The molar volume of silicon: sometimes materials are not what we would like them to be

PTB, IRMM, IMGC, NRLM and CSIRO have been cooperating on improving our knowledge of the Avogadro constant N/sub A/ and therefore of the molar volume of silicon. This is done by performing measurements of the interatomic distance a/sub 0/ (in metre) of the Si atoms in a near-perfect Si single crystal, of the density /spl rho/ (in kg/m/sup 3/) of such a crystal and of the molar mass M (in kg/mol) of the Si in the crystal. This paper proposes that, rather than assume perfection of the artefacts (i.e. the single crystals which could include some with voids) and cast doubt on the reliability of the measurement procedures for a/sub 0/, /spl rho/ and M (as a result of observed deviations), it seems to be the other way round: the measurement procedures seem to be more reliable at the stated (small) uncertainty than the variations observed for the molar volume. The measurement procedures with their small uncertainties, enabled to uncover real differences in the crystals.