To calculate defect formation energies, and hence predict defect concentrations, it is necessary to define the accessible ranges of chemical potentials for the elemental species involved in the formation of each defect (cf. eq. ( )). In most defect studies, the relevant region of chemical potential space is constrained by the thermodynamic stability limits of the system under study with respect to competing phases . Li 3 OCl, however, is not thermodynamically stable, but is metastable with respect to formation of Li 2 O and LiCl . Here, we follow the approach of Emly et al. and assume that the degradation of Li 3 OCl to form Li 2 O is kinetically suppressed, and therefore construct a metastable phase diagram with Li 2 O removed as a competing phase . The assumption that Li 3 OCl is metastable with respect to decomposition to Li 2 O and LiCl is supported by a recent analytical and force-field-based atomistic modelling study that predicts excellent kinetic stability of Li 3 OCl with respect to this decomposition pathway , although experimental synthesis of phase-pure crystalline Li 3 OCl remains extremely challenging .