  Since oxide materials like Sr$_2$FeMoO$_6$ are usually applied as thin films,
we studied the effect of biaxial strain, resulting from the substrate, on the
electronic and magnetic properties and, in particular, on the formation energy
of point defects. From our first-principles calculations, we determined that
the probability of forming point defects - like vacancies or substitutions - in
Sr$_2$FeMoO$_6$ could be adjusted by choosing a proper substrate. For example,
the amount of anti-site disorder can be reduced with compressive strain in
order to obtain purer Sr$_2$FeMoO$_6$ as needed for spintronic applications,
while the formation of oxygen vacancies is more likely for tensile strain,
which improves the functionality of Sr$_2$FeMoO$_6$ as a basis material of
solid oxide fuel cells. In addition, we were also be able to include the oxygen
partial pressure in our study by using its thermodynamic connection with the
chemical potential. Strontium vacancies become for example more likely than
oxygen vacancies at a pressure of 1$\,$bar. Hence, this degree of freedom might
offer in general another potential method for defect engineering in oxides
besides, e.g., experimental growth conditions like temperature or gas pressure.
