Ferroelectric Transitions in KH2PO4 and KD2PO4 Studied by Infra‐Red Polarized Reflection

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Abstract
The polarized infra‐red reflection spectra (250 to 4000 cm−1) of single crystals of KH2PO4 and KD2PO4 were measured at various temperatures above and below the ferroelectric Curie temperature. The results were compared with previous absorption measurements and allow better assignment of the infra‐red active modes. The existence of strong coupling between certain PO4 modes and low frequency hydrogen modes is deduced from the abrupt spectral changes on crossing the ferroelectric phase transition. This coupling results from the extremely anharmonic nature of the potential wells along the hydrogen bonds, as is also demonstrated by the KD2PO4 results. The build‐up of the line strength of the transverse hydrogen mode was related to the establishing of the spontaneous polarization below Tc. The two phase transitions observed in the KD2PO4 reflection spectra lead to the conclusion that KH2PO4 crystalline nucleation exists on the surface of KD2PO4 single crystals.