Possible origin of stabilized monoclinic structure of KNbO₃ nanomaterials at room temperature

Highlights: • Providing the hydrothermal conditions for monoclinic and orthorhombic KNbO₃. • Observation of monoclinic structure at low temperature. • Suppressed hydroxyl defects at the monoclinic–orthorhombic structure transition. - Graphical Abstract: - Abstract: We have investigated the structural phase transition and phase stability of newly discovered monoclinic crystal structure of KNbO₃ nanomaterials. Through x-ray diffraction, Raman scattering, and neutron diffraction measurements, we have found that the KNbO₃ grown at low temperature undergoes a successive structural transition from monoclinic, tetragonal, to cubic, while the KNbO₃ grown at high temperature does from monoclinic, orthorhombic, tetragonal, to cubic with the increase of temperature. After heat treatment, the monoclinic structure of KNbO₃ is transformed to an orthorhombic structure at room temperature. Accompanying the structural transformation, the Raman peaks corresponding to surface and lattice hydroxyl groups are strongly suppressed. These results imply that the hydroxyl defects should play a crucial role for the monoclinic structure of KNbO₃ at room temperature.