Phonon dynamics of orientationally disordered mixed crystals (RbCN)x(RbBr)1−x

An extended three-body force shell model (ETSM) has been applied to investigate the phonon dynamics of (RbCN)x(RbBr)1−x mixed crystals. The investigation is carried out for the composition range 0⩽x⩽1 for temperature varying between 0 and 380 K. The phonon dispersion curves computed by us are in good agreement with the neutron scattering data at 300 K. The unusual feature of these curves is the upward curvature seen in some of the acoustic branches. This is a result of k-dependent softening of phonon due to translation–rotation coupling. The transverse acoustic branch is more soft near the zone center.