First-Principles Calculations of Structures and Electronic Properties of Solid Pentaerythritol under Pressure

Structures and electronic properties of the pentaerythritol （PE） crystal under volume compression up to 0.85V0 are studied by E - V fitting method using density functional theory （DFT）. The compression dependences of the cell volumes, lattice constants, and molecular geometries of solid PE are presented and discussed. It is found that the solid PE presents anisotropy along a- and c-axes, and the c axis is the most compressible. Decreasing anisotropy ratio （c/a） with elevating compression suggests an enhancement of the vdW interaction with increasing compression. The C-C and C-H bonds are significantly reduced under compression, which may be related to the sensitivity. The solid PE has indirect band gap （X - G） in the range of the researched compression and the band gap is decreased with compression.     

First-principles study of the pressure-induced phase transition in CaTiO3

An investigation into the phase stabilities of CaTiO₃ under high pressure was conducted using first-principles calculations based on density functional theory. We have identified three candidate structures of CaTiO₃, Pbnm, Pm3m and Cmcm, respectively. Our results demonstrate that a phase transition from orthorhombic (Pbnm) to cubic (Pm3m) is impossible for CaTiO₃ under high pressure at ambient temperature, and further predict that Pbnm-CaTiO₃ will transform to post-perovskite phase (Cmcm) at enough temperature and pressure.     

Structural, Electronic Properties and Chemical Bonding of Borate Li4CaB2O6 under High Pressure： an Ab Initio Investigation

We calculate structural, electronic properties and chemical bonding of borate Li4CaB2O6 under high pressure by means of the local density-functional pseudopotential approach. The equilibrium lattice constants, density of states, Mulliken population, bond lengths, bond angles as well as the pressure dependence of the band gap are presented. Analysis of the simulated high pressure band structure suggests that borate Li4CaB2O6 can be used as the semi-conductor optical material. Based on the Mulliken population analysis, it is found that the electron transfer of the Li atom is very different from that of other atoms in the studied range of high pressures. The charge populations of the Li atom decrease with the pressure up to 60 GPa, then increase with the pressure.     

First-Principles Calculations for Thermodynamic Properties of Perovskite-Type Superconductor MgCNi3

The ground state properties and equation of state of the non-oxide perovstdte-type superconductor MgCNi3 are investigated by first-principles calculations based on the plane-wave basis set with the local density approximation （LDA） as well as the generalized gradient approximation （GGA） for exchange and correlation, which agree well with both theoretical calculations and experiments. Some thermodynamic properties including the heat capacity, the thermal expansion coefficient and the Griineisen parameter for perovskite structure MgCNi3 are obtained. The dependences of these thermodynamic properties on pressure and temperature are given for the first time.     

A Strength Softening Phase Transition Observed in Shocked （Mg0.92,Fe0.08）SiO3 Perovskite at About 83 GPa

We report the experimental data of Hugoniot longitudinal sound velocity VL for natural （Mg0.92,Fe0.08）SiO3 enstatite sample at about 40-140 GPa, consisting of three new data and five previously reported data but revised by our new Hugoniot equation of state parameters. Three segments, separated by two discontinuities, appear in the VL-PH （shock pressure） plot. Analyses show that the first discontinuity at about 64 GPa, with a sharp increase of VL of about 21%, is judged to be a phase transition from enstatite to Pbnm perovskite （PV）; while the second one at about 83 GPa, with a dramatic decrease of VL of about 23%, is likely caused by a subtle structural change from Pbnm PV to tetragonal PV, accompanied by material strength softening due to melting of oxygen sublattices. This strength softening evidence is obtained first from shock wave experiments, and probably has profound implications for probing into the origin of low seismic velocity anomaly in the Earth＇s lower mantle and thus constraining the geophysical and geochemical models for the Earth＇s lower mantle.     

First-Principles Study of Structural and Electronic Properties of Layered B2CN Crystals

Based on the hexagonal BN structure, six possible layered B~ CN structures are constructed. Their total energies, lattice constants as well as electronic properties are calculated using the ab initio pseudopotential density functional method within the local density approximation. The calculated results show that the B2 CN-V configuration with AA stacking sequence is the most stable among the six B2CN layered structures. The characteristics of electronic structures indicate that the B2 CN-V shows metallicity, which mainly comes from -B1-C-B1-C- chains.     

Phase Transition and EOS of Marmatite （Zn0.76Fe0.23S） up to 623K and 17 GPa

In situ energy dispersive x-ray diffraction for natural marmatite （Zn0.76Fe0.23S） is performed up to 17. 7 GPa and 623 K. It is fit, ted by the Birch-Murnaghan equation of state （EOS） that Ko and α0 for marmatite are 85（3）GPa and 0.79（16）＊10^-4 K^-1, respectively. Fe^2＋ isomorphic replacing to Zn^2＋ in natural crystal is responsible for high bulk modulus and thermal expansivity of marmatite. Temperature derivative of bulk modulus （OK/OT）p for marmatite is fitted to be -0.044（23） GPaK^-1. The unambiguous B3-B1 phase boundaries for marmatite are determined to be Pupper（GPa）= 15.50 - 0.016T（℃） and Plower （GPa）=9.94-0.012T（℃） at 300-623K.     

First-Principles Study of Structural Stabilities, Electronic and Optical Properties of SrF2 under High Pressure

An investigation of structural stabilities, electronic and optical properties of SrF2 under high pressure is conducted using a first-principles calculation based on density functional theory （DFT） with the plane wave basis set as implemented in the CASTEP code. Our results predict that the second high-pressure phase of SrF2 is of a Ni2In- type structure, and demonstrate that the sequence of the pressure-induced phase transition of SrF2 is the fluorite structure （Fm3m） to the PbC12-type structure （Pnma）, and to the Ni2In-type phase （P63/mmc）. The first and second phase transition pressures are 5. 77 and 45.58 GPa, respectively. The energy gap increases initially with pressure in the Fm3m, and begins to decrease in the Pnma phases at 30 GPa. The band gap overlap metallization does not occur up to 210 GPa. The pressure effect on the optical properties is discussed.     

Th3Ni2B2N3 a possible new superconducting compound: insights from electronic band structure

La₃Ni₂B₂N₃, which is similar to YNi₂B₂C and related borocarbides, was earlier studied by the electronic structure calculations [D.J. Singh, W.E. Pickett, Phys. Rev. B 51 (1995) 8668.], and was predicted to be a 3-D metal. In search of new compounds of the borocarbide and related families to get higher T_C, we have studied the compound Th₃Ni₂B₂N₃, by the first principles full potential electronic structure calculations by the linear augmented plane wave method. We get similar band structure for Th₃Ni₂B₂N₃ as found for La₃Ni₂B₂N₃, and the various atom-split component density of states show similar properties. The total electron density of states at Fermi level has been increased to about 92 states per Ry per f.u. as compared to 57 states per Ry per f.u. in La₃Ni₂B₂N₃. The main increase is due to the increased hybridization of the 5f states as seen by the more prominent low energy tail in the Th-component density of states. Based on this enhancement we predict Th₃Ni₂B₂N₃ to be a high temperature superconductor with a T_c in excess of 30 K.     

First-principles electronic-structure calculation on the spin distribution and the half-metallic properties of the mixed valence iron compound Ba2F2Fe1.5S3

The first principles within the full potential linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation (GGA) approach were applied to study the new mixed valence compound Ba₂F₂Fe_1.5S₃. The density of states, the electronic band structure and the spin magnetic moment are calculated. The calculations reveal that the compound has an antiferromagnetic interaction between the Fe^III and Fe^II ions arising from the bridging S atoms, which validate the experimental assumptions that there is a low-dimensional antiferromagnetic interaction in Ba₂F₂Fe_1.5S₃. The spin magnetic moment mainly comes from the Fe^III and Fe^II ions with smaller contribution from S anion. By analysis of the band structure, we find that the compound has half-metallic property.     

Coupling of Magnetization and Structural Distortions in Multiferroic BiFeO3： an Ab Initio Density Functional Theory Study

The coupling between magnetism and structural distortions in BiFeO3 （BFO） is investigated using density functional theory by considering the spin-orbit effect. Computational results show that the resulting magnetization M is rotated by reversal of sense of rotation of the oxygen octahedra in the double cell. The resulting magnetization is determined by the antiferrodistortive （AFD） distortions and ferroelectric （FE） displacements. This work clarifies the previous view that magnetism is only coupled with, and determined by, FE displacements. The excellent ferroelectricity is attributed significantly to the anomaly of Born effective charge of Bi, which is caused by the stereochemically active long pair of Bi 6s.     

Shock Induced Emission from Sapphire in High-Pressure Phase of Rh2O3 （Ⅱ） Structure

A distinct optical emission from the Rh203 （Ⅱ） structural sapphire is observed under shock compression of 125, 132, and 143 GPa. The emission intensity continuously increases with the thickness of shocked sapphire. The colour temperature is determined to be about 4000 K, which is obviously smaller than the reported value of the alpha phase alumina at the pressures below 80 GPa. The present results suggest that the structural transformation will cause an obvious change of optical property in sapphire.     

Structural Stability of CaCuMn6O12 under High Pressure and Low Temperature

In situ high pressure energy-dispersive x-ray synchrotron radiation diffraction and resistance experiments are carried out on CaCuMn6O12. Its crystal structure is stable in the measured pressure range. The equation of state of CaCuMn6O12 is obtained from the V/Vo - P relationship （V and Vo are the volumes at pressure P and at atmosphere）. The bulk modulus Bo is calculated based on the Birch-Murnaghan equation. Low temperature x-ray diffraction shows no phase transition occurring down to 160K.     

Pressure and Temperature Induced Phase Transition of ZnS from First-Principles Calculations

The pressure induced phase transition of ZnS from the wurtzite （WZ） and the zincblende （ZB） structures to the rocksalt （RS） structure and the temperature induced phase transition from the ZB structure to the WZ structure are investigated by ab initio plane-wave pseudopotential density-functional theory （DFT）, together with the quasiharmonic Debye model. It is found that the zero-temperature transition pressures from the WZ-ZnS and the ZB-ZnS to the RS-ZnS are 17.20 and 17.37 GPa, respectively. The zero-pressure transition temperature from the ZB-ZnS to the WZ-ZnS is 1199 K. All these results are consistent with the available experimental data. Moreover, the dependences of the normalized primitive cell volume V/V0 on pressure and thermal expansion coefficient α on temperature are also obtained successfully.     

Li- Site and Metal-Site Ion Doping in Phosphate-Olivine LiCoPO4 by First-Principles Calculation

We present a first-principles investigation of the crystal and electronic structure as well as the average insertion voltage of the Li-site （by Na and Cr） and metal-site （by isovalent Ni, Zn, Ca, Mg and Mn and aliovalent Cu, Al, In, Mo and Zr） doped LiCoPO4. The results show that both the Li-site doping and metal-site doping may reduce the volume change of the material during Li extraction/reinsertion process. The metal doped at Li-site will block the path of Li ion diffusion. The doping by aliovalent transition metals will introduce defect levels in the energy band. It could influence the conductivity insertion voltage.     

Theoretical study of CuxAg1−xI alloys

We have performed first-principles calculations using full potential linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT) to investigate the fundamental properties of Cu_xAg_1−xI alloys. We used both GGA96 [J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865.] and EVGGA [E. Engel, S.H. Vosko, Phys. Rev. B. 47 (1993) 13164.] generalized gradient approximations of the exchange-correlation energy that are based on the optimization of total energy and corresponding potential. Quantities such as lattice constants, bulk modulus, band gap, density of occupied states and effective mass were calculated as a function of copper molar fraction x. These parameters were found to depend non-linearly on alloy composition x, except the lattice parameter, which follows Vegard's law. The microscopic origins of the gap bowing were explained using the approach of Zunger and co-workers; we have concluded that the band-gap energy bowing was mainly caused by the chemical charge-transfer effect and the volume deformation , while the structural relaxation contribute to the gap bowing parameter at smaller magnitude. The calculated phase diagram shows a broad miscibility gap for this alloy with a high critical temperature.     

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.     

Strain induced ferroelectricity in the SrZrO3 / SrTiO3 superlattice: First principles study

The lattice distortion strain induced ferroelectricity in SrZrO₃/ SrTiO₃ superlattice is studied using first principles density functional theory. Within the tetragonal symmetry , the lattice distortion from the lattice mismatch in the superlattice structure is determined through energy minimization. This kind of lattice distortion leads to the formation of spontaneous polarization in the superlattice, although neither SrZrO₃ nor SrTiO₃ is ferroelectric. From analysis of the relative displacements of the cations and anions, we have found that the SrZrO₃ cell may make a greater contribution to the polarization in the SrZrO₃/ SrTiO₃ superlattice than the SrTiO₃ cell. An extremely large polarization of 42.7 μC/cm² has been predicted.     

First-principles study on the electronic structure and magnetic properties of metallic antiferromagnet C7H3S2N2

First-principles full potential linearized augmented plane wave (FPLAPW) calculations have been performed to study the electronic structure and the magnetic properties of 3-Cyanobenzo-1,3,2-dithiazolyl,C₇H₃S₂N₂. The density of states (DOS), the total energy of the cell, and the spontaneous magnetic moment of C₇H₃S₂N₂ were all calculated. The calculations reveal that the low-temperature phase of the compound C₇H₃S₂N₂ has a stable metal-antiferromagnetic ground state, and there exists an antiferromagnetically coupled interactions between the dithiazolyl radical(1), which is in good agreement with experiment.     

First-principles prediction of coexistence of magnetism and ferroelectricity in rhombohedral Bi2FeTiO6

First principles calculations based on the density functional theory within the local spin density approximation plus U(LSDA + U) scheme, show rhombohedral Bi₂FeTiO₆ is a potential multiferroic in which the magnetism and ferroelectricity coexist. A ferromagnetic configuration with magnetic moment of 4μB per formula unit has been reported with respect to the minimum total energy. Spontaneous polarization of 27.3 μC/cm², caused mainly by the ferroelectric distortions of Ti, was evaluated using the berry phase approach in the modern theory of polarization. The Bi-6s stereochemical activity of long-pair and the ‘d⁰-ness’ criterion in off-centring of Ti were coexisting in the predicted new system. In view of the oxidation state of Bi³⁺, Fe²⁺, Ti⁴⁺, and O²⁻ from the orbital-resolved density of states of the Bi-6p, Fe-3d, Ti-3d, and O-2p states, the valence state of Bi₂FeTiO₆ in the rhombohedral phase was found to be Bi³⁺₂Fe²⁺Ti⁴⁺O₆.