Local structure and nature of phase transitions in KNbO3

The direction and magnitude of Nh displacements from the center of oxygen octahedron in orthorhombic, tetragonal and cubic phases of KNbO3 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 Nh displacement is very close to the polar axis. However, the disorder of Nh 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. (C) 1998 Elsevier Science Ltd. All rights reserved.