Structural and superionic properties of Ag -rich ternary phases within the AgI MI₂systems

Abstract

The effects of temperature on the crystal structure and ionic conductivity of the compounds Ag 2 CdI 4 , Ag 2 ZnI 4 and Ag 3 SnI 5 have been investigated by powder diffraction and impedance spectroscopy techniques. ε-Ag 2 CdI 4 adopts a tetragonal crystal structure under ambient conditions and abrupt increases in the ionic conductivity are observed at 407(2), 447(3) and 532(4) K, consistent with the sequence of transitions ε-AgThe ambient-temperature β phase of Ag 2 ZnI 4 is orthorhombic and the structures of β-Ag 2 CdI 4 and β-Ag 2 ZnI 4 can, respectively, be considered as ordered derivatives of the wurtzite (β) and zincblende (γ ) phases of AgI. On heating Ag 2 ZnI 4 , there is a 12-fold increase in ionic conductivity at 481(1) K and a further eightfold increase at 542(3) K. These changes result from decomposition of β-Ag 2 ZnI 4 into α-AgI + ZnI 2 , followed by the appearance of superionic α-Ag 2 ZnI 4 at the higher temperature. The hexagonal crystal structure of α-Ag 2 ZnI 4 is a dynamically disordered counterpart to the β modification. Ag 3 SnI 5 is only stable at temperatures in excess of 370(3) K and possesses a relatively high ionic conductivity (σ ≈ 0.19 −1 cm −1 at 420 K) due to dynamic disorder of the Ag + and Sn 2+ within a cubic close packed I − sublattice. The implications of these findings for the wider issue of high ionic conductivity in AgI-MI 2 compounds is discussed, with reference to recently published studies of Ag 4 PbI 6 and Ag 2 HgI 4 and new data for the temperature dependence of the ionic conductivity of the latter compound.

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

“…This paper forms the second part of this comprehensive study. The three compounds of composition x ¼ 0:8 (RbAg 4 I 5 , KAg 4 I 5 and KCu 4 I 5 ) are all superionic phases and have been discussed in the previous publication [15]. Indeed, RbAg 4 I 5 and KAg 4 I 5 have exceptionally high ionic conductivities at ambient temperature (s ¼ 0:21 (6) and 0.08(5) O À1 cm À1 , respectively [15]).…”

“…The three compounds of composition x ¼ 0:8 (RbAg 4 I 5 , KAg 4 I 5 and KCu 4 I 5 ) are all superionic phases and have been discussed in the previous publication [15]. Indeed, RbAg 4 I 5 and KAg 4 I 5 have exceptionally high ionic conductivities at ambient temperature (s ¼ 0:21 (6) and 0.08(5) O À1 cm À1 , respectively [15]). Whilst a number of issues prohibit their commercial exploitation within solid state devices, including the high cost of silver, it is important from a fundamental point-of-view to examine how the superionic properties are enhanced by M + doping to form RbAg 4 I 5 and KAg 4 I 5 .…”

“…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.…”

Ionic conduction and effect of immobile cation substitution in binary system (AgI)4/5–(PbI2)1/5

“…On heating, transition to superionic phase occurs at 420 K with an abrupt increase in the conductivity to a value over 1 Scm −1 (5). Doping the compound with homo-and heterovalent cations results in the lowering of phase transition temperature (6)(7)(8).…”