Temperature dependence of the optical phonon spectra of deuterated dihydrogen phosphate between 7 and 300 K, studied by infrared reflectivity spectroscopy

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Abstract
The infrared reflectivity spectra polarised along and perpendicular to the ferroelectric c axis of a 90% deuterated potassium dihydrogen phosphate are reported from millimetre waves up to 600 cm-1 and in the temperature range 7-300 K for both the ferroelectric and the paraelectric phases. The phonon responses are obtained by fitting the factorised form of the dielectric function to experimental data. The analysis shows the activity of a low-frequency overdamped soft mode and the softening of an internal v4 mode, for polarisations both parallel and perpendicular to the c axis, and suggests a displacive mechanism for the ferroelectric-to-paraelectric phase transition.