Mechanism of Instability of B1 Structure of Alkali Halides under Pressure

Abstract

Upon applying pressure on alkali halide model-crystals of B1 structure, the oscillation of TA[100] mode becomes unstable. Due to deformation of TA[100] mode, the repulsive force between the nearest neighbor ions is weakened, energy is lowered, and the instability sets in. For the discussion of onset of instability, the internal energy and Gibbs free energy lead to the same conclusion. For a finite deformation, the Gibbs free energy must be used.