About the existing discrepancy in the determinations of the Avogadro constant

The values for the Avogadro constant N-A, derived from lattice spacing, den sity, and molar mass of silicon single crystals at several metrological ins titutes, show significant differences. To illuminate this discrepancy, comp arison 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 c orrection for the influence of carbon and oxygen impurities. It is proved t hat the discrepancy in the determinations of the Avogadro constant cannot b e explained by vacancies or interstitials. Also indications for macroscopic bubbles or cavities could not be found.