Lattice Dynamics of Ionic Microcrystals

Abstract

The density of phonon states and infrared absorption of RbF microcrystals have been calculated using a rigid-ion model. The microcrystals were assumed to be rectangular solids containing up to 144 atoms. The density of states of the microcrystals, although shifted to lower frequency, was found to have the same shape as the density of states assuming cyclic boundary conditions. The fundamental infrared absorption could be characterized as three distinct bands due to edge modes, surface modes, and volume modes. These absorption bands split in crystals with noncubic shape, in surprisingly good agreement with a continuum theory. In addition to the fundamental absorption, the calculation displayed a weak low-frequency absorption which resembled the density of states.