Structural characterization of the beta ->alpha superionic transition in Ag2HgI4 and Cu2HgI4

The nature of the beta --> alpha superionic transition in Ag2HgI4 and Cu2Hg I4 has been investigated using temperature dependent powder neutron diffrac tion and impedance spectroscopy techniques. In the case of Ag2HgI4, the sup erionic beta --> alpha transition occurs at T-c = 326(2) K and is accompani ed by a 50-fold increase in the ionic conductivity. In the Cu+ analogue, wh ich has a lower conductivity for a given temperature, the corresponding val ues are T-c = 338(4) K and sigma(alpha)/sigma(beta) similar to 6 The ambien t temperature crystal structures of the two compounds are different (space group 1 (4) over bar for beta-Ag2HgI4 and 1 (4) over bar 2m for beta-Cu2HgI 4) but, in contrast to the most recent study, the high temperature polymorp hs are found to be isostructural (space group F (4) over bar 3m). Possible explanations for the different behaviour of the ionic conductivity of the t wo compounds are given.