Coexistence of Ferroelectric Triclinic Phases and Origin of Large Piezoelectric Responses in Highly Strained BiFeO3 films

TL;DR

This study investigates the structural phases in strained BiFeO3 films, revealing that phase transitions between two triclinic phases near the MPB are likely responsible for their large piezoelectric responses.

Contribution

It uncovers the coexistence of two triclinic phases in BiFeO3 films and links their phase transition ease to enhanced piezoelectricity, supported by experimental and first-principles analysis.

Findings

Mixed-phase regions consist of two heavily tilted triclinic phases.

Phase separation from a monoclinic state explains the triclinic phases.

Large piezoelectric response is due to easy phase transition between these phases.

Abstract

The structural evolution of the strain-driven morphotropic phase boundary (MPB) in BiFeO3 films has been investigated using synchrotron x-ray diffractometry in conjunction with scanning probe microscopy. Our results demonstrate the existence of mixed-phase regions that are mainly made up of two heavily tilted ferroelectric triclinic phases. Analysis of first-principles computations suggests that these two triclinic phases originate from a phase separation of a single monoclinic state accompanied by elastic matching between the phase-separated states. These first-principle calculations further reveal that the intrinsic piezoelectric response of these two low-symmetry triclinic phases is not significantly large, which thus implies that the ease of phase transition between these two energetically close triclinic phases is likely responsible for the large piezoelectric response found in the BiFeO3 films near its MPB. These findings not only enrich the understandings of the lattice and domain structure of epitaxial BiFeO3 films but may also shed some light on the origin of enhanced piezoelectric response near MPB.