Differential raman scattering from soft modes in mixed crystals of K1_xNaxTaO3and K1-xLixTaO3

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Abstract
Comparison between the spectrum of the derivative with respect to wavelength of the intensity of Raman scattered light from K1-x Na x TaO3 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 K1-x Li x TaO3 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.