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Indian Journal of Physics

, Volume 83, Issue 1, pp 31–47 | Cite as

How does contrasting dependence of impurity-atom diffusivity on the density of host disordered medium arise?

  • Manju Sharma
  • S. YashonathEmail author
Article

Abstract

We report results of molecular dynamics investigations into neutral impurity diffusing within an amorphous solid as a function of the size of the diffusant and density of the host amorphous matrix. We find that self diffusivity exhibits an anomalous maximum as a function of the size of the impurity species. An analysis of properties of the impurity atom with maximum diffusivity shows that it is associated with lower mean square force, reduced backscattering of velocity autocorrelation function, near-exponential decay of the intermediate scattering function (as compared to stretched-exponential decay for other sizes of the impurity species) and lower activation energy. These results demonstrate the existence of size-dependent diffusivity maximum in disordered solids. Further, we show that the diffusivity maximum is observed at lower impurity diameters with increase in density. This is explained in terms of the Levitation parameter and the void structure of the amorphous solid. We demonstrate that these results imply contrasting dependence of self diffusivity (D) on the density of the amorphous matrix, ρ. D increases with ρ for small sizes of the impurity but shows an increase followed by a decrease for intermediate sizes of the impurity atom. For large sizes of the impurity atom, D decreases with increase in ρ. These contrasting dependence arises naturally from the existence of Levitation Effect.

Keywords

Amorphous solid anomalous diffusion density impurity 

PACS Nos.

61.43.-j 66.30.hh 66.30.Ny 

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Copyright information

© Indian Association for the Cultivation of Science 2009

Authors and Affiliations

  1. 1.Solid Sate and Structural Chemistry UnitIndian Institute of ScienceBangaloreIndia

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