Extrapolation of elastic properties to high pressure in the alkali halides

A central force model incorporating an electrostatic term, a Born-Mayer nearest-neighbor repulsion, and a Lennard-Jones 6–12 potential between anions is used to derive equations for the elastic constants. The measured elastic constants and pressure derivatives are used to evaluate the model parameters. The force between anions is stronger than the force experimentally determined for the isoelectronic inert gases and depends on the cation as well as the anion. The shear elastic constant C44 is predicted up to high pressure and is used to predict a phase transition in KF between 70 and 95 kb and in NaF between 300 and 450 kb.