Abstract
The temperature dependence of infrared reflection spectra of betaine phosphate single crystal is reported for polarizations parallel to the a- and b-axes. The spectra were fitted with the factorized form of the dielectric function. A mode assignment is proposed on the basis of a comparison with the spectra of betaine phosphite and deuterated betaine phosphate. Some very asymmetric phonon lines are observed, and the importance of mode couplings is emphasized. A picture of a tendency towards transverse optical mode softening of the low-frequency spectrum upon approaching either side of the phase transition temperature emerges for the polarization parallel to the b-axis, consistent with the peak observed in the temperature dependence of the dielectric constant. On the other hand, the temperature dependence of the ionic effective charges does not display any appreciable discontinuity at the phase transition, contrary to what is observed at the temperature of the transition to the ferroelectric phase of the parent compound betaine phosphite. This is consistent with the very low level of the spontaneous polarization below in betaine phosphate. The similarities of the behaviours with those of the compounds of the KDP family are discussed.
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