Pressure induced phase transition in ZnS

The phase transition of ZnS from the zincblende (ZB) structure to the rocksalt (RS) structure is investigated by the ab initio plane-wave pseudopotential density functional theory method. It is found that the pressures for transition from the ZB structure to the RS structure are 17.5 GPa from total energy-volume data and 15.4 GPa from equal enthalpies, consistent with the experimental data. From the high pressure elastic constants obtained, we find that the ZB structure ZnS is unstable when the applied pressure is larger than 17 GPa. Moreover, the dependence of the normalized primitive cell volume V/V₀ on pressure P can also be successfully obtained.     

The optical properties of HgS under high pressures

The DOS (density of states) and the optical properties of HgS under high pressure are studied with the first-principle computations. The change of the imaginary part, ε₂(ω), of the dielectric function shows that HgS tends to metallization with increasing pressure, and this well agrees with the band gap calculations and the conductivities measurement results in the previous work. Under the pressures below 15 GPa, ε₂(ω) is relatively anisotropic and tends to be more anisotropic with increasing pressure; while under the pressures above 15 GPa, the anisotropy decreases and finally becomes almost absolutely isotropic after the phase transition. The behavior of ε₂(ω) is strongly related to the structure change in the cinnabar to rocksalt phase transition process under high pressure.     

Observation of self-regulating response in LixMyMn2−yO4 (M=Mn, Ni): A study using density functional theory

Density functional theory is used to understand the response of the transition metal-oxygen octahedra in Li_xMn₂O₄ and Li_xNi_0.5Mn_1.5O₄ to lithium intercalation and de-intercalation. Electronic structure computations on these compounds for x=0, 0.5 and 1 indicate that the 3d DOS of Mn is almost unaffected to variations in x. On the other hand, the oxygen 2p-DOS and to a lesser extent Ni 3d DOS are found to be sensitive to perturbation. The observations are explained on the grounds of self-regulating response, characteristic of systems having localized d states that communicate with a covalent manifold.     

High pressure phase transition and band structures of different phases in CeO2

We report a detailed theoretical calculation of the electronic band structure of CeO₂ in cubic and orthorhombic phases under pressure using a tight-binding linear muffin-tin orbital method (TB-LMTO) within local density approximation (LDA). The compressibility behavior of this compound was discussed in the light of the changes occurring in the electronic structure. Apart from the electronic band structure and structural stability calculations, the density of states (DOS) and Fermi energies (E_f) at various pressures are calculated. The calculated lattice parameter, transition pressure, bulk modulus and the pressure-volume relation are found out to be in good agreement with experimental results.     

Atomistic computer simulation studies of La1−xSrxVO3

Static computer simulation techniques have been employed for structural investigation of the La_1−xSr_xVO₃ series. Potential parameters for V³⁺-O²⁻ and V⁴⁺-O²⁻ have been derived which reproduces the crystal structures of end members with sufficient accuracy. Variations of lattice parameters and bond distances with Sr concentration have been studied. The calculated lattice parameters decrease with increase in the Sr concentration. A structural phase transition from orthorhombic to cubic is observed at 50% Sr doping level.     

Charge transport near pressure-induced antiferromagnetic quantum critical point in Magnéli-phase vanadium oxides

Resistivity (ρ) measurements on Magnéli phases V₇O₁₃ and V₈O₁₅ were performed under high pressures up to 3.5GPa. We have achieved a pressure-induced transition from an antiferromagnetic metal to a paramagnetic metal (PM) at critical pressures P_c≈3.4 and 3.3 GPa for V₇O₁₃ and V₈O₁₅, respectively. The critical behavior of ρ(T) near P_c turned out to be quite unusual in that no noticeable precursor effect was observed. This strongly contrasts with the canonical quantum critical point behavior observed in chemically modified systems such as Ni(S,Se)₂ and V₂O₃. We propose that the presence of two distinct Fermi surface segments is responsible for the observed unusual behaviors.     

Molecular dynamics simulation of P-V-T relationship of ZnO with rock-salt structure using pair-wise interactions

Molecular dynamics (MD) method is used to investigate the behavior of the pressure-volume-temperature (P-V-T) relationship, lattice constant and thermal expansivity for ZnO with rock-salt structure at high pressures and temperatures. The interionic potential is taken to be the sum of pair-wise additive Coulomb, van der Waals attraction, and repulsive interactions. The isothermal and isobaric properties are discussed from the corresponding P-V-T relationship, and it is shown that the MD simulation is successful in reproducing the measured volumes of ZnO over a wide range of temperature and pressure. Meanwhile, the equations of state parameters including lattice constant, linear thermal expansion coefficient, and isothermal bulk modulus are calculated and compared with the available experimental data and the latest theoretical results. At an extended pressure and temperature range, P-V-T relationship, lattice constant, and linear thermal expansion coefficient have been predicted. The structural and thermodynamic properties of ZnO with rock-salt structure are summarized in the pressure 0-100 GPa ranges and the temperature up to 3100 K.     

Electronic topological transition and phase transition in rhodium and iridium based inter metallic compounds

Of the Rh₃Y, Rh₃La, Ir₃Y and Ir₃La inter-metallic compounds, the compound Rh₃Y exists in hexagonal structure, Ir₃Y and Ir₃La exist in rhombohedral structure, whereas the compound Rh₃La exists in both hexagonal and rhombohedral structures. Based on our tight binding-linear muffin tin orbital (TB-LMTO) study of other rhodium and iridium-based Rh₃X and Ir₃X (where X=Ti, Zr, Hf, V, Nb, Ta and Sc) inter-metallic compounds of AuCu₃ type cubic structure, an attempt is made to examine whether the compounds Rh₃Y, Rh₃La, Ir₃Y and Ir₃La will undergo a structural phase transition to cubic structure from their experimentally reported structures. From our study, it is observed that the compounds Rh₃Y and Rh₃La undergo a structural phase transition to cubic phase at 4.5 and 10.1 GPa, respectively, from their experimentally reported hexagonal and rhombohedral phases. Further it is predicted that both the compounds Ir₃Y and Ir₃La can exist in the cubic phase itself at ambient condition, in contrary to the experimental observation. From the band structure outputs that have been plotted for the compounds under compression, it is observed that the compounds Rh₃La, Ir₃Y and Ir₃La undergo the Lifshitz type of transition which may change the Fermi surface topology and hence the physical properties of these compounds.     

Simulated equation of state of CaF2 with fluorite-type structure at high temperature and high pressure

The pressure-volume-temperature (P-V-T) equation of state (EOS), isothermal bulk modulus, and thermal expansivity of CaF₂ with cubic fluorite-type structure are investigated using the constant temperature and pressure shell model molecular dynamics (MD) method with effective pair potentials which consist of the Coulomb, dispersion, and repulsion interaction. It was shown that MD simulation is very successful in accurately reproducing the measured volumes of the CaF₂ over a wide range of pressures. The simulated P-V data matched X-ray diffraction experimental results up to 9.5 GPa at 300 K. In addition, volume thermal-expansion coefficient and isothermal bulk modulus were also calculated and compared with available experimental data and the latest theoretical results at ambient condition. At extended temperature and pressure ranges, The P-V EOS under different isotherms at selected temperatures, T-V EOS under different isobars at selected pressures, thermal expansivity, and isothermal bulk modulus were predicted up to 1500 K and 10 GPa. The detailed knowledge of thermodynamic behavior and EOS at extreme conditions are of fundamental importance to the understanding of the physical properties of CaF₂.     

The effect of pressure on the Raman spectrum of NH4Cl

The Raman spectra of NH₄C1 are reported over a very wide pressure range at room temperature and some features of the well-known disorder-order transition as well as the spectra of the ordered phase at high pressures are discussed. The mode Grüneisen parameter has been determined to be equal to 2.1 ± 0.03 for ν₅(TO) in this phase showing that the volume-dependent anharmonicity is relatively large. Above 110kbar, significant spectral changes take place, a large number of lattice modes appear and some internal modes also reflect changes. Since these features closely resemble the ones observed in the newly discovered V of NH₄I, it is concluded that phase V also exists in NH₄Cl. The structure of phase V as well as the mechanism of the IV–V transition are still largely unknown but it is shown that the IV–V transition pressures in the ammonium halides vary linearly with the anionic radii.     

Oxygen K-edge in vanadium oxides: simulations and experiments

Band-structure (BS) calculations of the density of states (DOS) using the full potential augmented plane waves code WIEN97 were performed on the four single-valence vanadium oxides VO, V₂O₃, VO₂ and V₂O₅. The DOS are discussed with respect to the distortions of the VO₆ octahedra, the oxidation states of vanadium and the orbital hybridisations of oxygen atoms. The simulated oxygen K-edge fine structures (ELNES) calculated with the TELNES program were compared with experimental results obtained by electron energy-loss spectrometry (EELS), showing good agreement. We show that changes in the fine structures of the investigated vanadium oxides mainly result from changes in the O-p DOS and not from the shift of the DOS according to a rigid band model. Received 17 December 2001 / Received in final form 19 June 2002 Published online 13 August 2002     

Superconductivity of Li under pressure

The superconductivity of Li under pressure is studied by a density functional method. Structural and elastic properties, transition temperature (T_c), density of states (DOS), are considered for the material at ambient and at higher pressures. The calculations, particularly of T_c and DOS as a function of pressure, are compared with other available results. This is in view of the wide difference between the previously predicted maximum T_c-value and those observed by both magnetic susceptibility and electrical resistivity experiments. The present calculations yield better estimates of T_c and other parameters with respect to measured values.     

Magnetic and structural instabilities in the hexagonal Laves hydride ErMn2H4.6 under high-pressure

The structural and magnetic properties of ErMn₂H_4.6 have been studied by X-ray and neutron diffraction up to the pressures of 15 and 6 GPa, respectively. In the pressure range 0<P<3 GPa we observe a first-order phase transition to new high-pressure (HP) phase. The HP phase has the same hexagonal unit cell as the ambient-pressure phase but smaller lattice parameters (ΔV/V=−5%). The structural transition results in suppression of the long-range antiferromagnetic order. Our results suggest that pressure changes positions of the hydrogen atoms in the metal host. We speculate that the new arrangement of hydrogen atoms induces spin frustration and, therefore, suppresses long-range magnetic order in the HP phase.     

Ab initio calculations of elastic constants and thermodynamic properties of Li2O for high temperatures and pressures

The elastic constants and thermodynamic properties of Li₂O for high temperatures and pressures are calculated by the ab initio unrestricted Hartree-Fock (HF) linear combination of atomic orbital (LCAO) periodic approach. The lattice constant, elastic constants, Debye temperature, and thermal expansion coefficient obtained are in good agreement with the available experimental data and other theoretical results. It is found that at zero pressure the elastic constants C₁₁, C₁₂ and C₄₄, bulk modulus B and Debye temperature Θ_D decrease monotonically over the wide range of temperatures from 0 to 1100 K. When the temperature , C₁₂ approaches zero, consistently with the transition temperature 1200 K. However, with increasing pressure, they all increase monotonically and the anisotropy will weaken.     

First-principles study of quasi-one-dimensional β-Na0.33V2O5

The electronic structure of β-Na_0.33V₂O₅ has been evaluated using the first-principle density functional theory approach. All energy bands near the Fermi surface (FS) disperse principally along the b-axis direction indicating the quasi-one-dimensionality of this system. The theoretical simulation of the optical property yields reasonable explanations for the notable features revealed in the measurements of the optical spectroscopy. Superconductivity appearing under a pressure of 8 GPa has been discussed in connection with the pressure-induced structural and bands alternations. It is suggested that the strong interchain coupling could play a key role in the appearance of superconductivity. The electron correlation effects on the electronic structure have also been calculated and discussed in comparison with photoemission data.     

Chemical bonding and pseudogap formation in D022- and L12-structure (V, Ti)Al3

To obtain a rigorous definition of the chemical bonds in binary transition-metal aluminides, topological analyses were performed for VAl₃ and TiAl₃ in the D0₂₂ and L1₂ structures. The analyses were based on the valence charge densities calculated with the ab initio density functional theory. To better understand the formation mechanism of the pseudogap in these compounds, the band structure, the density of states (DOS) and the band decomposed charge density (BDCD) were calculated. The topological analyses reveal that the interactions between the (V, Ti) and Al atoms are all pure shared-shell interactions, the bonds are covalent and clearly have π-bond character. The study of the band structure, DOS and BDCD shows that the formation of the pseudogap is due to the crystal field energy splitting of the (V, Ti)-3d orbitals combined with the inter-unit-cell orbital interaction.     

Comparative investigations of the P-V-T relationship of NaCl at high pressure and temperature

Two different potential models of molecular dynamics (MD) simulations have been applied to investigate the pressure-volume-temperature (P-V-T) relationship and lattice parameter of NaCl under high pressure and temperature. The first one is the shell model (SM) potentials in which due to the short-range interaction pairs of ions are moved together as is the case in polarization of a crystal due to the motion of the positive and negative ions, and the second one is the two-body rigid-ion Born-Mayer-Huggins-Fumi-Tosi (BMHFT) potentials with full treatment of long-range Coulomb forces. The P-V relationship at 300 K, T-V relationship at zero pressure, and lattice parameter a, have been obtained and compared with the available experimental data and other theoretical results. Compared with SM potentials, the MD simulation with BMHFT potentials is very successful in reproducing accurately the measured volumes of NaCl. At an extended pressure and temperature ranges, P-V relationship under different isotherms at selected temperatures, T-V relationship under different pressures, and lattice parameter a have also been predicted. The properties of NaCl are summarized in the pressure range 0-30 GPa and the temperature up to 2000 K.     

Theoretical investigations of structural, elastic and thermodynamic properties for PtN2 under high pressure

We have investigated structural and elastic properties of PtN₂ under high pressures using norm-conserving pseudopotentials within the local density approximation (LDA) in the frame of density-functional theory. Calculated results of PtN₂ are in agreement with experimental and available theoretical values. The a/a₀, V/V₀, ductility/brittleness, elastic constants C_ij, shear modulus C′, bulk modulus B, shear modulus G, Young's modulus E, Poisson's ratio σ and anisotropy factor A as a function of applied pressure are presented. Through the quasi-harmonic Debye model, we also study thermodynamic properties of PtN₂. The thermal expansion versus temperature and pressure, thermodynamic parameters X (X=Debye temperature or specific heat) with varying pressure P, and heat capacity of PtN₂ at various pressures and temperatures are estimated.     

The electronic structure and magnetic properties of metallic antiferromagnetic Co[(CH3PO3)(H2O)] studied by first-principles calculations

The electronic structure, the metallic and magnetic properties of metal phosphonate Co[(CH₃PO₃)(H₂O)] have been studied by first-principles calculations, which were based on the density-functional theory (DFT) and the full potential linearized augmented plane wave (FPLAPW) method. The total energy, the spin magnetic moments and the density of the states (DOS) were all calculated. The calculations reveal that the compound Co[(CH₃PO₃)(H₂O)] has a stable metallic antiferromagnetic (AFM) ground state and a half-metallic ferromagnetic (FM) metastable state. Based on the spin distribution obtained from calculations, it is found that the spin magnetic moment of the compound is mainly from the Co²⁺, with some small contributions from the oxygen, carbon and phosphorus atoms, and the spin magnetic moment per molecule is 5.000μ_B, which is in good agreement with the experimental results.     

Dynamics of plume and crater formation after action of femtosecond laser pulse

Using microinterferometric method, a transition in laser plume from the regime with spallation to the regime without spallation is experimentally studied for the first time. The transition occurs when the fluence F_inc of incident radiation exceeds a threshold of “evaporation” (F_inc)_ev. It has been shown previously that the spallation layer is formed at fluence above the ablation threshold (F_inc)_abl. Thus the spallation exists within the limits (F_inc)_abl<F_inc<(F_inc)_ev. A laser beam has a maximum fluence (F_inc)_c on the axis of the beam. The threshold F_ev separates two cases with qualitatively different morphology: (1) with unbroken shell covering the crater entirely if F_abl<F_c<F_ev, and (2) with the shell having an aperture in the center (like the volcano muzzle) if F_c>F_ev.