Raman scattering studies of ferroelectric transitions induced by lithium and niobium in KTaO3

Abstract

Abstract The low temperature phases produced by Nb and Li in K1-x Lix Ta1-y NbyO3 have been studied using Raman scattering and optical depolarization for x≲0.06 and y≲0.06. When x=0 and 0.008<y0.06, the low temperature phase has been shown to be rhombohedral and the phase transition is predominantly a soft mode transition. This behavior contrasts with the order-disorder transition to a tetragonal phase seen when y=O and 0.01≲x≲0.06. The unique feature of K1-x Lix TaO3 is that small ferroelectric domains lead to Raman spectra with no evidence of the anisotropy of the mixed mode phonon energy expected in a polar lattice. K1-xLixTaO3 cannot be poled after the temperature is reduced below Tc, but it is poled if the sample is cooled with an applied electric field along a cubic axis. In poled samples, the anisotropy of the energy of the mixed mode can be demonstrated directly in the Raman spectra by changing the experimental geometry so that the scattered phonons have different components of k along the polar a...