Determination of the formation energies of defects in materials requires knowledge of the individual elemental chemical potentials (equation 1) which are typically constrained by the thermodynamic stability of the host material relative to competing phases. As Li 3 OCl is metastable and not globally stable, calculating the chemical potentials requires either prior experimental knowledge about phase stability , or calculation of host and competing phases at finite temperatures. As the structure and composition of Li 3 OCl is not widely acknowledged, the latter is chosen in this instance, however it is important to note that studies exist showing Li 3 OCl to possess kinetic stabilty relative to LiCl and Li 2 O 29,62 , proposed to be due to the "sluggish" anion transport in Li 3 OCl 63 . In this study, the chemical potential limits analysis program (CPLAP ) is combined with the quasi-harmonic approximation (QHA) to determine the temperature dependent stability region of Li 3 OCl and thus unveil suitable chemical potentials without a-priori knowledge. This approach, whilst not widely used due to its increased computational expense, has shown recent success in determining the stability of Na 2 Ti 3 O 7 . From our results, Li