LOCAL-STRUCTURE AND NATURE OF PHASE-TRANSITIONS IN KNBO3

The direction and magnitude of Nh displacements from the center of oxy gen octahedron in orthorhombic, tetragonal and cubic phases of KNbO3 w ere determined by means of polarized X-ray absorption fine structure ( polarized XAFS). The XAFS spectra for Nb K-edge were obtained from pla te-like single crystals in a transmission mode. It is found that in th e orthorhombic phase the direction of the Nh displacement is very clos e to the polar axis. However, the disorder of Nh positions grows gradu ally with increasing temperature. All phase transitions are governed b y both displacive and order-disorder mechanisms. The rhombohedral-to-o rthorhombic phase transition is considered as essentially a displacive type, while the order-disorder component is dominant in the tetragona l-to-cubic phase transition. (C) 1998 Elsevier Science Ltd. All rights reserved.