ORDER-DISORDER TRANSITION IN THE SYSTEM CEH2+X

The ordering mechanism in the excess-hydrogen system of the interstiti al alloy CeH2+x is described based on the mean-field approximation of the static-concentration-waves theory and using the results or neutron -diffraction experiments. The computed order-disorder transition tempe ratures (normalized at c = 0.25) agree with the various available expe rimental data quite satisfactorily, in the concentration range 0.13 le ss than or similar to c less than or similar to 0.5, but seem to be ov erestimated for c > 0.5, probably due to the increasing role of the la ttice contraction with increasing c. The character of the transition c hanges progressively from first to second order above a critical conce ntration c(o) approximately 0.3.