In contrast to the monotonic increase in ionic conductivity produced by supervalent doping, subvalent doping causes the ionic conductivity to decrease at moderate effective dopant concentrations under all synthesis conditions. High dopant concentrations are needed to achieve a net increase in ionic conductivity relative to undoped Li 3 OCl through subvalent doping. This behaviour is a consequence of two factors. First, although subvalent doping increases the concentrations of all positivelycharged defect species, it also decreases the concentrations of all negatively-charged defect species, including lithium vacancies. Because, in the undoped systems, the concentration of lithium vacancies is significantly higher than that of lithium interstitials, low-to-moderate levels of subvalent doping decrease the concentration of lithium vacancies by a much greater extent than they increase the concentration of lithium interstitials (cf. eq. 9). In assynthesised Li 3 OCl, this effect gives a minimum in ionic conductivity when . The second cause of a reduced room-temperature ionic conductivity under subvalent doping is recombination of V Li + Li i Frenkel pairs when the system is cooled. The V Li + Li i Frenkel pair formation energy in Li 3 OCl is high (our calculated value is 2.6 eV), which indicates a strong enthalpic driving force for lithium vacancyinterstitial pairs to recombine. Because lithium vacancies and lithium interstitials recombine in a 1:1 ratio, this Frenkel-pair recombination is limited by the concentration of the minority lithium defect species. In undoped Li 3 OCl, the concentration of lithium interstitials is low and lithium vacancies are present in large excess. Frenkel-pair recombination therefore has a negligible effect on the lithium vacancy concentration and a corresponding negligible effect on the net ionic conductivity. Subvalent doping, however, under all synthesis conditions, increases the concentration of lithium interstitials and decreases the concentration of lithium vacancies. As the concentrations of these two defects become more equal, an increasing proportion of lithium vacancies are removed through Frenkel-pair recombination, and the ionic conductivity is progressively reduced. This effect is strongest when the concentrations of lithium vacancies and lithium interstitials are equal; nearly all the lithium vacancies and interstitials recombine, greatly reducing the concentrations of these charge carrying defects, and giving a sharp decrease in ionic conductivity. For effective dopant concentrations above this threshold, the concentration of lithium interstitials under synthesis conditions now exceeds that of lithium vacancies, and the room temperature lithium interstitial concentration and ionic conductivity both increase with effective dopant concentration.