Dynamical mechanism of ferroelectric phase transition in KDP/DKDP mixed crystals and distortion of PO4 tetrahedron

The x(zz)yx(zz)y Raman spectra of KDP/DKDP ((KH2PO4)1−x(KD2PO4)x) mixed crystals show the forbidden ν3ν3 and ν4ν4 modes of the PO4 tetrahedron with S4 site symmetry at the paraelectric phase. This indicates that the local site symmetry of the PO4 tetrahedron must be C2 and not S4 which is the average symmetry of the crystals. The half width of the forbidden ν4ν4 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 PO4 tetrahedron, the distortion of the PO4 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 PO4 tetrahedron.