It is also briefly worth considering some of the other native defects in CaCuP. The cation interstitials are high in energy, with Ca i significantly less likely to form than Cu i -this behaviour can be rationalised from the larger effective ionic radius of Ca 2+ compared to Cu + (100 pm and 77 pm respectively) . P i is much lower in energy, 1.95 eV in the neutral charge state. This is partly due to its smaller size, but a significant driving force is also the ease of formation of a dimeric P-P species with respect to, e.g., O-O interstitial dimers in metal oxides . The cation on anion antisites, Ca P and Cu P , are also high in energy, as significant electrostatic repulsion is generated by substituting a cation onto an anion site that is then surrounded by other cations. Once more, the larger Ca ion causes greater disruption to the local environment, and is therefore the higher energy species. Ca Cu is also a high energy n-type defect, again causing significant lattice distortion, whereas Cu Ca is p-type (effectively self-doping the 2+ site with a 1+ cation), and is separated from the Cu-P planes, resulting in only a small distortion to the local environment. It is worth noting that there is a reasonable extrinsic p-type doping window in CaCuP, and substituting a 1+ cation with a similar ionic radius to Cu + onto the Ca site could result in further generation of holes. Finally, the anion on cation antisites are considered, which are noticeably lower in energy than the cation on anion antisite species. This can be rationalised by noting that P is amphoteric (can act as both a cation and an anion), and so can exist surrounded by P anions by adopting a positive oxidation state. Both the P Ca and P Cu species remain in the neutral charge state (equivalent to P adopting the +5 oxidation state) across all Fermi levels.