About the existing discrepancy in the determinations of the Avogadro constant

The values for the Avogadro constant N-A, derived from lattice spacing, density, and molar mass of silicon single crystals at several metrological institutes, show significant differences. To illuminate this discrepancy, comparison measurements of lattice spacing and density were performed. Positron annihilation measurements were used to estimate the vacancy concentration in Si samples. All crystals lead to nearly the same lattice spacing after correction for the influence of carbon and oxygen impurities. It is proved that the discrepancy in the determinations of the Avogadro constant cannot be explained by vacancies or interstitials. Also indications for macroscopic bubbles or cavities could not be found.