  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.
