Reinvestigating atomic ordering in KNaNbOand its impact on ferroelectric properties.

In the present work, we reinvestigate the atomic ordering of a Pb-free Morphotropic Phase Boundary (MPB) composition viz., K0.5Na0.5NbO3(KNN50) and its vicinity at various length scales using high-resolution X-ray diffraction and pair distribution function data. We have observed a monoclinic phase (Space Group: Pm) at long/short ranges differing from a very recent report by Saha et al. [2024 Journal of Physics: Condensed Matter] [1]. Moreover, the ferroelectric (polarisation) dominance of short-range ordering (SRO) over long-range ordering (LRO) has been observed and quantified for the very first time using the amplitude of the ferroelectric frozen phonon mode (Γ4-) (corresponding to the high symmetry cubic phase), thereby structure is linked with ferroelectric (or polarisation) property for a widely studied MPB system viz., KxNa(1-x)NbO3(KNNx for x = 0.40, 0.50, and 0.60). Two uniquely identified monoclinic phases has been observed for SRO (MSRO) and LRO (MLRO) for all the compositions. The amplitude of ferroelectric frozen phonon mode (Γ4-) corresponding to MSRO is significantly higher (≈ 150 % - 180 %) than MLRO. A peak is observed in the amplitude of Γ4-and intensity of prominent Raman peaks (ν1and ν5) for x = 0.50, which is held responsible for high physical properties viz., dielectric permittivity, piezoelectric coefficient, remnant polarisation, electromechanical coupling coefficient, and many more widely reported in literature for KNN50.