The superionic properties of the compounds RbAg 4 I 5 , KAg 4 I 5 and KCu 4 I 5 have been investigated by powder neutron di4raction and complex impedance spectroscopy. RbAg 4 I 5 and KAg 4 I 5 have room-temperature ionic conductivities of β«Ψβ¬ 0.21(6) and 0.08(5)
Ψ 1 cm Ψ 1 , respectively, which increase gradually on increasing temperature. KCu 4 I 5 is only stable in the temperature range between 515(5) K and its melting point of 605 K, and its ionic conductivity is β«Ψβ¬ 0.61(8) Ψ 1 cm Ψ 1 , at T β«Ψβ¬ 540 K. At lower temperatures, KCu 4 I 5 disproportionates into KI Ψ 4CuI and the ionic conductivity falls by over three orders of magnitude. Leastsquares re5nements of the powder neutron di4raction data for RbAg 4 I 5 at ambient temperature con5rm the reported structure (space group P4 1 32, Z β«Ψβ¬ 4, a β«Ψβ¬ 11.23934(3) A > ), though with some di4erences in the preferred locations of the mobile Ag Ψ . KAg 4 I 5 and KCu 4 I 5 are found to adopt the same basic structure as RbAg 4 I 5 , with the I Ψ forming a -Mn-type sublattice, with the K Ψ located in a distorted octahedral environment and the Ag Ψ (Cu Ψ ) predominantly distributed over two sites which are tetrahedrally co-ordinated to I Ψ . The implications for the conduction mechanism within these compounds are discussed, using a novel maximum entropy di4erence Fourier technique to map the distribution of the Ag Ψ (Cu Ψ ) within the unit cell.
2002 Elsevier Science (USA)