Lattice dynamics of the para-electric phase of PbHPO4

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Abstract
The low-frequency Raman spectrum of ferroelectric PbHPO4 has been measured at temperatures between 250 and 410 K (TC approximately=310 K). The measurements extend much further into the para-electric phase than in the authors earlier work. The frequencies, linewidths and intensities of several lattice vibrations exhibit anomalies at temperatures near TC. These anomalies are variously ascribed to lattice contraction effects and PO4 group restructuring. The A'(Bu)-symmetry soft mode observed in the ferroelectric phase is essentially independent of temperature in the para-electric phase, while the low-frequency wing feature that appears at T approximately=TC-100 K becomes very weak near TC and is independent of the temperature thereafter. The microscopic details of the phase transition are discussed in terms of these results. The Raman results indicate that the crystal structure remains acentric even at T approximately=TC+100 K and that the transition is driven by proton ordering.