The silicon lattice parameter -: an invariant quantity of nature?

The lattice parameter of silicon plays an important role in the determination of the Avogadro constant and the line-structure constant. Today, three values of the d(220) silicon lattice spacing are available, measured at the Physikalisch-Technische Bundesanstalt (PTB, Germany), the Istituto di Metrologia "G. Colonnetti" (IMGC, Italy) and the National Research Laboratory of Metrology (NRLM, Japan) and based on the metre scale. Using the PTB lattice spacing comparator, the spacings of the different materials were measured and compared with one another in order to check the possibility of combining the results to form a common "best" value. The characterization of the relevant impurities and crystal defects and their number densities are discussed and corrections applied for their influences on the interatomic distances. The NRLM data are in disagreement with those of the PTB and the IMGC, from which a lattice parameter for a hypothetical, ideal, perfect Si crystal was derived with a relative uncertainty of about 1 x 10(-8).