Structural description of the superionic behaviour in the system (AgI) x–(PbI 2) 1− x, 2/3≤ x≤4/5

Abstract

The crystal structure and ionic conductivity of the Ag +-rich phases in (AgI) x –(PbI 2) 1− x have been investigated by powder neutron diffraction and impedance spectroscopy studies of samples with x=2/3 and x=4/5. The phase diagram in this region appears to be somewhat simpler than that presented previously with a single phase stable at T≥420 K ( x=2/3) and T≥400 K ( x=4/5). This is a superionic phase, with σ~0.1 Ω −1 cm −1 at T=420 K, in which the anions form a face centred cubic ( f.c.c.) sublattice of lattice parameter a~6.34 Å. We therefore label this phase as f.c.c.-Ag x Pb 1− x I 2− x . The structure adopts the centrosymmetric space group Fm 3 ̄ m, with the majority of cations (over 90%) located in the octahedral 4( b) cavities and the remainder within the tetrahedral 8( c) interstices. Whilst this sixfold co-ordination to I − is to be expected for Pb 2+, it is relatively unusual for Ag +, especially within a superionic phase. To highlight this behaviour, comparison is made with the f.c.c.-structured superionic phase α-Ag 2HgI 4. On cooling, the f.c.c.-Ag x Pb 1− x I 2− x phase with x=4/5 dissociates at T≈400 K to form AgI and PbI 2. The former adopts its zinc blende-structured γ phase whilst, unusually, the latter is stabilised in its 6 R polytype rather than the normal 2 H modification. The γ-AgI+6 R-PbI 2 mixture appears to be stable at ambient temperature indefinitely. For the x=2/3 case, 6 R-PbI 2 appears separately on cooling at ≈420 K and, on further cooling, complete dissociation to form a two-phase mixture of γ-AgI and 6 R-PbI 2 occurs by ≈390 K.