Determination of the local structure of NbO6 octahedra in the orthorhombic phase of a KNbO3 crystal using EXAFS

A new method proposed by us to determine the displacement direction of B atoms from centrosymmetric positions in ABO₃ crystals is used to study the local atomic structure of KNbO₃ in the orthorhombic phase. It is shown that the conventional treatment of the EXAFS yields serious errors in determinations of the local distortions of NbO₆ octahedrons. To eliminate these errors, it is suggested that diffraction data on the average atomic displacements should be combined with the results of direct calculations of the Nb K-EXAFS of KNbO₃ in the orthorhombic phase. This approach was used to establish that the preferential direction of displacement of Nb in the orthorhombic phase of KNbO₃ is in the direction of the polar twofold axis. Fiz. Tverd. Tela (St. Petersburg) 40, 1097–1101 (June 1998)