The Na Ba substitution is approximately 0.5 eV higher in energy than K Ba , and undergoes self-compensation by Na int species, as highlighted at point B; similarly, Li Ba is selfcompensated by Li int at point C. This is observed for Na and Li because their ionic radii are reasonably small (1.02 Å and 0.76 Å compared to the 1.61 Å of Ba), greatly reducing the energy of the interstitial defect species and increasing the energy of the substitution species due to the size mismatch. In the case of K, the interstitial defect is several eV higher in energy (and the substitution lower) due to its larger ionic radius of 1.38 Å and therefore does not self-compensate. These thermodynamic preferences for Ba site substitution are reflected in the values for t ASnO 3 in Table -the greater the deviation from a perfect t, the more significant the energy penalty. Substitution onto the Sn site is unlikely for all three dopants due to the prohibitively high formation energies, and in any case undergo significant charge compensation from various interstitial species, as well as the oxygen vacancy.