Random lattice strain and its relaxation towards the morphotropic phase boundary of Na0.5Bi0.5TiO3-based piezoelectrics: Impact on the structural and ferroelectric properties

We demonstrate that the lead-free piezoelectric compound Na0.5Bi0.5TiO3 (NBT) exhibits random lattice strain in the ferroelectric phase, and that this feature primarily dictates the way the system evolves towards the morphotropic phase boundary in the unpoled state of NBT-based piezoelectrics. Investigations on two different morphotropic phase boundary (MPB) systems, namely Na0.5Bi0.5TiO3 - K0.5Bi0.5TiO3 (NBT-KBT) and Na0.5Bi0.5TiO3 - BaTiO3 (NBT-BT), revealed that the coupled structural-polar evolution towards the MPB is primarily driven by the necessity to minimize this strain. Our study suggests that the random lattice strain originates in the random stacking of the in-phase tilt and antiphase octahedral tilted regions, and that the system is able to minimize it by adopting a sequential stacking of the two tilt types, leading to a long-period modulation in the octahedral tilt configuration over large parts of the sample volume. This hinders the development of long-range ferroelectric order as the MPB is approached. We also demonstrate that the composition showing the maximum piezoelectric coefficient corresponds to a structural state wherein considerable polar-structural disorder coexists with the field-stabilized long-range rhombohedral ferroelectric order after poling, and not coexistence of two ferroelectric phases (tetragonal and rhombohedral), generally believed.