Journal of Physics, Condensed Matter 2012-07-11

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

B J Morgan, P A Madden

Index: J. Phys. Condens. Matter 24(27) , 275303, (2012)

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Abstract

Extreme room temperature conductivity enhancements have been reported for nanocrystalline AgI of up to  × 10(4) relative to bulk β-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 β (wurtzite) and γ (zincblende) 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. The 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.