Dynamics of the ferroelectric transition in PbHPO4and PbDPO4

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Abstract
The results of a comprehensive Raman scattering study of the dynamics of the ferroelectric transition in PbHPO4 and PbDPO4 are reviewed. The lattice vibrations and PO4 internal modes are quite similar in the two crystals. Several modes that lose intensity with increasing temperature remain visible, albeit weakly, above Tc . This fact indicates that the crystal structure remains acentric above Tc . The low-frequency dynamics of the two crystals are not identical; a soft mode and a strange temperature dependent wing feature are observed. The Raman results show that the wing feature is not associated with a tunneling mode and is probably due to some relaxational mode. It is also concluded that the transition is driven by proton/deuteron ordering. These findings are compared with recent results from other techniques, which have provided contradictory information.