Differential Raman scattering in mixed crystals of K_1-x Na_x TaO₃

Wavelength derivative Raman spectra of five mixed crystals of K_1-xNa_xTaO₃ have been measured at room temperature (RT) and liquid Nitrogen temperature (LNT). The results show three types of structures: a) two structures resulting from first order Raman scattering made allowed by the displacement of Na from the center of inversion site. One of these structures strongly softens as the crystal temperature approaches the ferroelectric phase transition. b) two structures that correspond to first order Raman scattering by the LO₃ and TO₄ zone center modes. These are observed only in K_0.35Na_0.65TaO₃ at LNT. c) structures that correspond to second order Raman scattering. Except for four lines the main effect of Na concentration is to shift the energy of the lines. Comparison of the Na concentration and pressure dependences of the energies of these lines shows that the average change in the volume of the unit cell resulting from the replacement of K by Na has a dominant effect on the energy shift of the lines. Four of the lines undergo a large change in shape and intensity. At present, no definite explanation of this result can be made.