Hydrogen in thin epitaxial Nb films

Abstract The solubility isotherms of hydrogen in epitaxial Nb(100) films have been determined via in situ X-ray lattice-parameter measurements. The Nb films were grown by molecular-beam epitaxial techniques on R-Al 2 O 3 substrates with different thicknesses varying from 34 to 204 nm. Similar to Nb(110) films, the maximum out-of-plane lattice expansion scales inversely with Nb(100) film thickness. Furthermore, the critical temperature T c for the α-α′ phase transition as derived from the solubility isotherms depends strongly on the film thickness. For both orientations we determine similar T c ’s but for film thicknesses below 30 nm, T c in Nb(100) films are higher by about 80 K as compared to Nb(110) films. The influence of finite size scaling and adhesion of the films to the substrates on the solubility isotherms will be discussed.