Unit-cell dimension measurements from pairs of X-ray diffraction lines

A method is described for precise unit-cell dimension measurements based on the accurate recording of the angular separation between two neighbouring orders of reflection which may correspond to different wavelengths. A number of pairs of reflections at high diffraction angles are used and a least-squares refinement is performed after taking into account the angular dependence of systematic aberrations. The method has been tested on Ge and several selenides (InSe, In2Se3, GaSe) and the accuracy of the results is discussed.