To calculate the SCFL of ZnSb 2 O 6 , a python based implementation of sc-fermi was used (which can be found at ). The required inputs are an electronic density of states of the defect-free system, a temperature, the total cell volume and number of electrons, and the thermodynamic transition levels for all defect species. The SCFL is calculated by recognising that the overall charge of a system must be equal to zero, which must be equal to the concentration of all of the charged defects plus any positive holes and negative electrons. We can construct simultaneous equations using the Fermi-Dirac distribution and the formation energy of a defect (Equation ), and therefore find the Fermi level that gives overall charge neutrality. A rigorous description of this problem, and how it is implemented in the code, can be found in the original paper. In our analysis, we calculated the SCFL at the synthesis temperature of ZnSb 2 O 6 and froze the defect concentrations at this temperature. We then recalculated the SCFL at room temperature, allowing the concentration of the individual charge states of each defect to change, in order to predict the total electron concentration at the temperature at which our experiments were performed.