P. R. Vyas

An improved lattice mechanical model for FCC transition metals

Abstract A lattice mechanical model for transition metals, recently proposed by Antonov et al. has been improved. This improved model has been tested against its ability to reproduce a large number of lattice mechanical properties like binding energy, P–V relations, elastic constants and their pressure derivatives, interatomic interactions, phonon dispersion curves, temperature variation of Debye temperature, Debye–Waller factor and temperature variation of mean square displacement with fairly good success.

High temperature and pressure thermodynamics of strontium: A macroscopic approach

Close proximity of d-bands (above) to the Fermi level (E F) makes the heavy alkaline earth metals (Ca, Sr and Ba) fairly sensitive to external influences like temperature and pressure. Softening of some of the phonon modes at high temperatures and/or pressures implies that anharmonic effects can play an important role in determining lattice dynamics and related properties. In the conventional approach, phonon density of states (p-dos) have to be calculated at each volume to compute free energy and thereby the other thermodynamic properties, which is computationally quite demanding. Using an alternative technique, the mean-field potential (MFP) approach was combined with the relatively soft local pseudopotential to obtain the free energy at different temperatures and pressures. The results for phonon frequency shifts at finite temperatures using the MFP approach and those calculated from p-dos within the quasiharmonic approximation are very similar. This validates the use of the MFP approach coupled with the local pseudopotential to estimate vibrational response of the system at high-temperature and high-pressure environments. The present scheme was used to study various thermophysical properties for elemental strontium at elevated temperatures and pressures, including the high-pressure melting curve and temperature along the shock Hugoniot. Computed results are affirmatively compared and analyzed with other reported data. The present scheme completely bypasses traditional cumbersome calculations, and it is computationally convenient yet accurate.

Volume variation of Gruneisen parameters offcc transition metals

The volume variation of the Gruneisen parameters of tenfcc transition metals, up to 40% compression, has been studied on the basis of a model approach proposed by Antonovet al. The results are reasonably good for six metals except for Rh, Ag, Au and Ni when compared with available experimental and other theoretical values. The model requires an appropriate modification for Rh, Ag, Au and Ni.

The temperature-dependent electrical transport properties of liquid Sn using pseudopotential theory

We present the calculations of electrical resistivity, thermo-electric power and thermal conductivity based on the self-consistent approximation. The pseudopotential due to Hasegawa et al. [J. Non-Cryst. Solids 117/118, 300 (1990)] for full electron–ion interaction, which is valid for all electrons and contains the repulsive delta function to achieve the necessary s-pseudisation, was used in the calculation. Temperature dependence of structure factor is achieved through temperature-dependent potential parameter in the pair-potential. The outcome of the present study is discussed in the light of other such results and with predictions of Wiedemann and Franz law up to moderately high temperature. Specially, high-temperature resistivity data necessitates the careful investigation of electron energy dispersion close to the Fermi level and possible metal to non-metal transition while going from dense-fluid to low density-fluid state. In the absence of experimental data at high temperature, these findings may serve as future guideline.

Temperature dependent atomic transport properties of liquid Rb

A simple analytical model for atomic motion is used to obtain velocity autocorrelation function (VACF) for liquid Rb. The modified empty-core potential due to Hasegawa et al., which represents the orthogonalisation effect due to s-core states in such sp-bonded metals, is used for electron–ion interaction. The potential parameter rc is determined at different temperatures from the knowledge of structure factor. We find quantitative explanation for the density and temperature dependence of VACF and self-diffusion coefficients. The coherent behaviour of liquid Rb in terms of the dynamic structure factor employing viscoelastic theory has also been studied. Intrinsic temperature effects have been studied through damping term in the pair potential. The predicted results for VACF, cosine power spectrum, mean square displacement, diffusion and viscosity coefficients have been compared with recent available molecular dynamics (MD) data and a good agreement has been achieved.

The temperature dependent collective dynamics of liquid sodium

Liquid alkali metals show, near the melting point, an upward bending of the dispersion relation at small momentum transfer values. This so-called positive dispersion can be described within generalized hydrodynamics as a visco-elastic reaction of the liquid. There is a speculation that long-living clusters could be the physical reason behind this phenomenon. To shed light on this question a treatment of pseudopotential theory on liquid sodium was performed at different temperatures starting at the melting point. In the present study, we used the modified empty core potential due to Hasegawa et al. (J. Non-Cryst. Solids, 117/118 (1990) 300) along with a local field correction due to Ichimaru-Utsumi (IU) to explain electron-ion interaction. The potential used is composed of a full electron-ion interaction and a repulsive delta function, which represents the orthogonalisation effect due to the s core states. The temperature dependence of pair potential is calculated by using the damping term exp(-πkBTr/2kF)....

THERMODYNAMIC PROPERTIES OF 4f- AND 5f-SHELL METALS AT FINITE TEMPERATURES: MEAN-FIELD POTENTIAL APPROACH IN CONJUNCTION WITH LOCAL PSEUDOPOTENTIAL

The thermodynamic properties of 4f- and 5f-shell metals have been studied at high temperatures using mean-field potential approach. The MFP seen by the lattice ion is constructed in terms of the total energy-volume relation using local pseudopotentials due to Pandya et al. [Physica B 307, 138 (2001)]. We have calculated static compression, shock-wave compression, volume thermal expansion, isothermal and adiabatic bulk moduli (BT and BS), specific heats (CV and CP), thermodynamic Gruneisen parameter (γth), anharmonic contribution to the specific heat and temperature along shock Hugoniot for 4f (γ-Ce)- and 5f (fcc-Th)-shell metals. The results are well compared with the other theoretical and experimental findings, which ensure the use of pseudopotentials for studying thermodynamic properties at higher temperatures in case of lanthanides and actinides.

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.

The study of anharmonic properties and s-p-d hybridization in barium at extreme environment

ABSTRACT Theory of pseudopotential has been used in the present study to carry out computation of various thermodynamic parameters of barium. The role of anharmonic effect due to vibrations of lattice ions has been accounted by coupling local pseudopotential with mean field potential which has been computed using second-order perturbation theory. Contribution due to thermally excited electrons has been accounted by Mermin functional. The excellent agreement of presently computed pressure with experimental result has also been observed at which body centered cubic to hexagonal close packed structure phase transition occurs. Such success leads to conclude that the s-p-d hybridization and anharmonic effects are included properly in the presently used conjunction scheme with additional advantage of its computational simplicity.

Mean-field potential approach for thermodynamic properties of lanthanide: Europium as a prototype

In the present paper, a simple conjunction scheme [mean-field potential (MFP) + local pseudopotential] is used to study the thermodynamic properties of divalent lanthanide europium (Eu) at extreme environment. Present study has been carried out due to the fact that divalent nature of Eu arises because of stable half-filled 4f-shell at ambient condition, which has great influence on the thermodynamic properties at extreme environment. Due to such electronic structure, it is different from remaining lanthanides having incomplete 4f-shell. The presently computed results of thermodynamic properties of Eu are in good agreement with the experimental results. Looking to such success, it seems that the concept of MFP approach is successful to account contribution due to nuclear motion to the total Helmholtz free energy at finite temperatures and pressure-induced inter-band transfer of electrons for condensed state of matter. The local pseudopotential is used to evaluate cold energy and hence MFP accounts the s–p–d–f hybridization properly. Looking to the reliability and transferability along with its computational and conceptual simplicity, we would like to extend the present scheme for the study of thermodynamic properties of remaining lanthanides and actinides at extreme environment.