Understanding the strain mechanisms in BiFeO3-BaTiO3 piezoelectric ceramics near the morphotropic phase boundary

Origins of the unipolar strain, strain hysteresis, and remnant strain of Mn-modified BF-BT ceramics near the pseudo-cubic and rhombohedral (PC-R) phase boundary, before and after poling treatments, were investigated by high-energy synchrotron X-ray diffraction. The largest unipolar strain of BF-BT ceramics was found to occur at the composition with a single pseudo-cubic phase, rather than the phase boundary composition, owing to the synergetic contributions from the reversible phase transition of PC-R phase, non-180° domain switching, and lattice distortion. It is interesting to find that the phase boundary composition exhibits an irreversible PC-R phase transition, where its unipolar strain after poling is attributed to the lattice distortion. The lowest strain hysteresis and remnant strain were observed in BF-BT ceramics with rhombohedral phase, due to the fact that the reversible lattice distortion dominates the strain level. These new findings on BF-BT solid solutions are expected to provide new insights on the strain mechanisms of perovskite-structured ferroelectric materials.