Trapping the Excess Electron of FeB-Type CeSi by Hydrogenation ─ Formation of Ferromagnetic Hydrides CeSiHx (TC ≈ 24 K).

Two new hydrides, respectively deuterides, of the FeB-type CeSi were synthesized, and their reaction as well as magnetic behavior and structures were studied. Hydrogenation leads to two phases, a kinetic product retaining the FeB structure with hydrogen-filled tetrahedra (LaGeH type, Pnma, P-phase: CeSiD0.82(2)) and the thermodynamically preferred hydrogen-filled CrB-type structure (ZrNiH type, Cmcm, C-phase: CeSiD0.96(1)). Both hydrides are ferromagnets and feature Curie temperatures of 22.4(1) K (C-phase) and 24.3(1) K (P-phase), which are exceptionally high for cerium intermetallics. The effective magnetic moments of μ(Ce3+) = 2.53 μB are very close to the free ion value of 2.54 μB. The compounds were studied using in situ neutron powder diffraction and magnetic characterization and by density functional theory and chemical bonding analysis.