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In this study, tantalum carbide (TaC) samples were placed in a diamond anvil cell to study the equation of state at room temperature and high pressure using synchrotron radiation X-ray diffraction. By fitting the data at ambient pressure and up to the highest pressure of 38.5GPa, we obtained the bulk modulus and first derivative of TaC as $K_0=290.7$ (6.8) GPa and $K_0^{\prime }=3.05$ (0.49), respectively. In addition, we calculated the bulk modulus and band structure of TaC under high pressure using density functional theory. The obtained bulk modulus is 267 (3)GPa. TaC is metallic in nature throughout the entire pressure range. We studied the high-pressure deviatoric stress of TaC using linewidth analysis method. We found that TaC can support a maximum differential stress of up to 18.6GPa at the highest pressure of 38.5GPa.

The crystal structure and vibrational spectra of CoFe₂O₄ ferrite were studied using X-ray diffraction and Raman spectroscopy over a pressure range of 0–35GPa. A structural phase transition from the cubic $Fd\bar{3}m$ phase to the post-spinel orthorhombic Bbmm phase occurs at a pressure of approximately 23GPa through a two-phase region. Pressure-induced changes in the structural parameters, lattice distortion, and vibrational modes of the studied ferrite were investigated in detail. Lattice parameters, bond lengths, compressibility, and bulk modulus for both the cubic and orthorhombic phases of CoFe₂O₄ were determined.

By first-principles study, we propose a hypothetical crystal (with space group P42/MNM) Krypton dioxide (KrO₂) which might be synthesized at a pressure over 95GPa. At a pressure around 95GPa noticeable charge transfer from Kr atom to O atom occurs and chemical bonds are formed. Mechanism of this phenomena is analyzed using the electron-hopping mechanism. The formed Kr–O bond at high pressure can exist with retreated pressure down to 5GPa. The electron hopping mechanism might be applicable to the fabrication of other noble gas compounds.

We present the polarized reflection spectra of several MPcs (M≡Co, Ni, Cu, Zn, Pb) in the Q-band region and interpret them based on conventional exciton theory. We compare the polarized reflection spectra of the phthalocyanine radical salts NiPc(AsF₆)_0.5, H₂Pc(AsF₆)_0.67 and LiPc and interpret the new absorption band near the Q-band using the relationship between the degree of oxidation and the intensity of this new band. Based on the pressure dependence of this new band and the diagnostic phonon modes, we prove a pressure-induced charge transfer in NiPc(AsF₆)_0.5. A metal–insulator phase transition is predicted from the analysis of the plasmon absorption and is confirmed by the high-pressure electrical resistivity. We propose the origin of the pressure-induced charge transfer and the mechanism of the metal–insulator transition. We find the optical transition associated with the 3d_z2 band in the reflection spectrum of CoPc(AsF₆)_0.5, which is proved by comparison with the mixed crystals Co_xNi_{1 − x}Pc(AsF₆)_0.5. A new weak intermolecular optical transition is found through the resonance enhancement of a local phonon in the mixed crystals Co_xNi_{1 − x}Pc(AsF₆)_0.5.

The (Mg, Fe)O solid solution is one of the major lower mantle minerals, and studying its properties and structures under high pressure is a fundamental step toward understanding Earth's deep interior. Here within the framework of density functional theory, we first discuss the relationship between the total energy and iron doped positions of (Mg, Fe)O, and find that the doped iron favors to be dispersive. Then the pressure-induced phase transitions of (Mg, Fe)O from NaCl-type (B1) to CsCl-type (B2) are probed. It is found that the phase transition pressure of (Mg, Fe)O decreases with damped oscillation, as the increase of iron concentration. This phenomenon is essentially determined by the iron concentration as well as iron doped positions. The electronic structures of MgO and (Mg_0.75Fe_0.25)O at 436 GPa are calculated, and the results show that the doped irons play a crucial role in the metallicity of (Mg_0.75Fe_0.25)O. Our results are in agreement with the experimental counterparts. This study would provide some useful information for understanding the behavior of pressure-induced phase transition and geoscience.

In this work, the structural, electronic and absorption properties of 2-methyl-2H-naphtho-[1,8-de]triazine in the pressure ranges of 0–250GPa are studied in detail (hereinafter referred to as 2-methyl crystal). Density functional theory (DFT) is used to calculate the lattice constants, bond lengths and bond angles of 2-methyl under different pressures. The results show that the crystals undergo complex transformations under compression, and the major structural transformations occur at pressures of 90GPa and 210GPa with repeated formations and disconnections. In addition, the $a$- and $c$-directions of the 2-methyl are stiffer than the $b$-direction, which indicates that the compressibility of the crystal is anisotropic. From the specific analysis of the bandgaps of 2-methyl, we can know that the crystal is converted from semiconductor to metal at 90GPa. The absorption spectrum of the crystal also indicates that 2-methyl has a relatively high optical activity with the increasing pressure.

In this work, detailed DFT calculations of the structural, mechanical and electronic properties of crystalline CaSi₂ with four different structures in the pressure range of 0–50 GPa are performed by GGA-PBE. It is found that the Enthalpy differences imply that the R$\bar{3}$m phase $\to$ I4₁/amd phase $\to$ P6/mmm phase transition in CaSi₂ occur at $P_1=2.5$GPa, $P_2=33.5$GPa by using the XC of GGA, which is consistent with previous experiments and theoretical conclusions. Besides, the elastic stability criterion is used to study the change of the elastic constant of CaSi₂ under pressures. In particular, the bulk modulus $B$, shear modulus $G$, Young’s modulus $E$, sound velocity $v$ and brittleness and toughness properties of CaSi₂ under pressures are comprehensively studied for the first time. Finally, the changes of the anisotropy factor of CaSi₂ are studied under different pressures, and the electronic structure is studied in detail.

In this paper, the structural, electronic and optical absorption properties of $m$-aminobenzoic acid crystals (hereinafter referred to as $m$-amino) in the pressure range of 0–300GPa are calculated by density functional theory (DFT). The changing trend of the lattice constant of $m$-amino under different pressures is analyzed. We find that the crystal undergoes complex transformation. Furthermore, it can be seen that the structure of $m$-amino along the $b$-axis is stiffer than that along the $a$-axis and $c$-axis, suggesting that the crystal has anisotropic compressibility. Through the analysis of the band gap and density of states of $m$-amino, it is found that the electronic properties of $m$-amino are transformed from semiconductor phase to metal phase at 100GPa, then jump into the semiconductor phase and maintain the metal phase again in the pressure range of 150–250GPa. Repeated phase transitions indicate that the structure of $m$-amino becomes more unstable as the pressure increases. Besides, from the absorption spectra, the optical activity of $m$-amino is relatively high with the increase of pressure, and two obvious structural transitions are observed at 70 and 270GPa, respectively.

In this paper, density functional theory (DFT) is used to study the structure, electron and absorption properties of 6-Amino crystal in the pressure range of 0–300GPa. The variations of the lattice constants, bond lengths and bond angles show that they undergo complex transformations under different pressures. More narrowly, the covalent bonds of the benzene ring and the uracil ring in the planar intramolecular structure are broken, and then the new covalent bonds between the adjacent intermolecular structure are reshaped at about 80, 100 and 160GPa. The analysis results of band gap and DOS of 6-Amino indicate that the electronic properties of 6-Amino repeatedly transform from the semiconductor system into the metal phase at 80, 180 and 260GPa. The absorption spectra show two important structural changes at 100 and 180GPa, and present the high optical activity with the change of pressure.

We have investigated physical properties of the valence fluctuating compound YbInCu₄ at high pressures and low temperatures. The first-order valence transition temperature T_V ~ 42 K at ambient pressure is completely suppressed for pressures above 2.49 GPa. Present electrical resistivity data in the pressure-stabilized high-temperature (HT) phase exhibit successive shoulders at around 20 and 2.4 K. The former can reasonably be compared with the Kondo temperature T_K ~ 25 K estimated from the static-susceptibility data at ambient pressure. For pressures above 2.27 GPa, the ac-susceptibility showed a clear peak at 2.4 K, which can be explained as the onset of a long-range ferromagnetic ordering. ⁶³Cu pure-quadrupole-resonance in the HT phase showed that the value of spin-lattice relaxation time T₁ and the almost T-independent behavior are hardly affected with pressure up to 1.3 GPa, in contrast to the large increase in quadrupole frequency.

The electrical resistivity of single crystalline HoNi₂B₂C has been measured at high pressure and magnetic fields. The three anomalies in the magnetoresistance due to metamagnetic transitions are observed both at ambient and high pressures. It is found that the metamagnetic transition fields increase with increasing pressure. The temperature dependence of electrical resistivity is strongly dependent on magnetic field. Non Fermi liquid behavior is observed near the metamagnetic transition fields. But the normal Fermi liquid behavior recovers after completing the phase transition. The Grüneisen parameters are also calculated to examine the stability of electronic state.

A diamond anvil cell (DAC) made of reinforced plastic has been developed for magneto-photoluminescence experiments in pulsed high magnetic fields. Our DAC has a standard and simple structure equipped with a stainless steel gasket. We have made magneto-photoluminescence experiments of CdTe/Cd_0.8Mn_0.2Te multiple quantum wells up to 41 T at 4.2 K in the pressure range 0 to 2.3 GPa. We found that the effect of the eddy current heating of the gasket can be negligible small when we use the pulsed field whose duration is a few tens of milliseconds or longer. We have also found that the exciton Zeeman shift strongly depends on the pressure, which can be a manifestation of the enhancement of the sp-d and d-d exchange interactions in the Gd_0.8Mn_0.2Te layer by applying high pressures.

Chlorine-containing (Sr,Ca)₃Cu₂O_4+δCl_2-y (0212-Cl) superconductor is the high-T_cCu-oxide superconductor (HTSC) with T_c=80 K and was synthesized under high-pressure and high-temperature. The 0212-Cl has double [CuO₂] planes and the apical oxygen are replaced by Chlorine. We measured the resistance of 0212-Cl under high pressure using a diamond-anvil cell up to 13.3 GPa and two-terminal ac-method to investigate the pressure dependence of T_c. T_c of 0212-Cl increases gently with increasing pressure.

The electronic structure of the Iridium based L1₂ intermetallic compounds (A₃B) such as Ir₃Ti, Ir₃Zr, Ir₃Hf, Ir₃V, Ir₃Nb and Ir₃Ta, which have wide applications as high temperature structural materials are studied by means of Self-Consistent Tight Binding Linear Muffin Tin Orbital (TB-LMTO) method. These compounds are found to crystallize in the Cu₃Au type structure. The total energies are calculated as a function of volume and fitted to Birch equation of state to find the equilibrium lattice parameter and the bulk modulus. They are tabulated and compared with the available experimental and other theoretical data. The partial number of electrons at A and B sites of these compounds are calculated as a function of volume. We find that there is a continuous transfer of d-electrons from A-site to B-site. The band structure and density of states histograms are plotted. From the DOS histograms, we find that the plots are similar for all the compounds except for Ir₃V. In the case of Ir₃V, we find that there are hybridization between Ir-d like and V-d like states at the Fermi level. Hence, it is predicted that under compression there may be a Lifshitz type of transition in Ir₃V. The cohesive energy, heat of formation and the electronic specific heat coefficient of the compounds are also computed.

High pressure-high temperature electrical resistivity study on composition-controlled Nd_0.9Ca_0.1Ba₂Cu₃O_7-δ high T_c superconductor (HTSC) is carried out by a four-probe technique using Bridgman anvils. A simple heating coil arrangement is used for heating the samples. Electrical resistivity behavior under pressure (up to a maximum of 8 GPa) at various temperatures (up to a maximum of 523 K) were studied and reported in this paper. Simulation of energy dispersive X-ray diffraction confirms substitution of calcium at the Nd site of Nd-123. Variation of the electrical resistivity under pressure is compared with that of the structural changes and the bulk modulus was determined.

An effective interionic interaction potential is developed to study the pressure-induced phase transitions from zinc blende (B3) to rock salt (B1) structure in diluted magnetic semiconductors Zn_1-xMn_xSe (x=0.08 and 0.15). As a first step, the elastic constants, including the long-range Coulomb, van der Waals (vdW) interaction and the short-range repulsive interaction up to second-neighbor ions within the Hafemeister and Flygare approach, are derived. Assuming that both the ions are polarizable, the Slater–Kirkwood variational method is employed to estimate the vdW coefficients. The estimated values of the phase transition pressure (P_t) increase with Mn concentration. The vast volume discontinuity in the pressure volume phase diagram identifies the structural phase transition from zinc blende to rock salt structure. The variation of second-order elastic constants with pressure resembles that observed in some binary semiconductors. It is noticed that the vdW interaction is effective in obtaining the thermodynamical parameters such as Debye temperature, Gruneisen parameter, and thermal expansion coefficient. However, the inconsistency in the value of pressure derivative of the theoretical and experimental value of C₄₄ is attributed to the fact that we have derived the expressions by assuming that the overlap repulsion is significant only up to nearest neighbors, as well as neglecting thermal effects.

The advancement in new materials processing and fabrication techniques has made it possible to better control the atomistic level of structures in a way, which was not feasible only a decade ago. If one can couple this atomic control with a good understanding of the relationship between structure and properties, this will in the future lead to a significant contribution to the synthesizing of tailor-made materials. In this paper we have focused on, the structurally related nanolayered ternary compounds M_N+1AX_N, (MAX) where N = 1, 2 or 3, M is an early transition metal, A is an A-group (mostly IIIA and IVA) element, and X is either C and/or N, which has attracted increasing interest owing to their unique properties. The general relations between the electronic structure and materials properties of MAX phases have been elaborated based on ab initio calculations.

The improved Tosi–Fumi pair potential has been employed to simulate the melting behavior and Grüneisen parameters of sodium chloride (NaCl) using molecular dynamics (MD) method at constant volume. The melting curve of NaCl is compared with the experimental data and other calculations in a pressure range from 0 to 144 GPa. The simulated results validate the amount of 20% superheating of the NaCl solid and yield an approximate power law dependence of the Grüneisen parameter (γ) on compression γ = γ₀(V/V₀)^q, with q ≈ 1.078, in the temperature range from 298 to 1073 K and pressure range from 0 to 60 GPa.

The structural phase transitions, mechanical properties and electronic structures of OsO₂ under high pressure are systemically investigated by the first-principles plane-wave basis pseudopotential calculations. The possible pressure-induced transition sequence in OsO₂ may be the rutile, pyrite and fluorite phases, and the stable CaCl₂ structure is not found. The fluorite phase has high bulk modulus (355.3 GPa), large shear modulus G (267.9 GPa), and huge elastic constant C₄₄ (292.7 GPa), and consequently is an excellent candidate of superhard materials. Crystal structures, valence electron densities, band structures, DOS and PDOS of the rutile, pyrite and fluorite phases of OsO₂ have also been carefully analyzed to elucidate their mechanical properties.

In the present paper, a new relationship for the pressure dependences of elastic constants is developed by using a new expression for the pressure dependence of bulk modulus and a method developed by Grover et al. The proposed relationship is applied to study elastic constants of MgO, NaCl, CaF₂, and CaO. The results obtained for elastic constants are found in good accordance with the experimental and first-principle results.