Phase transitions of hydrogen in quasi-two-dimensional vanadium lattices

The influence of the strain state on the thermodynamics of hydrogen in quas i-two-dimensional potentials is reported. The host lattice is V embedded in Fe in the form of a Fe/V(001) superlattice, which represents a strongly co nfined absorption potential extending over just 13 monolayers. Delta(H) ove r bar (H) and Delta(S) over bar (H) for the continuous solubility region ar e calculated from the measured isotherms. Two phase transitions are observe d for atomic ratios up to c = 1 in the temperature region 50-300 degreesC. The first transition occurs in the range 0.03 less than or equal to c less than or equal to 0.07, and shows a Curie-Weiss behaviour. No corresponding phase boundary exists in bulk vanadium hydrides. The second transition, at c similar or equal to 0.35 and T < 150 <degrees>C, exhibits large hysteresi s and involves an ordering not previously observed in thin vanadium layers. The site blocking at low concentrations scales linearly with the initial s train and yields a blocking concentration of c = 0.083(1) at zero strain, a s compared to 0.415(9) in bulk V. This difference is ascribed to the finite -size of the host lattice.