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Wikipedia: Chemical structure and properties of palladium hydride
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.
The bulk adsorption and rapid diffusion mean that the hydrogen atoms can tunnel from the centre of one octahedral site to the next with a relatively high amplitude. If metal impurities (say, a Calcium atom) also like to occupy the same insterstices that hydrogen propagates in, then the interaction cross-section of the hydrogen with the impurity atoms would be enhanced with respect to the gas phase.

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by Migeru (migeru at eurotrib dot com) on Sat Jan 12th, 2013 at 12:47:53 PM EST
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