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

The structures and phase transitions of AgNbO3 were investigated using neutron powder diffraction and restricted single-crystal x-ray diffraction. Both methods have revealed the high temperature M-3-O-1, O-2-T and T-C phase transitions but have not given any significant evidence of low temperature M-1-M-2 and M-2-M-3 ones. The refinements of neutron diffraction patterns allowed us to determine the symmetry, space group and crystal structure for all phases except the 01 one. The existence of structural disorder in the T and probably 02 phases was found. The high temperature paraelectric phase transitions can be interpreted on the basis of consecutive condensation of oxygen octahedron tilts around the main axis. The ferroelectric and antiferroelectric behaviour has been associated with Ag and Nb cations. The reason why phase transitions between low temperature ferroelectric and antiferroelectric phases are not detectable by diffraction methods is discussed. The sequence of phase transitions in AgNbO3 can then be understood in the framework of a long range and/or local order-disorder type arrangement.