An X-ray study of coupling between acoustic and optic modes at the ferroelastic phase transition in As2O5

Abstract

The intensity of the 006 reflection of As2O5 has been measured as a function of temperature by single-crystal X-ray diffraction across the proper ferroelastic phase transition at Tc=578 K. The intensity is proportional to mod Tc-T mod in the low-temperature (P212121) phase corresponding to second-order Landau behaviour, and vanishes above Tc confirming the previously postulated high-temperature symmetry P41212. The structure factor of the 006 reflection has been expressed in terms of the amplitude of a cooperative atomic displacement pattern arising from coupling between the critical acoustic instability and optic phonons.