Ionic layered PbFCl-type compounds under high pressure

X-ray diffraction experiments under high pressure have been carried out on BaFCl, BaFBr, acid BaFI that belong to the family of layered ionic crystals with the matlockite PbFCl structure. Experiments up to 30 GPa with differe nt pressure transmitting media and different powder proportions were undert aken to determine the anisotropic stress component contribution. The bulk a nd linear moduli were determined for each compound and structural phase tra nsitions have been observed near 21 and 27 GPa for BaFCl and BaFBr, respect ively. The difference in bond strength due to the layered properties of the se compounds results in clear differences in compressibilities along the tw o principal directions, perpendicular and parallel to the C-4 axis of the t etragonal matlockite structure. The anisotropic behavior of the unit-cell p arameters observed under compression is attributed to the anisotropic coord ination of the highly polarizable halogen anion. This phenomenon created a large static dipole at the halogen and involves an energy contribution that seems to stabilize the layered structure.