Electron momentum density, band structure, and structural properties of SrS

The electron momentum density, the electronic band structure, and the structural properties of SrS are presented in this paper. The isotropic Compton profile, anisotropies in the directional Compton profiles, the electronic band structure and density of states are calculated using the ab initio periodic linear combination of atomic orbitals method with the CRYSTAL06 code. Structural parameters of SrS—lattice constants and bulk moduli in the B1 and B2 phases—are computed together with the transition pressure. The computed parameters are well in agreement with earlier investigations. To compare the calculated isotropic Compton profile, measurement on polycrystalline SrS is performed using 5Ci-²⁴¹Am Compton spectrometer. Additionally, charge transfer is studied by means of the Compton profiles computed from the ionic model. The nature of bonding in the isovalent SrS and SrO compounds is compared on the basis of equal-valenceelectron-density profiles and the bonding in SrS is found to be more covalent than in SrO.     

Effect of high pressure on polymorphic phase transition and electronic structure of XAs (X=Al,Ga, In)

The structural and electronic properties of XAs (X = Al, Ga, In) under pressure have been investigated using ab-initio pseudo-potential approach within local density approximation in B3→B1→B2 phases. The values of phase transition pressures show reasonably good agreement with the experimental data and better than others. The B1→B2 phase transition in InAs is not seen. The volume collapse computed from equation of state (EOS) is found to be in good agreement with the experimental values. Under ambient conditions, the energy of B3 phase is lowest as compared to other phases, while at high pressures beyond B1→B2 phase transition, the energy of B2 phase is found to be lower than that of B1 phase showing correct stability of the phases. There is relatively smaller enthalpy associated with B3→B1 transition as compared to B3→B2 transition. The electronic structures have also been computed at different pressures. We have also reported the effect of pressure on energy gap and valence band width.     

Structural,electronic and mechanical properties of alkaline earth metal oxides MO (M=Be,Mg, Ca,Sr, Ba)

The structural, electronic and mechanical properties of alkaline earth metal oxides MO (M=Be, Mg, Ca, Sr, Ba) in the cubic (B1, B2 and B3) phases and in the wurtzite (B4) phase are investigated using density functional theory calculations as implemented in VASP code. The lattice constants, cohesive energy, bulk modulus, band structures and the density of states are computed. The calculated lattice parameters are in good agreement with the experimental and the other available theoretical results. Electronic structure reveals that all the five alkaline earth metal oxides exhibit semiconducting behavior at zero pressure. The estimated band gaps for the stable wurtzite phase of BeO is 7.2 eV and for the stable cubic NaCl phases of MgO, CaO, SrO and BaO are 4.436 eV, 4.166 eV, 4.013 eV, and 2.274 eV respectively. A pressure induced structural phase transition occurs from wurtzite (B4) to NaCl (B1) phase in BeO at 112.1 GPa and from NaCl (B1) to CsCl (B2) phase in MgO at 514.9 GPa, in CaO at 61.3 GPa, in SrO at 42 GPa and in BaO at 14.5 GPa. The elastic constants are computed at zero and elevated pressures for the B4 and B1 phases for BeO and for the B1 and B2 phases in the case of the other oxides in order to investigate their mechanical stability, anisotropy and hardness. The sound velocities and the Debye temperatures are calculated for all the oxides using the computed elastic constants.     

氧化钒晶体的半导体至金属相变的理论研究

本文利用第一性原理方法研究了金红石相和单斜相VO₂晶体的电子结构和热力学性质.在计算中采用局域密度近似结合Hubbard U模型(LDA+U)描述电子的局域强关联效应,同时也利用微扰密度泛函方法计算了两种相结构的声子谱.计算结果表明V原子3d电子轨道中x²-y²轨道能级分裂决定了VO₂晶体在不同相结构下的金属和绝缘体特性.零温状态方程计算揭示了在68 GPa时可以发生从单斜结构 关键词： 2')" href="#">VO₂ 相变 第一性原理     

Ab initio lattice dynamics and thermodynamic properties of SrO under pressure

We present an ab initio calculation of lattice dynamics and thermodynamic properties of SrO in the NaCl (B1) and CsCl (B2) structured phases under pressure. We employ the density-functional perturbation theory (DFPT) within the local density approximation (LDA) in conjunction with the quasi-harmonic approximation. At zero pressure, the calculated dispersions agree well with the measured ones for the B1 phase. A pressure-induced soft transverse acoustic (TA) phonon mode is identified at the zone boundary X point of the Brillouin zone. The B2 phase is stable at high pressure. Our analysis suggest that this soft transverse mode is responsible for the pressure induced structural phase transition. The increase of the electronic dielectric function with pressure is attributed to the closure of the p–d gaps. The thermal expansion coefficient decreases with pressure and does not show any negative behavior. The predicted transition pressure decreases with temperature.     

Electronic,mechanical, phase transition,and thermo-physical properties of TMC (TM = V,Nb, and Ta): high pressure ab initio study

The structural, electronic, mechanical, phase transition, and thermo-physical properties of refractory carbides, viz. VC, NbC, and TaC have been computed in stable B1 and high pressure B2 phases by means of two different ab initio calculations using pseudo- and full-potential schemes. These materials have mixed covalent-, metallic-, and ionic-type bonding. The calculations of elastic constants show the mechanical stability of these materials in B1 phase only. The brittle nature and anisotropy is observed in these materials in B1 phase. Non-central forces are present in both the phases. Elastic wave velocities and Debye temperature have also been calculated. The present results on structural, phase transition, elastic, and other properties are in reasonably good agreement with the available experimental and theoretical data. The calculations in high pressure phase need experimental verification.     

High pressure structural phase transition in uranium monochalcogenides

The pressure induced phase transition in uranium monochalcogenides, UX (X = S, Se, and Te) is studied by two-body potential approach. It is found that US, USe and UTe undergo a structural phase transition from NaCl (B1) type to CsCl (B2) type at 78.5, 21 and 9.5 GPa, respectively, which is in good agreement with the recent experimental data. In addition, second-order elastic constants (SOECs) (C ₁₁, C ₁₂ and C ₁₄) have been calculated which can be used to establish the nature of the forces in these materials. The present study shows that the considered two-body potential model can be used to predict the phase transition pressure in UX compounds provided the strength and hardness parameters in B1 and B2 phases are different.     

Pressure-induced phase transition and electronic properties of AlN nanowires: an ab initio study

The structural stability of AlN nanowires have been analyzed in wurtzite (B4), zincblende (B3), rocksalt (B1) and CsCl (B2) type phases using density functional theory based ab initio approach. The total energy calculations have been performed in a self-consistent manner using local density approximation as exchange correlation functional. The analysis finds the B4 type phase as most stable amongst the other phases taken into consideration and observes the structural phase transition from B4?→?B3, B4?→?B1, B4?→?B2, B3?→?B1 and B3?→?B2 at 42.7, 76.54, 142, 30.4 and 108.9?GPa respectively. Lattice parameter, bulk modulus and pressure derivatives of AlN nanowires have also been calculated for all the stable phases. The electronic band structure analysis of AlN nanowires shows a semiconducting nature in its B4, B3 and B1 type phases, whereas the B2 type phase is found to be metallic.     

UV filter 2‐ethylhexyl 4‐methoxycinnamate: a structure,energetic and UV–vis spectral analysis based on density functional theory

2‐Ethylhexyl 4‐methoxycinnamate (EHMC) is a very commonly used UVB filter that is known to isomerize from the (E) to the (Z) isomer in the presence of light. In this study, we have performed high level quantum chemical calculations using density functional theory (DFT) with the B3LYP density functional and extended basis sets to study the gas‐phase molecular structure of EHMC and its energetic stability. Calculations were also performed for related smaller molecules cinnamic acid and 4‐methoxycinnamic acid. Charge delocalization has been analyzed using natural charges and Wiberg bond indexes within the natural bond orbital analysis and using nucleus independent chemical shifts. Density functional theory calculations reveal that the (E) isomer of EHMC is more stable than the (Z) by about 20 kJ mol^?1 in both the gas and aqueous phases. The enthalpy of formation in the gas phase of (E)‐EHMC was derived from an isodesmic bond separation reaction. Long‐range corrected DFT calculations in implicit water were made in order to understand the excited state properties of the (E) and (Z) isomers of EHMC. Copyright © 2013 John Wiley & Sons, Ltd.     

Study of the effect of precipitant on the high-temperature phase stability of SrO-ZrO2 prepared by n-butanol soft-template method

SrO doped zirconia (20%) was synthesized by n-butanol soft-template method using both NaOH and ammonia solution as precipitants. The high-temperature phase stability was investigated following further heat treatment at 1000°C for 2 h. XRD and Raman spectra were used to characterize the crystal form of zirconia. In addition, TEM was used to characterize the dispersibility of SrO doped zirconia. The results indicated that the concentration of OH^? introduced into the ZrO₂ lattice was the main factor controlling the crystal form of nanosized zirconia. The NaOH solution precipitant could improve the dispersibility of SrO doped t-ZrO₂, and could also prevent the phase transformation of zirconia from t-ZrO₂ to m-ZrO₂ effectively.     

Electron scattering studies of magnetic isovector transitions in 12C at high momentum transfer

The inelastic electron scattering cross sections for the M1 transition to the 15.11 MeV (1⁺, T = 1) level and for the M2 transition to the 16.58 MeV (2^?, T = 1) level in ¹²C have been measured in the momentum transfer region q = 0.4–3.0 fm^?1, with emphasis on precise data at high momentum transfers. Additionally, a broad state near 15.4 MeV excitation has been observed and its excitation energy and natural width have been established as 15.44 ± 0.04 MeV and 1.5 ± 0.2 MeV, respectively. The Fourier-Bessel technique for determining the Mλ transition current density has been applied to the M1 and M2 transitions. Particular attention has been paid to the Coulomb corrections required to deduce the PWBA form factors. The M1 radiative width is Γ_γ0 = 38.5 ± 0.8 eV.     

A high pressure Raman study of TeO2 to 30 GPa and pressure-induced phase changes

The effect of pressure on the Raman modes in TeO₂ (paratellurite) has been investigated to 30GPa, using the diamond cell and argon as pressure medium. The pressure dependence of the Raman modes indicates four pressure-induced phase transitions near 1 GPa, 4.5 GPa, 11 GPa and 22 GPa. Of these the first is the well studied second-order transition fromD ₄ ⁴ symmetry toD ₂ ⁴ symmetry, driven by a soft acoustic shear mode instability. The remarkable similarity in the Raman spectra of phases I to IV suggest that only subtle changes in the structure are involved in these phase transitions. The totally different Raman spectral features of phase V indicate major structural changes at the 22GPa transition. It is suggested that this high pressure-phase is similar to PbCl₂-type, from high pressure crystal chemical considerations. The need for a high pressure X-ray diffraction study on TeO₂ is emphasized, to unravel the structure of the various high pressure phases in the system.     

Phase transitions in [Ni(NH3)6](NO3)2

Electric permittivity ^* = ′ − i″ of nickel-hexammino nitrate (NHN) has been measured within the range of temperature from 9 to 300 K at a frequency of 8.8 GHz (X-band). It has been found that the phase transitions at T_k1 = 247 K and T_k2 = 90 K are discontinuous structural transitions between centrosymmetric phases, whereas the transition at T_c = 63 K is a continuous phase transition (glass?).     

Adsorption of O2 on the CaO and SrO (1 0 0) surfaces: A first-principles study

First-principles calculations based on density functional theory (DFT) have been performed to investigate the adsorption of O₂ on the CaO and SrO (1 0 0) surfaces. The present results indicate that the bridge-top site for both the CaO and SrO (1 0 0) surfaces is the most favorable site for O₂ adsorption, with predicted adsorption energies of 1.437 eV and 1.236 eV, respectively. Detailed analysis of density of states, Mullikan population and vibration frequency are performed. The calculated results imply the possible formation of a peroxo (O₂²⁻) when O₂ is adsorbed with the bridge-top mode on both CaO and SrO (1 0 0) surfaces.     

Equation of state and thermal expansivity of LiF and NaF

The high-pressure and high-temperature behaviors of LiF and NaF have been studied up to 37 GPa and 1000 K. No phase transformations have been observed for LiF up to the maximum pressure reached. The B1 to B2 transition of NaF at room temperature was observed at ～28 GPa, this transition pressure decreases with temperature. Unit-cell volumes of LiF and NaF B1 phase measured at various pressures and temperatures were fitted using a P–V–T Birch–Murnaghan equation of state. For LiF, the determined parameters are: α₀ = 1.05 (3)×10^?4 K^?1, dK/dT = ?0.025 (2) GPa/K, V ₀ = 65.7 (1) Å³, K ₀ = 73 (2) GPa, and K′ = 3.9 (2). For NaF, α₀ = 1.34 (4)×10^?4 K^?1, dK/dT = ?0.020 (1) GPa/K, V ₀ = 100.2 (2) Å³, K ₀ = 46 (1) GPa, and K′ = 4.5 (1).     

Pressure induced phase transformations in alkali metal hydrides

Abstract

A three body potential approach(Singh and Singh, Phys. Rev. B39,761(1989)) has been described and applied to investigate the pressure induced phase transformation from B1 (NaCl-Structure) to B2 (CsCl-Structure) phases for the partially ionic alkali-metal hydrides (LiH, NaH and KH). Our calculated values of the phase transition pressures and the associated volume collapses for NaH and KH are in reasonably good agreement with their experimental data. The variations of the relative volumes (V(P)/V(O)) with pressure for both B1 and B2 phases, have shown no phase transition upto 140 GPa in LiH. This feature is in agreement with the experimental observations.     

Formation of the high pressure graphite and BC8 phases in a cold compression experiment by Raman scattering

We use Raman scattering to study phase transition in the graphitic g‐BC₈ phase and graphite at high pressure up to 84 GPa. The E_2g Raman active mode of graphite (G peak) can be detected up to 84 GPa. We demonstrate that there is (1) a phase transition in g‐BC₈ and in graphite at 35 GPa and (2) that above 35 GPa, the g‐BC₈ and graphite transform under high pressure to possibly fully sp³‐bonded, disordered hp‐BC₈, and hp‐C phases. Below the phase transition, a polynomial fit to the G peak position versus pressure data yielded a quadratic relation; above the phase transition, it demonstrates linear behavior. The phase transition at high pressure in BC₈ system and graphite is reversible. Quenched hp‐BC₈ and hp‐C phases have the Raman spectrum typical to that of the graphitic phases. Copyright © 2013 John Wiley & Sons, Ltd.