Direct observation of relaxation modes in KNbO3 and BaTiO3 using inelastic light scattering.

Cubic and tetragonal ${\mathrm{BaTiO}}_{3}$ and orthorhombic ${\mathrm{KNbO}}_{3}$ have been studied by Raman scattering, using an iodine filter to eliminate elastically scattered light. In addition to the usual phonon features, such as the soft E mode in tetragonal ${\mathrm{BaTiO}}_{3}$ and the soft ${B}_{2}$ mode in orthorhombic ${\mathrm{KNbO}}_{3}$, central components have been observed in both materials for the first time. These components have the linewidths, line shapes, and thermal properties of relaxation modes, and symmetry properties consistent with an eight-site order-disorder model of the successive phase transitions. Additionally, the data provide direct evidence that clusters of precursor order are present in cubic ${\mathrm{BaTiO}}_{3}$ and the model provides estimates of the temperatures at which these clusters are present in both ${\mathrm{BaTiO}}_{3}$ and ${\mathrm{KNbO}}_{3}$. These values are in good agreement with the results of linear birefringence and refractive-index measurements.