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

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