Manganese-doping of lead magnesium niobium titanate: Chemical control of dielectric properties

An investigation has been carried our into the acceptor-doping of lead magnesium niobate titanate (PMNT), 0.95 {Pb(Mg1/3Nb2/3)O-3}-0.05 PbTiO3, through the introduction of manganese ions. These are added by co-reaction of PMNT powder with MnCO3 and PbO in a single-stage reaction to sintered ceramics. The technique is found to enhance the sinterability of PMNT in less oxidising CO2 atmospheres. Furthermore, the dielectric response of the Mn-doped ceramics depends strongly on the sintering atmosphere employed. In air, a rapid transition from relaxor to normal ferroelectric properties takes place upon addition of manganese. However, in CO2, relaxer properties persist up to concentrations of approximately 8 mol% manganese. This difference in behaviour is attributed to varying oxidation states of the manganese ions, it being proposed that that air-sintered samples contain predominantly Mn3+-ions. In CO2-sintered samples, by comparison, the manganese ions exist in both +2 and +3 oxidation states. The relevance to the formation of dipoles, and to an understanding of relaxer ferroelectric ceramics is brought out, employing arguments based on crystal chemistry. Attention is also given to unresolved questions and to areas of future work. (C) 1998 Elsevier Science Limited. All rights reserved.