Pyroelectric properties and orthorhombic to rhombohedral electric field induced phase transition in relaxor-ferroelectric single crystals

The pyroelectric coefficients of 0.72Pb(Mg₁3/Nb₂3/)O₃-0.28PbTiO₃ (PMN-28PT), 0.67Pb(Mg₁3/Nb₂3/)O₃-0.33PbTiO₃ (PMN-33PT) and 0.92Pb(Zn₁3/Nb₂3/)O₃-0.08PbTiO₃ (PZN-8PT) single crystals, oriented and poled along the [001]_c, [101]_c and [111]_c directions (C: pseudocubic) have been measured using a dynamic method. In otherwise pseudoorthorhombic samples of PZN-8PT, a room temperature rhombohedral phase can be stabilized by poling along the [111]_c direction at sub zero temperatures. By strain measurement and in situ polarized light microscopy, the corresponding orthorhombic to rhombohedral electric-field induced phase transition has been observed in [111]c-oriented PZN-SPT upon application of a unipolar field. This last result reiterates the universality of the electric field induced phase transition in ferroelectric materials, especially in such crystals near the morphotropic phase boundary where phases are nearly degenerate.