A LOW-TEMPERATURE EXTENDED X-RAY ABSORPTION STUDY OF THE LOCAL ORDER IN SIMPLE AND COMPLEX PEROVSKITES .1. POTASSIUM NIOBATE

EXAFS study of potassium niobate was performed at the niobium K edge between 300 and 4.5 K. It is shown that there is no significant modification of the local environment of the niobium atoms within their oxygen octahedra with temperature. In the orthorhombic phase as well in the rhombohedral phase, each niobium is shifted by about 0.216 angstrom from the central position in the octahedron, along a [111] axis. This confirms the so called 'eight-site model' used for ferroelectric perovskites. The Nb-Nb and Nb-K distances are also considered. The complementarity of x-ray or neutron diffraction techniques sensitive to the long-range order and EXAFS spectroscopy sensitive to the local order is discussed.