Infrared spectroscopy of Kh2Po4-type ferroelectrics

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Abstract
The peculiar profiles of infrared reflectivity spectra of KDP-type crystals and the inadequacy of the classical Born-Huang dielectric function model to fit them already recognized for a long time, gave rise to many investigations for some 30 years. Most recent works have shown that the factorized form of the dielectric function is able to fit all spectra satisfactorily. The analysis of complete spectra has been performed and allows the assignment of all vibrational modes. Among recent findings, this review emphasizes the essential role played by the μ4c internal polar mode which is destabilized by the onset of intersite proton motions upon heating in the ferroelectric phase, and softens. It couples with the lower-frequency external main polar mode and the system evolves to get the Cochran's polarization mode pattern at Tc . The eigenvectors of the μ4c polar mode are such that they transmit to the direction of the FE axis the instability that appears in the plane perpendicular to the FE axis.