Calculation of Phonon Dispersion Curves and Mode Grüneisen Parameters in RbI

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Abstract
The phonon dispersion curves of RbI are calculated using the breathing shell model, allowing for the force constants of the isotropic deformation to be unequal to the shell‐core force constants. The model is extended to include the proper third order anharmonic contributions. The mode Grüneisen parameters are calculated within this model using first order perturbation theory. The parameters entering the calculation of the mode Grüneisen parameters are the isothermal pressure derivatives of the harmonic breathing shell model parameters and are obtained from the pressure derivatives of the long wavelength measurements in a similar way as are the harmonic parameters. Using this model the specific heat and the linear thermal expansion coefficient as functions of temperature are calculated.