximum at P ≈ 13 GPa, whereas experimental studies provide two separate branches of the melting curve, which demonstrate the opposite behavior. Analysis of the results obtained raises open issues in developing machine learning potentials for covalent materials and stimulates further experimental and theoretical studies of melting behavior in BP. [ABSTRACT FROM AUTHOR]

Experiments on the dynamic isentropic compression of solid CO2 samples by the megabar pressure induced by the superstrong magnetic field of an explosive magnetic generator have been performed with the X-ray diffraction detection of the state of the compressed samples. The generator operation is based on the fast compression of the initial magnetic flux in the cavity of the generator by a conducting cylindrical liner accelerated by the products of the explosion of a cylindrical explosive charge. Two points at pressures of 349 and 459 GPa on the compressibility diagram of CO2 have been determined in the experiments, where the degree of compression ρ/ρ0 of CO2 has reached currently highest values of 3.90 and 4.02, respectively. Comparison has shown that theoretically calculated equations of states for crystal phases of CO2 almost completely reproduce the experimental results, which confirms a high accuracy of theoretical predictions and the identity of the experimental and theoretical equations of states of CO2 modifications stable at high pressures. [ABSTRACT FROM AUTHOR]

Methanol as a basic liquid and the simplest alcohol is widely used in industry and scientific experiments. However, there are still no reliable data on the thermodynamic properties of methanol at high pressure. Here, we present an experimental and computational study of the thermodynamic properties of liquid methanol under high pressure up to 15 kbar, which significantly exceeds previously reported pressures. A temperature response to a small adiabatic change in pressure has been measured using a piston–cylinder apparatus. We have compared our experimental results with the literature data for lower pressures and NIST approximations. We find that all existing experimental data do not agree with each other and with our experiments. The NIST approximations are mainly based on low pressure data and appear to be unreliable in the high pressure region, giving even qualitatively wrong results. OPLS and COMPASS force field models have been used in the method of molecular dynamics. The agreement of molecular simulation with our experimental data is definitely unsatisfactory, which means that the most common computational models of methanol are not sufficiently good. We hope that these experimental data and approximations will help in developing better computational models. [ABSTRACT FROM AUTHOR]

The stability of CO2 phases at pressures up to 1600 GPa is confirmed using evolution methods for predicting crystal structures. Stable CO2 phases are as follows: (to 279 GPa), (279–952 GPa), Pbcn (952–1018 GPa), and (above 1018 GPa). The equations of state for stable CO2 phases up to pressures of about 1600 GPa are calculated for the first time using ab initio methods and high-temperature calculations within the quasi-harmonic approximation. It is shown that high-pressure , , and phases have rather high bulk moduli (290–415 GPa). Phases with sixfold coordination of carbon atoms (Pbcn and ) have higher coefficients of thermal expansion in comparison with the phase. [ABSTRACT FROM AUTHOR]

The volume and conductivity of nonstoichiometric chalcogenide glass g-As3Te2 have been investigated at high hydrostatic pressures (up to 8.5 GPa), and results have been compared with earlier data for stoichiometric chalcogenide glass g-As2Te3. Structural and Raman studies of g-As3Te2 glass have revealed a greater significance of As–As pair correlations in the range of medium-range order compared with "classic" chalcogenide glass g-As2Te3. Even at such a large excess of arsenic, a high concentration of "improper" Te–Te neighbors has been observed because of chemical disorder. Under normal conditions, the thermal gap (0.43–0.48 eV) and resistivity (>104 Ω cm) of glass g-As3Te2 are greater than those of g-As2Te3. The elastic behavior of g-As3Te2 glass, as well as of g-As2Te3, under compression has been observed at pressures up to 1 GPa, the initial values of bulk moduli for these glasses being nearly coincident. Polyamorphic transformation in g-As3Te2 (with softening of relaxing bulk modulus) is more diffuse and extends to higher pressures (from 1.5 to 4.0 GPa). The metallization process in g-As3Te2 is also more diffuse: metallic conductivity is reached at pressures of 5.5–6.0 GPa. As in the case of the stoichiometric glass, the baric dependences of the bulk modulus exhibit a kink in the pressure range 4–5 GPa. Up to maximal pressures, the volume and resistivity relax logarithmically in time with roughly the same rate as in the case of g-As2Te3. The residual densification of g-As3Te2 after pressure release is roughly twice as high as for g-As2Te3 and equals 3.5%, the conductivity of the compacted glass is about three orders of magnitude higher than that of the as-prepared sample. Under normal conditions, a considerable relaxation of the volume and resistivity has been observed. As for densified g-GeS2 glass, the logarithmic kinetics of this relaxation has been successfully described in terms of our earlier model based on the concept of relaxation self-organized criticality with the activation energy (1.3 eV) remaining unchanged up to 5 × 106 s. [ABSTRACT FROM AUTHOR]

In this brief report, we propose the ab initio prediction of a new polymer of carbon monoxide, previously overlooked in extensive searches for its phases. This polymer (named γ-graphene monoxide) is more energetically favorable at ambient pressure than molecular CO, and possesses a set of unusual properties. Both first principles calculations and straightforward crystallochemistry analysis suggest that it is metallic at ambient pressure, and even superconducting with a critical temperature Tc = 4–20 K. [ABSTRACT FROM AUTHOR]

The relaxation of the optical absorption edge and of the linear sizes of stoichiometric GeS2 glass densified at a high hydrostatic pressure have been studied. Precision data have been obtained on the pressure dependence of the specific volume and the bulk compression modulus upon an increases and decrease of pressure in the range up to 8 GPa, including long-term volume relaxation at room temperature after the removal of pressure. The residual densification of glass after the action of pressure of 8.3 GPa with hold-up for 20 h was 9.5% immediately after the removal of pressure and 8% at the beginning of optical measurements with a corresponding absorption edge shift by 170 nm. The relaxation of the optical absorption edge and of the linear sizes has been investigated at room temperature as well as in a series of subsequent annealings right to the return of glass to its initial state. Two processes that have been singled out are both linear in logarithmic time with close relaxation curve slopes, but with rates differing by two orders of magnitude. The relaxation activation energy lies in interval 1.60–1.77 eV, increasing with temperature. A possible mechanism of logarithmic relaxation of glass properties is considered. [ABSTRACT FROM AUTHOR]