Phase transitions and piezoelectricity enhancement around MC → O phase transition in PMN-0.34PT single crystals

Phase transitions of Pb(Mg_1/3Nb_2/3)O₃-0.34PbTiO₃ (PMN-0.34 PT) have been studied by using dielectric constant variations, polarization-electric field hysteresis loops, and polarizing light microscopy (PLM). Even without the assistance of electric field, orthorhombic (O) phase exists in the [001]_C-oriented PMN-0.34 PT single crystals during the heating process, which is confirmed by the polarizer/analyzer extinction angle from full-angle in situ PLM observation combined with the grayscale image analysis. The phase transition sequence is monoclinic (M_C) → O → M_C → tetragonal (T) → cubic (C), which is different from the commonly accepted M_C → T → C sequence. The existence of the O phase accompanies with much enhanced dielectric and piezoelectric performance, which indicates a new mechanism to produce large piezoelectricity in PMN-PT single crystals.