Phase transitions in Pb₃(P_1-xAs_xO₄)₂: influence of the central peak and flip mode on the Raman scattering of hard modes

Abstract

The integrated intensities and full width at half maximum (FWHM) of the 80 cm-1 Raman bands (hard modes) of ferroelastic Pb3(PO4)2 and Pb3(P0.77As0.23O4)2 were investigated in both the ferro and 'intermediate' phases for different polarisations as functions of temperature. The observed behaviour of these quantities has been interpreted in terms of interactions of hard modes with the central peak, flip mode and soft mode. The consequences of the three-component order parameter interpretation of the phase transition are also discussed. The integrated intensity curves in the 'intermediate' phase revealed a 'precursor order parameter' type of dependence. The numerical value of this 'precursor order parameter' at Tc is also estimated for Pb3(PO4)2. From the FWHM versus temperature curves in the 'intermediate' phase, the flip mode frequency is estimated to be 3*1010 Hz.