Inclusion Of On-site Coulomb Interaction Research Articles

The electronic structure and the oxygen adsorption at BaO terminated surface of GdBaCo2O5.5: A first principles study

The double perovskites GdBaCo2O5.5 (GBCO) is candidate cathode materials for application in intermediate temperature solid oxide fuel cells (IT-SOFCs), attributing to excellent electrical conduction properties and oxidation activity. The electronic structure and the oxygen adsorption at BaO terminated surface of GBCO are investigated using projector augmented-wave (PAW) method based on spin-polarized density functional theory (DFT) with the inclusion of on-site Coulomb interaction. The BaO-terminated surface is adopted according to recent experimental results that this termination is energetically favorable for the double perovskites GBCO. We predict that G-AFM configuration is the most stable structure. The calculated results of oxygen vacancy energy indicate that the lowest energy vacancy positions are in the GdO layers. Charge transfer for the adsorbed oxygen and the surface ions is also examined by Bader charge analysis. The oxygen vacancies can facilitate oxygen adsorption and catalyze the bond cleavage. The existence of oxygen vacancy strengthens surface activity by acting as an active site at the surface. These results at the atomic level can contribute to the detailed understanding of oxygen reduction reaction mechanism.

Electronic structure and magnetic properties of PbMO3 (M = Fe, Co, Ni) magnetic perovskites: An ab initio study

We present the electronic, magnetic and structural properties of the magnetic transition metal oxides PbMO 3 (M = Fe , Co , Ni ) in cubic perovskite structure. The calculations are based on the density functional theory (DFT) within plane-wave pseudopotential method and local spin density approximation (LSDA) of the exchange-correlation functional. On-site Coulomb interaction is also included in calculations (LSDA+ U ). The systems are considered in ferromagnetic (FM) and G-type antiferromagnetic (G-AFM) order. FM structures are energetically more favored than G-AFM and than non-magnetic states for all the systems studied. The spin-polarized electronic band structures show that all the structures have metallic property in FM order without Hubbard-U interaction (U eff = 0). However, the inclusion of on-site Coulomb interaction (U eff = 7 eV ) opens a semiconducting gap for majority spin channel of PbFeO 3 and of PbNiO 3 resulting in a half-metallic character. PbCoO 3 system remains as metallic with LSDA+ U scheme. Bonding features of all structures are largely determined by the hybridizations between O–p and d-states of transition metal atoms. The partial magnetic moment of Fe atom in PbFeO 3 is enhanced by inclusion of Hubbard-U interaction (2.55 μB ⇒ 3.78 μB). Total magnetic moments of half-metallic PbFeO 3 and of PbNiO 3 compounds are very close to integer values.

Density functional theory study on the interaction of CO with the Fe3O4(0 0 1) surface

The adsorption properties of CO on the nondefective and defective (with an oxygen vacancy) B-layer Fe3O4(001) surfaces (an octahedral environment of iron ions) at different coverages are studied using the spin-polarized density functional theory with the inclusion of on-site Coulomb interaction by introducing Hubbard U parameter (DFT+U) method. It is found that the CO adsorption energy in general increases with the CO coverage. Both types of B-layer Fe3O4(001) surfaces have the excellent ability for CO oxidation. Comparatively, the defective B-layer Fe3O4(001) surface has a much stronger CO adsorption ability and a lower CO oxidation ability than the nondefective B-layer Fe3O4(001) surface. The density of state reveals the bonding mechanism of CO on the two surfaces. The effects of the ambient conditions on the CO adsorption processes are analyzed.

Energetics and electronic structure of GaN codoped with Eu and Si

First principles calculations using pseudopotentials and generalized gradient approximation (GGA) for the exchange--correlation energy show that addition of Si makes Eu doping in GaN energetically favorable. It breaks local symmetry around Eu ions and leads to shallow states below the conduction band that could facilitate intra-4$f$ shell transitions. Silicon atoms on Ga sites act as intrinsic donors transforming Eu from a 3$+$ to a 2$+$ state. The half-filled 4$f$ states with a 7 ${\ensuremath{\mu}}_{\mathrm{B}}$ magnetic moment on each Eu ion lie within the band gap of GaN and are narrower compared with the only-Eu doping case due to reduced hybridization with the host states. There is a tendency for clustering of Eu ions with ferromagnetic coupling and the \ensuremath{\sim}5-\AA{} interatomic distance, but EuN phase formation is unfavorable. Further effects of the inclusion of onsite Coulomb interaction U within GGA+U formalism on the electronic structure are discussed.

Spin states and hyperfine interactions of iron in (Mg,Fe)SiO 3 perovskite under pressure

With the guidance of first-principles phonon calculations, we have searched and found several metastable equilibrium sites for substitutional ferrous iron in MgSiO 3 perovskite. In the relevant energy range, there are two distinct sites for high-spin, one for low-spin, and one for intermediate-spin iron. Because of variable d-orbital occupancy across these sites, the two competing high-spin sites have different iron quadrupole splittings (QS). At low pressure, the high-spin iron with QS of 2.3–2.5 mm/s is more stable, while the high-spin iron with QS of 3.3–3.6 mm/s is more favorable at higher pressure. The crossover occurs between 4 and 24 GPa, depending on the choice of exchange-correlation functional and the inclusion of on-site Coulomb interaction (Hubbard U). Our calculation supports the notion that the transition observed in recent Mössbauer spectra corresponds to an atomic-site change rather than a spin-state crossover. Our result also helps to explain the lack of anomaly in the compression curve of iron-bearing silicate perovskite in the presence of a large change of quadrupole splitting, and provides important guidance for future studies of thermodynamic properties of this phase.

Metal-insulator transition in double cobaltites RBaCo2O5.5 (R = Eu, Gd): Specific features of their optical properties

The temperature dependence of the optical properties of GdBaCo2O5.5 in the region of the metal-insulator transition was studied in the spectral range λ = 4.0–0.258 μm (E = 0.3–4.8 eV) by the spectroscopic ellipsometry method. The electronic structure of the crystal at T = 300 K was calculated by the linearized muffin-tin orbital method in the local-density approximation with inclusion of on-site Coulomb interaction (LDA + U). The general features of the behavior of the optical properties of GdBaCo2O5.5 and EuBaCo2O5.5 are discussed. It is shown that the optical response on the metal side of the metal-insulator transition is determined by the redistribution of the optical-conductivity spectral weight in going from high to low energies and is significantly incoherent in character. The optical band gap width of the GdBaCo2O5.5 is experimentally determined to be Eg = 0.26 eV, which is close to a theoretical estimate. The type of interband transitions is determined from analyzing the temperature dependence of the optical functions for absorption bands.

Theoretical investigation of magnetic order inRFeAsO(R=Ce,Pr)

Density-functional theory calculations are carried out on $R\text{FeAsO}$, $R=\text{Ce}$ and Pr, the parent compounds of the high-${T}_{c}$ superconductors $R{\text{FeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$, in order to determine the magnetic order of the ground state. It is found that the magnetic moments on the Fe sites adopt an antiferromagnetic order, with a stripelike pattern similar to the case of LaFeAsO. Within the generalized gradient approximation along with Coulomb on-site repulsion $(\text{GGA}+U)$, we show that the $R$ magnetic moments also adopt an antiferromagnetic order for which, within the $R\text{O}$ layer, the same spin $R$ sites lie along a zigzag line perpendicular to the Fe spin stripes. While within GGA the $R\text{ }4f$ band crosses the Fermi level, upon inclusion of on-site Coulomb interaction on the $R$ sites the $4f$ band splits and moves away from the Fermi level. If on-site Coulomb interaction is also included on the Fe sites, $R\text{FeAsO}$ is shown to change from a semimetal to a Mott insulator.

DFT+<em>U</em> Study on the Interaction of Water Molecule and Ceria (111) Surface

The interaction of water molecule and a ceria (111) surface was investigated using DFT+U (density functional theory with the inclusion of on-site Coulomb interaction by introducing Hubbard U parameter) method. The results show that water molecules adsorb on the oxidized ceria (111) surface through a single H-bond configuration and they do not decompose. On a reduced ceria (111) surface, the water molecules adsorb through a non-H-bond configuration. Furthermore, water molecules prefer to dissociate and form a hydroxyl surface. The hydroxyl surface is much more stable than the physisorption state of H2 on the oxidized ceria (111) surface. In other words, reoxidation of the reduced ceria (111) surface through the dissociation of the hydroxyl surface and the generation of H2 molecules is an endothermic process. Therefore, there are mainly two adsorption states for water molecules on the reduced ceria (111) surface: i) chemisorption through a non-H-bond configuration and ii) dissociative adsorption with a hydroxyl surface. The hydroxyl surface may dissociate under certain conditions and reoxidize the reduced ceria (111) surface.

Facilitated vacancy formation at Zr-doped ceria(1 1 1) surfaces

The effects of Zr doping on the atomic and electronic properties of the ceria(1 1 1) surface are studied using first-principles density functional theory with the inclusion of on-site Coulomb interaction. The atomic structures, electronic structure, and the vacancy formation energies of the Zr-doped and undoped ceria(1 1 1) surfaces are compared. It is found that (i) Zr doping induces a severe distortion of the unreduced surface structure; (ii) at the reduced Zr-doped ceria(1 1 1) surface, the oxygen anions around the oxygen vacancy show much larger displacements than those on the pure CeO 2(1 1 1) surface; (iii) an oxygen vacancy is more easily formed around the Zr dopant, and the reduction energy is lowered by about 0.5 eV; (iv) the excess electrons left by the removed oxygen atom localize on the two Ce cations neighboring the vacancy and thus brings about the reduction of the two Ce 4+ ions; and (v) the atomic structure modification induced by the Zr doping plays a vital role in facilitating the reduction of the ceria–zirconia solid solution as compared to the pure ceria.

The adsorption of Au on Zr-doped CeO2(110) surface: A first-principle study

The adsorption of Au on the (110) surfaces of CeO2 and Zr-doped CeO2 were studied using projector-augmented wave (PAW) method based density-functional theory (DFT) within the gradient approximation (GGA) and with the inclusion of on-site Coulomb interaction (DFT+U). It is found that, due to the doping of Zr, the adsorption energies of Au are increased and the strongest adsorption geometry is altered. The doping of Zr results in larger distortion in the structure of the substrate, and enhances the catalytic activity of the Au/CeO2(110) system and the oxidization of Au. These results may lead to a better understanding for the Au/Zr-doped CeO2 catalysts and give some clues for improving the efficiency of the three-way catalysts (TWC).

Structural and electronic properties of NM-doped ceria(NM = Pt,Rh): a first-principles study

The effects of noble metal (NM) dopants(NM = Pt,Rh) on the structural and electronic properties of ceria are studied using a densityfunctional theory (DFT) method, with the inclusion of on-site Coulomb interaction(DFT+U). It is found that these NM dopants give rise to large perturbations of the atomic andelectronic structures of ceria and induce metal-induced gap states at the Fermi levelsuitable for accommodating extra electrons, thereby lowering the reduction energy of ceriaand making the NM-doped cerias more reducible than pure ceria. This mechanism forfacilitating the reduction of ceria is different from that of Zr-doped ceria where theincreased reducibility is largely due to the structural distortions and not to electronicmodifications.

The electronic and reduction properties of Ce 0.75Zr 0.25O 2(1 1 0)

The electronic and chemical reduction properties of the Ce 0.75Zr 0.25O 2(1 1 0) surface are presented based on DFT + U calculations, i.e. density functional theory with the inclusion of on-site Coulomb interaction. Experimental studies in the literature have shown that redox activity and thermal stability of ceria are significantly enhanced by the presence of zirconia dopants. In the present theoretical study it is found that mixing zirconia into ceria leads to important structural and chemical consequences such as non-equivalent O atoms at the surface, lower reduction energy and larger surface relaxation. The electronic mechanisms behind these modifications are analysed.

Oxygen vacancy formation energy in Pd-doped ceria: A DFT+U study

Using the DFT+U method, i.e., first principles density functional theory calculations with the inclusion of on-site Coulomb interaction, the effects of Pd doping on the O vacancy formation energy (E(vac)) in CeO(2) has been studied. We find that E(vac) is lowered from 3.0 eV in undoped ceria to 0.6 eV in the Pd-doped compound. Much of this decrease can be attributed to emerging Pd-induced gap states above the valence band and below the empty Ce 4f states. These localized defect states involve the Pd ion and its nearest neighbors, which are also the main acceptors of the extra electrons left on reduction. The effect of the Pd dopant on the geometric structure is very modest for CeO(2) but considerable for CeO(2-x).

First-principles study of thePt∕CeO2(111)interface

The adsorption of platinum on the ceria (111) surface may change the reduction and redox properties of $\mathrm{Ce}{\mathrm{O}}_{2}$. The density functional theory with the inclusion of on-site Coulomb interaction has been used to study the properties of Pt adsorbed on the (111) surfaces of stoichiometric and reduced ceria. In all cases, strong interactions are found between the Pt and the surfaces. The Pt-O-Ce bonds may be formed between the Pt adatom and the surface in the preferred adsorption mode. The adsorption on the reduced surface is stronger than on the unreduced surface. The formation energy of an O vacancy at the $\mathrm{Pt}∕\mathrm{Ce}{\mathrm{O}}_{2}(111)$ interface is lower than on the clean $\mathrm{Ce}{\mathrm{O}}_{2}(111)$ surface due to the existence of the lower lying partially occupied metal induced gap states including mainly the $\mathrm{Pt}\phantom{\rule{0.2em}{0ex}}4d$ states below the empty $\mathrm{Ce}\phantom{\rule{0.2em}{0ex}}4f$ band.

Oxygen vacancy formation energy at the Pd/CeO2(111) interface

The atomic and electronic structure of Pd adsorption on the unreduced and reduced ceria (111) surfaces is studied using first principles projector-augmented-wave (PAW) method based density functional theory (DFT) within generalized gradient approximation (GGA) and with the inclusion of on-site Coulomb interaction (GGA + U). It is found that there exist different adsorption features for Pd on different surfaces (unreduced and reduced): (1) on the unreduced CeO2(111) surface, Pd prefers to be adsorbed on the O-bridge site (O2-site); (2) on the reduced CeO2(111) surface, the adsorption is much stronger than that on the unreduced CeO2(111) surface; (3) the formation of an oxygen vacancy is easier at the Pd/CeO2(111) interface than that on the clean CeO2(111) surface, indicating that the Pd adatom enhances the oxygen storage capacity (OSC) of ceria, and the interaction between the palladium catalyst and the ceria support enhances the lower temperature activity of the catalyst. The mechanisms for this promotion in the OSC of ceria at the Pd/ceria interface are discussed.

The Interaction between Pd and CeO2(111) surface: A first-principle study

The adsorption properties of Pd on CeO2(111) surface are studied using the first principle projector-augmented-wave (PAW) method based density functional theory (DFT) within generalized gradient approximation (GGA) and with the inclusion of on-site Coulomb interaction (DFT+U). It is found that there exist different adsorption features for different coverages of Pd: (1) For one monolayer (ML) Pd adsorption on CeO2(111) surface, Pd prefers to be adsorbed on the atop O site leaning toward the Ce-bridge site; while, for the 1/4 ML adsorption, Pd prefers to be adsorbed on the O-bridge site leaning toward the atop subsurface O site. (2) The interaction between the adsorbed Pd atoms is very strong when the coverage is one ML; on the other hand, there is almost no interaction between the Pd atoms for the 1/4 ML Pd adsorption, correspondingly, the interaction between Pd adatoms and CeO2(111) substrate is stronger for lower coverage adsorption (1/4 ML) than that for higher coverage adsorption (1 ML). (3) The adsorption of Pd disturbes the CeO2(111) surface structure in the vicinity of the adsorption site. (4) The Pd adsorption makes the Pd/CeO2 more active as compared with the clean CeO2(111) and bulk Pd metal, and there may exist some active sites at the Pd/CeO2 interface. These studies may lead to a better understanding for the Pd/CeO2 catalysts and give some hints to improve the efficiency of TWC.

Effects of Zr doping on stoichiometric and reduced ceria: A first-principles study

The Zr doping in CeO(2) may change the reduction properties and therefore the redox properties of CeO(2). Using first-principles density functional theory with the inclusion of on-site Coulomb interaction for a 96-atom supercell, these effects are studied by comparing the differences in atomic structures, electronic structures, and reduction energies of the doped CeO(2) and those of the nondoped CeO(2). It is found that (1) Zr doping of the ceria structure results in important modifications involving nonequivalent O atoms; (2) the oxygen anions (still four-coordinated) next to the doping center show considerably lower reduction energies (by 0.6 eV) and larger displacements ("higher mobilities"); (3) an O vacancy is most easily created close to the Zr centers, therefore the Zr-doping centers might serve as nucleation centers for vacancy clustering; and (4) the electrons left by the released oxygen localize on two Ce cations neighboring the vacancy, which results in the reduction of two Ce(4+) ions.