ORDERING PROCESSES IN MONOPHASE CEH2+X

The hydrogen ordering processes in the beta phase of the rare-earth hy dride system CeH2+x are analyzed. It is assumed that, within the discu ssed concentration range (x < 0. 75), the stable states of the metal-h ydrogen system are homogeneous monophase solutions with fixed concentr ations. The description of ordering is based upon the mean-field appro ximation of the static-concentration-waves theory. The relevant energy constants describing the H-H interactions are determined using availa ble experimental data corresponding to the order-disorder transformati on temperatures. It is shown that in the system under consideration th ere exists (in the concentration range x greater-than-or-equal-to 0.35 ), besides the usual order-disorder transformation, an additional orde r-order transition at low temperatures which, in some cases (for x > 0 .44), has a two-step character.