A determination of the Avogadro constant NA by the X-ray crystal density (XRCD) method is described, where the density p of silicon crystals has been determined with a relative uncertainty of 1.1×10^<-7> by the interferometric measurements of the diameters of 1-kg silicon spheres. Recent results on the molar volume M/p of silicon crystals suggest that a large number of microscopic defects are contained in the crystals.
