Field-induced shift of morphotropic phase boundary and effect of overpoling in (1-x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ piezocrystals

The domain structure and phase symmetry of the (1-x) Pb (Mg13 Nb23) O3 -xPbTi O3 piezoelectric crystals with a composition close to the morphotropic phase boundary (MPB) between the rhombohedral R3m and monoclinic Pm phases (x0.3) have been studied by polarized light microscopy. It is found that poling by an electric field parallel to the 〈001〉 pseudocubic direction shifts the MPB toward the lower x side. This shift is the origin of the overpoling effect which degrades the performance of the materials, e.g., the piezoelectric coefficient (d33) and dielectric constant decrease with increasing poling field. In optimally poled crystals, d33 can reach as high as 3100 pCN.