Manifestation of a ferroelectric-antiferroelectric phase transition in Li0.12Na0.88Ta0.4Nb0.6O3 in its Raman scattering spectra

Abstract

The ferroelectric-antiferroelectric phase transition in the Li0.12Na0.88Ta0.4Nb0.6O3 ceramic solid solution has been studied by the Raman scattering technique. As the temperature approached the transition point from below, we observed an appreciable broadening of the lines associated with the vibrations of the cations occupying octahedral and cubooctahedral cavities of the structure and with the oxygen network vibrations (which implies a substantial increase in disorder on the cation sublattices), as well as a decrease to zero intensity of the 875-cm−1 line corresponding to stretch vibrations of the bridging oxygen in the BO6 octahedral anion in the vicinity of the transition. The temperature dependence of the 875-cm−1 line intensity near the transition was used to study the behavior of the phase transition order parameter η. The behavior of η was found to disagree markedly with the Landau theory of second-order phase transitions. It is shown that discrepancies originate from the increase in disorder in the niobium and tantalum sublattices in the Li0.12Na0.88TayNb1-yO3 solid solution system with increasing y. The order of the transition is lowered.