Absence of a space-charge–derived enhancement of ionic conductivity in beta|gamma-heterostructured 7H and 9R-AgI

Extreme room temperature conductivity enhancements have been reported for nanocrystalline AgI of up to ×104 relative to bulk β-AgI. 1 These samples were identified as possessing 7H and 9R polytype structures, which can be considered as heterostuctures composed of thin, commensurate layers in the β (wurtzite) and γ (zinc blende) phases. It has been proposed that space charge layer formation at β|γ-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 β- and γ-AgI and mixed β|γ superlattices, to study the effect of heterostructuring on intrinsic defect populations and Ag+ transport. Ionic conductivities and Ag+ diffusion coefficients vary as β > 7H ≈ 9R ≈ 10L > γ. The β|γ-heterostructured polytypes show no enhancement in defect populations or Ag+ mobilities relative to the β-AgI phase, and instead behave as simple composites of β- and γ-AgI. This contradicts the proposal that the extreme conductivity enhancement observed for 7H and 9R polytypes is explained by extensive space charge formation.

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