Accurate determination and correction of the lattice parameter of LaB₆ (standard reference material 660) relative to that of Si (640b)

X-ray powder diffraction and synchrotron radiation have been used to determine the lattice parameter of the NIST standard reference material (SRM 660) LaB₆ as 4.156468 Å with an accuracy of 12 parts per million (p.p.m.), calibrated relative to the lattice parameter of the Si powder standard [a₀ = 5.430940 (11) Å, Si 640b]. A discrepancy of 0.00048 (5) Å, or nine standard deviations from the NIST reference, is observed between the currently accepted lattice spacing of LaB₆ and the measured value. Twelve different measurements of the lattice parameter were made at beam energies between 10 and 20 keV. The observed discrepancy in the lattice parameter is consistent for the different energies used. The absolute values of the mean difference between the measured and calculated 2θ centroids, , are highly consistent, between 0.0002 and 0.0004° for energies from 5 to 14 keV, and between 0.0005 and 0.0008° for energies from 15 to 20 keV. In order to determine the peak positions with high precision, account must be taken of the observed peak asymmetry. It is shown that significant asymmetry is due to peak broadening and must be taken into account in order to determine accurate peak locations and lattice spacings. The approach shows significant advantages over conventional analysis. The analysis of peak broadening is compared with models used in Rietveld analysis.