Far infrared reflectivity spectra of a KH2PO4 single crystal

Conclusion The experimental results presented in this paper show that two heavily damped modes, due to proton tunneling, are active in the two phases of KDP: the well-known soft mode which attains a hard-core frequency at the transition temperature and a low frequency mode polarized in a direction perpendicular to the c axis. These modes are responsible for the temperature variation of the low frequency dielectric permittivity in the two crystallographic directions. Due to the size of the samples we do not obtain a single domain crystal, and it was impossible to distinguish the B₁ and B₂ modes. The 200–300 cm^–1 spectral range is very sensitive to the orientation of the incident wave, and certainly corresponds to the hydrogen bonds vibrations.