ABSOLUTE MEASUREMENT OF LATTICE SPACING D(220) SILICON CRYSTAL IN FLOATING-ZONE

The lattice spacing d(220) of a silicon crystal of National Research Laboratory of Metrology has been measured with a new combined X-ray and optical interferometer, with relative uncertainty of 0.16 ppm. This value is in good agreement with other reported values, whereas the ratio of molar mass M to density rho measured for this crystal shows discrepancy of around 3 ppm from previously reported ratios. It seems that the conventional route to determining the Avogradro constant from M, rho and d(220) Will require a new characterization technique to estimate the number of silicon atoms in a unit cell volume.