Anisotropy oxygen diffusion

Although the correlation between ionic porosity and diffusivity is imperfect, there is a rough trend that oxygen diffusivity in the minerals increases with increasing IP. The trend is useful in qualitative estimation of closure temperature (among other applications). Extending the relation to metallic systems, one prediction is that diffusion in face-centered cubic structure (25.95% free space) is slower that that in body-centered structure (31.98% free space) of the same metal composition. To avoid the issue of anisotropy, it would be worthwhile to reexamine the relations between diffusivity and ionic porosity using only isometric minerals. 

Ingrin J., Pacaud L., and Jaoul O. (2001) Anisotropy of oxygen diffusion in diopside. Earth Planet. Sci. Lett. 192, 347-361. 

P.-C. Lin, U. Kreutzer, and T. Jue. Anisotropy and temperature dependence of myoglobin translational diffusion in myocardium implication for oxygen transport and cellular architecture../. Am. Chem. Soc., 56 658-666, 1934. 

As a further complication the data previously discussed was reported for bulk materials and takes no account of the anisotropic structure of the K2Nip4 materials. It has been well documented that the oxygen interstitial in these types of oxides is located in the rocksalt layers and that the most likely conduction path for interstitial mobility is through the ab plane [30-32]. Hence it is likely that the attractive diffusion performance reported for the nickelate based materials could be further improved through the orientation of the cathode layer. In order to characterize the anisotropy of both diffusion and surface exchange, Bassat et al [19] have investigated isotopic exchanges... 

A more complex example is La2Cu04. The doping dependence of oxygen tracer diffusion coefficients in La2Cu04 and their anisotropy are illustrated in Fig. 6.19. Whilst the behaviour at low x-values is in accordance with simple defect chemistry, the interactions and structmral changes at high doping concentrations lead to deviations from the ideal mass action laws (see Fig. 5.55). 

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