Was it necessary to introduce the notion "relaxor ferroelectrics"?: The problem of phase transitions in (Pb,Li1/2-La1/2)(Zr,Ti)O3, (Pb,La)(Zr,Ti)O3, Pb(Mg1/3Nb2/3)O3, Pb(In1/2Nb1/2)O3.: And related materials.: 1.: Model conceptions

A phenomenological description of the effects originating in the coexistence of FE and AFE phases is presented. The behaviour of the discussed systems is analyzed in the frame of the Landau phenomenological model on the basis of the thermodynamic potential taking into account the order parameters of the both mentioned phases. The interaction of domains of FE and AFE phases is taken into consideration. The system described by the said potential possess the following main properties: - fulfillment of the Fogel-Fulcher law; - an increase of the measuring AC field amplitude E-0 leads to an increase of epsilon'(T)-maximum and to a linear decrease of T-m; - their hysteresis loops have a specific form: narrow dielectric hysteresis loops with a small residual polarization and narrow quadratic loops of electrooptical hysreresis; - existence of polar phase domains at temperatures essentially higher than T-m; - crystal structure inhomogeneity which has similar to100 A(0) scale at temperatures lower than T-m; - dependence of their properties on the sample's prehistory; - a long-duration relaxation; - a high degree of paraelectric phase transition diffuseness.