Effect of biaxial elastic constraints on H-H interactions in ultrathin vanadium

The hydrogen (H) solubility isotherms and the relative partial enthalpy- and entropy of H in a MoV/V(001) superlattice in the range 0less than or equal to[H/V]less than or equal to1 [atomic ratio] are addressed. The difference in solubility between MoV and V assures that H only resides in the 22Angstrom thick V layers, and the use of MoV instead of Mo increases the range of reversibility of the uptake, for the actual strain state. The biaxial elastic constraints, imposed by the H-free layers and the substrate, are shown to have a big influence on the H-H interaction. At low H content, the mean H-H interaction is close to zero, while for [H/V]greater than or similar to0.25, the interaction becomes attractive. We show that for a film constrained to expand in one dimension, the elastic dipole interaction is zero when the H-induced local strain field is isotropic, whereas for an anisotropic local strain field the interaction energy can be attractive, if the strain axes are parallel aligned with the direction of the expansion. Based on the symmetries of the respective local strain fields, we interpret the onset of the attractive H-H interaction at [H/V]approximate to0.25 as a self-induced change of H site occupancy, from tetrahedral to octahedral sites.