Comment on the interpretation of Raman spectrum of KDP

ABSTRACT

Controversial views of the Raman spectrum related to the mechanism of the ferroelectric phase transition of KDP (KH₂PO₄) are discussed. In the model proposed by Tominaga etal. (Solid St. Commun. 48 (1983) 265), it was concluded that PO₄ tetrahedra are distorted to polar C₂ symmetry already above T_c. Here, it is shown that the C₂ symmetry is simply that of a local ${\mathrm{H}}_2{\text{PO}}_4^{-}$ ion. Therefore, the Raman spectra do not give any evidence of the "PO₄ orientational order-disorder model" in which protons are not involved at all in the phase transition. The assignment of the overdamped soft mode in this model is also discussed.

KEYWORDS:

• KDP
• Raman
• phase transition
• symmetry
• PO₄
• proton

Acknowledgment

The author thanks Kohji Abe for valuable discussions.