SOLUBILITY ISOTHERMS OF HYDROGEN IN EPITAXIAL NB(110) FILMS

The solubility isotherms of hydrogen in epitaxial Nb(110) films have b een determined via in situ x-ray lattice-parameter measurements. The N b films were grown by molecular-beam epitaxial techniques on Al2O3 (<1 1(2)over bar 0>) substrates with thicknesses ranging from 32 to 527 nm . All isotherms can be well described by the standard expression for t he solubility of a monatomic lattice gas. Aside from the metal-hydroge n interaction, which appears to be bulklike, all other properties exhi bit a strong thickness dependence, including the hydrogen-hydrogen int eraction, the critical temperature for the alpha-alpha' transition, an d the maximum lattice-parameter change that can be reached in saturati on. For the critical temperature we find a scaling with the film thick ness, which most Likely is due to the elastic boundary condition provi ded by the rigid substrate. The clamping of the epitaxial film to the substrate hinders critical fluctuations of macroscopic hydrogen densit y modes from developing, thereby lowering the critical temperature for the alpha-alpha' transition.