Structural characterization of thebetatoalphasuperionic transition in Ag2HgI4and Cu2HgI4

The nature of the superionic transition in Ag2 HgI4 and Cu2 HgI4 has been investigated using temper dependent powder neutron diffraction and impedance spectroscopy techniques. In the case of Ag2 HgI4 , the superionic transition occurs at Tc = 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 Tc = 338(4) K and / ~ 6. The ambient temperature crystal structures of the two compounds are different (space group I for -Ag2 HgI4 and I 2m for -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.