Equation of state of a classical anharmonic oscillator

Static properties of a particle moving in an anharmonic potential in equilibrium with a temperature bath and an external field are discussed in the framework of classical statistical mechanics. This model system represents the basic unit in current theories of structural phase transitions. Hierarchies of equations for the correlations (cumulants) and irreducible vertices are derived from the equilibrium condition. Approximate solutions are obtained from the hierarchies by truncation. Alternatively, one can write the equilibrium condition as differential equation which may be solved exactly, if appropriate initial conditions are known. Both methods have been worked out for a single- and a double-well potential. By truncation of the hierarchies one obtains as quantitatively correct result only a low-temperature expansion for the single-well potential.     

Predicting XAFS scattering path cumulants and XAFS spectra for metals (Cu,Ni, Fe,Ti, Au) using molecular dynamics simulations

The ability of molecular dynamics (MD) simulations to support the analysis of X‐ray absorption fine‐structure (XAFS) data for metals is evaluated. The low‐order cumulants (ΔR, σ², C₃) for XAFS scattering paths are calculated for the metals Cu, Ni, Fe, Ti and Au at 300 K using 28 interatomic potentials of the embedded‐atom method type. The MD cumulant predictions were evaluated within a cumulant expansion XAFS fitting model, using global (path‐independent) scaling factors. Direct simulations of the corresponding XAFS spectra, χ(R), are also performed using MD configurational data in combination with the FEFFab initio code. The cumulant scaling parameters compensate for differences between the real and effective scattering path distributions, and for any errors that might exist in the MD predictions and in the experimental data. The fitted value of ΔR is susceptible to experimental errors and inadvertent lattice thermal expansion in the simulation crystallites. The unadjusted predictions of σ² vary in accuracy, but do not show a consistent bias for any metal except Au, for which all potentials overestimate σ². The unadjusted C₃ predictions produced by different potentials display only order‐of‐magnitude consistency. The accuracy of direct simulations of χ(R) for a given metal varies among the different potentials. For each of the metals Cu, Ni, Fe and Ti, one or more of the tested potentials was found to provide a reasonable simulation of χ(R). However, none of the potentials tested for Au was sufficiently accurate for this purpose.     

Theoretical interpretation of the vibrational spectrum of bicyclo[1.1.0]butane in terms of an ab initio anharmonic model

The anharmonic vibrational IR and Raman spectra of the bicyclo[1.1.0]butane molecule have been calculated in the range of up to 4000 cm^?1 using a numerical and analytical realization of the van Vleck second-order operator perturbation theory. Cubic and quartic force constants in normal coordinates, as well as cubic surfaces of the dipole moment and polarizability, have been found by numerical differentiation of the corresponding first and second derivatives calculated by the MP2/cc-pVTZ quantum-mechanical method. In order to increase the prediction accuracy of vibrational transitions, corresponding harmonic frequencies have been obtained by the CCSD(T)/cc-pVTZ high-precision quantum mechanical method. The anharmonic intensities of the IR and Raman spectra have been calculated using canonical transformations of the operators of the dipole moment and polarizability expanded into a Taylor series around the equilibrium configuration. The assignment of experimental vibrational bands in the IR and Raman spectra has been analyzed. It has been shown that the anharmonic calculation based on the above-described procedure of combining more exact harmonic frequencies with the anharmonic force field obtained with a more economical method makes possible the reliable interpretation of the majority of spectral bands, including Fermi and Darling-Dennison resonances.     

高温高压下闪锌矿相GaN结构和热力学特性的分子动力学研究

利用分子动力学方法和Buckingham经验势模型对重要半导体材料GaN立方闪锌矿相的晶格常数、相变压力(从闪锌矿到岩盐结构)、热膨胀、等温体模量、定压热容等结构和热力学特性在300—3000K的温度范围和0—65GPa的压力范围内进行了研究.研究表明，闪锌矿相GaN常态下的结构和热力学参数的模拟结果与实验数据及其他理论结果相符.同时在所选作用势模型可靠性检验的基础上，对等温体模量、定压热容诸非谐性参量在高温高压下的热力学行为进行了预测.所得结果在材料科学等领域的研究中具有一定的应用背景和参考价值. 关键词： GaN Buckingham势 分子动力学模拟 高温高压     

On the neglecting of higher‐order cumulants in EXAFS data analysis

The cumulant expansion is one of the most powerful and useful methods for EXAFS data analysis, in which the higher‐order cumulants allow to consider deviations from a simple Gaussian distribution. In this work, analytical expressions have been derived to show the effects of neglecting higher‐order cumulants in EXAFS analysis by the ratio method. The errors in the best‐fitting procedure owing to the omission of the higher‐order cumulants, as well as of the coordination number, can be determined.     

热膨胀动态过程的理论和实验研究

 本文列出了一维点阵非谐振动的非线性微分方程组，并求出了这组方程在相应边值条件下的解析解。据此算出了该点阵的热膨胀系数和热膨胀动态过程的特征时间，同时说明了它的物理意义。关于金属箔的实验结果，可以作为这一理论预言的一个验证。这些结果还告诉我们，在高速加热时，即使温度均匀而且膨胀自由，热应力仍然存在，不过将很快弛豫。     

Thermal Expansion and Deformation of Graphene

Taking into consideration short-atomic-range interactions and anharmonic effects,we calculate the thermal expansion coefficients,Gruneisen parameters,the elastic modulus of graphene varying with temperature and the phonon frequency.The anharmonic effects associated with the graphene deformation are also discussed.The results show that the value of thermal expansion coefficient is negative in the moderate temperature range,and it becomes positive when the temperature grows to be higher than a certain value.The change rate of elastic modulus with respect to temperature and pressure are calculated,and phonon frequencies are estimated.In the process of graphene thermal expansion,it is accompanied with the change of bond length and the rotation around the axis normal to the plane.Our results indicate that the effects due to the bond change are more significant than that of the rotation.We also show that if anharmonic effects are ignored,the thermal expansion coefficient and the Gruneisen parameters are zero,and the elastic modulus and the phonon frequency are constant.If anharmonic effects are considered up to the second term,these values will vary with temperature,and become closer to the experimental value.The higher the temperature is,the more significant the anharmonic effects become.     

Stereochemical analysis of ferrocene and the uncertainty of fluorescence XAFS data

Methods for the quantification of statistically valid measures of the uncertainties associated with X‐ray absorption fine structure (XAFS) data obtained from dilute solutions using fluorescence measurements are developed. Experimental data obtained from 10 mM solutions of the organometallic compound ferrocene, Fe(C₅H₅)₂, are analysed within this framework and, following correction for various electronic and geometrical factors, give robust estimates of the standard errors of the individual measurements. The reliability of the refinement statistics of standard current XAFS structure approaches that do not include propagation of experimental uncertainties to assess subtle structural distortions is assessed in terms of refinements obtained for the staggered and eclipsed conformations of the C₅H₅ rings of ferrocene. Standard approaches (XFIT, IFEFFIT) give refinement statistics that appear to show strong, but opposite, preferences for the different conformations. Incorporation of experimental uncertainties into an IFEFFIT‐like analysis yield refinement statistics for the staggered and eclipsed forms of ferrocene which show a far more realistic preference for the eclipsed form which accurately reflects the reliability of the analysis. Moreover, the more strongly founded estimates of the refined parameter uncertainties allow more direct comparison with those obtained by other techniques. These XAFS‐based estimates of the bond distances have accuracies comparable with those obtained using single‐crystal diffraction techniques and are superior in terms of their use in comparisons of experimental and computed structures.     

Structural investigations on an amorphous Se90Te10 alloy produced by mechanical alloying using EXAFS, cumulant expansion and RMC simulations

We investigated an amorphous Se(90)Te(10) alloy produced by mechanical alloying using two different approaches. First, we used extended x-ray absorption fine structure (EXAFS) spectroscopy and the cumulant expansion method using the Einstein model for the temperature dependence of the cumulants to obtain the cumulants C(*)(1), C(*)(2), and C(*)(3). From these, we found information about the structure of the alloy as well as the thermal and structural disorder, anharmonicity of the effective interatomic pair potentials, thermal expansion of the Se-Se and Se-Te bonds and asymmetry of the partial distribution functions g(Se-Se)(r) and g(Se-Te)(r). The cumulants C(*)(1), C(*)(2), and C(*)(3) also allowed us to reconstruct the g(EXAFS)(ij)(r,T) functions from EXAFS. Then, we made reverse Monte Carlo (RMC) simulations using the total structure factor S(K) obtained from synchrotron x-ray diffraction and the EXAFS oscillations χ(k) on the Se K edge as input data to obtain the g(RMC)(ij)(r) functions. Both methods furnished very similar g(ij)(r) functions, and the structural data obtained from them were also very similar. The results obtained from both methods showed the presence of Se-Te pairs indicating that there is alloying at the atomic level. In addition, we could not find any evidence of the presence of Te clusters in the alloy.     

Thermodynamic properties of cubic boron nitride based on an analytic mean field approach

We present a detailed derivation of the analytic expressions for the equation of state (EOS) and internal energy of Morse model solids based on an analytic mean field potential (AMFP) method. The formalism is applied to cubic boron nitride (c-BN). One set of potential parameters are determined by fitting the experimental P-V-T data of c-BN up to 160 GPa at 295 K and 80 GPa in the range 500–900 K. Various physical quantities including the isothermals, thermal expansion, isochoric heat capacity, Helmholtz free energy and internal energy are calculated and analyzed. The theoretical results are consistent with the available experimental data and those calculated by others. These results presented in this paper verify that the AMFP method is a useful approach to consider the anharmonic effect at high temperature. Numerous reasonable predictions and the change trend of the properties for c-BN at extreme conditions have been given.     

Experimental determination of vibrational anharmonic contributions

This article deals with the experimental determination of separate vibrational anharmonic contributions to the self‐energy of phonons in any crystals based on temperature dependence of their Raman spectra. We propose a new approach to find each anharmonic contribution by using special temperature points. We apply the approach to diamond, silicon, and crystalline α‐S₈ and show that our results for summarized anharmonicity in diamond and silicon are in good agreement with the values obtained previously for these systems by other researchers. Copyright © 2013 John Wiley & Sons, Ltd.     

Physical properties of molybdenum monoboride: Ab-initio study

The Ab initio investigations on structural, electronic, optical and thermal properties of MoB have been reported using full potential linearised-augmented plane wave method within the framework of density functional theory. The exchange and correlation potentials were calculated using the Perdew-Burke-Ernzerhof-Sol generalised gradient approximation. The calculated equilibrium lattice constants and cell volume are in excellent agreement with the experimental results as compared to the available theoretical data. Electronic band structure shows that MoB is metallic in nature. From the partial densities of states of MoB it has been found that major contribution on the Fermi level is due to Mo-4d states. Among the reported optical parameters the large value of reflectivity at low energy shows that MoB can be used as a coating material in IR region. Maximum absorption in extreme UV region shows that it can be used in production of electricity through solar power in space vehicles. Various thermal properties have been calculated in a wide temperature range at high pressures. Change in thermal expansion coefficient with respect to temperature shows that anharmonic effect in MoB is very weak at high temperature. The optical and thermal properties of MoB are presented for the first time in this work.     

Thermophysical properties of iridium at finite temperature

The bulk properties of materials in an extreme environment such as high temperature and high pressure can be understood by studying anharmonic effects due to the vibration of lattice ions and thermally excited electrons.In this spirit,in the present paper,anharmonic effects are studied by using the recently proposed mean-field potential(MFP) approach and Mermin functional which arise due to the vibration of lattice ions and thermally excited electrons,respectively.The MFP experienced by a wanderer atom in the presence of surrounding atoms is constructed in terms of cold energy using the local form of the pseudopotential.We have calculated the temperature variation of several thermophysical properties in an extreme environment up to melting temperature.The results of our calculations are in excellent agreement with the experimental findings as well as the theoretical results obtained by using first principle methods.We conclude that presently used conjunction scheme(MFP+pseudo potential) is simple computationally,transparent physically,and accurate in the sense that the results generated are comparable and sometimes better than the results obtained by first principle methods.Local pseudopotential used is transferable to extreme environment without adjusting its parameters.     

2,3-二氯吡嗪的红外、拉曼光谱和密度泛函研究

应用KBr压片法、熔融法分别测定了2,3-二氯吡嗪(2,3-DCP)结晶相和液相下400～4 000 cm-1范围内的傅里叶变换红外光谱(FTIR),及其600～4 000 cm-1内的傅里叶变换拉曼光谱(FT-Raman)。采用密度泛函(DFT)理论之B3LYP方法在6-311++G(2df,2pd)基组水平上优化了该分子的平衡几何结构,基于此结构应用谐性力场计算获得了2,3-DCP的振动频率、红外强度和拉曼活性并进一步计算了直到四阶的非谐性力场,将该力场带入标准旋振哈密顿量并利用二阶微扰理论获得了更加准确的振动频率,相应的红外、拉曼光谱。通过非谐力场获得的振动频率位置结合谐性强度与实验结果比对,对2,3-DCP的各振动带进行了详细指认,采用简正坐标分析方法得到各振动频率的势能分布(PED),首次对2,3-DCP的振动光谱进行了全面归属。结果同时显示：考虑非谐性效应后的理论结果大大提高了振动频率的预测性,用其获得的振动频率能很好的再现实验基频,其与实验值差异大多保持在10 cm-1以下,即使在谐振预期很差的高频区域,考虑非谐效应后这种差异也迅速降低到19 cm-1以下,这对正确归属和预期振动光谱是十分有帮助的。目前的结论也可推广应用到其他分子体系。     

Dynamics of the phase-change material GeTe across the structural phase transition

Results of inelastic neutron scattering experiments and ab initio molecular dynamics simulations for GeTe – the parent compound of phase-change materials are reported. The inelastic neutron spectra of GeTe powder samples have been determined in the temperature range extending from 300 to 700 K. The phonon peaks undergo thermal shifts resulting from anharmonic effects being weaker for acoustic than optic modes. A small concentration of free charge carries arising from the presence of Ge-vacancies was found not to affect significantly the neutron weighted phonon densities of states of GeTe. The spectral pattern changes qualitatively across the structural phase transition, but the local structure of GeTe remains hardly affected, as confirmed by the analysis of temperature dependence of the pairdistribution function obtained from ab initio molecular dynamics investigations. The present theoretical studies support in a wide extent our experimental observations and also those provided by local probe methods.     

金属橡胶热物理性能理论与试验研究

针对金属橡胶材料在高温环境下的热稳定性能和热传导性能, 基于金属橡胶的内部基本组分以及特殊的编织制作工艺, 构建了两种典型排列微元体结构, 并以此描述微元体在三种接触状态下的热膨胀特性, 提出了金属橡胶热膨胀Schapery分析模型, 从理论上解释了金属橡胶热膨胀的产生机理.根据金属橡胶基本组成单元的传热模式, 研究了金属橡胶的传热过程.利用热电比拟法和有限元法, 分析微元体的导热性能, 结合微元体的分布形式, 提出了金属橡胶的导热分析模型.试验测试了不同相对密度金属橡胶的热膨胀和热传导性能, 验证了金属橡胶热膨胀和导热特性分析理论模型的适用性.所得到的金属橡胶的热膨胀和热传导性能理论模型和试验结果, 为金属橡胶材料在高温环境下的热物理特性分析提供了理论基础和计算分析依据. 关键词： 金属橡胶 热膨胀 热传导 金属丝螺旋卷     

MgO和Cu的弹性常数随温度及压强变化关系的研究

从Anderson-Grüneisen参数定义式和热膨胀系数定义式出发，根据Tallon的普遍化理论,推导出了一个用于计算弹性常数的理论模型,用所得到的理论模型分别计算了MgO和Cu两种固体材料的弹性常数 、 和 随压强以及温度变化的理论数值，并将理论计算结果与Kumar模型计算结果以及相关实验数据进行了比较，对所得到的理论模型计算结果与Kumar模型计算结果之间的差异性进行了分析和探讨。研究结果表明：所推导的理论模型计算结果与实验数据更加符合，并且由于所用的推导方法不依赖于晶体的结构,因此该模型具有更好的合理性和普适性。     

MgO和Cu的弹性常数随温度及压强变化关系的研究

从Anderson-Grüneisen参数定义式和热膨胀系数定义式出发,根据Tallon的普遍化理论,推导出了一个用于计算弹性常数的理论模型,用所得到的理论模型分别计算了MgO和Cu两种固体材料的弹性常数 、 和 随压强以及温度变化的理论数值,并将理论计算结果与Kumar模型计算结果以及相关实验数据进行了比较,对所得到的理论模型计算结果与Kumar模型计算结果之间的差异性进行了分析和探讨。研究结果表明：所推导的理论模型计算结果与实验数据更加符合,并且由于所用的推导方法不依赖于晶体的结构,因此该模型具有更好的合理性和普适性。     

MgO和Cu的弹性常数随温度及压强变化关系的研究

从Anderson-Grüneisen参数定义式和热膨胀系数定义式出发,根据Tallon的普遍化理论,推导出了一个用于计算弹性常数的理论模型,用所得到的理论模型分别计算了MgO和Cu两种固体材料的弹性常数C_(11)、C_(12)和C_(44)随压强以及温度变化的理论数值,并将理论计算结果与Kumar模型计算结果以及相关实验数据进行了比较,对所得到的理论模型计算结果与Kumar模型计算结果之间的差异性进行了分析和探讨.研究结果表明:所推导的理论模型计算结果与实验数据更加符合,并且由于所用的推导方法不依赖于晶体的结构,因此该模型具有更好的合理性和普适性.