Reversible transition between coherently strained BiFeO₃ and the metastable pseudotetragonal phase on (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7 (001)

Coherently strained BiFeO₃ epitaxial films deposited on (001)-oriented (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7 have a tetragonal crystal form, a stress-distorted version of the rhombohedral phase. A conversion from coherent BiFeO₃ to a new, tilted pseudotetragonal phase with the c/a ratio exceeding 1.2 is observed beyond the critical thickness of 60 nm. X-ray reciprocal space maps display that this highly elongated metastable structure is monoclinically distorted by ∼0.2° and exhibits an out-of-plane tilt of ∼3°. These observations are at odds with traditional understandings that a coherent epilayer should turn into its parent structure upon increasing the thickness, providing a new insight into the strain relaxation mechanism of epitaxial films. We show that in the heating and cooling cycles, the transition between these two phases is completely reversible and is associated with the alleviation of thermal stress. Our results reveal that the coherent BiFeO₃ epilayer with tetragonal symmetry stabilized by moderate compressive strain behaves as a structural bridge that links the thermally stable rhombohedral phase and the metastable tetragonal-like phase with a giant axial ratio. Moreover, the finding of a BiFeO₃ phase mixture in our study extends the threshold in-plane strain of the stress-driven morphotropic phase boundary to a value as low as −2.3%.