Stable competing phases within the Zn-Sb-O chemical potential space were relaxed using the PBE0 functional, a plane-wave energy cut-off of 500 eV and a converged k-point mesh until the forces were reduced to below 1 × 10 -2 eV Å-1 . The ground state energies were used to identify the chemical potential limits that bound the thermodynamic stability of ZnSb 2 O 6 , using the program cplap. The chemical potential of oxygen was set as the dependent variable in the cplap analysis, as this can be most readily changed in experiment through the use of partial pressures. The chemical potential limits from the thermodynamic stability calculations can be found in Table . ), one occupied by Zn (pale orange) and the other by Sb (dark orange) in ZnSb 2 O 6 , and two distinct anion sites (pale and dark blue), both occupied by O. Two interstitial candidate sites were identified: "i1" on the 8h Wyckoff site between two Zn and two Sb atoms, and "i2" on the 4c Wyckoff site between two Zn atoms. All supercells were optimised to reduce forces below 1 × 10 -2 eV Å-1 , keeping the lattice vectors constant while allowing ionic coordinates to move. Supercell calculations were performed using a Γ-centred 2 × 2 × 2 k-point mesh.