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An equilibrium phase diagram satisfying the Gibbs phase rule is computed for the PbZrO3-PbTiO3 (PZT) system from a low-order Landau expansion in the approximation of the theory of regular solutions. We show that in the equilibrium phase diagram, miscibility gaps replace the morphotropic phase boundary and the paraelectric to ferroelectric transition lines of the “diffusionless” phase diagram. The decomposition occurs by a peritectoid reaction and the miscibility gaps expand significantly with increasing positive values of the atomic exchange interaction parameter. Diffusional processes are estimated to be sufficiently fast to achieve two-phase equilibria at normal rates of cooling used to process PZT materials. [ABSTRACT FROM AUTHOR]