LOW-TEMPERATURE STATES IN THE BETA-CEH2+X PHASE

The low temperature ordering processes in the single-phase system beta -CeH2-x are analysed using the mean-field approximation of the static concentration wave theory. It is applied to the subsystem of the x hyd rogen atoms located in the octahedral interstices;of the f.c.c. metal lattice. It is shown that, depending upon the value of x, these hydrog en atoms form, in the low temperature, limit two different ordered sta tes, in agreement with the neutron diffraction data. In addition, the boundary values x(1) and x(2) (where both ordered states are equally f avourable) coincide closely with the concentrations of the low tempera ture ''pseudocubic'' phases observed in the X-ray experiments.