DFT modeling of silver disorder and mobility in the semiconductor cluster [Ag₂₈S₂₆(P(O)PhOMe)₁₂(PPh₃) ₁₂]

Disorder of silver atoms and high cation mobility are commonly observed and closely coupled features in silver chalcogenides, The ligand-stabilized cluster [Ag₂₈(μ₆-S)₂(ArP(O)S₂) ₁₂-(PPh₃)₁₂] (1) (Ar = 4-anisyl), with a total of 666 atoms, displays in its X-ray structure highly localized disorder at two core silver atoms. To explore the nature of this disorder, we have applied density functional methods to its internal structure and flexibility. The pseudo-S₆ symmetry of the cluster provides six equivalent pockets to place the pair of silver atoms, and with the exception of populating neighboring sites, all permutations relax to structures with similar cores. The barrier to concerted motion of the central silver atoms from one set of pockets to the next of the C_i-symmetric conformer is estimated to be less than about 26 kJ mol^-1. Cluster 1 can be considered a model for bulk phase cation mobility.