Absolute measurement of lattice spacing d(220) silicon crystal in floating zone

The lattice spacing d220 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 ρ measured for this crystal shows discrepancy of around 3 ppm from previously reported ratios. It seems that the conventional route to determining the Avogadro constant from M, ρ and d220 will require a new characterization technique to estimate the number of silicon atoms in a unit cell volume.