Study on ferroelectric phase transition of KH2PO4 by Raman scattering — objection to the proton tunneling model

The dynamics of ferroelectric phase transition of KH2PO4 (KDP) have been clarified by means of Raman scattering experiments. The locally and momentarily distorted PO4 tetrahedrons play the main role on the ferroelectric phase transition in KH2PO4. The broad mode below ∼150 cm-1 in x(yx)y spectrum above the transition temperature Tc and the corresponding sharp mode at about 150 cm-1 below Tc have been assigned to the libration modes of the PO4 tetrahedrons. From this result the very low frequency central component (≤20 cm-1) of x(yx)y Raman spectra at T≥Tc has been explained by the order-disorder dynamics of the distorted PO4 tetrahedrons. These results exclude the usual role of the proton dynamics on the KDP problem.