On a small error in SRM640, SRM640a and SRM640b lattice parameters

A small error has been found in the lattice parameters of the NIST (National Institute of Standards and Technology) [previously known as the National Bureau of Standards (NBS)] certified silicon standard reference materials (SRMs) SRM 640, SRM 640a and SRM 640b. The internal standards used to certify these silicon standard reference materials appear to have been incorrectly scaled using the wavelengths of Cu Kα₁ radiation in use in 1973 and 1962 instead of the wavelengths of Ni Kα₁ and Co Kα₁ radiations. The following lattice parameters are proposed as being more correct: SRM 640, 5.430898 (39) Å; SRM 640a, 5.430854 (40) Å; SRM 640b, 5.430969 (39) Å. This corresponds to an increase of 0.000018 Å for SRM640 and of 0.000029 Å for SRM640a and SRM640b. These values are uncorrected for refraction. The uncertainties include those of the original NBS/NIST certification and an additional X-ray wavelength scaling uncertainty not previously accounted for.