ELECTRONIC-STRUCTURE OF CE AND SM IN HYDRIDES AND THE 4F COLLAPSE IN YBHX

The population of the 4f, 5d, and 6s shells of rare-earth atoms in RH, hydrides (R=Ce, Sm, Yb; x approximate to 2-3) has been studied by the x-ray line-shift method. The population of the 5d and 6s shells of Ce acid Sm atoms, and the charge on them in metals and hydrides, were de termined from experiment and calculated within the Hartree-Fock-Dirac (Koopmans) model. The decrease of the charge on Ce and Sm revealed upo n transition from the metal to the hydride argues unambiguously for th e anionic (hydride) model. In YbHx with x greater than or equal to 2, the structural transition is accompanied by a strongly pronounced elec tronic transition from divalent to a noninteger-valence state [(m) ove r bar(x = 2.25-2.708) = 2.66+/-0.02]. (C) 1998 American Institute of P hysics. [S1063-7834(98)00108-7].