Contribution of optical phonons to the thermal expansion of α'-PdHxand α'-PdDx

From thermal expansion measurements between 10 and 250K on alpha '-PdHx and alpha '-PdDx with 0.63<or=x<or=0.70 the authors determine the average optical phonon frequencies omega H and omega D of PdHx and PdDx. The average volume Gruneisen parameter of the optical phonons is gamma opt=3.5+or-0.2 and that of the acoustic phonons is gamma ac=2.5+or-0.2. The Debye temperature for the acoustic phonons is equal to that of pure palladium within the experimental uncertainties.