Structural Phase Transitions to 2D and 3D Oxygen Vacancy Patterns in a Perovskite Film Induced by Electrical and Mechanical Nanoprobing.

Oxygen vacancy migration and ordering in perovskite oxides enable manipulation of material properties through changes in the cation oxidation state and the crystal lattice. In thin-films, oxygen vacancies conventionally order into equally spaced planes. Here, it is shown that the planar 2D symmetry is broken if a mechanical nanoprobe restricts the chemical lattice expansion that the vacancies generate. Using in situ scanning transmission electron microscopy, a transition from a perovskite structure to a 3D vacancy-ordered phase in an epitaxial La Sr MnO film during voltage pulsing under local mechanical straining is imaged. The never-before-seen ordering pattern consists of a complex network of distorted oxygen tetrahedra, pentahedra, and octahedra that, together, produce a corrugated atomic structure with lattice constants varying between 3.5 and 4.6 Å. The giant lattice distortions respond sensitively to strain variations, offering prospects for non-volatile nanoscale physical property control driven by voltage and gated by strain.