# Controllable Defect Driven Symmetry Change and Domain Structure Evolution in BiFeO3 with Enhanced Tetragonality

## TL;DR

This study demonstrates how ion implantation-induced defect engineering can controllably induce a true super-tetragonal phase with high tetragonality in BiFeO3 thin films, enabling reversible phase transitions and potential polarization enhancement.

## Contribution

It introduces a method to precisely control defect concentrations to achieve and manipulate a super-tetragonal phase in BiFeO3, surpassing previous tetragonality limits.

## Key findings

- Achieved a c/a ratio of ~1.3 in BiFeO3.
- Demonstrated reversible phase transition with memory effect.
- Established defect engineering as a pathway to enhance ferroelectric properties.

## Abstract

Defect engineering has been a powerful tool to enable the creation of exotic phases and the discovery of intriguing phenomena in ferroelectric oxides. However, accurate control the concentration of defects remains a big challenge. In this work, ion implantation, that can provide controllable point defects, allows us the ability to produce a controlled defect-driven true super-tetragonal (T) phase with enhanced tetragonality in ferroelectric BiFeO3 thin films. This point defect engineering is found to drive the phase transition from the as-grown mixed rhombohedral-like (R) and tetragonal-like (MC) phase to true tetragonal (T) symmetry. By further increasing the injected dose of He ion, we demonstrate an enhanced tetragonality super-tetragonal (super-T) phase with the largest c/a ratio (~ 1.3) that has ever been experimentally achieved in BiFeO3. A combination of morphology change and domain evolution further confirm that the mixed R/MC phase structure transforms to the single-domain-state true tetragonal phase. Moreover, the re-emergence of R phase and in-plane stripe nanodomains after heat treatment reveal the memory effect and reversible phase transition. Our findings demonstrate the control of R-Mc-T-super T symmetry changes and the creation of true T phase BiFeO3 with enhanced tetragonality through controllable defect engineering. This work also provides a pathway to generate large tetragonality (or c/a ratio) that could be extended to other ferroelectric material systems (such as PbTiO3, BaTiO3 and HfO2) which may lead to strong polarization enhancement.

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Source: https://tomesphere.com/paper/1901.09817