Electron Paramagnetic Resonance and X-ray Studies of the Phase Transformation in Pb3P2O8

Abstract

The compound Pb3P2O8 has a low temperature monoclinic phase with space group C2/c and a high temperature phase with space group [Formula: see text], both with two crystallographically inequivalent cationic sites. X-ray data taken at five temperatures have indicated that an F-centered monoclinic cell is developed prior to its transition to the rhombohedral phase. The EPR spectra at room temperature show that Mn2+ substitutes only at the cationic site on the twofold axis. A phase transition occurs at 185 °C and approaching this temperature from below the magnetic axes in the ac plane rotate continuously, by about 40°. At 185 °C the Z axis coincides with the c axis of the hexagonal cell (a* axis of the monoclinic cell). The crystal field splitting parameter b20 increases from −423 G to −222 G while b22 goes from 280 G to zero. A model based on the theory of Blume and Orbach for the zero-field splitting and with point charges as the source of the electric field is used to relate the EPR parameters to the atomic displacements.