Chemical and strain driven morphotropic phase boundaries in bismuth ferrite
Date of Issue2015
School of Materials Science and Engineering
Morphotropic phase boundary (MPB), a highly sought-after concept in piezoelectric materials, is the key factor for large electromechanical response. Unfortunately, mainstream piezoelectric materials all contain lead, a toxic element that is incompatible to the ever-growing demands for global environmental and ecological conservation. In the new wave of searching for lead-free replacements, BiFeO3 (BFO) rises as a promising candidate due to the existence of both chemical- and strain-driven MPBs. In the chemical-driven MPB, the role of “chemical pressure” remains debated. To gain more insight into this issue, La-doped BFO films were systematically investigated in terms of structural, ferroelectric, dielectric and piezoelectric properties. It is found that under the weak chemical pressure induced by La, BFO still undergoes the ferroelectric – paraelectric phase transition with an intermediate metastable antipolar state that produces double hysteresis behavior. However, there is no enhancement in the piezoelectric measurement close to the MPB region, probably because the increase of the relative permittivity is small compared to those of BFO doped with rare-earth elements with smaller ionic radii. Furthermore, the chemical substitution is introduced into the highly-strained BFO films to combine the chemical- and strain-driven MPBs in this unique system, aiming to create multiple phase boundaries with ultrahigh piezoelectric response. Universal polarization rotation behavior is observed in rare-earth-element doped tetragonal-like BFO, analogous to those found in rhombohedral-like phase. However, a global polar instability is absent at high doping concentration before the tetragonal-like BFO lattice collapses due to epitaxial breakdown. The preliminary results of La-doped tetragonal-like BFO films show some degree of enhancement in the piezoelectric response, which requires further experimental validation.