| Abstract
| - Hydrogenation of La3Pd5Si leads to the formation of three hydride phases. The α-phase contains isolated [Pd−D−Pd] fragments, which are joined into polymeric (−Pd−D−Pd−)nzig-zag chains in the β-phase. In the γ-phase a partial interruption of the zig-zag chains and the formation of isolated [D−Pd−D−Pd] and [D−Pd−D−Pd−D] fragments occur that is attributed to the onset of repulsive Si−D and D−D interactions and the competition between silicon and deuterium which act as transition metal ligands.
- Pressure-composition isotherm measurements show that the ternary lanthanum palladium silicide phase La3Pd5Si absorbs reversibly up to 5 hydrogen atoms per formula unit at 550 K and 14 bar hydrogen pressure. In-situ synchrotron and neutron powder diffraction reveals three phases, an α-phase having the limiting composition La3Pd5SiD∼1.6 at low deuterium pressure (at up to 9.5 bar D2 and 550 K), a β-phase La3Pd5SiD∼2.30−4 at intermediate deuterium pressure (<9.5 bar D2 and 550 K), and a relatively unstable γ-phase La3Pd5SiD∼5 at high deuterium pressure (obtained at 75 bar D2 and 293 K). While the α and β phases retain the symmetry of the H-free La3Pd5Si (space group Imma), the γ-phase undergoes a symmetry lowering (aγ∼aβ, bγ∼3bβ and cγ∼cβ, Vγ∼3Vβ, space group Pmnb). The structure of the α-phase contains isolated [Pd−D−Pd] fragments, which are joined into polymeric (−Pd−D−Pd−)nzig-zag chains in the β-phase. In the γ-phase some D sites depopulate, while new D sites are occupied, thus leading to a partial interruption of the zig-zag chains and the formation of isolated [D−Pd−D−Pd] and [D−Pd−D−Pd−D] fragments. This unexpected behavior can be attributed to the onset of repulsive Si−D and D−D interactions (Si−D > 3.0 Å, D−D > 2.1 Å) that divide the structure into Si-poor slabs that absorb hydrogen and Si-rich slabs that do not. The competition between silicon and deuterium which act as a transition metal ligand is further underlined by the fact that Pd atoms having one Si ligand are capable of forming Pd−D bonds, whereas Pd atoms having two Si ligands are not.
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