A structural study on the PbFCl-type compounds MFCl (M=Ba, Sr and Ca) and BaFBr under high pressure

High-pressure X-ray diffraction studies on BaFCl and BaFBr up to 50 GPa, on SrFCl up to 42 GPa, and on CaFCl up to 27 GPa were performed at room temperature using a diamond anvil cell and synchrotron radiation. Data for the equations of state are given. The c/a ratios of the tetragonal structure of these compounds show different variations under pressure and structural phase transitions are observed for BaFCl and BaFBr at transition pressures of about 21 GPa and 27 GPa, respectively. A simple hard-sphere model is presented, which describes the observed structural variations almost quantitatively and accounts for the structural phase transition.