The total dielectric constant, used to obtain the finite-size correction term E corr in Equation 1, was calculated to be 13.99, 12.16 and 11.61 ϵ 0 along the a, b and c directions using amset. Herein, the Kumagai-Oba (eFNV) finite-size charge correction scheme was applied due to the anisotropic character of Sb 2 O 5 . 46 O i is the next lowest energy defect, which has a formation energy of 3.89 eV under Opoor conditions (Figure ). It is stable as a neutral dimer across all Fermi levels, with an O-O bond length of 1.40 Å. This is known as peroxide species which has also been found in other n-type oxides. cuts the CBM at around 5.0 eV, and so is much less likely to form. High formation energies for V Sb were also observed in ZnSb 2 O 6 , 10 and can be attributed to the high oxidation state (+5) of Sb in this compound. However, under Sb-poor conditions, the formation energy of V Sb at the CBM drops by ∼2 eV to ∼3 eV, becoming one of the lowest energy native defects under these conditions. Notably, V Sb here is found to be a rare four-electron negative-U center where only the (0/-4) charge transition level occurs within the bandgap -indicating that the equilibrium charge state of antimony vacancies changes directly from neutral to -4 as the Fermi level approaches the CBM. Negative-U behavior is typically related to structural reconstruction and lattice distortion, and so the structures of V Sb 0 and V Sb -4 were investigated in detail. As shown in Figure , an O-O-O trimer is formed in V Sb 0 , similar to the Se-Se-Se trimer found for V Sb in Sb 2 Se 3 which also results in 4-electron negative-U behavior. This trimer geometry was identified using ShakeNBreak where the energy was found to be around 4 eV lower than the structure obtained with a standard (unperturbed) defect geometry relaxation (Figure ). On the other hand, for V Sb in +3, +4 and +5 charge states we find a split-vacancy configuration to be the preferred arrangement (Figure ). A split-vacancy configuration can be thought of as a divacancy and interstitial cluster, as a neighbouring host Sb atom displaces toward the vacancy position (creating an additional vacancy and interstitial in the process). This type of vacancy structure is often formed in cation vacancies with high charge states, having also been reported in α-Al 2 O 3 and β-Ga 2 O 3 . The thermodynamic analysis of intrinsic point defects in Sb 2 O 5 above demonstrates that the oxygen vacancies are the lowest formation energy defects. However, they are still moderately high in energy and the donor levels are deep in the bandgap (0.9 -0.6 eV below the CBM). Therefore, there will not be any significant intrinsic n-type doping in this system. However, Sb 2 O 5 has a large n-type doping window of around 5.0 eV. Doping window is defined as the formation energy of the lowest energy compensating defect species at the corresponding band edge, as this sets an upper limit to the formation energy of dopants which could push the Fermi level close to the band edge without being negated by ionic charge compensation from intrinsic defects. O i does not charge compensate as it stays neutral across the bandgap. In this case, the donor doping is charge-compensated by the native acceptor V Sb -4 , cutting the CBM at around 5.0 eV under O-poor conditions. Compared with ZnSb 2 O 6 , 10 Sb 2 O 5 has a wider n-type doping window since the dominant intrinsic acceptor (cation vacancy V Sb ) here has higher energy than V Zn because of the higher oxidation state of Sb.