Phase diagrams and interaction between polar and antiferrodistortive modes in the PZT and NBT-BT systems

Thanks to the combination of dielectric and anelastic spectroscopies, it has been possible to probe the structural transitions involving the antiferrodistortive (AFD) tilt modes of the octahedra better than before in PZT [1] and NBT-BT [2]. In PZT close to the antiferroelectric phase, a new border TIT is identified with disordered tilting before the final long range ordered tilting of the R3c phase is established below TT. Such a line merges with TC, presumably giving rise to a combined ferroelectric-tilt transition. On the high-Ti side of our series of large grain ceramic samples, TT does not cross the morphotropic phase boundary (MPB), but merges with it. On the other hand, in NBT-BT it had already been established that the instability to the ferroelectric state Td coincides with the tilt transition between tetragonal a0a0c+ and rhombohedral a-a-a- (in Glazer notation) over an extended range of Ba compositions. Figure 1 Phase diagrams of PZT and NBT-BT where the combined polar-AFD transitions are vertically shaded. The tilt patterns are given in Glazer notation S/LRO = short/long range order. These lines of combined polar-tilt.transitions are vertically hatched in Fig. 1, and it is suggested that they are a consequence of cooperative interaction between the polar and the AFD modes, as proposed for the trigger-type transitions in the multiferroic BiFeO3 [3]. References [1] F. Cordero, F. Trequattrini, F. Craciun and C. Galassi, J. Phys.: Condens. Matter 23, 415901 (2011). [2] F. Cordero, F. Craciun, F. Trequattrini, E. Mercadelli and C. Galassi, Phys. Rev. B 81, 144124 (2010). [3] I.A. Kornev and L. Bellaiche, Phys. Rev. B 79, 100105 (2009)].