The determination of unit-cell parameters from a Laue diffraction pattern

Single‐crystal Laue diffraction patterns have not hitherto been used for the precise determination of unit‐cell dimensions, although axial ratios and interaxial angles have frequently been derived. Here, we use an attenuator such as 0.1 mm palladium foil in the white synchrotron‐radiation beam incident on the crystal; the absorption edge (e.g. palladium at 0.509 Å) then sharply defines the minimum wavelength present. With gnomonic projections in the manner described by Carr, Cruickshank & Harding [J. Appl. Cryst. (1992), 25, 294–308], the unit‐cell dimensions on an absolute scale can be derived. Three tests show that these dimensions are correct to circa 0.5%.