Heteroepitaxy of Tetragonal BiFeO₃ on Hexagonal Sapphire(0001)

Highly elongated BiFeO₃ is epitaxially grown on hexagonal sapphire(0001) substrate within a rather narrow synthesis window. Both X-ray reciprocal space maps and Raman characterizations reveal that it is of true tetragonal symmetry but not the commonly observed <i>M</i>_<i>C</i> type monoclinic structure. The tetragonal BiFeO₃ film exhibits an island growth mode, with the island edges oriented parallel to the ⟨10–10⟩ and ⟨12–30⟩ directions of the sapphire substrate. With increasing deposition time, a transition from square island to elongated island and then to a continuous film is observed. The metastable tetragonal phase can remain on the substrate without relaxation to the thermally stable rhombohedral phase up to a critical thickness of 450 nm, providing an exciting opportunity for practicable lead-free ferroelectrics. These results facilitate a better understanding of the phase stability of BiFeO₃ polymorphs and enrich the knowledge about the heteroepitaxial growth mechanism of functional oxides on symmetry-mismatched substrates