Absence of a space-charge-derived enhancement of ionic conductivity in β|γ-heterostructured 7H-and 9R-AgI

Extreme room temperature conductivity enhancements have been reported for nanocrystalline AgI of up to x10(4) relative to bulk beta-AgI (Guo et al 2005 Adv. Mater. 17 2815-9). These samples were identified as possessing 7H and 9R polytype structures, which can be considered as heterostructures composed of thin, commensurate layers in the beta(wurtzite) and gamma (zincblende) phases. It has been proposed that space-charge layer formation at beta vertical bar gamma-interfaces causes near complete disordering of the Ag+ sublattice in these polytypes, resulting in a massive intrinsic enhancement of ionic conductivity. We have performed molecular dynamics simulations of beta- and gamma-AgI and mixed beta vertical bar gamma superlattices, to study the effect of heterostructuring on intrinsic defect populations and Ag+ transport. The ionic conductivities and Ag+ diffusion coefficients vary as beta > 7H approximate to 9R approximate to 10L > gamma. The beta vertical bar gamma-heterostructured polytypes show no enhancement in defect populations or Ag+ mobilities relative to the beta-AgI phase, and instead behave as simple composites of beta- and gamma-AgI. This contradicts the proposal that the extreme conductivity enhancement observed for 7H and 9R polytypes is explained by extensive space-charge formation.