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

The influence of the strain state on the thermodynamics of hydrogen in quasi-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 confined absorption potential extending over just 13 monolayers. Delta(H) over bar (H) and Delta(S) over bar (H) for the continuous solubility region are calculated from the measured isotherms. Two phase transitions are observed 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 hysteresis and involves an ordering not previously observed in thin vanadium layers. The site blocking at low concentrations scales linearly with the initial strain and yields a blocking concentration of c = 0.083(1) at zero strain, as compared to 0.415(9) in bulk V. This difference is ascribed to the finite-size of the host lattice.