HYDROGEN ORDERING IN SUPERSTOICHIOMETRIC RARE-EARTH HYDRIDES FOR A SYSTEM WITH AN ENERGY-CONSTANTS RATIO P=V-2 V-1-LESS-THAN-1 - LAH2+X/

The processes of hydrogen ordering in the beta phase of rare-earth hyd rides RH(2+x) depend on the hydrogen concentration in the set of octah edral interstices x and are determined by the value of the parameter p = V-2 /V-1, the ratio of energy constants characterizing the interact ions between the octahedral H atoms. Applying the mean-field approxima tion of the static-concentration-waves theory we had analyzed the case s of p=1 and p>1 in our previous publications which corresponded, resp ectively, to the systems TbH2+x and CeH2+x. In the present article, we have investigated the processes of H ordering in the La-H system, rep resentative of the case p<1. An important result is the prediction of two transition temperatures corresponding to the separate vanishing of the two order parameters present. The theoretical treatment is applie d to recent neutron-diffraction data by Udovic et nl. on LaD2.25 and c ompared to other earlier obtained experimental results obtained on LaH 2+x compounds.