Evolution of orthorhombic domain structures during the tetragonal-to- orthorhombic phase transition in the layered perovskite Sr2-x Lax Mn O4

When Sr2+ ions in Sr2 Mn O4 containing only Mn4+ ions were partially replaced by La3+, a new phase having orthorhombic symmetry appeared around an La content of x=0.15 between the tetragonal I4/mmm (T) phase and the charge and orbital ordered (COO) phase, accompanying the introduction of Mn3+ ions. Our in situ observation using a transmission electron microscope revealed that the orthorhombic (O) phase could be identified as an orbital ordered state without charge ordering, and that its microstructure is characterized by an alternating array of two banded-shape variants with different orthorhombicities, OI and OII. It was also found that the T -to-O phase transition exhibited a unique evolution of domain structures, which resulted in the above-mentioned banded microstructure. In particular, the domain-structure evolution consisted of three steps: the appearance of the (T+ OI) and then the (OI + OII) coexisting states, followed by the annihilation of the interface between the OI and OII variants. The evidence suggests that this unique pattern of evolution is due to coupling between the short-wavelength Jahn-Teller (JT) distortion, associated with the Mn3+ ion, and the long-wavelength O distortion.