X-ray spectra show evidence of unexpected substitutional alloying under high pressure

In classical metallurgy, there exists a very famous rule known as Hume-Rothery's rule, which describes the conditions necessary for the formation of a solid solution from two independent metals. In order to have a substitutional crystalline solid solution in which the atoms of one element randomly substitute for atoms of another element in a crystal structure, the components must have an atomic size within 15% and electronegativity within 0.4 of each other. According to this rule, a Ce-Al solid solution cannot be obtained. Recently, a research team led by Professor H.K. Mao (Carnegie Institution of Washington) and Professor R. Ahuja (Uppsala University) found during high pressure research on the intermetallic compound of Ce₃Al that a solid solution is formed in a Ce-Al system. The differences in radii and electronegativity of Ce and Al were diminished by applying pressure. Both synchrotron X-ray studies (XRD and X-ray absorption spectroscopy) and ab initio calculations showed the same cause for bringing the two elements closer in radii and electronegativity, resulting in the new alloy phase. Even after the release of pressure, this substitutional alloy remained. During in-situ X-ray absorption measurements at the Ce LIII edge, conspicuous changes in the sharpness of the absorption, correlated to delocalization of 4f electrons, were observed. For more information, see the paper, "Substitutional alloy of Ce and Al", Q-S.Zeng et al., Proceedings of National Academy of Science, 106, 2515-2518 (2009) Published online before print February 2, 2009, doi: 10.1073/pnas.0813328106