Relations between ionic order or disorder and classical or, relaxer ferroelectric behaviour in two lead-free TKWB-type ceramics

Two ceramics of Ba2NaNb5O15 and BaNa2Nb5O14F compositions, which have a tetragonal tungsten bronze (TKWB)-type structure, are synthesized and characterised by dielectric measurements. Their ferroelectric properties an correlated with the ionic order or disorder in the crystalline network. The oxide is a classical ferroelectric, whereas the oxyfluoride shows a relaxer ferroelectric behaviour in agreement with cationic disorder of Ba2+ and Na+ in the same 15 C.N. crystallographic site. The relaxer characteristics are compared to those of well-known perovskites. BaNa2Nb5O14F is the first lead-free oxyfluoride relaxer of TKWB type. (C) 2001 Elsevier Science B.V. All rights reserved.