Title: Structural, Magnetic, and Electronic Properties of the Co-Fe-Al Oxide Spinel System: Density-Functional Theory Calculations

A systematic study of nine binary and ternary spinel oxides formed from Co, Al, and Fe is presented by means of density functional theory. Analysis of the structural, magnetic, and electronic properties through the series of materials is carried out. Preference for the octahedral spinel sites are found in the order Fe<Co<Al. The electronic band gaps of Co{sub 3}O{sub 4} and Fe{sub 3}O{sub 4} are shown to remain largely unchanged as Al is substituted into the lattice forming M{sub 2}AlO{sub 4} (M=Fe,Co), but increase greater than 1 eV for MAl{sub 2}O{sub 4} as the octahedral M metal sites are lost.more » However, for stoichiometric FeAl{sub 2}O{sub 4}, the unsatisfied valence state of Fe results in partial occupation of the conduction band. The results and chemical trends are discussed in terms of atomic site and orbital energies, and in relation to potential photoelectrolysis activity for the splitting of water as a renewable means of hydrogen production.« less

Gadolinium-oxide clusters in various sizes and stoichiometries have been systematically studied by employing the density functional theory with the generalized gradient approximation. The clusters in bulk stoichiometry are relatively more stable and their binding energies increase with the increasing size. Stoichiometric (Gd{sub 2}O{sub 3}){sub n} clusters of n = 1–3 prefer cage-like structures, whereas the clusters of n = 4–30 prefer compact structures layered by wedge-like units and exhibit a rough feature toward the bulk-like arrangement with small disorders of atomic positions. The polyhedral-cages analogous to carbon-fullerenes are stable isomers yet not the minimum energy configurations. Their stabilities can bemore » improved by embedding one oxygen atom or a suitable cage to form core-shell configurations. The mostly favored antiferromagnetic couplings between adjacent Gd atoms are nearly degenerated in energy with their ferromagnetic couplings, resulting in super-paramagnetic characters of gadolinium-oxide clusters. The Ruderman-Kittel-Kasuya-Yosida (RKKY)-type mechanism together with the superexchange-type mechanism plays cooperation role for the magnetic interactions in clusters. We present, as a function of n, calculated binding energies, ionization potential, electron affinity, and electronic dipole moment.« less

First-principle calculations of structural, electronic, elastic and phonon properties of SnMg{sub 2}O{sub 4}, SnZn{sub 2}O{sub 4} and SnCd{sub 2}O{sub 4} compounds are presented, using the pseudo-potential plane waves approach based on density functional theory (DFT) within the generalized gradient approximation (GGA). The computed ground state structural parameters, i.e. lattice constants, internal free parameter and bulk modulus are in good agreement with the available theoretical results. Our calculated elastic constants are indicative of stability of SnX{sub 2}O{sub 4} (X=Mg, Zn, Cd) compounds in the spinel structure. The partial density of states (PDOS) of these compounds is in good agreement with themore » earlier ab-initio calculations. The phonon dispersion relations were calculated using the direct method. Phonon dispersion results indicate that SnZn{sub 2}O{sub 4} is dynamically stable, while SnMg{sub 2}O{sub 4} and SnCd{sub 2}O{sub 4} are unstable.« less

Photoelectron spectroscopy has been conducted for a series of (CrO3)n- (n = 1-5) clusters and compared with density functional calculations. Well-resolved photoelectron spectra were obtained for (CrO3)n- (n = 1-5) at 193 nm (6.424 eV) and 157 nm (7.866 eV) photon energies, allowing for accurate measurements of the electron binding energies, low-lying electronic excitations for n = 1 and 2, and the energy gaps. Density functional and molecular orbital theory (CCSD(T)) calculations were performed to locate the ground and low-lying excited states for the neutral clusters and to calculate the electron binding energies of the anionic species. The experimental andmore » computational studies firmly establish the unique low-spin, non-planar, cyclic ring structures for (CrO3)n and (CrO3)n- for n ≥ 3. The structural parameters of (CrO3)n are shown to converge rapidly to those of the bulk CrO3 crystal. The extra electron in (CrO3)n- (n ≥ 2) is shown to be largely delocalized over all Cr centers, in accord with the relatively sharp ground state photoelectron bands. The measured energy gaps of (CrO3)n exhibit a sharp increase from n = 1 to n = 3 and approach to the bulk value of 2.25 eV at n = 4 and 5, consistent with the convergence of the structural parameters.« less

The structural, electronic, and phonon properties of Li{sub 2}O and Li{sub 2}CO{sub 3} solids are investigated using density functional theory (DFT) and their thermodynamic properties for CO{sub 2} absorption and desorption reactions are analyzed. The calculated bulk properties for both the ambient- and the high-pressure phases of Li{sub 2}O and Li{sub 2}CO{sub 3} are in good agreement with available experimental measurements. The oxygen atoms in the ambient phase of the Li{sub 2}CO{sub 3} crystal are not equivalent as reflected by two different sets of C-O bond lengths (1.28 and 1.31 {angstrom}) and they form two different groups. When Li{sub 2}CO{submore » 3} dissociates, one group of O forms Li{sub 2}O, while the other group of O forms CO{sub 2}. The calculated phonon dispersion and density of states for the ambient phases of Li{sub 2}O and Li{sub 2}CO{sub 3} are in good agreement with experimental measurements and other available theoretical results. Li{sub 2}O(s)+CO{sub 2}(g) {rightleftharpoons} Li{sub 2}CO{sub 3}(s) is the key reaction of lithium salt sorbents (such as lithium silicates and lithium zircornates) for CO{sub 2} capture. The energy change and the chemical potential of this reaction have been calculated by combining DFT with lattice dynamics. Our results indicate that although pure Li{sub 2}O can absorb CO{sub 2} efficiently, it is not a good solid sorbent for CO{sub 2} capture because the reverse reaction, corresponding to Li{sub 2}CO{sub 3} releasing CO{sub 2}, can only occur at very low CO{sub 2} pressure and/or at very high temperature when Li{sub 2}CO{sub 3} is in liquid phase.« less