Internal modes and the local symmetry of PO4 tetrahedra in K(H1−xDx)2PO4 by Raman scattering

The v 3 and v 4 internal modes of PO 3− 4 tetrahedra are observable in KH 2 PO 4 even above the transition temperature in the scattering geometry x ( zz ) y contrary to the selection rule of the site symmetry S 4 in the D 2 d space group. Tominaga et al. discussed that the origin of this broken selection rule is due to the momentary site symmetry C 2 of these tetrahedra, and that the mechanism of the phase transition is of the order-disorder type of these distorted tetrahedra. Parallel studies compatible with their results have been carried out on mixed crystals K(H 1− x D x ) 2 PO 4 and the same conclusion has been obtained as to the momentary site symmetry. These internal modes increase in intensity with deuteration, and the origin of this increase is discussed.