Structural description of the superionic behaviour in the system (AgI)x–(PbI2)1−x, 2/3≤x≤4/5

The crystal structure and ionic conductivity of the Ag+-rich phases in (AgI)x–(PbI2)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.-AgxPb1−xI2−x. The structure adopts the centrosymmetric space group Fm3̄m, 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 Pb2+, 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-Ag2HgI4. On cooling, the f.c.c.-AgxPb1−xI2−x phase with x=4/5 dissociates at T≈400 K to form AgI and PbI2. The former adopts its zinc blende-structured γ phase whilst, unusually, the latter is stabilised in its 6R polytype rather than the normal 2H modification. The γ-AgI+6R-PbI2 mixture appears to be stable at ambient temperature indefinitely. For the x=2/3 case, 6R-PbI2 appears separately on cooling at ≈420 K and, on further cooling, complete dissociation to form a two-phase mixture of γ-AgI and 6R-PbI2 occurs by ≈390 K.