Competing structural phase transition scenarios in the giant tetragonality ferroelectric BiFeO3-PbTiO3: Isostructural vs multiphase transition

A systematic x-ray and neutron powder diffraction study of the giant tetragonality multiferroic (1−x) BiFeO3-(x) PbTiO3 have revealed that the compositions close to the morphotropic phase boundary present two different structural phase transition scenarios on cooling from the cubic phase: (i) cubic → tetragonal (T2) + tetragonal (T1) → tetragonal (T1) and (ii) cubic →  tetragonal (T2) + tetragonal (T1) + rhombohedral (R3c) → tetragonal (T1) + rhombohedral (R3c). The comparatively larger tetragonality (c/a − 1) of the T1 phase as compared to the coexisting isostructural T2 phase is shown to be a result of significantly greater degree of overlap of the Pb/Bi-6s and Ti/Fe-3d with the O-2p orbitals as compared to that in the T2 phase. The formation/suppression of the minor metastable rhombohedral phase seems to be governed by subtle play of local kinetic factors. In the scenario when the minor rhombohedral (R) phase is formed along with the tetragonal phases it is able to accommodate the large transformation stress in the system due to formation of the tetragonal phases, and prevent the solid from disintegration into powder after sintering. When the metastable rhombohedral phase is not formed, the large transformation strain ruptures the grain boundaries leading to fragmentation of the dense solid to powder.