Lattice dynamics of the ferroelectric transition in lead hydrogen phosphate

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Abstract
The Raman spectrum of ferroelectric PbHPO4 (TC ≈ 310K) has been measured at temperatures between 10 and 320 K. The frequencies, linewidths and intensities of the hard modes show no drastic change at TC . From a self-energy model analysis, the low-frequency Raman spectrum provides evidence for two temperature-dependent components; a resonance-like soft mode and another mode at very low frequency. A direct comparison with structural data shows that the frequency and the intensity of the soft mode are directly related to the heavy-atom displacement. The low-frequency wing may correspond to the proton soft mode predicted by theory.