This gives a measure of how many additional defects of defect species X are introduced or removed per dopant. Figure shows calculated values of η X for V Li and O Cl with q = -1 and for V Cl and V Li with q = +1 for an effective dopant concentration of r [M r ] = ±10 18 cm -3 for both supervalent and subvalent doping. The calculated values of η VLi and η Lii reflect the strong asymmetry in the doping response of the lithium vacancy and interstitial. Supervalent doping under lithium-poor conditions strongly increases the concentration of lithium vacancies, indicated by η VLi ≈ 1. For supervalent doping under lithium-rich conditions, η VLi remains high, but is now smaller than η O Cl , reflecting the preferential formation of O Cl antisites. For subvalent doping, η Lii < 10 -8 under all conditions; at these dopant concentrations, subvalent doping has a negligible effect on the concentration of lithium interstitials and instead principally increases the concentration of Cl vacancies (η V Cl ≈ 1). In the case of Li 3 OCl, then, the doping-response efficiency, η X , provides an intuitive description of the strongly asymmetric response to supervalent versus subvalent doping. One practical consequence of the asymmetric doping response of Li 3 OCl is the qualitatively different response of the ionic conductivity under supervalent versus subvalent doping. Supervalent doping produces a monotonic increase in lithium vacancy concentration, and a corresponding monotonic increase in ionic conductivity. Subvalent doping, in contrast, produces a decrease in ionic conductivity at low-to-moderate dopant levels due to two complementary effects. First, at low-to-moderate subvalent doping levels the primary effect of subvalent doping is to decrease the concentration of lithium vacancies, rather than increase the concentration of lithium interstitials (cf. fig. ). Second, by increasing the number of lithium interstitials formed under synthesis conditions, a greater number of lithium vacancies are removed when cooling to room temperature due to recombination of V Li as [V Li + Li i ] Frenkel pairs. For sufficiently high dopant levels, subvalent doping is predicted to give an increase in ionic conductivity relative to undoped Li 3 OCl. This net positive contribution to the ionic conductivity, however, only occurs when the effective dopant concentration is much larger than the lithium vacancy concentration in the undoped material, r [M r ] [V Li ] undoped . The strongly asymmetric doping response of Li 3 OCl with respect to supervalent versus subvalent doping can be understood as a consequence of the low formation energy of positively charged V Cl defects with respect to Li i defects (fig. ); equivalently, this effect can be considered to be a consequence of the dominant [V Li + V Cl ] Schottky disorder that characterises the lithium defect chemistry in this system. Under Li-poor conditions, the Fermi energy is pinned by the [V Li + V Cl ] pair; V Li and V Cl are present in approximately equal concentrations, and Li i concentrations are several orders of magnitude lower, corresponding to a classical Schottky-disordered system. Under Li-rich conditions, the Fermi energy is pinned by the [O Cl + V Cl ] pair and the dominant defect species are O Cl and V Cl rather than lithium disorder. Under these conditions, however, the concentration of V Li is still several orders of magnitude larger than the concentration of Li i , and the lithium defect chemistry can still be considered within the schema of [V Li + V Cl ] Schottky disorder.