Hydrogen in metals structure, diffusion and tunnelling

Hydrogen ions can diffuse into the interstitial wells between atoms in many metals to form solid solutions MH, or ordered metal hydride phases. Such materials are technically useful for the storage and purification of hydrogen, but absorbed H may also lead to unwanted brittleness in steel and other construction metals. The interesting material problems and the possible technical applications have led to large efforts over many years in studies of hydrogen absorbed in metals and its diffusion mechanisms. Lately, the promise of cheap energy from cold fusion of deuterium in metal hydrides has made most materials scientists aware of this field of research. 

We will, in a simple way, review some properties of absorbed hydrogen in metals and especially the quantum mechanical tunnelling that often determines diffusion and other dynamic effects. For more details we refer to several reviews and a vast literature. 

Dissolved hydrogen in metals may be thought of as single ions that are located on certain interstitial sites and that have given their electrons to the conduction bands. However, the electronic band picture is not very good for such solid solutions since protons distributed at random on sites destroy the periodicity of the lattice. There must also be a shielding electronic cloud near each H . Calculations of the electronic properties of the dissolved H and its heat of solution, AH, relative to that of ( )H2 in the gas phase, are therefore difiicult and uncertain. Interactions between H ions will also influence AH and make it concentration-dependent. 

Experimentally, AH/k is negative and of order — 5000 K for H in Pd, V, Nb, Ta, Ti, etc. Consequently, these metals readily absorb large 

The most stable hydride of palladium is PdHo.e, but hydrogen contents up to nearly PdH can be made with electrolytic charging. 

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