Relationship of structure and ionic mobility in solid MAg4I5

Abstract

The crystal structure of MAg₄I₅(M = K, Rb, NH₄) is cubic with a= 11.2 Å, z= 4 and space group of the enantiomorphic pair P4₁32 and P4₃32. The locations of the I^– and M⁺ ions have been deduced from symmetry and packing considerations. The Ag⁺ ions do not occupy distinct single sites, but are distributed randomly over a number of interstitial positions. This model is consistent with the expectionally high mobility of the Ag⁺ ions and the high entropy of formation of the compound. The absence of Na or Cs analogues is explained by the steric requirements of the proposed structure.