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.