BiFeO₃ is one of the most studied multiferroic materials. Both its magnetic and ferroelectric properties can be influenced by doping. A large body of work on the doped material has been presented in the past couple of years. In this paper we provide a perspective on general doping concepts and their impact on the material's functionality.

We have investigated structural phase transitions across a ferroelectric-to-antiferroelectric phase boundary in epitaxial (001) oriented ${\text{Bi}}_{\left(1-\text{x}\right)}{\text{Sm}}_{\text{x}}{\text{FeO}}_3$ thin films. For the ${\text{Sm}}^{3+}$ concentration of $0.1\le x\le 0.14$, we observe short-range antiparallel cation displacements, verified by the appearance of localized $\frac{1}{4}\left\{011\right\}$ weak reflections in the selected area electron diffraction patterns. At the critical composition of $x=0.14$, the system adopts a complex nanoscale domain mixture with appearance of $\frac{1}{4}\left\{011\right\}$, $\frac{1}{2}\left\{011\right\}$, $\frac{1}{2}\left\{010\right\}$, and $\frac{1}{2}\left\{111\right\}$ reflections and an incommensurate phase bridging the rhombohedral and orthorhombic phases. For compositions $0.14<x<0.2$, orientational twin domains coupled with antiphase oxygen octahedral tilts, identified by $\frac{1}{2}\left\{hkl\right\}$ weak superstructure are observed. The above systematic changes in the microstructure as a function of ${\text{Sm}}^{3+}$ doping are linked to the macroscopic functional properties.