Lattice Dynamics of Alkali Halide Crystals. II. Experimental Studies of KBr and NaI

Abstract

Dispersion curves for the lattice vibrations propagating in the [$00\ensuremath{\zeta}$], [$\ensuremath{\zeta}\ensuremath{\zeta}0$], and [$\ensuremath{\zeta}\ensuremath{\zeta}\ensuremath{\zeta}$] directions in NaI at 100\ifmmode^\circ\else\textdegree\fi{}K and in KBr at 90\ifmmode^\circ\else\textdegree\fi{}K have been measured using neutron spectrometry, and the results compared with calculations based on a simple shell model. Both substances obey the Lyddane-Sachs-Teller relation. In addition, some measurements were made on KBr at 400\ifmmode^\circ\else\textdegree\fi{}K; most frequencies showed a decrease of a few percent. At this temperature, the acoustical modes show no significant energy broadening, the transverse optical modes show slight broadening, and the longitudinal optical modes show very considerable broadening. The anomalous broadening of the LO modes is not yet understood and requires further study. It appears to be specimen-dependent as well as temperature-dependent.