ORDERING IN THE SYSTEM BETA-TBD2+X

The ordering process in the hydrogen subsystem of the interstitial all oy TbD2+x is described with the mean-field approximation to static-con centration wave theory and with the results of neutron-diffraction exp eriments. It is shown that the measured temperature dependence of the intensity for the superstructure reflections in the alloy with composi tion TbD2.18 is well represented by the calculated temperature depende nce of the square of the order parameter. Furthermore, the computed tr ansition temperatures for alloys with different concentrations x in th e range 0 less-than-or-equal-to x less-than-or-equal-to 0. 25 are clos e to those determined from resistivity and neutron-scattering experime nts.