Evidence of phase coexistence in hydrothermally synthesized K0.5Na0.5NbO3 nanofibers

The argument of phase coexistence in lead-free piezoelectric perovskite K0.5Na0.5NbO3 (KNN) has led to significant discrepancy in the inteperation of their exact crystalline structures in literature. In this work, the structural details of hydrothermally prepared KNN nanofibres and microcubes were investigated by a combination of high resolution transmission electron microscope (TEM), solid-state 23Na and 93Nb nuclear magnetic resonance (NMR) spectroscopy and two-dimensional 23Na 3QMAS NMR. Despite powder X-ray diffraction yields an average orthorhombic structure for both morphologies, high resolution TEM revealed mixed phases between orthorhombic (O) and monoclinic (M) symmetry with a growth direction of [001] and [110], respectively, in individual KNN nanofibres. Consistent with the TEM analysis, solid-state 23Na NMR indicates a distribution of multiple Na sites in the KNN nanofibers, which are not observed in the KNN microcubes. This study provides microscopic evidence of phase coexistence in KNN nanofibers and suggested two-phase concomitant state might be highly dependent on the size and morphologies of the KNN nanostructures.