HIGH-PRESSURE ISOSYMMETRIC PHASE-TRANSITION IN ORTHORHOMBIC LEAD FLUORIDE

Orthorhombic alpha-PbF2 was investigated at high pressure by angle-dis persive, x-ray powder diffraction. A phase transition between the cotu nnite-structured alpha phase (alpha-PbCl2-type, space group Pnam, Z = 4) and a second orthorhombic gamma phase (Co2Si-related, space group P nam, Z = 4) was observed beginning at close to 10 GPa. The experimenta l evidence is consistent with a first-order, isosymmetric transition w ith a volume change of 2%. There are only minor changes to the structu ral topology at the phase transition; however, the cation coordination number increases from nine to ten. This structure type represents a t hird possible high-pressure, postcotunnite structure in ionic AX(2) co mpounds, the choice of which is determined by the cation-anion radius ratio.