Site Selectivity of Hydrogen in Metals and Alloys

Site Selectivity of Hydrogen in Metals and Alloys.—Entropy Data for PdjH. The partial molar entropy (5h) of H in Pd and its alloys is of great interest because of the information which can be obtained concerning the nature and distribution of H atoms in the metal lattice. At low temperatures hydrogen atoms are randomly distributed over the octahedral sites in the lattice, of which there is 1 per Pd atom. However, at high temperatures it is possible that both tetrahedral and octahedral sites are partially occupied. 

To summarize the arguments on which this proposal is based it is convenient to separate the experimental entropy Sh into individual entropy terms as follows 

Magerl, N. Stump, H. Wipf, and G. Alefeld, J. Phys. Chem. Solids, 1977, 38, 683. 

the electronic term, has been calculated from the electronic specific heat of Pdto be — lOJK mol . Boureau et al. point out that the temperature dependence of the electronic specific heat means that this value overestimates S2 by a factor of 2 or 3. They calculate S2 to be —6.7 JK mol but use an average value of —8.3 JK mol  

6 JK mol . Boureau and Kleppa point out that equally good agreement between experimental and calculated entropies can be obtained by setting S,(Pd) = 

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