Thesis - Oztek_Muzaffer_T_200508_MA

Figure 4. CaCu5-type P6/mmm structure of LaNi 5 [51]. Gray spheres indicate La atoms.
Pure metals and alloys usually have simple crystal structures like body centered
cubic, face centered cubic or hexagonal close packed structures. Hydrogen atoms usually
occupy either tetrahedral or octahedral interstices in those structures [52]. Interstitial sites
occupied by hydrogen in the host lattice result in expansion of the crystal lattice. Each
hydrogen interstitial causes displacements of the metal atoms from their regular sites and
the resulting crystal lattice distortions cause physical property changes. Volume
expansions of 20% are typical for the solution of one hydrogen atom per metal atom [52].
This phenomenon is responsible for the lower terminal H solubility in block samples of
LaNi 5 , compared to powder samples, because of the limited expansibility of block
samples with larger crystal grains as opposed to the ready stress relaxation of smaller
particles [53].
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