Local structural mechanism for phase transition and ferroelectric polarization in the mixed oxide K_0.5Na_0.5NbO₃

(K_xNa_1-x)NbO₃ (KNN) and its solid solutions are considered as potential alternatives to traditional Pb-based piezoelectric materials. Recent investigations indicated that the local structure in KNN is much more complex than its long-range average structure. Here, we have undertaken a systematic investigation of the temperature-dependent evolution of local and average structures of KNN5 [(K_xNa_1-x)NbO₃,x=0.5] using time of flight neutron scattering. Rietveld analysis of neutron diffraction patterns indicated that the average structure of KNN5 changes from monoclinic to tetragonal to cubic upon heating from 100 to 773 K. In contrast, analysis of neutron pair distribution function (PDF) indicated that the local structure stays monoclinic within the above temperature range. Based on comparative analysis of local and average structures, it is proposed that the average structural phase transitions in KNN5 are partly derived from a local ordering of the polar units, which are correlated over a length of 10∼15 Å. A clear indication of order-disorder type transition at 473 K is evident from the temperature-dependent PDF patterns. Additionally, based on local structural analysis, it is shown that large electrical polarization in KNN5 can be attributed to displacements of both A-site and B-site atoms. These structural insights may help in further optimization of electrical properties of Pb-free KNN piezoceramics.