Structural characterization of thebetatoalphasuperionic transition in Ag₂HgI₄and Cu₂HgI₄

Abstract

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.

Crystal structures and ionic conductivities of ternary derivatives of the silver and copper monohalides—II: ordered phases within the (AgX)x–(MX)1−x and (CuX)x–(MX)1−x (M=K, Rb and Cs; X=Cl, Br and I) systems

“…In this vein, the authors are pursuing a comprehensive study of the crystal structures and ionic conductivities of ternary derivatives of the AgX and CuX (X ¼ Cl; Br, I) compounds (see, for example, Refs. [11][12][13][14][15]). …”

Mechanochemical Synthesis and Temperature‐Dependent Optical Properties of Thermochromic (Ag1−xCux)2HgI4

“…Ag + and Cu + cations occupy the tetrahedral voids (Figure a, b) . These structure can be derived from chalcopyrite (Cu 2 Fe 2 S 4 ) by re‐arranging four Ag/Cu and two Hg atoms over the eight cationic sites . β‐Ag 2 HgI 4 further undergoes a phase transition from tetragonal to cubic structures at 50 °C where the cubic structure is of the disordered cubic zinc blend type (space group; F4‐3m ) .…”

“…Ag 2 HgI 4 and Cu 2 HgI 4 are well known thermochromic materials for decades . These materials by virtue of possessing mobile Ag + and Cu + cations at the tetrahedral site of the crystal structure show high ionic conductivity value albeit at higher temperatures …”

Phase transition study in a [Cu2HgI4: 0·xAgI] mixed composite system

“…Rietveld refinements of the diffraction data show the lattice parameters as a = 6⋅0672 Å and c = 12⋅2266 Å at room temperature (Hull and Keen 2000). The structure of AgI is a hexagonal wurtzite and zincblende-type structure (Burley 1962).…”