Structural Study of Layered PbFCl-Type Compounds

Abstract

The pressure dependence of the longitudinal acoustic mode corresponding to the elastic constant C₃₃ of the matlockite layered ionic compounds BaFX (X= Cl, Br or I) has been studied by Brillouin spectroscopy in a diamond anvil cell. All the measurements have been performed up to 20 GPa. The difference of the elastic response between the BaFX compounds is attributed to the distortion of the X electron charge density of the highly polarizable anions. This phenomenon is related to the large static dipole at the X anion and involves an energy contribution U_pol which seems to stabilize the layered structure.