The crystal structure and ionic conductivity of Ag+-rich phases in (AgI)x–(PbI2)1−x were investigated by performing powder neutron diffraction and impedance spectroscopic studies on samples with x = 2/3 or 4/5. The phase diagram in this region appeared to be simpler than that with a single phase that was stable above 420K (x = 2/3) or 400K (x = 4/5). This was a superionic phase (0.1S/cm at 420K) in which the anions formed a face-centred cubic sub-lattice with a lattice parameter, a, of about 6.34Å. This phase was labelled as being face-centered cubic AgxPb1−xI2−x. The structure adopted the centro-symmetrical space group, Fm¯3m, with most cations (over 90%) being located in octahedral 4(b) cavities, and the remainder in tetrahedral 8(c) interstices. This six-fold coordination to I− was expected for Pb2+, but was relatively unusual for Ag+; especially within a superionic phase. A comparison was made with the face-centered cubic superionic phase, α-Ag2HgI4. Upon cooling, the face-centered cubic AgxPb1−xI2−x phase, with x = 4/5, dissociated at about 400K to form AgI and PbI2. The former adopted a zincblende-structured γ-phase while the latter stabilized in its 6R polytype rather than the normal 2H modification. The γ-AgI+6R-PbI2 mixture appeared to be stable indefinitely at ambient temperature. In the x = 2/3 case, 6R-PbI2 appeared separately, upon cooling, at about 420K. Upon further cooling, complete dissociation to form a 2-phase mixture of γ-AgI and 6R-PbI2 occurred by about 390K.

Structural Description of the Superionic Behaviour in the System (AgI)x–(PbI2)1−x, 2/3 ≤ x ≤4/5. S.Hull, D.A.Keen, P.Berastegui: Solid State Ionics, 2002, 147[1-2], 97-106