Baric Dependences Research Articles

A search for pressure-induced phase transition in the layered perovskite-like Pr2Ti2O7

In our study, we present comprehensive findings on the structural properties of Pr2Ti2O7 across a broad pressure range of 0–30 GPa. Neutron diffraction experiments, conducted under ambient conditions, offer crucial structural insights into the initial monoclinic phase with the P21 space group. As pressure increased, a significant phase transition to the monoclinic P21/m phase occurred at 13.8 GPa. Structural data analysis from X-ray diffraction highlights the essence of this transition, identifying it as the tilting of Ti-O6 octahedra. High-pressure Raman spectroscopy data unequivocally confirms the phase transition, detecting anomalies in the baric dependencies of some vibration modes of Pr2Ti2O7 and the emergence of new modes in the Raman spectra within the pressure region associated with the phase transition. The analysis of these novel vibration modes points to alterations in the Ti-O6 octahedra, emphasizing the pivotal role played by Ti4+ and O2- ions in the mechanism of the pressure-driven phase transition.

VIBRATIONAL AND X-RAY MEASUREMENTS OF La2Ti2O7 UNDER HIGH PRESSURE CONDITION

The present study focuses on investigating the impact of high-pressure conditions on the crystal structure and vibration spectra of the perovskite-like layered compound La2Ti2O7. To this end, X-ray diffraction and Raman spectroscopy techniques were employed to observe the behavior of the compound under pressure levels up to 30 GPa. The neutron diffraction at ambient pressure confirms that the structure of La2Ti2O7 is monoclinic P21. The results indicate that when the pressure attains 17.3 GPa, the original monoclinic phase of P21 symmetry undergoes a phase transition to the monoclinic phase of P2 symmetry. The observed pressure-induced phase transitions in La2Ti2O7 are associated with anomalous changes in both the unit cell parameters and the vibrational modes. Additionally, the study obtained the baric dependences of lattice parameters, unit cell volume and vibration frequencies, which were used to calculate the bulk modules for the initial and pressure-induced phases of La2Ti2O7.

Change in the melting temperature baric dependence during the transition from macro to nanocrystal

In this work we present new analytical (i.e., without computer modeling) method for calculating the dependence of the nanocrystal melting temperature both on pressure and on the size (number of atoms N) and surface shape of the nanocrystal. This method is based on the paired Mie–Lennard-Jones interatomic interaction potential, and takes into account the dependence of both the state equation and other lattice properties on the size and shape of the nanocrystal. For the first time, the dependences of the melting temperature on the pressure P, size N, and shape f of the nanocrystal were obtained. Calculations have been performed for gold, platinum and iron. It is shown that at any pressure, the melting point Tm(P, N, f) decreases both with an isomorphic-isobaric (f, P – const) decrease in the number of N atoms, and with an isomeric-isobaric (N, P – const) deviation of the nanocrystal shape from the energy-optimal shape. It is shown that the value of the baric derivative of the melting temperature Tm′(P) for a nanocrystal at low pressures is greater, and at high pressures less than the Tm′(P) value for a macrocrystal. At this, the dependence of the Tm′(P) function on the nanocrystal size is insignificant, i.e., at constant N-f-arguments, the baric dependences Tm(P, ∞) and Tm(P, N, f) are practically parallel. It is indicated how this method can be applied for experimental evaluation of the pressure under which a nanocrystal is located in a refractory matrix.

Study of the electrophysical and magnetic properties of a Dirac 3D semimetal Cd3As2 with nanogranules of MnAs

We report on the main results of studying the electrical and magnetoresistance (MR) of a composite material consisting of 70 % mol. Dirac semi-metal Cd3As2 and 30 % mol. ferromagnet MnAs at pressures up to 50 GPa in a diamond anvil cell with a «rounded cone-flat» type anvils, as well as magnetization at hydrostatic pressures up to 6 GPa in a toroid-shaped high-pressure cell, both at room temperature and in the temperature range of 180 – 350 K at atmospheric pressure. A mixture of methanol and ethanol in a ratio of 4:1 was used as a pressure transmitting medium. Elemental analysis of Cd3As2 + 30 % mol MnAs composites showed that much of the volume is occupied by the Cd3As2 phase. The proportion of MnAs phase inclusions is less than 5 %. The feature of Cd3As2 + MnAs is the presence of a significant region of non-mixing of the Cd3As2 and MnAs phase melts. A negative MR was revealed with increasing pressure in the entire studied baric zone. The maximum negative MR is observed in the baric zone of 22 – 26 GPa. Further increase in the pressure up to the maximum level result in the appearance of several extrema on the ΔR/R0(P) curve, with negative MR not exceeding 4 %. Upon pressure release from 50 GPa, the baric dependence of ΔR/R0(P) is characterized by an inversion of the MR sign: at pressures around 40 GPa, a negative MR is replaced by a positive MR, and at around 20 GPa, the maximum value of positive MR of ~5.3 % is observed. Signs of the instability of the monoclinic structure of Cd3As2 resulted from its partial decomposition upon decompression were revealed. The results obtained can be used in spintronics when using appropriate composite materials.

Compressibility, Metallization, and Relaxation in Nonstoichiometric Chalcogenide Glass g-As3Te2 at High Hydrostatic Pressure versus “Classic” g-As2Te3 Glass

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.

Study of the melting temperature baric dependence for Au, Pt, Nb

In this work we present new analytical (i.e., without computer modeling) method for calculating the dependence of the melting temperature (Tm) on pressure (P) for a single-component crystal. The method is based on the paired four–parameter Mie–Lennard-Jones interatomic interaction potential, and the delocalized criterion of melting. This method was used to calculate the baric dependences of the melting temperature Tm(P) and its pressure derivative Tm′(P) for gold, platinum and niobium in the pressure range: P = 0–1000 GPa. It has been shown that the dependences calculated by this method for gold and platinum are in better agreement with experimental data than the dependences obtained by computer modeling methods: the molecular dynamics simulations and the ab initio Z-method calculations. For niobium, the calculated Tm(P) dependence turned out to be steeper, i.e. the Tm′(P) values turned out to be greater than the experimental data obtained in [D. Errandonea et al. Communications Materials 1, 1 (2020)]. It has been indicated that this deviation can be caused both by a decrease in the Lindemann parameter with an increase in pressure, and by the redistribution of electrons in the s-to-d orbitals during compression of transition metals with a bcc structure. Furthermore, the discrepancy may also be due to the highly-correlated chain-like self-diffusion, which is inherent in bcc metals before melting.

ВЛИЯНИЕ ВЫСОКИХ ДАВЛЕНИЙ НА ЭЛЕКТРИЧЕСКИЕ СВОЙСТВА ОКСИДА CE0.8CU3V4O12

The electrical properties of the oxide Ce0.8Cu3V4O12 were studied at pressures up to 50 GPa. The samples have been synthesized by barothermal method. These compounds crystallize in the cubic symmetry with a perovskite-type structure. At ambient pressure in the temperature range from 10 to 300 K Ce0.8Cu3V4O12 - oxide has a metallic conductivity. Electrical resistance of the examined compounds measured at room temperature decreases when the pressure is raised from 16 to 50 GPa. There are two interval, where the resistance first falls rapidly with the pressure increase, and then the rate of the drop in resistance slows sharply. The baric dependencies of the magnetoresistance were obtained at the magnetic field up to 1 T. The values of the magnetoresistance are negative and amount 2% at pressure 22 GPa. The pressure range PT was determined, where a significant change of properties takes place. The results obtained for Ce0.8Cu3V4O12-oxide and the data of compounds ACu3V4O12 (A - Gd, Tb, Er, Tm) have been compared. A shift of the PT regions towards higher pressures with increasing serial number A was found. The manifestation of the effect of chemical contraction may be associated with a change in the crystal and electronic structure is assumed.

ВЛИЯНИЕ ВЫСОКОГО ДАВЛЕНИЯ НА ТЕРМОЭЛЕКТРИЧЕСКИЕ СВОЙСТВА ФУЛЛЕРЕНА С70

Research has been performed into baric dependences of thermo-EMF and thermal conductivity derivative of C70 crystalline fullerene at pressures up to 46 GPa and room temperature. Pressure were created in a chamber with conducting diamond anvils. Structural transformations were recorded by changes in thermo-EMF and thermal conductivity. The pressure ranges in which there is a significant change in the thermoelectric properties of fullerene C70 are determined. The nature of the baric dependences of thermo-EMF and thermal conductivity derivative indicates irreversible structural transformations at pressures of the order of 32 GPa, due to the destruction of the structure of fullerene molecules. At pressures above 32 GPa, the thermo-EMF values become close to zero. This may be due to the formation of graphite-like carbon.

РАСЧЕТ БАРИЧЕСКИХ КОЭФФИЦИЕНТОВ ТЕПЛОПРОВОДНОСТИ СМЕШАННЫХ РАСТВОРОВ ЭЛЕКТРОЛИТОВ

In the paper presents the calculated optimal values of the baric thermal conductivity coefficients corresponding
 to the minimum of the quadratic functional in comparison with the baric dependences of aqueous solutions of
 ethylene glycol and electrolytes in the cooling systems of marine internal combustion engines. For the pressure
 dependences of electrolyte solutions, an equation was obtained that makes it possible to calculate the thermal
 conductivity of mixed solutions at any pressure with an average error of 1.2%. Scientific interest lies in the
 study of the nature of the dependence of the baric coefficients of thermal conductivity of mixed electrolyte
 solutions on temperature when using the latter along with ethylene glycol coolants

A structural phase transition in La2Ti2O7 at high pressure

High-pressure effects on the crystal structure and vibration spectra of the perovskite-like layered La2Ti2O7 compound were studied using X-ray diffraction and Raman spectroscopy at pressures up to 30 GPa. The crystal structure of the compound was measured by means of a neutron diffraction at room temperature and ambient pressure. At P = 17.3 GPa, phase transition from the initial monoclinic phase of P21 symmetry to the monoclinic phase of P2 symmetry has been observed. The pressure-induced phase transitions in La2Ti2O7 are accompanied by anomalies in the pressure dependences of the unit cell parameters, as well as the vibrational modes. The baric dependences of lattice parameters, unit cell volume and vibration frequencies were obtained; the bulk modules for initial and pressure-induced phases of La2Ti2O7 were calculated.

Изменение температуры плавления металлов с ростом давления

A new analytical (i.e., without computer modeling) method for calculating the dependence of the melting temperature (Tm) of a single-component crystal on pressure (P) is proposed. The method is based on the delocalization melting criterion and does not contain fitting constants. The baric dependences of the melting temperature Tm(P) and its pressure derivative Tm(P) for gold, platinum and niobium in the pressure range: P = 0 – 1000 GPa were calculated by this method. It was shown that the dependences calculated by this method for gold and platinum agree better with the experimental data than the dependences obtained by computer simulation methods. For niobium, the calculated dependence Tm(P) turned out to be steeper, i.e., the value Tm(P) turned out to be larger than in the experiment. It was indicated that this discrepancy might be due both to a decrease in the Lindemann parameter with increasing pressure and to a redistribution of electrons on the s-d-orbitals during compression of transition metals with a BCC structure.

Change in the melting temperature of metals with an increase in pressure

A new analytical (i.e., without computer modeling) method for calculating the dependence of the melting temperature Tm of a single-component crystal on pressure P is proposed. The method is based on the delocalization melting criterion and does not contain fitting constants. The baric dependences of the melting temperature Tm(P) and its pressure derivative T'm(P) for gold, platinum and niobium in the pressure range: P=0-1000 GPa were calculated by this method. It was shown that the dependences calculated by this method for gold and platinum agree better with the experimental data than the dependences obtained by computer simulation methods. For niobium, the calculated dependence Tm(P) turned out to be steeper, i.e., the value T'm(P) turned out to be larger than in the experiment. It was indicated that this discrepancy might be due both to a decrease in the Lindemann parameter with increasing pressure and to a redistribution of electrons on the s-d-orbitals during compression of transition metals with a BCC structure. Keywords: melting point, pressure, interatomic interaction, gold, platinum, niobium.

Development and Laboratory Test of the Gelling Composition for the Selective Isolation of Formation Waters

The paper discusses the development of a silicon-dioxide based back plugging material with a controllable gelling period, which provides selective isolation of highly permeable areas of the bottomhole zone (BHZ) in production wells. The optimal composition, concentration and mixing procedure of the components for the gelling process based on the reaction between sodium silicate (Na2SiO3) and hydrochloric acid (HCl) were determined. Also gel formation, beginning and end of solidification, temperature and baric barometric dependences of the process were studied at the best power mixture ratios. In addition, the effect of the gelling composition with the optimal composition on the permeability of the layers was studied by checking the layered formation model. It was determined that in certain concentrations and ratios of Na2SiO3 and HCl solutions, a gelling composition is formed, and it is possible to use this composition for the selective isolation of reservoir waters. Keywords: Well bottom zone; Gel; composition; Reservoir model; Hydrochloric acid; Liquid glass; Water flooding; Permeability; Selective isolation.

High pressure effects on the crystal and magnetic structure of ScMnO3

The crystal and magnetic structures of hexagonal manganite ScMnO3 have been studied by means of neutron diffraction at high pressures up to 2 GPa in the temperature range 10–300 К. At ambient pressure below TN = 130 K the triangular antiferromagnetic (AFM) state of mixed Г1 + Г2 irreducible representation is formed. Upon application of high pressure, the symmetry of magnetic structure persists up to 2 GPa. A significant suppression of the ordered magnetic moment and gradual spin reorientation were revealed under pressure. The baric dependence of Neel temperatures TN was obtained with the average linear pressure coefficient dTN/dP= 6.9 K/GPa, which is significantly larger with respect to related compounds like YMnO3. For the data obtained, one may expect modification of the magnetic ground state symmetry to the pure Г1 representation at pressures about 6 GPa.

Electronic structure, optical and elastic properties of AgAlS2 crystal under hydrostatic pressure

In this work, the influence of hydrostatic pressure on the structural, electronic, optical, and mechanical properties of chalcopyrite AgAlS2 crystal was investigated by the first-principles density functional theory calculations. The lattice parameters and unit cell volume decrease with the hydrostatic pressure leading to an increase in the total energy of the crystal. Shown, the bandgap value increases with the pressure while the general peculiarities of the band structure remain almost unchanged. The baric dependences of the real and imaginary parts of the dielectric function are constructed and their analysis is carried out. Several elastic parameters of the material are calculated, such as elastic coefficients Cij, modulus of elasticity B, shear modulus G, Young's modulus E, Poisson's ratio v. Calculated bulk modulus is consistent with previous theoretical study. The spatial distribution of elastic coefficients and their planar projections are constructed and it is shown that hydrostatic pressure leads to an increase in their anisotropy. It is shown that the modulus of elasticity B has the smallest anisotropy, while the largest is observed for the shear modulus G.

Temperature-baric dependence of the effective thermal conductivity of amorphous and polycrystalline rocks

The paper presents the results of experimental studies of the temperature-baric effective thermal conductivity of a number of sandstones and granites. For the presentation we have chosen four sandstone samples from Dagestan and Tyumen deposits, two of which have a prevailing crystalline ordering, and two – a structure close to amorphous, and two samples of granite from Dagestan and Kola deposits. The experiment was provided by the absolute stationary compensation method of flat plates in the temperature range of 273–523 K in the range of hydrostatic pressures from atmospheric to 400 MPa. The results confirmed the proposed low-parameter model. The measured values of the thermal conductivity of the samples at 300 K were in the range from 0.7 to 3.5 W/m K. The temperature dependence in the considered range in all cases is well described by the power-law dependence. In addition, we have found a fairly significant correlation between the baric dependence of the effective thermal conductivity itself in the normalized dimensionless representation and the baric dependence of the exponent in the temperature dependence. This correlation can be traced for all studied samples of granites and sandstones and it makes possible to reduce the number of independent parameters. We have also shown that for sandstones with an amorphous structure, a simultaneous increase in temperature and pressure can lead to a rather significant total contribution. If taking into account the temperature-baric dependence of the effective thermal conductivity for rocks with a more common predominantly crystalline order, as noted by a number of authors, is not necessary due to the compensation of the pressure and temperature contributions, then for significantly amorphous rocks these contributions are summed up, and the total contribution may turn out to be significant.

Studies of the Effective Thermal Conductivity of Sandstone Under High Pressure and Temperature

We discuss the baric and temperature dependences of the effective thermal conductivity of rocks (including sandstones) naturally occurring in the earth’s crust. We present the results of our experimental studies of the effective thermal conductivity of natural sandstone under hydrostatic pressure (up to 400 MPa) in the temperature range (273–523 K), performed by the absolute steady-state method of parallel plates with an uncertainty of 3–4% and summarize numerous our previous results as well as results by the authors. We have analyzed the general regularities of the thermal conductivity temperature dependence for crystalline and amorphous dielectrics and rocks and compared them with the data of other authors. The temperature dependence of thermal conductivity is derived to be described by the power law, where the degree ranges from − 0.5 to + 0.5 and it can be used as an indicator of orderliness. The experimental data show that the pressure affects the nature of the temperature dependence. The heat transfer in dielectrics and rocks is revealed to occur due to increment of the maximum frequency of atomic acoustic vibrations and the associated Debye temperature. Based on the Landau and Lifshitz theories, we also discuss the possibility of reversible second-order phase transitions. In brittle and blocky crystalline solids this effect is more pronounced because they acquire a new quality—plasticity, whereas in plastic dielectrics and rocks only insignificant quantitative change is observed. The rocks without irreversible structural changes restore the original volume when decreasing the pressure. The results contribute to theoretical models describing heat transfer in complex disordered structures which is important for solving a number of tasks in Earth sciences.

ФАЗОВЫЕ ПЕРЕХОДЫ В ТВЕРДЫХ РАСТВОРАХ НА БАЗЕ CDAS2 ПРИ ДАВЛЕНИЯХ ДО 50 ГПА

Research has been performed into baric dependences between electrical resistance and thermo-EMF of eutectic solid solutions based on cadmium diarsenide of various compositions (Cd0.97Zn0.03As2 and Cd0.95Zn0.05As2) at pressures from 16 to 50 GPa and room temperature. Pressures were created in a chamber with conducting diamond anvils that served as contacts with the sample. Structural changes were recorded by changing electrical resistance and thermo-EMF. When replacing cadmium atom, zinc forms closer bonds with arsenic, which must result in the crystal lattice strengthening and higher pressures of phase transitions. Solid solutions preserve phase transitions observed in pure cadmium diarsenide. An increase in the crystal lattice stability of the solutions is confirmed compared to initial cadmium diarsenide. It is shown that with an increase in the zinc concentration pressures of phase transitions are shifted into the zone of higher pressures. The compounds preserve electronic conductivity in the range of pressures under consideration. At pressures higher than 30 GPa the thermo-EMF values become close to zero. This may be due to both an increase on concentrations of minority charge carriers and the impact of additional donating levels in the forbidden zone of solid solutions.

Theoretical study of the size dependencies of the thermodynamic properties of tungsten at various pressures and temperatures

The parameters of the paired 4-parametric interatomic potential of Mie-Lennard-Jones for bcc-W were obtained. The equation of state, baric and size dependencies of the coefficient of thermal volume expansion, elastic modulus, specific isobaric heat capacity and also specific surface energy of tungsten macro- and nano-crystal were calculated using obtained parameters of interatomic potential. The pressure derivatives of these dependencies were studied also.

Iron Oxidation in a Mixture with Polycarbonate after Plastic Deformation under High Pressure

Oxidation of a mixture of Fe powder with polycarbonate based on bisphenol-A was studied on a Q500 thermogravimetric analyzer (TGA) after plastic deformation under a pressure of 10 and 25 kb in dynamic and isothermal heating modes. Analysis of the TGA curves of the resulting metal–polymer composite showed the presence of two sections: a section of weight loss and growth. Their temperature and weight values have been determined, and the baric dependence of these parameters has been established, including the oxidizability of the Fe powder. After programmed annealing of the metal–polymer mixture activated by deformation, a porous metal-oxide-polymer nanocomposite is formed, consisting of nanoparticles coated with a thin PC nanolayer (nanowires and hematite nanolayers), which grow vertically from iron particles reinforcing the surrounding polymer matrix. The presence of polycarbonate and its decomposition products on the surface of oxidized Fe particles was established. Removal of the formed “coating” increases the oxidizability of the metal. A decrease in the temperature of thermal decomposition of polycarbonate (from 400 to 270°C) during its interaction with the oxidized surface of iron particles is shown, which is explained by the high catalytic activity of one-dimensional hematite nanostructures formed during oxidation of iron particles.