Mode Grüneisen parameter dispersion relation of RbI determined by neutron scattering

Inelastic-neutron-scattering measurements of 43 mode Grüneisen parameters for the [$00\xi$], [$\xi \xi 0$], and [$\xi \xi \xi$] directions of RbI are reported. These "mode $\gamma$'s" were determined to an accuracy of 2 to 10% of the Grüneisen-parameter range (about -2 to +3). Comparison is made and good agreement is found with the results obtained using a breathing shell model which was extended to include the third-order anharmonic interactions. The third-order parameters including those due to electronic polarization effects are determined.