POLARIZED RAMAN STUDY OF THE FERROELASTIC TRANSITION IN LANTHANUM PENTAPHOSPHATE

Polarized Raman spectra in LaP5O14 have been studied in the low frequency range (0-85 cm-1) between 78 and 575 K across the 398 K ferroelastic transition temperature. By contrast to previous results, two softening modes with different symmetries are observed, only one of them having the same B2g symmetry as the transition's order parameter. An anomalous behaviour is pointed out for the linewidths of these modes in the ferroelastic phase. It is also shown that higher order coupling terms between the ferroelastic spontaneous shear and the B2g soft optic mode in addition to linear terms are necessary to explain the experimental results. © 1979.