Structural investigation of AgNbO3 phases using x-ray and neutron diffraction

Abstract

The structures and phase transitions ofAgNbO3 were investigated using neutron powder diffraction and restricted single-crystalx-ray diffraction. Both methods have revealed the high temperatureM3–O1,O2–T and T–C phase transitions but have not given any significant evidence of low temperatureM1–M2 and M2–M3 ones. The refinements of neutron diffraction patterns allowed us to determinethe symmetry, space group and crystal structure for all phases except theO1 one. The existence of structural disorder in the T and probablyO2 phases was found. The high temperature paraelectric phase transitions can beinterpreted on the basis of consecutive condensation of oxygen octahedron tiltsaround the main axis. The ferroelectric and antiferroelectric behaviour hasbeen associated with Ag and Nb cations. The reason why phase transitionsbetween low temperature ferroelectric and antiferroelectric phases are notdetectable by diffraction methods is discussed. The sequence of phase transitions inAgNbO3 can then be understood in the framework of a long range and/or local order–disorder typearrangement.