Differential raman scattering from soft modes in mixed crystals of K_1-x, Na_xTaO₃ and K_1-xLi_xTaO₃

Comparison between the spectrum of the derivative with respect to wavelength of the intensity of Raman scattered light from K_1-x, Na_xTaO₃ and the spectrum from pure KTaO₃, shows that two of the lines have been induced by the introduction of the Na impurity. The energy of one line (A) is about 60 cm^-1 and changes only slightly with x. The energy of the second line (B) changes between 93 and 110 cm^™1 as x changes from 0.1 to 0.65. Similar measurements on K_1-xLi_xTaO₃ show a line similar to line A but with an energy of 39 cm^-1 at room temperature. Line A shows a very significant softening from 60 cm^™1 to < 20 cm^-1 as the temperature is reduced towards the ferroelectric transition temperature. The results are interpreted in terms of impurity modes around the Na or Li impurities which are displaced from the centre of inversion even at room temperature.