Electrical anisotropy in single crystals of Bi-layer structured ferroelectrics

Single crystals of PbBi2Nb2O9 (PBN), PbBi4Ti4O15 (PBT) and Pb2Bi4Ti5O18 (P2BT), belonging to the bismuth layer-structured ferroelectric compound, were grown and dielectric, conducting and ferroelectic properties were evaluated separately in directions of a(b)-axis and c-axis. The relationship between crystal structure (the number of oxygen octahedra in the perovskite block m) and electrical properties was discussed comprehensively. The dielectric permittivity at Tc in the a(b)-axis direction was more than 20 times larger than for the c-axis direction. The anisotropy depended on m and was larger in the compound with even-number m than that with odd-number m. The DC conductivity increased with increasing m in both a(b)-axis and c-axis directions. The spontaneous polarization in the c-axis direction was not recognized for PBN and PBT with even-number m but observed for P2BT with odd-number m, although the value of remanent polarization was much smaller than for the a(b)-axis direction.