In solid electrolytes that contain more than one chargecarrying defect species, increasing the concentration of the minority defect species may have little practical effect on the overall ionic conductivity. Alternatively, where there are strong interactions between dopant atoms and oppositely charged mobile defect species, these mobile defects may be kinetically trapped, resulting in a net decrease in ionic conductivity . These potential complications mean that aliovalent doping of solid electrolytes is not guaranteed to produce a significant increase in ionic conductivity, and predicting the effectiveness of a specific aliovalent doping strategy requires characterising the response to doping of all relevant defects in the system of interest. When considering the response of specific solid electrolytes to aliovalent doping, it is often simply assumed that the introduction of aliovalent dopants is principally charge-compensated by the formation of oppositely charged mobile defect species ; i.e., in lithiumion solid electrolytes, supervalent (donor) doping will principally increase the concentration of lithium vacancies and subvalent (acceptor) doping will principally increase the concentration of lithium interstitials. In practice, however, aliovalent doping may instead cause the preferential formation of immobile defects within the host-framework substructure , giving much smaller changes in concentrations of the targeted mobile defect species than otherwise would be expected. To accurately predict how defect populations respond to aliovalent doping it is therefore necessary to go beyond simple "chargecompensation" defect schemes, and instead consider a more complete thermodynamic model that accounts for a full range of native defects .