Valence-contrast studies by resonant diffraction with synchrotron radiation; the rôle of X-ray absorption measurements

The X-ray absorption spectra of GaCl₂, GaAlCl₄ and GaCl₃ have been measured in the vicinity of the Ga K edge and values of f'' and f' estimated for Ga^I and Ga^III from the latter two spectra by using the optical theorem and the Kramers-Kronig relation. The resulting f' values are compared with those previously determined from anomalous-scattering measurements with synchrotron X-rays on the compound GaCl₂ [Wilkinson, Cheetham & Cox (1991). Acta Cryst. B47, 155-161] and found to be in good agreement. The use of anomalous scattering methods for distinguishing oxidation states is discussed in the light of these results and others found in the literature.