Lattice Instability in AgCl Probed with Thermal Diffuse Scattering

Silver chloride, AgCl, adopts the rocksalt crystal structure at ambient conditions of pressure and temperature. AgCl was observed to undergo at 6.6 GPa a structural phase transition to a monoclinic structure. Computational work attributed this transition to a lattice-dynamical instability of the transverse-acoustic (TA) phonon branch under pressure. In contrast to plentiful computational work, corresponding high-pressure experimental data are conspicuously lacking. We ask for 9 shifts of beamtime on ID15B to study AgCl single crystals with thermal diffuse scattering (TDS) as a function of pressure. The principal aim of this work is to determine the complete phonon dispersion relations of AgCl and their variation with pressure, with particular interest in the predicted instability of a TA phonon branch. These results will yield crucial experimental data and provide insight into the role that a phonon instability may play in driving the structural phase transition in AgCl near 7 GPa.