  Ordering of mobile defects in functional materials can give rise to
fundamentally new phases possessing ferroic and multiferroic functionalities.
Here we develop the Landau theory for strain induced ordering of defects (e.g.
oxygen vacancies) in thin oxide films, considering both the ordering and
wavelength of possible instabilities. Using derived analytical expressions for
the energies of various defect-ordered states, we calculated and analyzed phase
diagrams dependence on the film-substrate mismatch strain, concentration of
defects, and Vegard coefficients. Obtained results open possibilities to create
and control superstructures of ordered defects in thin oxide films by selecting
the appropriate substrate and defect concentration.
