A 7H polytype of AgI (characterized by its stacking-fault arrangement) was detected at the interface between β-AgI and γ-Al2O3. This could be the main, or unique, AgI constituent at high Al2O3 contents. The driving force for formation of the 7H phase was assumed to be the interaction of Ag+ with the alumina phase, as in AgCl and AgBr composites. There, due to the relative stability of the rock-salt structure, only ideal semi-infinite space-charge layers occurred. It was found that considering the 7H structure to be a heterostructure of γ- and β-phases (β/γ/β/γ/…) led to an explanation that was consistent with all of the observed features. These included the extreme Ag+-conductivity (that could not be explained by semi-infinite space charges), the phase transition behaviour and the qualitative similarities with AgCl:Al2O3 and AgBr:Al2O3. Because the layer separation was of sub Debye-length scale, a mesoscopic effect upon ionic conductivity was expected. This was supported by the conductivity anomalies of β/γ 2-phase mixtures.

A Mesoscopic Heterostructure as the Origin of the Extreme Ionic Conductivity in AgI:Al2O3. J.S.Lee, S.Adams, J.Maier: Solid State Ionics, 2000, 136-137, 1261-6