Present State of the avogadro constant determination from silicon Crystals with natural isotopic compositions

A determination of the Avogadro constant from two selected silicon single-crystals with natural isotopic compositions is described. The density, molar mass, and lattice spacing of the two crystals were measured at NMIJ, PTB, IRMM, IMGC, and NIST. When all the data are combined, they lead to a value of the Avogadro constant of 6.022 1353 (18)/spl times/10/sup 23/ mol/sup -1/ with a relative combined standard uncertainty of 3.1/spl times/10/sup -7/.