Crucial role of octahedral untilting R3m/P4mm morphotropic phase boundary in highly piezoelectric perovskite oxide

Large piezoelectricity is usually found at morphotropic phase boundary (MPB), a composition-induced phase boundary between two ferroelectric phases. However, only a small fraction of MPBs (e.g., in PZT and PMN-PT) show large piezoelectricity. It remains a long-standing puzzle why not all MPBs yield large piezoelectricity. Here, by a comparative study of two MPB systems of BaZr0.2Ti0.8O3-PbTiO3 (BZT-PT) and Bi0.5Na0.5TiO3-PbTiO3 (BNT-PT), we find that the symmetry relation between the two ferroelectric phases at MPB plays a crucial role in piezoelectric activity. BZT-PT ceramic, having an R3m/P4mm MPB, exhibits strong piezoelectricity of d33 = 500 pC/N, whereas BNT-PT ceramic, having an R3c/P4mm MPB, exhibits much weaker piezoelectricity of d33 = 150 pC/N. The sharp difference between the two types of R/T MPBs is shown to arise from a significant difference in free energy barrier associated with oxygen octahedral tilting at MPB. An oxygen octahedral tilting/untilting transition at R3c/P4mm MPB leads to a high free energy barrier and results in low piezoelectric activity, whereas R3m/P4mm MPB with octahedral untilting produces large piezoelectricity due to low free energy barrier at such MPB. This feature show quite general in most of present MPBs systems and may become a dependable guideline for designing highly piezoelectric Pb-free materials.

Comparison of typical phase diagrams between (a) high-performance R3m/T MPB with octahedral untilting and (b) low-performance R3c/T MPB with octahedral tilting.171