were dried and ground together in near stoichiometric molar ratios. The powder was heated for 12 hours at 600 °C in a 400 mbar Ar atmosphere in a sealed quartz tube to pre-react to form the ZnSb 2 O 4 phase. The 1-2-4 polycrystalline powder was then extracted, reground and heated in an air atmosphere at 800 °C for a further 12 hours for oxidation into the ZnSb 2 O 6 (1-2-6) phase. This two-step process was necessary to control antimony evaporation during baking, and we typically observed less than 1% Sb loss during the synthesis which could be accurately accounted for by adding excess Sb 2 O 3 in the starting materials.