Hydrogen absorption in epitaxial bcc V (001) thin films analysed by statistical thermodynamics

Andersson, Aits and Hjörvarsson of Uppsala University measured hydrogen uptake in epitaxial bcc (body centred cubic) vanadium (V) (001) thin films of thickness, 50 nm and 100 nm, over temperature range between 443 K and 513 K. The reported equilibrium pressure–temperature–composition (P–T–C) relationships for the epitaxial bcc V (001) thin films showed appreciable extent of enhancement of H solubility compared with that for bulk bcc V. In this work, the reported equilibrium P–T–C relationships for the epitaxial bcc V(001) thin films by Andersson et al. were analysed in terms of statistical thermodynamics for H2 gas partial pressure p(H2) up to 100 Pa and H/V mole atom ratio x in VHx up to 1. The present analysis results showed that, up to x = 0.75, the state of H in the V lattice was comparable to that in bulk VHx specimen but that, in the range of x higher than 0.75, state of H in the thin film with the constrained basal plane condition was evidently distinguishable from that in non-constrained bulk VHx. This was concluded to be the consequence of the tetragonal distortion of the bcc lattice with biaxially constrained condition at the bottom surface of the VHx (001) thin film in the range of x exceeding 0.75.