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.     

Vibrational and thermal properties of ScN and YN: quasi-harmonic approximation calculations and anharmonic effects

The structural, vibrational and thermal properties of rocksalt ScN and YN are investigated by using a first-principles plane-wave approach. The results are discussed in comparison with the similarly calculated results for rocksalt MgO and zincblende AlN. The thermal expansivity (α(V)) computed within the quasi-harmonic approximation shows that there are significant anharmonic effects in ScN and YN, which are comparable to those in MgO. Since no experimental results are available for α(V) of either ScN or YN, the anharmonic effects are accounted for by a variant of the very recently introduced effective semiempirical ansatz (Phys. Rev. B 2009 79 104304) for calculating anharmonic free energy, which does not require any input from experiment. The validity of this very simple approach is demonstrated first by applying it to MgO. For the considered phase of AlN, the quasi-harmonic approximation is valid up to very high temperatures, and the thus obtained α(V) is in good agreement with experiment. The values of α(V) for semiconductor transition metal nitrides that crystallize in the rocksalt phase are higher than those for the zincblende phase of group-IIIB nitrides, and a major part of these differences is due to the crystal structure.     

Thermodynamic properties of rhodium at high temperature and pressure by using mean field potential approach

The thermophysical properties of rhodium are studied up to melting temperature by incorporating anharmonic effects due to lattice ions and thermally excited electrons. In order to account anharmonic effects due to lattice vibrations, we have employed mean field potential (MFP) approach and for thermally excited electrons Mermin functional. The local form of the pseudopotential with only one effective adjustable parameter r_c is used to construct MFP and hence vibrational free energy due to ions – F_ion. We have studied equation of state at 300 K and further, to access the applicability of present conjunction scheme, we have also estimated shock-Hugoniot and temperature along principle Hugoniot. We have carried out the study of temperature variation of several thermophysical properties like thermal expansion (β_P), enthalpy (E_H), specific heats at constant pressure and volume (C_P and C_V), specific heats due to lattice ions and thermally excited electrons (C_V^ion and C_V^el, isothermal and adiabatic bulk moduli (B_T and B_s) and thermodynamic Gruneisen parameter (γ_th) in order to examine the inclusion of anharmonic effects in the present study. The computed results are compared with available experimental results measured by using different methods and previously obtained theoretical results using different theoretical philosophy. Our computed results are in good agreement with experimental findings and for some physical quantities better or comparable with other theoretical results. We conclude that local form of the pseudopotential used accounts s-p-d hybridization properly and found to be transferable at extreme environment without changing the values of the parameter. Thus, even the behavior of transition metals having complexity in electronic structure can be well understood with local pseudopotential without any modification in the potential at extreme environment. Looking to the success of present scheme (MFP + pseudopotential) we would like to extend it further for the study of liquid state properties as well as thermophysical properties of d and f block metals.     

Lattice heat capacities of solid argon

An anharmonic rigid-atom model has been used to study the heat capacities of solid argon. The model derives the interatomic forces by BuckinghamCorner potential and takes account of all neighbour interaction. The contribution of the cubic and quartic terms of the potential energy expansion to the heat capacities have been accounted for through Helmholtz-free energy by perturbation theory. It is concluded that the model is most suitable among the ones designed for the class of solids to which argon belongs.     

P-V relation for cuprous halides: Pseudopotential approach

Cuprous halides are found to show significantly different behaviour from that of alkali halides. This difference is mainly due to the presence of outermost d-electrons present in Cu⁺ ion. In the present paper, the ab initio pseudopotential formalism incorporating the effect of d-electrons is used to generate the equation of state (EOS) for group I-VII binary compounds from first-principles calculations. The expression for the EOS is derived for the first time within the pseudopotential framework. The computed isothermal compression curves for cuprous halides are compared with compression curves from other EOSs belonging to different classes and categories and have also been tested for the prediction of end point.     

A model pseudopotential for noble metals

A simple model pseudopotential has been proposed for noble metals and used to calculate the form factor, band gap, liquid metal resistivity and phonon dispersion curves for gold. A reasonably good agreement is found between the computed and experimental results.     

Ionoluminescence in the presence of ionizing radiation

An increase in the ionoluminescence intensity from a ZnS-CdS:Ag sample by a factor of more than 20 upon additional electronic excitation by UV light has been found. The effect decreases in magnitude with an increase in the energy of bombarding H₂⁺ ions and peaks at an energy below 175 eV. This effect manifests itself in samples having a system of shallow electron traps and is due to the relaxation of highly excited anharmonic atomic vibrations, caused by the ion impact, through the electronic channel. A mechanism is proposed and computed for the phenomenon revealed.     

On the Model Pseudopotential Approach to Calculate the Lattice Mechanical Properties of Thorium

The applicability of model pseudopotential approach to investigate lattice mechanical properties of Thorium, a typical f-shell metal, has been examined in light of some recent experimental and theoretical studies. It is found that presently available model potentials do not account for s-d-f hybridization adequately. The equation of state is greatly affected by s-d-f hybridization. It is also observed that a model potential giving reasonably good phonon dispersion curves may not reproduce equally good density of phonon states and hence the related lattice mechanical properties. However, it is possible to have a model potential, which may give fairly good estimate of lattice dynamical properties.     

Electronic structure and lattice properties of zinc-blende InN under high pressure

Dependencies of electronic structure and lattice properties of InN with zinc-blende structure on hydrostatic pressure are presented based on band structures computed using the empirical pseudopotential method. The pressure behavior of the pseudopotential form factors have been analyzed. The effect of pressure on the density of states has been examined. Trends in bonding and ionicity under pressure are also discussed. Our results show as well that the absolute value of the Fourier transform of the valence charge density might be useful in the prediction of the phase transition in zinc-blende materials. Received 25 May 2001 and Received in final form 16 January 2002     

Finite-temperature thermophysical properties of fcc-Ca

At an extreme environment, such as high-temperature and high-pressure, harmonic theory has obvious limitations, where the anharmonic effects are influential in determining bulk properties of the materials. In this regard, necessity for incorporating anharmonicity through vibrational contribution and thermally excited electrons to the total free energy at finite temperatures is illustrated taking an example of divalent fcc-Ca. In this regard, we have employed a coupling scheme of combining recently proposed mean-field potential (MFP) with the local pseudopotential to obtain vibrational contribution to the total free energy. To access the applicability of the present coupling scheme, we have calculated temperature variation of several thermodynamical properties. Static EOS, shock Hugoniot and temperature along principal Hugoniot are also estimated. Results are satisfactorily compared with the other theoretical and experimental data and the use of local pseudopotential in conjunction with the MFP approach is justified.     

Anharmonic oscillation of ions trapped in a rf trap with light buffer gas

The effect of a light buffer gas on the anharmonic oscillation of ions trapped in a rf trap is studied. The rf resonance absorption signals showed a change of the signal height and the hysteresis with the sweep direction of the dc voltage or the probing frequency due to the anharmonicity of the pseudopotential well of a rf trap. It was found that the signals changed drastically or even disappeared depending on the pressure of buffer gas, although almost the same number of ions were trapped. These effects indicate that the sensitivity of detection of the trapped ions can be improved by appropriately choosing the pressure of the buffer gas and the sweep direction. The trapped ions could be detected until 76 h 20 min and the storage time of 1.3×10⁵ s was determined when these parameters were optimized.     

High pressure phase transition and elastic behaviour of lanthanum monochalcogenides

The Phase transition and elastic properties of La-monochalcogenides have been investigated under pressure by means of a modified charge-transfer potential model which incorporates the Coulomb interaction modified by Coulomb screening due to the delocalization of electron of rare-earth atom leading to many-body interactions, covalency effect and overlap repulsion extended up to second-nearest neighbours. Under high pressure the coordination increases and they transform from rock-salt to CsCl structure. The calculated values of cohesive energy, lattice constant, phase transition pressure, relative volume collapse, harmonic and anharmonic elastic moduli and their first- and second-order pressure derivatives agree well with the available measured data and better than those computed by earlier workers. Present model is capable of explaining the Cauchy’s discrepancy correctly.     

Mg掺杂ZnO所致的禁带宽度增大现象研究

采用第一性原理的超软赝势方法，研究了纤锌矿ZnO及不同量Mg掺杂ZnO合金的电子结构.理论计算表明，Mg的掺杂导致ZnO晶体的禁带宽度增大.研究发现，Zn 4s态决定导带底的位置，Mg的掺入导致Zn 4s态向高能端的偏移是导致禁带宽度增大的根本原因. 关键词： 密度泛函理论 赝势 Mg掺杂ZnO     

Path integral computation of phonon anharmonicity

The partition function of an oscillator disturbed by a set of electron particle paths has been computed by a path integral formalism which permits to evaluate at any temperature the relevant cumulant terms in the series expansion. The low temperature cutoffs in the anharmonic cumulant series are determined fulfilling the constraint of the third law of thermodynamics. The general method here proposed has been applied to the semiclassical Su-Schrieffer-Heeger model whose time dependent source current is linear in the oscillator displacement field. We find that this peculiar current induces large electron-phonon anharmonicities on the phonon subsystem. As a signature of anharmonicity the phonon heat capacity shows a peak whose temperature location strongly varies with the strength of the e-ph coupling. Since the electron hopping potential provides a sizeable background in the low and intermediate temperature range, such a peak is partly smeared in the total heat capacity. Low energy oscillators are more sensitive to anharmonic perturbations.Received: 7 January 2004, Published online: 3 August 2004PACS: 71.20.Rv Polymers and organic compounds - 31.15.Kb Path-integral methods - 63.20.Kr Phonon-electron and phonon-phonon interactions     

Interaction of rare gas atoms with metal surfaces: A pseudopotential approach

The pseudopotential formulation of the helium-metal surface interaction due to Harris and Liebsch has been generalized to treat the heavier rare gases. The atom-surface repulsion is calculated to first-order in the pseudopotential and is combined with a damped van der Waals interaction to produce the total physisorption potential. Reasonable results are obtained for all the rare gases considered, with a systematically increasing potential-well depth through the rare gas sequence. A single surface-atom model is also considered and its limitations discussed.     

Y3Al5O12的热输运性质的第一性原理研究

基于密度泛函微扰理论（DFPT）结合模守恒赝势方法进行晶格动力学模拟.得到了钇铝石榴石（YAG）的声子态密度、分波声子态密度和声子的色散谱.利用第一Brillouin区的特殊点取样方法,计算了YAG的比热容和布局数平均的声子群速度.在非谐相互作用下,利用Fermi黄金公式结合第一Brillouin区的特殊点取样方法,得出了YAG非谐声子平均自由程.综合考虑了两种声子散射机制,得到了YAG陶瓷的热导率.结果表明,对于YAG陶瓷,在低温时,晶界散射将对热阻起主要作用;在高于一定温度时,三声子相互作用对热阻的贡献将占主导地位.同时也从理论上证明了Sato等提出的在室温以上,YAG陶瓷与单晶的热导率的差异可以忽略的观点.所得到的热导率、比热容随温度的变化与实验结果很好地符合. 关键词： 声子平均自由程 密度泛函微扰理论 3Al₅O₁₂声子结构')" href="#">Y₃Al₅O₁₂声子结构 热导率     

Improved local pseudopotential for gold

Phonon frequencies, binding energy and liquid metal resistivity of gold have been calculated by using a local model pseudopotential in conjunction with a dielectric function in the non-interacting band scheme. A good agreement is found between the computed and experimental results.     

Electron transfer in proton-hydrogen collisions in dense semi-classical hydrogen plasma

Quantum mechanical calculations have been accomplished to study the dynamics of the reaction: p + H(1s) → H(nlm) + p in dense semi-classical hydrogen plasma. Interactions among the charged particles in plasma are represented by a pseudopotential which takes care of the collective effects at large distances and quantum effect of diffraction at small distances. Various capture cross sections are computed for the incident proton energy lying within 10 to 500 keV by applying a distorted wave method which uses a variationally determined closed-form wave function of hydrogen atom. Moreover, an inclusive study is made to explore the effects of screening of plasma and quantum diffraction on various capture cross sections for a wide range of thermal Debye length and de Broglie wave length. It has been found that various cross sections suffer considerable changes due to varying Debye length and de Broglie wave length.     

Lattice dynamics of thorium

In the present work, a local model pseudopotential has been proposed to study the lattice dynamics of thorium. The model potential depends on the core and ionic radii, and accounts for the s-d-f hybridization effects in a phenomenological way. When this form of potential is applied to derive the phonon dispersion curves of Th, sufficiently good agreement is found between the computed and experimental results.