Structural Phase Transition of Ruthenium Cluster Hydrides

We present a study of the structural effects of hydrogen adsorption on Ru₁₄^– investigated by a combination of trapped ion electron diffraction and density functional computations. While the bare Ru₁₄^– forms a double layer hexagonal structure, the adsorption of hydrogen initiates an evolution of the metal core toward an icosahedral structure. The structural rearrangement is driven by the difference in mean hydrogen adsorption energies, gradually stabilizing the icosahedral cluster core structure for higher hydrogen coverage. Detailed temperature dependent measurements reveal a crossover between the two structure motifs and indicate a structural phase equilibrium. Accompanying free energy computations confirm the chemical equilibrium and identify a hydrogen coverage instability region where the cluster hydrides decompose into the two core isomers with different hydrogen coverage.