Evaluation methodology of diffusion coefficient of guest substances associated with angstrom-scale open spaces in materials by slow positron beam.

The knowledge of elemental migration associated with angstrom-scale open spaces is a prerequisite for sophisticated material design with respect to nanodispersion as well as a better understanding of molecular migration in a geological environment. Here, we show the methodology for evaluating the diffusion coefficient of guest substances focusing on angstrom-scale open spaces, where the depth-variation of open spaces caused by elemental diffusion in the micrometer-scale surface region is examined by the variable-energy slow positron beam. The present methodology determines a low diffusion coefficient of ∼10−14 cm2 s−1 for potassium at 533 K in SiO2 glass within the framework of Fick’s principle providing the evidence that ∼2.5 A open spaces intrinsically present in the glass matrix govern the diffusion process.