Calculated equation of state of al, cu, ta, mo, and W to 1000 GPa

Calculated Hugoniots and 293-K isotherms at pressures up to 1 TPa (10 Mbar) for the five reference metals Al, Cu, Ta, Mo, and W are reported using the classical mean-field approach where both the cold and the thermal parts of the Helmholtz free energy are derived entirely from the 0-K total energies and electronic density of states calculated with the full-potential linearized augmented plane wave method within the generalized gradient approximation to exchange correlational functional. Our approach permits efficient computation and invokes no empirical parameters. Both the experimental Hugoniots and 293-K isotherms are reproduced excellently.     

First-principles study of structural,mechanical, lattice dynamical and thermal properties of nodal-line semimetals ZrXY (X=Si,Ge; Y=S,Se)

Abstract

The nodal-line semimetals are new and very promising materials for technological applications. To understand their structural, mechanical, lattice dynamical and thermal properties in detail, we have investigated theoretical study of ZrXY (X = Si,Ge; Y = S,Se) using Density Functional Theory for the first time. Obtained lattice parameters are in excellent agreement with previous experimental data. These nodal-line semimetals obey the mechanical stability conditions for tetragonal structure. We obtain Bulk modulus, Shear modulus, Poisson’s ratio, Pugh ratio, sound velocities and thermal conductivity using elastic constant. All the materials behave in brittle manner. Poisson’s ratio data and Bader charge analysis results indicate that the ionic bonding characters are dominant. Next, the lattice dynamical properties are calculated. Phonon density of states shows that nodal-line semimetals ZrXY are also dynamically stable in the tetragonal structure. Raman and IR active phonon modes are determined. Highest optical mode at gamma point corresponds to A_2u (IR active) and E_g (Raman active) modes for ZrXSe and ZrXS, respectively. Based on phonon density of states, thermal properties such as Helmholtz free energy, entropy, heat capacity at constant volume and Debye temperature are also presented and discussed.     

第一性原理对金的高压相变和零温物态方程的计算

在密度泛函理论下，用缀加平面波加局域轨道方法，分别采用广义梯度近似(GGA)和局域密度近似(LDA)对金的面心立方晶格结构(fcc)、体心立方晶格结构(bcc)和六角密堆积结构(hcp)的结构能量进行了计算.在GGA下，计算得出fcc向hcp和hcp向bcc的相变分别发生在380GPa 和1250GPa；而LDA下相变分别发生在490GPa和790GPa.当计算压强达到2TPa时，bcc在这两种近似下仍然保持稳定的结构.根据不同体积下不同结构的电子态密度的特征，对发生相变的物理原因进行了定性的分析，在此基础上得到了金的零温状态方程. 关键词： 缀加平面波方法 固态相变 电子态密度 物态方程     

A coarse-grained model of the effective interaction for charged amino acid residues and its application to formation of GCN4-pLI tetramer

ABSTRACT

We present a simple coarse-grained model of the effective interaction for charged amino acid residues, such as Glu and Lys, in a water solvent. The free-energy profile as a function of the distance between two charged amino acid side-chain analogues in an explicit water solvent is calculated with all-atom molecular dynamics simulation and thermodynamic integration method. The calculated free-energy profile is applied to the coarse-grained potential of the effective interaction between two amino acid residues. The Langevin dynamics simulations with our coarse-grained potential are performed for association of a small protein complex, GCN4-pLI tetramer. The tetramer conformation reproduced by our coarse-grained model is similar to the X-ray crystallographic structure. We show that the effective interaction between charged amino acid residues stabilises association and orientation of protein complex. We also investigate the association pathways of GCN4-pLI tetramer.     

高压下Al的零温电子结构和物态方程

 用LMTO法（线性Muffin-Tin轨道法），计算了金属铝的超高压电子结构及零温物态方程。Al的压缩比到10，压力达10 TPa。根据第一原理计算结果，在带结构方面，s带总是处于Fermi能以下，随着压力的增加，s、p和d的带宽增加，随后则杂化程度增加，所以s、d轨道的电子占据数连续变化。上述变化对压力的影响也是连续的，换言之，从s→d转变没有理由说明Al在0.5 TPa附近冲击Hugoniot的斜率的拐弯现象。而这一点则是与Altshuler的观点不同。物态方程的第一原理计算结果表明，bcc结构比fcc结构要软，但因差别不很大，即使发生fcc→bcc的转变，也不会引起Hugoniot的质的变化（拐弯）。Al的LMTO物态方程还表明，我们以前所采用的半经验的冷压误差可达30%。为此，根据LMTO结果，给出新的冷压表达式p=∑_i=0⁵a_iδ^i/3，δ=Ω₀/Ω，其中a₀=-7.327 79，a₁=18.754 3，a₂=10.209 7，a₃=-2.523 53，a₄=-4.787 2，a₅=6.065 94，拟合误差小于3%。     

Lattice-dynamical functions of solids at very high pressures

Abstract

Vibrational spectra of face-centered cubical (f.c.c.) solids are investigated at the pressure region, where the quantum-statistical model for the electron energy is applicable. Phonon dispersion curves are obtained for different values of the specific volume. The integration of the phonon spectra over the Brillouin zone yields the zero-point vibrational energy, the Debye temperature, the Gruneisen coefficient, mean-square amplitude of zero-point and thermal vibration, the melting temperature. All quantities under consideration are calculated from the universal functions of the reduced volume, using the scaling relations for given atomic number 2.     

A comparison between the mechanical and thermal properties of single-walled carbon nanotubes and boron nitride nanotubes

Carbon nanotubes (CNTs) are semimetallic while boron nitride nanotubes (BNNTs) are wide band gap insulators. Despite the discrepancy in their electrical properties, a comparison between the mechanical and thermal properties of CNTs and BNNTs has a significant research value for their potential applications. In this work, molecular dynamics simulations are performed to systematically investigate the mechanical and thermal properties of CNTs and BNNTs. The calculated Young’s modulus is about 1.1 TPa for CNTs and 0.72 TPa for BNNTs under axial compressions. The critical bucking strain and maximum stress are inversely proportional to both diameter and length-diameter ratio and CNTs are identified axially stiffer than BNNTs. Thermal conductivities of (10, 0) CNTs and (10, 0) BNNTs follow similar trends with respect to length and temperature and are lower than that of their two-dimensional counterparts, graphene nanoribbons (GNRs) and BN nanoribbons (BNNRs), respectively. As the temperature falls below 200 K (130 K) the thermal conductivity of BNNTs (BNNRs) is larger than that of CNTs (GNRs), while at higher temperature it is lower than the latter. In addition, thermal conductivities of a (10, 0) CNT and a (10, 0) BNNT are further studied and analyzed under various axial compressive strains. Low-frequency phonons which mainly come from flexure modes are believed to make dominant contribution to the thermal conductivity of CNTs and BNNTs.     

Equations of State for Regular Solids

Equations of State (EOS) for solids under strong compression and wide ranges in temperature are most commonly represented by "parametric" EOS forms using temperature dependent parameters for the volume V 0 , the bulk modulus K 0 , and its pressure derivative K_{0}^{\prime} for the given ambient (or zero) pressure. Therefore, various common "parametric" EOS forms are compared at first with the recently proposed [1,2] A dapted P olynomical expansion AP2, and in the second part with the Mie-Grüneisen approach, which uses one of the common EOS forms for the pressure of the static lattice or for the zero temperature isotherm, p ZT ( V ), and a detailed modelling of the additional thermal pressure, p th ( V , T ), in the form p(V,T) = p_{\rm ZT}(V) + p_{\rm th}(V,T) . Thereby, it is shown, that "intrinsic" anharmonicity effects have to be taken into account and between the two schemes differences of the order of a few percent in pressure are noticed for regular solids, like Cu, Ag, and Au. These differences are discussed with respect to the present uncertainties in a practical pressure scale for wide ranges in pressure (up to several TPa) and in temperature (up to 1500 K and above).     

Parabolic equation formulation via a singular perturbation technique and its application to scattering from irregular surfaces

Abstract

This paper consists of two parts. In the first part, the solution of the Helmholtz equation under forward-scattering or propagation conditions is sought as a uniform asymptotic perturbation expansion using the method of multiple scales. It is then shown that the parabolic wave equation (PWE) solution is the zeroth-order term in this expansion. In the second part, the electric-field integral equation and the magnetic-field integral equation, derived under the PWE approximation, are solved for surface currents induced on a sinusoidal surface. The scattered fields produced by these currents are then calculated using the appropriate radiation integrals. Results are compared to those obtained using the method of ordered multiple interactions developed by Kapp and Brown.     

Optical absorption of the F center as function of the temperature

Abstract

The temperature shift of the F absorption band for KBr and KCl single crystals was experimentally and theoretically determined. The theoretical calculations are based on the pseudopotential method using different wave functions and assumed a linear thermal expansion coefficient for the crystalline lattice. The change of F-center absorption energy due to collisions with the thermally vibrating lattice is calculated. This contribution is small. The measurements agree fairly well with the results obtained using the present model.     

High pressure physics research at BARC

Abstract

High pressure physics research at BARC spans a period of more than two decades and covers the area of both static and dynamic pressures on the one hand and both theoretical and experimental aspects on the other. The experimental facilities available include Diamond Anvil Cells for X-ray diffraction and Raman spectroscopy and a light gas gun. Phase transformations investigated in metallic systems includethose in Cd-Hg, transition metals, rare earths and actinides and these investigations have been supported by detailed band structure calculations and studies of the mechanismof phase transformations. Extensive equation of state studies over a wide range of pressures include a new model for the intermediate pressure range between 0.5 and 10 TPa and interpretation of experimental shock Hugoniots.     

Review: high pressure generation techniques beyond the limit of conventional diamond anvils

ABSTRACT

Current anvil designs and problems associated with various efforts to generate static high pressures beyond the limit of conventional diamond anvil cells (DACs) (～400?GPa) are reviewed. Pressures of up to 1?TPa have been reported by one research group using the double-stage DAC (ds-DAC) technique, but no other research group has successfully reproduced this high pressure result. Some research groups have used toroidal anvils, achieving pressures of >400?GPa. We have conducted numerous ds-DAC experiments and investigated the problems associated with such experiments. They include problems associated with various pressure scales in the multi-megabar region, difficulties in obtaining reliable X-ray diffraction patterns from micron-sized samples, and physical property measurements of tiny materials that may be harder than diamond. Each of these problems is discussed, following the summary of various experiments.     

High pressure phase diagrams of binary lanthanide alloys between La,Ce and Pr

Abstract

In comparison with the “regular” binary alloy phase diagrams between “regular” trivalent lanthanide metals, binary intralanthanide alloys of Ce exhibit many irregularities typical for Ce under pressure due to its f electron delocalization. For comparison with the La-Ce and Ce-Pr high pressure phase diagrams also the more regular La-Pr data are presented for the pressure range up to 40GPa.     

Synthesis experiments on B-Sb,Ge-Sb,and Xe-Pd systems using a laser heated diamond anvil cell

Abstract

In an effort to synthesize B-Sb, Ge-Sb and Xe-Pd compounds under high pressure, the respective system was laser-heated in a diamond anvil cell at temperatures above 2500 K and up to a maximum pressure of 51 GPa. The product was characterized by X-ray diffraction using rotating anode and synchrotron radiation X-ray sources. No reaction was observed in any of these systems up to pressures of 32, 20 and 51 GPa, respectively. In the case of Ge-Sb, new peaks were observed in the pressurequenched samples, but they were identified with the known metastable phases of Ge. In this regard our results are contrary to the earlier work on Ge-Sb.     

Intramolecular association within the SAFT framework

A general theory for modelling intramolecular association within the SAFT framework is proposed. Sear and Jackson [Phys. Rev. E. 50 (1), 386 (1994)] and Ghonasgi and Chapman [J. Chem. Phys. 102 (6), 2585 (1995)] have previously extended SAFT to include intramolecular association for chains with two sites. We show that the resulting equations from the two approaches are equivalent, and use their work as a basis for developing a new general theory. The approach used by Ghonasgi and Chapman is based on mass balances and an infinite dilution result and provides the equations needed to determine the contribution to the Helmholtz free energy from association (inter- as well as intramolecularly) at equilibrium. Sear and Jackson rederived the contribution to the Helmholtz free energy from association from the theory by Wertheim [J. Stat. Phys. 42 (3–4), 459 (1986)] with inclusion of intramolecular association, and using this approach we obtain an expression for the Helmholtz free energy that is valid also at non-equilibrium states (with respect to hydrogen bonds), which is very useful when calculating derivatives.     

Thermodynamic properties of ZnSe under pressure and with variation in temperature

The thermodynamic properties of Zn Se are obtained by using quasi-harmonic Debye model embedded in Gibbscode for pressure range 0–10 GPa and for temperature range 0–1000 K. Helmholtz free energy, internal energy, entropy,Debye temperature, and specific heat are calculated. The thermal expansion coefficient along with Gruneisen parameter are also calculated at room temperature for the pressure range. It is found that internal energy is pressure dependent at low temperature, whereas entropy and Helmholtz free energy are pressure sensitive at high temperature. At ambient conditions,the obtained results are found to be in close agreement to available theoretical and experimental data.     

Magnetization distribution in a helicoidal structure containing a point defect

The helicoid that forms in an external magnetic field in the presence of a point defect is modeled in the framework of the Landau theory of second-order phase transitions. A general solution to the nonlinear problem retaining all terms in the Helmholtz free-energy functional is obtained by means of Green functions. The magnetization distribution in the plane perpendicular to the helicoid axis and to the external field is calculated.     

Representation of the Helmholtz equation solution in the form of a series based on backscattering multiplicity

Abstract

The representation of the Helmholtz equation solution in the form of a series based on backscattering multiplicity is considered. New methods for calculating wave fields propagating in inhomogeneous media, which have been developed on the basis of this series, are presented. The results that have been obtained using these new methods are briefly described.     

Ammonium ion dynamics in NH4Br at high pressure - type of reorientation

Abstract

Proton NMR relaxation measurements were performed for ammonium bromide at pressures up to 800 MPa. The correlation functions for ammonium ion reorientation were calculated by Monte Carlo method. Knowledge of these functions allow calculation of the NMR relaxation times T₁. Comparison between experimental and calculated values of relaxation times gives some insight into the model of ammonium ion reorientation.     

Study of elastic properties of CeO2 by statistical moment method

The purpose of the present paper is to investigate the temperature and pressure dependences of the elastic properties of cerium dioxide using the statistical moment method (SMM). The equation of states of bulk CeO₂ is derived from the Helmholtz free energy, and the pressure dependences of the elastic moduli like the bulk modulus, B_T, shear modulus, G, Young’s modulus, E, and elastic constants (C11, C12, and C44) are presented taking into account the anharmonicity effects of the thermal lattice vibrations. In the present study, the influence of temperature and pressure on the elastic moduli and elastic constants of CeO₂ has also been studied, using three different interatomic potentials. We compare the results of the present calculations with those of the previous theoretical calculations as well as with the available experiments.