Chemical Structure and Properties
The absorption of hydrogen produces two different phases, both of which contain palladium metal atoms in a face centered cubic (fcc, rocksalt) lattice, which is the same structure as pure palladium metal. At low concentrations up to PdH0.02 the palladium lattice expands slightly, from 3.889 Å to 3.895 Å. Above this concentration the second phase appears with a lattice constant of 4.025 Å. Both phases coexist until a composition of PdH0.58 when the alpha phase disappears. Neutron diffraction studies have shown that hydrogen atoms randomly occupy the octahedral interstices in the metal lattice (in an fcc lattice there is one octahedral hole per metal atom). The limit of absorption at normal pressures is PdH0.7, indicating that approximately 70% of the octahedral holes are occupied. The absorption of hydrogen is reversible, and hydrogen rapidly diffuses through the metal lattice. Metallic conductivity reduces as hydrogen is absorbed, until at around PdH0.5 the solid becomes a semiconductor.
Read more about this topic: Palladium Hydride
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