Raman Spectrum of K${\mathrm{Ta}}_{0.64}$${\mathrm{Nb}}_{0.36}$${\mathrm{O}}_3$

Raman scattering from the mixed crystal K${\mathrm{Ta}}_{0.64}$ ${\mathrm{Nb}}_{0.36}$ ${\mathrm{O}}_3$ ($T_c\approx 10$°C) has been investigated as a function of temperature in the paraelectric and single-domain-ferroelectric phases. The phonon spectrum was found to correspond predominantly to that of the host lattice of KTa${\mathrm{O}}_3$. Significant effects due to the mixed nature of the crystal were found. In the ferroelectric phase, first-order lines were observed with symmetry properties forbidden for an ideal perovskite structure. Also, two Raman lines in the ferroelectric phase are attributed to local modes associated with the Nb ions in the lattice. In the paraelectric phase, lines observed at 202 and 279 ${\mathrm{cm}}^{-1\mathrm{}}$ persisted as first-order structures. The behavior of the 279-${\mathrm{cm}}^{-1\mathrm{}}$ line was found to be consistent with a reduction of the phonon symmetry from $F_{2u} \mathrm{to} F_2$, which could be caused by a relaxation of the inversion symmetry of the cubic lattice. The persistence of the 202-${\mathrm{cm}}^{-1\mathrm{}}$ line cannot be accounted for by the loss of inversion symmetry and may be due to the relaxation of translational symmetry. The shape of the 202-${\mathrm{cm}}^{-1\mathrm{}}$ peak shows a Fano-type interference. First-order structures occurred also at 175 and 436 ${\mathrm{cm}}^{-1\mathrm{}}$, and are attributed to zone-boundary phonons. First-order scattering was induced in the paraelectric phase by the application of an electric field to the sample. For comparable values of polarization, the intensity of the induced first-order scattering measured just above the transition temperature was found to be substantially less than the intensity of the spontaneous first-order scattering measured just below the transition temperature. Existing theory predicts that the spontaneous and induced scattering intensities should be nearly equal for these conditions.