Structural characterization of the superionic transition in Ag₂HgI₄ and Cu₂HgI₄

The nature of the superionic transition in Ag₂ HgI₄and Cu₂ HgI₄has been investigated using temper dependent powder neutron diffraction and impedance spectroscopy techniques. In the case of Ag₂ HgI₄ , the superionic transition occurs at T_c = 326(2) K and is accompanied by a 50-fold increase in the ionic conductivity. In the Cu⁺analogue, which has a lower conductivity for a given temperature, the corresponding values are T_c = 338(4) K and / ~ 6. The ambient temperature crystal structures of the two compounds are different (space group I for -Ag₂ HgI₄and I 2mfor -Cu₂ HgI₄ ) but, in contrast to the most recent study, the high temperature polymorphs are found to be isostructural (space group F 3m ). Possible explanations for the different behaviour of the ionic conductivity of the two compounds are given.