Interaction potentials for AgCl and AgBr

Abstract

We present interaction potentials for AgCl and AgBr derived from quantum-mechanical approximations and fitted to the experimental lattice constant and energy, elastic and dielectric constants, and special phonon frequencies. The expressions are simple enough for atomistic calculations of crystal defects and molecular dynamics while sufficiently representing the peculiar features of silver halides. The outstanding finding is the dominance of strong van der Waals interaction. Two-body van der Waals forces determine the observed small lattice constant, large lattice energy, and small elastic constant $c_{44}$, while three-body van der Waals forces contribute mostly to the violation of the Cauchy relation and the large bulk modulus. We could not find convincing indications of partial covalency in silver halides nor of an easy quadrupole deformability of the ${\mathrm{Ag}}^{+}$ ion.