The impact of single-axis lattice expansion on the optical response of $\mathrm{BiFe}{\mathrm{O}}_3$ films is examined. Low-energy He implantation is used to tailor morphotropic phases of $\mathrm{BiFe}{\mathrm{O}}_3$ films and study changes in their optical spectra with continuously increasing lattice expansion. He ion implantation of epitaxial rhombohedral (R)- and tetragonal (T)-like $\mathrm{BiFe}{\mathrm{O}}_3$ films induces uniaxial out-of-plane strain that, on R-like films, eventually leads to a complete R-T phase transition. This approach allows us to provide insights into the optical response of $\mathrm{BiFe}{\mathrm{O}}_3$ films. Strain doping of T-like films leads to a significant redshift of the optical absorption spectra that is theoretically explained by a lowering of Fe $3d{t}_{2g}$ states. R-like films, on the other hand, show a less-pronounced sensitivity to uniaxial strain and a blueshift of about 250 meV at the strain-induced R-T transition. The results demonstrate that strain doping allows a deeper examination of the optical properties of epitaxial phases that are otherwise impossible to access by standard epitaxy.

• Received 22 May 2019
• Revised 27 August 2019

DOI:https://doi.org/10.1103/PhysRevMaterials.3.094410