Cation disordering in Ag2HgI4 and Cu2HgI4

The cation order-disorder transitions in Ag₂HgI₄ and Cu₂HgI₄ are first order. This is unusual, since in other superionic conductors the cation disordering is gradual with temperature if there is no structural phase transition. These two materials are also unique in that they have two disordering cations rather than one. A study of a two species lattice gas model shows that this extra degree of freedom is responsible for the first order nature of this transition on the fcc lattice.