STRUCTURE DISORDER AND RAMAN SCATTERING IN BaTiO₃ AND KNbO₃

Abstract
We have shown [1] that the Raman scattering spectra observed in the cubic paraelectric phase of BaTiO₃ by different investigators [2], [3] cannot be due to either impurity related effects or to ordinary second order process even if one of the two phonons participating in such a process were soft. The KNbO₃ spectra are very similar. In particular, they also show prominent broad peaks at 260 and 560 cm^-1 analogous to those found at about 270 and 530 cm^-1 in BaTiO₃. In both cases the spectrum does not disappear at the ferroelectric phase transition and its intensity begins only to decrease at quite higher temperatures (200 °C for BaTiO₃ and 600 °C for KNbO₃). Such an effect can be related to the structure disorder which has been proposed on the basis of X Ray diffuse scattering experiments [4]. This disorder can be described in terms of off-center atomic displacements : the structure has no local inversion symmetry center and thus Raman scattering can be observed even in the cubic phase. The Raman scattering can be interpreted as a second order process involving a phonon and a pseudo-spin excitation. The decrease with temperature of the Raman cross section must be correlated with the evolution of the disordered structure.