K. V. Namjoshi

Line shape and temperature dependence of the first order Raman spectrum of diamond

Abstract It is shown that a modified Voigt profile best represents the first order Raman line shape of diamond. The temperature dependence of the peak position and the width of this spectrum has been measured in the temperature range of 25–1000 K. The shift of the peak position can be almost fully accounted for by thermal expansion alone. A small contribution due to anharmonic effects becomes evident for temperatures above 700 K. The half-width data agree well with a two-phonon decay process.

Mode‐Grüneisen Parameters and Thermal‐Expansion Coefficient of NaCl, CsCl, and Zinc‐Blende‐Type Crystals

The temperature dependence of the coefficient of thermal expansion of a number of ionic and partially ionic crystals is calculated using a modified rigid ion model and a shell model of lattice dynamics. The model parameters are obtained from long‐wavelength optic mode frequencies, elastic constants and the high‐and low‐frequency dielectric constants. Using the pressure derivatives of these quantities, mode‐Gruneisen parameters as functions of wave vectors are next obtained. Thermal expansion coefficient is calculated from the Gruneisen parameters in the quasiharmonic approximation. The calculated values compare well with experimental data.

Rigid ion model of lattice dynamics - A re-evaluation☆

Abstract A rigid ion model using an effective ionic charge gives reasonable agreement with phonon dispersion of crystals with NaCl, CsCl and zinc-blende structures. Model parameters are obtained from three elastic constants and two long-wavelength optic phonon frequencies. If the pressure dependence of these quantities is also available, the model may be readily used for the evaluation of mode Gruneisen parameters, Gruneisen constant and coefficient of thermal expansion.

Infrared absorption by alkali halides in the transparent regime and its temperature dependence

Abstract The problem of residual absorption in the transparent regime of ionic solids is discussed in terms of multiphonon density of states functions. Calculated values of the absorption coefficient for NaC l follow the expected exponential frequency dependence. Absolute values of the absorption coefficient and its temperature dependence also agree with experimental results.

Phonon Dispersion, Thermal Expansion and the Melting Curves of Inert Gas Solids

A volume dependent lattice dynamical model is presented for the inert gas solids. The model, which incorporates first and second neighbor interactions, utilizes the three elastic constants to predict the phonon dispersion. The volume dependences of the individual phonon frequencies are deduced with the use of a Lennard‐Jones potential. The coefficient of thermal expansion is then calculated from the associated mode‐Gruneisen parameters. Finally, the melting curves for solid argon, neon and xenon are calculated with the aid of Lindemanns criterion and the Grneisen equation of state. The calculated results indicate fair agreement with available experimental data.

Comparison of Various Lattice Dynamical Models for the Calculation of Thermal Expansion of Solids

A critical comparison of the Debye continuum, the modified rigid ion, the simple shell and the breathing shell models in the calculation of lattice dynamics of ionic solids is made. With the help of the Mie‐Grűneisen equation of state under the quasi‐harmonic approximation, the coefficient of thermal expansion can be calculated if the pressure dependence of lattice dynamics is available. All the above models may be adapted for this purpose. More elaborate lattice dynamical theories of thermal expansion involving anharmonicity are shown to be equivalent to this simple approach. The relative significance of different normal mode phonons in connection with anomalous thermal expansion, pressure induced phase transition and melting is pointed out.