Raman study of the soft mode in ferroelectric PbDPO4

Abstract
The low-frequency Raman spectrum of PbDPO4 has been studied as a function of temperature between 10 and 484K (TC approximately=450K). A soft mode was observed whose frequency exhibits a weak temperature dependence (from 79.5 cm-1 at 10K to about 67 cm-1 at TC). Comparison with results from earlier measurements of the equivalent mode in PbHPO4 shows that it cannot be the tunnelling mode predicted by theory. The authors conclude that the soft mode does not drive the ferroelectric transition in either PbHPO4 or PbDPO4.