Local structure and nature of phase transitions in KNbO 3

Abstract

The direction and magnitude of Nb displacements from the center of oxygen octahedron in orthorhombic, tetragonal and cubic phases of KNbO 3 were determined by means of polarized X-ray absorption fine structure (polarized XAFS). The XAFS spectra for Nb K-edge were obtained from plate-like single crystals in a transmission mode. It is found that in the orthorhombic phase the direction of the Nb displacement is very close to the polar axis. However, the disorder of Nb positions grows gradually with increasing temperature. All phase transitions are governed by both displacive and order-disorder mechanisms. The rhombohedral-to-orthorhombic phase transition is considered as essentially a displacive type, while the order-disorder component is dominant in the tetragonal-to-cubic phase transition.