Phonons in the mixed crystal K_0.5Rb_0.5I

Abstract

Abstract Phonons propagating in the symmetry directions Δ, ∑, and A in K 0.5 Rb 0.5 I are obtained by inelastic scattering of thermal neutrons at 300 K. The acoustic phonons are well defined and no anomalous broadening is observed, but the transverse zone centre phonon, the only observed optic phonon, has a width about twice the instrumental resolution. The phonon frequencies in the mixed crystal are generally below the averaged frequency values of KI and RbI. The softening of modes is more prominent near the zone boundaries. The results are analysed in terms of the shell model and the rigid ion model for ionic crystals, and compared with similar calculations for KI end RbI. The elastic constants obtained from the eleven parameter shell model in units of 10 11 dyn/cm 2 are C 11 = 3.04, C 12 = 0.12, and C 44 = 0.50.