Effect of hydrogenation on the structure and magnetic properties of an iron oxide cluster

The structure and properties of the Fe₈O₁₂H_n clusters (n = 0-18) are computed using the all-electron density functional theory with the generalized gradient approximation for the exchange-correlation potential. The ground state of Fe₈O₁₂ is found to be a singlet state having a bi-capped octahedron geometry. Upon hydrogenation, the octahedral framework of Fe is retained in Fe₈O₁₂H_n up to n < 7, beyond which point the iron octahedron transforms into a cube. Hydrogen atoms are bound to oxygen atoms up to n = 12, but they bind to the faces of the Fe₈ cube when n > 12. The total spin magnetic moment of a Fe₈O₁₂H_n cluster is larger than 6 μ_B for 1 ≤ n ≤ 18, except for n = 8 and 10, where the lowest total energy states are antiferromagnetic singlets. The reason for this deviation from the general behavior in the Fe₈O₁₂H_n series is attributed to the collective superexchange phenomenon. Surprisingly, the total spin magnetic moment of a Fe₈O₁₂H_n cluster is found to be substantially larger than the total spin magnetic moment of the bare Fe₈ cluster when n = 12-18. All of the Fe₈O₁₂H_n clusters are stable with respect to an abstraction of a single hydrogen atom but are unstable toward the abstraction of an H₂ dimer when n =10 and n = 14-18.