Defect induced fluctuations in the paraelectric phase of KTa_0.94Nb_0.06O₃

First order Raman lines observed in the paraelectric phase of KTa_0.94Nb_0.06O₃ are shown to be the result of odd symmetry microdistortions or fluctuations with a correlation length that extends to several unit cells. The possibilities that the lines result from impurity induced or disorder induced Raman scattering are considered and discarded. From simple theoretical considerations it is shown that the integrated intensity of distortion induced first order Raman lines, normalized by the corresponding phonon occupation numbers, is proportional to the average distortion intensity. This proportionality is independent of the correlation length of the distortion field provided it is larger than a certain minimum. Using this relation and the experimentally measured integrated intensity of the TO₂ line we find that the average distortion intensity is proportional to T/(T_c-T). This result is compared to two models which qualitatively predict the existence of microdistortions or fluctuations in the paraelectric phase. The experimentally observed temperature dependence is found to be consistent with the defect induced fluctuations model and inconsistent with continuous phase transitions models.