H. C. Gupta

Noncentral forces in the study of lattice dynamics of metals

A comparative study of angular force models proposed by Clark et al (1964) and deLauney (1956) including the electron-ion interaction on the lines of Behari and Tripathi (1970) as applied to the cubic metals has been made. The forces are considered up to second neighbours only. Analytically it is found that the two models are equivalent for bcc structures but they give different results for fcc metals. The phonon dispersion curves for three transition metals (V,Nb,Ta) have been computed and the numerical results from both the models are found to be exactly coincident and also in good agreement with the experimental data.

An analytical expression for Eg and Ag modes of pyrochlores

A general analytical expression has been derived for Eg and A1g modes in terms of stretching and bending force constant for cubic pyrochlore structure compounds. The expression has been applied to different pyrochlore oxides accounting six stretching and four bending force constants. The calculation of Eg and A1g mode has been carried out by using the earlier reported values of force constants for titanates, hafnates and stannates. The analytical expression can be applied to any pyrochlore structure compounds for evaluating and assigning Eg and A1g mode if the interatomic force constants are already known.

A model pseudopotential for noble metals

Abstract 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.

A study of the interatomic interaction in oxide spinel MnCr2O4

Abstract The strengths of interatomic interaction in oxide spinel have been investigated by using a short-range force constant model. The interatomic ionic interaction between Cr-O in MnCr2O4 dominates the other interatomic interactions. The zone-centre phonon frequencies thus calculated are found to be in agreement with the observed results.

Study of zone-center phonons in lithium manganese oxide

Abstract The zone-center phonons of spinel structure lithium manganese oxide have been investigated by applying de Launey angular force constant model. The calculation is extended to lithium extracted phases Li 0.5 Mn 2 O 4 and Li 0.015 Mn 2 O 4 also. The lattice phonons thus obtained are compared with available experimental results. The lithiation effects on phonon modes are also discussed.

Zone centre phonons in high TcBa2YCu3O7

Abstract The zone centre phonons have been investigated in Ba2YCu3O7 using an angular force model. The theoretical values are in reasonable agreement with the available single crystal micro Raman and far-infrared transmission spectra measurements. It indicates that the observed transmission minima are due to phonon absorption.

Phonon dispersion relations of graphite and first stage alkali metal-graphite intercalation compounds

Abstract The phonon dispersion relations of graphite and first stage alkali metal-graphite intercalation compounds (KC 8 ) have been calculated using an angular force model approach. Initially, for graphite, the investigations were made by considering the interactions up to two neighbours in the plane and one neighbour between the planes. We present the first calculation of the phonon dispersion relations of graphite where the recent experimental measurement A 2u out of plane mode at 868 cm −1 is satisfactorily explained as output data, whereas earlier workers have presented the phonon spectrum considering this mode as input data. We have extended the analysis to the evaluation of the phonon spectrum and elastic constants of first stage potassium-graphite intercalation compounds. Good agreement has been observed between theory and the available experimental results. An inference is drawn that the angular forces play a vital role in explaining the acoustic modes of the phonon spectra.

Angular forces in the lattice dynamics of diamond crystal lattices

Abstract The lattice dynamics of the diamond lattice has been investigated using a model in which the interatomic forces include, in addition to central forces, the angular forces of the type employed by de Launay and Clark, Gazis and Wallis. The 2 models have been applied to silicon and germanium to investigate the phonon dispersion curves in the three symmetry directions of the Brillioun zone. The results are found to be qualitatively in good agreement with experiments.

Local mode behaviour in Rb1−xKx

Abstract A simple model has been developed to study the phonons in binary alloys showing local mode behaviour. A nonrandomness parameter λ, to represent the clustering effect is introduced in order to explain the phenomenon of local mode behaviour in binary alloys of the type A1−xBx When λ is reduced to zero, the model gives one mode behaviour. The lattice dynamics of Rb1−xKx alloys with x=0.06, 0.18 and 0.29 have been reported exhibiting satisfactorily the local-mode behaviour, when compared with the available experimental results.

Phonon dispersion in thorium: A pseudopotential approach

Abstract A one parameter model pseudopotential is used in investigating the phonon dispersion in thorium. A very good agreement has been obtained between theory and experiment, especially, in transverse branches. This is attributed to the removal of the discontinuity in the pseudopotential in r -space.