Structural characterization of further high temperature superionic phases of Ag₂HgI₄and Cu₂HgI₄

Abstract

The high temperature crystal structure of the superionic compounds Ag2HgI4 and Cu2HgI4 has been investigated using powder neutron diffraction. In addition to the widely studied β→α superionic transitions observed in both compounds just above ambient temperature, we have characterized the transitions to phases (labelled δ) in Ag2HgI4 and Cu2HgI4 which occur at ~410 and ~578 K, respectively. Prior to melting, Ag2HgI4 undergoes an additional transition at ~445 K to a phase labelled ε. The crystal structure of the δ phases comprises a slightly distorted h.c.p. anion sublattice with the two cation species dynamically disordered over the octahedral and tetrahedral interstices. In ε-Ag2HgI4 the cations are distributed over the tetrahedral and trigonal interstices formed by a b.c.c. anion sublattice and can, therefore, be considered to be a `cation-deficient' α-AgI-type superionic phase. The implications of these experimental findings in the wider context of the family of copper- and silver-based superionic conductors and for previous suggestions of `unusual' behaviour of the α↔δ transition are discussed.

High-temperature thermal expansion and structural behaviour of stromeyerite, AgCuS

“…Hence, the superionic transitions in stromeyerite have a different origin. According to [15], there are relatively few examples of structural phase transitions between two superionic phases accompanied by a rearrangement of the rigid anion sublattice. A model for ionic conduction in α-and δ-AgCuS was derived from powder diffraction data: similar to Cu 2 S [16], the cation conductivity in α-AgCuS is two dimensional and occurs in slabs perpendicular to the c-direction, whereas it is likely that cations jump in 'skewed' 100 directions between nearest-neighbour tetrahedral sites via the peripheries of octahedral cavities in δ-AgCuS.…”

“…In the newest work on the structure of stromeyerite by Baker et al [9], for convenience, following the analogy with silver iodide, the low-temperature phase was designated as γ -AgCuS, the RT phase as β-AgCuS, and the cation-conducting high-temperature phase as α-AgCuS. Nevertheless, it is not clear whether the hexagonal or the cubic high-temperature phase was denoted as α-AgCuS by Baker et al To clarify this matter by following the notation accepted for other superionic conductors (see for instance [15]), the hexagonal structure of AgCuS should be denoted as α-AgCuS and the cubic phase at the temperature immediately above the α-phase should be denoted as δ-phase. Results of Rietveld refinements for selected temperatures for β-, α-and δ-AgCuS are illustrated in figure 2 and summarized in tables 1-3.…”

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]). …”

Superionic Materials: Structural Aspects

“…Extensive ionic conductivity and diffraction studies at elevated temperatures have been performed. Seven superionic phases have been identified, formed with bcc (a-Ag 2 CdI 4 [58] and e-Ag 2 HgI 4 [59]), hcp (d-Ag 2 HgI 4 [59] and a-Ag 2 ZnI 4 [58]) and fcc (a-Ag 2 HgI 4 [60], Ag 3 SnI 5 [58] and Ag 4 PbI 6 [61]) anion sublattices. These are analogous to the superionic phases found in the binary Ag þ and Cu þ halides, but with two cations (Ag þ /Cu þ ) replaced by a single divalent one (M 2 þ ) and a vacancy.…”

“…These are analogous to the superionic phases found in the binary Ag þ and Cu þ halides, but with two cations (Ag þ /Cu þ ) replaced by a single divalent one (M 2 þ ) and a vacancy. In several cases, addition of the M 2 þ dopant ions lowers the superionic transition temperature compared to pure a-AgI, but at the cost of a significant reduction in the ionic conductivity [59].…”