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

An investigation has been carried out 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 cobalt ions. These are added by co-reaction of PMNT powder with CoO 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 Co-doped ceramics depends on the sintering atmosphere employed. In air, a transition from relaxer to normal ferroelectric properties takes place upon addition of cobalt. However, in CO2, relaxer properties persist at all levels of cobalt-substitution. This difference in behaviour is attributed to varying oxidation states of the cobalt ions, it being proposed that that air-sintered samples consist of a mixture of Co2+ and Co3+ ions. In CO2-sintered samples, by comparison, the cobalt ions exist only in the +2 oxidation state. The relevance to the formation of dipoles, and to an understanding of relaxer ferroelectric ceramics is brought out, comparing the observed behaviour with that of PMNT doped with manganese. (C) 1998 Elsevier Science Limited. All rights reserved.