Reversible structural change and thermodynamic properties of hydrogen in thin vanadium films

The absorption of H in 50 nm V(001) films under pressures of 1-10(4) Pa H-2 and at temperatures between 310 and 530 K was studied using in situ combined x-ray diffraction (XRD) and electrical resistivity measurements. The H-induced XRD peak shifts clearly indicate a fully reversible and nondeteriorating first-order phase transformation for the film. The H-induced residual resistivity changes in the V film demonstrate several extreme points (maxima and minima) apparently associated with some special points on the pressure isotherms of the film both below and above the critical temperature (T-c=460 +/- 20 K). Correlating these extreme points with the known V(bulk)-H phase diagram indicates that, in addition to the first-order transformation, two additional reversible second-order transitions exist in the film. The enthalpy and entropy of this transformation, obtained from the temperature dependence of the plateau pressures, are -39 +/- 1 kJ/mol degrees K, and -60 +/- 2 J/mol, respectively. The thermodynamic constants and the lattice parameters of the phases for the first-order transformation in the film closely correspond to the alpha ->beta phase transformation in bulk V.