Soft Mode and Long-Range Polar Order in KTa sub 1--x Nb sub x O sub 3 for Low Niobium Concentrations

The nature of the structural phase transitions in KTa sub 1--x Nb sub x O sub 3 solid solutions for low niobium concentrations has recently been the object of many controversial discussions. In order to help clarify the situation Raman scattering results from room temperature to 8K are presented, performed on crystals with x = 0.008, 0.012 and 0.020. The data are interpreted in the framework of a dynamical model used for calculations of the densities of phonon states and of second-order Raman processes. Conclusions on the nature of the phase transitions in the KTN system are given. 20 ref.--AA