Superionic conductors are solids that reach a liquid-like ionic conductivity at a temperature T s before reaching their melting point . This makes superionic con-ductors promising candidate solid electrolytes for applications such as fuel cells and all-solid-state batteries . The fundamental physics of superionic phasetransitions, and the mechanistic details of ion transport in the superionic regime, remain poorly understood, which limits the scope for the rational design and optimisation of superionic solid electrolytes for specific applications. A better understanding of the physical basis of superionic phase-transition behaviour and of the factors that cause some materials to exhibit superionic phases is therefore desirable both as an intellectual endeavour and as the basis for developing improved solid electrolytes for practical applications . Superionic conductors that exhibit a superionic transition can be classified type-I or type-II according to the nature of the transition . Type-I superionic conductors, such as AgI, exhibit a discontinuous superionic transition, accompanied by a first-order structural phase transition at T s from a poorly-conducting phase to the superionic phase . In contrast, type-II superionic conductors, such as β-PbF 2 , exhibit a continuous superionic transition, accompanied by a second-order phase transition associated with a constant-pressure heat capacity peak at T s . For both type-I and type-II superionic conductors, the superionic transition is associated with a marked increase in ionic conductivity as well as a decrease in ionic conductivity activation energy, which is often interpreted as evidence of a qualitative change in diffusion mechanism of the mobile ionic species across the superionic transition.