Lattice dynamics of the ferroelectric transition in PbDPO4: comparison with PbHPO4

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Abstract
The Raman spectrum of ferroelectric PbDPO4 has been measured at temperatures between 10 and 484 K (TC approximately 450 K). It is found that the crystal symmetries are the same above and below TC in PbDPO4 as well as in PbHPO4. The frequencies, linewidths and intensities of several hard modes show weak anomalies at temperatures near TC. A soft mode is observed whose frequency exhibits a much weaker temperature dependence compared with the equivalent mode in PbHPO4. Comparison with the PbHPO4 results shows that this mode is not the deuteron/proton-tunnelling mode. A low-frequency wing appears near 295 K and its intensity grows with increasing temperature as T to TC. The wing is not associated with a tunnelling mode and is probably due to some relaxational mode.