A NEW FRAMEWORK FOR UNDERSTANDING RELAXOR FERROELECTRICS

A new theoretical framework is presented for understanding the dielectric response of relaxor ferroelectrics. The theory is developed by reference to the dielectric response of lead magnesium niobate, a well-known relaxor material. The existence is proposed of a ferroelectric network linking niobium-oxygen coordination octahedra, where the temperature at which a given octahedron becomes decoupled from this network depends on its local coordination by other niobium-oxygen octahedra. This dependence permits the definition of a Curie point distribution function, which expresses the number of octahedra being decoupled from the network at a given temperature, as a function of temperature. The real part of the permittivity, episilon'-r, is simulated by convoluting the Curie point distribution function with a simple dielectric response function, phi-E(T), and an explanation is given for the frequency dependence of epsilon'-r. The imaginary part of the permittivity, epsilon'-r, is interpreted by regarding the decoupling of NbO6 octahedra from the ferroelectric network as a relaxation process. The role of ferroelectric microregions and microregion walls is discussed, and the contribution of the theoretical framework to the future development and exploitation of relaxor materials is assessed.