Polarization mechanisms of morphotropic phase boundary lead barium niobate (PBN) compositions

Investigations of the polarization mechanisms in tungsten bronze relaxor ferroelectrics, Pb_1-xBa_xNb₂O₆ (PBN[(1-x) %]), in particular, of the morphotropic phase boundary compositions have been carried out for both ceramics and bulk crystals by means of electrical, thermal, optical, and electron-microscopic methods. Relations of phase-transition behavior with compositional and polarizational fluctuations have been carefully studied. Instead of a sharp boundary composition, it is found that a rather wide composition range, 1-x≊0.63±0.03 in Pb _1-xBa_xNb₂O₆, corresponds to the orthorhombic and tetragonal morphotropic phase boundary. It is demonstrated with the help of optical conoscopic interference figures that for the near-morphotropic phase boundary single crystal PBN of tetragonal symmetry, the polarization vector can be switched orthogonally to an orthorhombic phase simply by the application of an electric field, which authenticates optically the 90°polarization rotation. A transmission electron microscope study in the temperature range from -180 °C to room temperature revealed the presence of incommensurate ferroelastic domains in the morphotropic phase boundary composition PBN crystals. In light of the above observed experimental data, the polarization mechanisms of the morphotropic phase-boundary compositions are discussed in terms of qualitative thermodynamics.