Non-validity of Bragg's additivity law for rare-earth compounds

Mass attenuation coefficients have been measured for rare-earth compounds at averaged photon energies 5.947 and 8.118 keV. The measured values are compared with theoretical calculations. The disagreement between experiment and theory is larger than the experimental error when the incident photon energy is close to the L shell absorption edge. Further away from the edge the agreement between experiment and theory is within 3%. The breakdown of the mixture rule close to L absorption edges is discussed.