For defect formation energy calculations, a single oxygen vacancy defect was introduced into 5 distinct oxygen positions (O1-O5) of the primitive Bmab unit cell structures of La8Ni6O20±1, Pr8Ni6O20±1 and the lowest energy structure of La4Pr4Ni6O20±1 found from the convex hull analysis. An oxygen interstitial defect was also introduced into each structure within the (La,Pr)O rocksalt layer. The atomic positions and lattice parameters for each structure were allowed to fully relax to a force tolerance of 0.01 eV Å -1 . The chemical potential of oxygen (𝜇 𝑂 2 ) in the defect calculations was referenced to O2 gas via the calculation of the energy of and isolated triplet O2 molecule (𝐸 𝑂 2 ) in a box (12×12×12 Å). To correct for the known over binding of O2 molecules with the PBE functional, a correction of +1.374 eV was added to the energy of O2, based on the procedure previous described by Wang et al. The impact of temperature on the oxygen chemical potential, 𝜇 𝑂 2 (𝑇, 𝑝 𝑂 2 ), was approximated as :