Monoclinic phases and stress-relief conditions in (1 − x)Pb(Mg1/3Nb2/3)TiO3-xPbTiO3 solid solutions

A comparative study on heterophase states in perovskite-type solid solutions of (1 − x)Pb(Mg1/3Nb2/3)TiO3-xPbTiO3 is carried out for compositions near the morphotropic phase boundary. The conditions for mechanical stress relief at elastic matching of phases are analysed at x = const in a wide temperature range. The heterophase states concerned with the presence of the intermediate monoclinic phase are interpreted using the domain state-interface diagrams calculated for x = 0.28, 0.32 and 0.34. It is shown that optimum volume fraction parameters of the domains in the monoclinic phase of the B type are varied in relatively wide ranges and promote complete stress relief with cubic-monoclinic phase coexistence. Two scenarios of stress relief at x = 0.32 are considered in connection with different heterophase states (either tetragonal-monoclinic of the B type or tetragonal-monoclinic of the C type) in a wide temperature range. Possibilities of elastic matching of two polydomain phases (tetragonal-monoclinic of the B type) with almost equal relative widths of the domains in these phases are shown for x = 0.34. The active role of domains of the monoclinic phases in stress relief and forming the planar unstrained interfaces is discussed.