Structural phase transitions of ionic layered PbFX (X = Cl⁻or Br^–) compounds under high pressure

The PbFX (X = Cl^–or Br^–) compounds crystallize in tetragonal structure with space group P4/nmm. High pressure X-ray diffraction studies carried out on PbFCl compound reveals that it undergoes pressure induced structural transitions at ∼18 GPa and ∼38 GPa to orthorhombic and monoclinic (P2₁/m) phases respectively. Like PbFCl, a similar phase transition from tetragonal to orthorhombic phase is observed in PbFBr at intermediate pressure. These phase transitions seem to be similar to the transitions involving other matlockite structure compounds such as BaFX (X = Cl^–, Br^–or I^–). PbFCl has a larger structural stability range compared to BaFCl and is attributed to the large anisotropic coordination of the Pb²⁺ and Cl^–ions