Hydrogen induced lattice expansion and effective H-H interaction in single phase PdHc

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Abstract
The lattice expansion of Pd during the absorption of hydrogen has been investigated with a combined technique in which the length changes and hydrogen concentration were measured simultaneously and on the same sample in the single-phase region of the PdHc phase diagram above the critical point. For atomic ratios up to c=0.65, the hydrogen induced relative volume changes are found to be Delta V/V0=0.200 c-0.0493c2. This concentration dependence of the lattice expansion, as well as that of the partial molar enthalpy Delta HH of hydrogen in Pd, can be described successfully by taking into account the volume and concentration dependences of the characteristic band-structure energy (EF-Es) introduced in the semi-empirical heat of formation model of Griessen and Driessen. Using this model the thermally broadened density of states in PdHc, as a function of band fillings above the Fermi level in pure Pd, is deduced from the experimental data and it is shown that the strong concentration dependence of Delta V/V0 in PdHc originates from the difference between the volume dependences of the almost completely filled d band at low hydrogen concentrations and the occupied s-p band at higher hydrogen concentrations. In NbHc where such a d band to s-p band 'transition' does not occur the concentration dependences of Delta HH and Delta V/V0 are dominated by the volume dependence of the partially filled d band.