Dynamical mechanism of ferroelectric phase transition in KDP/DKDP mixed crystals and distortion of PO 4 tetrahedron

Abstract

The x ( z z ) y Raman spectra of KDP/DKDP ((KH 2PO 4) 1− x (KD 2PO 4) x ) mixed crystals show the forbidden ν 3 and ν 4 modes of the PO 4 tetrahedron with S 4 site symmetry at the paraelectric phase. This indicates that the local site symmetry of the PO 4 tetrahedron must be C 2 and not S 4 which is the average symmetry of the crystals. The half width of the forbidden ν 4 mode becomes narrow with increasing deuterium concentration. Assuming that the reciprocal half width of the forbidden mode is the degree of local distortion of the PO 4 tetrahedron, the distortion of the PO 4 tetrahedron gradually increases with increasing D-concentration and is strongly correlated with the ferroelectric phase transition temperature of the KDP/DKDP mixed crystals. This means that the mechanism of the ferroelectric phase transition of these KDP/DKDP mixed crystals agrees with the order–disorder mechanism of the locally distorted PO 4 tetrahedron.