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

The structure and properties of the Fe8O12Hn 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 Fe8O12Hn 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 Fe8O12Hn cluster is larger than 6 µB 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 Fe8O12Hn series is attributed to the collective superexchange phenomenon. Surprisingly, the total spin magnetic moment of a Fe8O12Hn 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 Fe8O12Hn 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.

A comparative study of small 3d-metal oxide (FeO)n, (CoO)n, and (NiO)n clusters.

Geometrical and electronic structures of the 3d-metal oxide clusters (FeO)n, (CoO)n, and (NiO)n are computed using density functional theory with the generalized gradient approximation in the range of 1 ≤ n ≤ 10. It is found that the cluster geometries are similar in the (FeO)n and (CoO)n series but noticeably different in the (NiO)n series for several values of n. All of the lowest total energy states are found to possess relatively small spin multiplicities and are either antiferromagnetic or ferrimagnetic except for the states of (NiO)3, (NiO)4, (NiO)9, and (NiO)10, which are ferromagnetic. The computed polarizabilities per atom undergo a steep decrease when compared to the atomic values of the MO monomers (M = Fe, Co, and Ni). Surprisingly, the polarizability does not strongly depend on either M or n in all the considered series when n varies from 3 to 10. The binding energies per atom are the largest in the (FeO)n series, followed by the binding energies of (CoO)n and (NiO)n.

Structure and Properties of Polyfluoride Fn(-) Clusters (n = 3-29).

The electronic and geometrical structures of the neutral Fn and singly negatively charged Fn(-) polyfluorides (n = 3-29) are studied using three levels of theory: density functional theory (DFT) with generalized gradient approximation, hybrid Hartree-Fock-DFT, and hybrid HF-DFT with long-range corrections. For n > 4, each polyfluoride possesses a number of states with different geometries that are closely spaced in total energy. The geometrical structures of the lowest total energy states follow different patterns for the even-n and odd-n Fn(-) anion branches with a preference for higher symmetry geometries. The largest F29(-) anion considered is found to possess Oh symmetry. All the anions beginning with F3(-) are found to possess adiabatic and vertical electron detachment energies exceeding the electron affinities of halogen atoms and are therefore superhalogen anions. Electron affinities, energies of formation, and binding energies show oscillatory behavior as functions of the number n of fluorine atoms. The neutral Fn species are found to be barely stable and are bound by polarization forces. The Fn(-) anions, on the contrary, are quite stable toward the loss of F, F(-), and F2(-), but not to the loss of F2.

An all-electron density functional theory study of the structure and properties of the neutral and singly charged M12 and M13 clusters: M = Sc-Zn.

The electronic and geometrical structures of the M12 and M13 clusters where M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn along with their singly negatively and positively charged ions are studied using all-electron density functional theory within the generalized gradient approximation. The geometries corresponding to the lowest total energy states of singly and negatively charged ions of V13, Mn12, Co12, Ni13, Cu13, Zn12, and Zn13 are found to be different from the geometries of the corresponding neutral parents. The computed ionization energies of the neutrals, vertical electron detachment energies from the anions, and energies required to remove a single atom from the M13 and M13(+) clusters are in good agreement with experiment. The change in a total spin magnetic moment of the cation or anion with respect to a total spin magnetic moment of the corresponding neutral is consistent with the one-electron model in most cases, i.e., they differ by ±1.0 µ(B). Exceptions are found only for Sc12(-), Ti12(+), Mn12(-), Mn12(+), Fe12(-), Fe13(+), and Co12(+).

Dimerization of defect fullerenes and the orientational phase transition in oxidized C60 fullerite.

Oxidized fullerite was obtained by heating a fullerite sample intercalated with oxygen, (O2)0.44C60, up to 300 degrees C. Orientational phase transitions in the oxidized fullerite are studied using differential scanning calorimetry (DSC) and have been found to possess a specific enthalpy whose value is lower by 25% than in the initial (O2)0.44C60 sample. In order to find possible reasons for hindrance to the buckyball rotations, we performed optimizations of defect buckyball fullerenes C60-n with different distributions of vacancies along with the dimers C60-n-C60-n and C60-C60-n for n = 1-4 using density functional theory with generalized gradient approximation. We found that the dimerization energy ranges from 1.07 eV (C58-C58) to 6.56 eV (C56-C56) and from 1.81 eV (C60-C58) to 4.29 eV (C60-C56), respectively. The formation of such dimers, which could in addition interact with defect buckyball cages and form larger aggregates, is to be related to the lowering of the orientational transition enthalpy.