  We use first-principles calculations to investigate the interplay between
strain and the charge state of point defect impurities in complex oxides. Our
work is motivated by recent interest in using defects as active elements to
provide novel functionality in coherent epitaxial films. Using oxygen vacancies
as model point defects, and CaMnO$_3$ and MnO as model materials, we calculate
the changes in internal strain caused by changing the charge state of the
vacancies, and conversely the effect of strain on charge-state stability. Our
results show that the charge state is a degree of freedom that can be used to
control the interaction of defects with strain and hence the concentration and
location of defects in epitaxial films. We propose the use of field-effect
gating to reversibly change the charge state of defects and hence the internal
strain and corresponding strain-induced functionalities.
