ABSTRACT
A large electromechanical response in ferroelectrics is highly desirable for developing high-performance sensors and actuators. Enhanced electromechanical coupling in ferroelectrics is usually obtained at morphotropic phase boundaries requiring stoichiometric control of complex compositions. Recently it was shown that giant piezoelectricity can be obtained in films with nanopillar structures. Here, we elucidate its origin in terms of atomic structure and demonstrate a different system with a greatly enhanced response. This is in non-stoichiometric potassium sodium niobate epitaxial thin films with a high density of self-assembled planar faults. A giant piezoelectric coefficient of â¼1900 picometer per volt is demonstrated at 1 kHz, which is almost double the highest ever reported effective piezoelectric response in any existing thin films. The large oxygen octahedral distortions and the coupling between the structural distortion and polarization orientation mediated by charge redistribution at the planar faults enable the giant electric-field-induced strain. Our findings demonstrate an important mechanism for realizing the unprecedentedly giant electromechanical coupling and can be extended to many other material functions by engineering lattice faults in non-stoichiometric compositions.
ABSTRACT
High-performance piezoelectric materials are critical components for electromechanical sensors and actuators. For more than 60 years, the main strategy for obtaining large piezoelectric response has been to construct multiphase boundaries, where nanoscale domains with local structural and polar heterogeneity are formed, by tuning complex chemical compositions. We used a different strategy to emulate such local heterogeneity by forming nanopillar regions in perovskite oxide thin films. We obtained a giant effective piezoelectric coefficient [Formula: see text] of ~1098 picometers per volt with a high Curie temperature of ~450°C. Our lead-free composition of sodium-deficient sodium niobate contains only three elements (Na, Nb, and O). The formation of local heterogeneity with nanopillars in the perovskite structure could be the basis for a general approach to designing and optimizing various functional materials.
ABSTRACT
We employ first-principles methods to study the mechanism controlling the electrical conduction in BiFeO3 (BFO). We find that under oxygen-rich conditions, Bi vacancies (V(Bi)) have lower defect formation energy than O vacancies (V(O)) (-0.43 eV vs. 3.35 eV), suggesting that V(Bi) are the acceptor defects and control the conductivity of BFO, making it a p-type semiconductor. In order to obtain further insight into the conduction mechanism, we calculate the effect of donor (Sn(4+)) and acceptor (Pb(2+)) impurities in BFO. Results indicate that Sn impurities prefer to substitute Fe sites to form shallow donor defects, which compensate the acceptor levels derived from V(Bi). Meanwhile, Pb atoms favour the substitution of Bi sites to form acceptor defects, reducing the overall concentration of holes (h(+)). Theoretical findings were later surveyed by current-voltage characteristics of Sn- or Pb-doped BFO nanofibers. This study is of general interest in carrier transport in charge compensation semiconductors, and of particular relevance within the context of defect-mediated conductivity in BFO.
