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

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. Nous avons mesure le spectre de reflection infra-rouge polarise (250 a 4000 cm−1) dans des mono-cristaux de KH2PO4 et KD2PO4 a differentes temperatures au-dessus et au-dessous de la temperature Curie ferroelectrique. Nous avons compare les resultats avec lee mesures precedentes des spectres d'absorption ce qui permet une meilleure determination des modes actifs infra-rouges. Ou deduit l'existence d'un fort couplage entre certains modes du groupe PO4 et des modes d'hydrogene a basse frequence, des changements brusques dans les spectres en passant le changement de phase ferroelectrique. Ce couplage est le resultat du caractere tres anharmonique des puits potentiels au long des liens a hydrogene comme le montrent egalement les resultats de KD2PO4. Le renforcement du spectre du mode d'hydrogene transvertal est relie a l'etablissement de la polarization spontanee au dessous de Tc. Les deux changements de phase observes dans les spectres de reflection KD2PO4, menent a la conclusion qu'il existe une nucleation cristalline KH2PO4 sur la surface de mono-cristaux KD2PO4.