Oxygen Orbitals Research Articles

DFT+ U study of self-trapping, trapping, and mobility of oxygen-type hole polarons in barium stannate

The charge-transfer insulating perovskite oxides currently used as fuel cell electrolytes undergo, at high temperature, an oxidation reaction 1 2 O 2 (g) + V • • O → O X O + 2h • , that produces oxygen-type holes. Understanding the nature and mobility of these oxygen-type holes is an important step to improve the performance of devices, but presents a theoretical challenge since, in their localized form, they cannot be captured by standard density functional theory. Here, we employ the DFT+U formalism with a Hubbard correction on the p orbitals of oxygen to investigate several properties of these holes, in the particular case of BaSnO 3. We describe the small oxygen-type hole polarons, the self-trapping at their origin, and their trapping by trivalent dopants (Ga, Sc, In, Lu, Y, Gd, La). Strong similarities with protonic defects are observed concerning the evolution of the trapping energy with ionic radius of the dopant. Moreover, we show that long-range diffusion of holes is a complex phenomenon, that proceeds by a succession of several mechanisms. However, the standard implementation of DFT+U within the projector augmented-wave (PAW) formalism leads to use very large, unphysical values of U for the O-p orbital. We propose here a slightly modified DFT+U scheme, that takes into account the fact that the O-p is truncated in usual DFT+U implementation in PAW. This scheme yields more physical values of U than the ones traditionally used in the literature, and describes well the properties of the hole polaron.

Study of electronic and optical properties of ZnO clusters using TDDFT method

We present a systematic computational study of the electronic and optical properties of (ZnO)n clusters with –16, 24 and 36 using time dependent density functional calculations (TDDFT). The calculations bring out the role of dimensionality of clusters, multiple connectivity of Zn–O network and origin of blue-shift in the absorption spectra. This is accompanied by large value of optical gap in 2D clusters as compared with 3D structures. The observed optical gap are strongly related to the size, geometrical shape, symmetry and dimensionality of the structures. Our calculations also shows the blue shift of the spectral lines from planar and ringlike structures to 3D spheroidal clusters due to the occurrence of the dipole-forbidden transitions in the optical gap for highly symmetric structures. A microscopic analysis of the molecular orbitals and eigenvalue spectra show that the electronic transitions occur between the non-bonding p-states of Oxygen and unoccupied sp-hybrid orbitals of Zinc atoms. We additionally use many body perturbation theory (MBPT) method to validate the TDDFT results.

Intramolecular hydrogen bonding in malonaldehyde and its radical analogues.

High level Brueckner doubles with triples correction method-based ab initio calculations have been used to investigate the nature of intramolecular hydrogen bonding and intramolecular hydrogen atom transfer in cis-malonaldehyde (MA) and its radical analogues. The radicals considered here are the ones that correspond to the homolytic cleavage of C-H bonds in cis-MA. The results suggest that cis-MA and its radical analogues, cis-MARS, and cis-MARA, both exist in planar geometry. The calculated intramolecular O-H⋯O=C bond in cis-MA is shorter than that in the radical analogues. The intramolecular hydrogen bond in cis-MA is stronger than in its radicals by at least 3.0 kcal/mol. The stability of a cis-malonaldehyde radical correlates with the extent of electron spin delocalization; cis-MARA, in which the radical spin is more delocalized, is the most stable MA radical, whereas cis-MARS, in which the radical spin is strongly localized, is the least stable radical. The natural bond orbital analysis indicates that the intramolecular hydrogen bonding (O⋯H⋯O) in cis-malonaldehyde radicals is stabilized by the interaction between the lone pair orbitals of donor oxygen and the σ* orbital of acceptor O-H bond (n → σ*OH). The calculated barriers indicate that the intramolecular proton transfer in cis-MA involves 2.2 kcal/mol lower barrier than that in cis-MARS.

Observation of momentum-dependent charge excitations in hole-doped cuprates using resonant inelastic x-ray scattering at the oxygen K edge

We investigate electronic excitations in La2-x(Br,Sr)xCuO4 using resonant inelastic x-ray scattering (RIXS) at the oxygen K edge. RIXS spectra of the hole-doped cuprates show clear momentum dependence below 1 eV. The spectral weight exhibits positive dispersion and shifts to higher energy with increasing hole concentration. Theoretical calculation of the dynamical charge structure factor on oxygen orbitals in a three-band Hubbard model is consistent with the experimental observation of the momentum and doping dependence, and therefore the dispersive mode is ascribed to intraband charge excitations which have been observed in electron-doped cuprates.

Oxygen defects formation and optical identification in monolayer borophene

Surface reactions with oxygen are a fundamental cause of the degradation of borophene, which has greatly limited the application in nanoelectronic and optoelectronic devices. Density functional theory calculations disclose that there is an energy release of about 3 eV for each oxygen atom adsorbed onto borophene. The hybridization between pz orbitals of oxygen and p orbitals of neighboring B atoms lead to minor distortions of the lattice and low energy metastable forms, finally to form different oxygen defects. The mechanism for oxidation involving reactive dangling oxygen atoms is proposed, and absorption spectra are suggested to identify the types of oxygen defects in borophene.

Multiband model for the electronic structure of Sr2TiO4

AbstractWe introduce and investigate the multiband d–p model for a TiO layer, such as realized in SrTiO, with all and orbitals (at titanium ions) and orbitals (at oxygen ions). Complementary density functional theory ab initio computations are employed to determine the actual electron number per TiO unit and one finds perfect Sr ionization with Sr ions and charged (TiO) layer. This system is predicted to be a robust nonmagnetic insulator, in agreement with experiment. The above charge distribution is crucial and when we deviate from it, even by a small amount, the system becomes conducting or very close to conducting and various magnetic structures compete with one another. This finding is generic, that is, it holds in a broad range of d–p Hamiltonian parameters. As expected, d–p hybridization strongly redistributes electrons and leads to titanium ions between and ionic configurations. Surprisingly, SrTiO is not a simpleminded system but instead electron densities are finite and roughly the same in all different orbitals (of and symmetry) and the electron densities within O() orbitals are within the range 5.4–5.9. By selecting the charge‐transfer energy, , we reproduce the experimental band gap of 3.8 eV. We emphasize that a realistic treatment of electronic distribution requires local Coulomb interactions at oxygen orbitals and we show that the Coulomb interactions at titanium ions are strongly renormalized when the Coulomb interactions at oxygen ions are neglected.

(Invited) Discovering Wide Band Gap Oxide Semiconductors By Engineering Strong Electron Correlations

Wide gap semiconductors are often designed based on energy separation of the cation and oxygen orbitals. In strongly correlated semiconductors, however interacting electrons create effective transport gaps based on the occupancy of orbitals. I will discuss some recent examples from our laboratory on discovery of new electronic phases that have gaps of the order of 3eV that have been designed by electron doping of perovskite nickelates. In the un-doped case, these systems possess small gaps of the order of 0.1eV, and adding electrons to the partially filled d-orbitals, enhances the gap to 3eV taking advantage of strong correlations in the eg orbitals of Ni sites. This new strategy allows us to create wide gaps in semiconductors that are nominally metallic or nearly-metallic like in the pristine state and could be expanded to a broader range of materials systems. Besides the gap opening that in itself is fascinating, there are several opportunities to tune these materials to achieve multiple resistance states, continuous tuning of resistance by anion-site disorder and create multi-functionality. I will then give some examples of their applications in neural science and engineering, photonics and evolutionary biology.

Superconductivity Induced by Oxygen Doping in Y2 O2 Bi.

When doped with oxygen, the layered Y2 O2 Bi phase becomes a superconductor. This finding raises questions about the sites for doped oxygen, the mechanism of superconductivity, and practical guidelines for discovering new superconductors. We probed these questions in terms of first-principles calculations for undoped and O-doped Y2 O2 Bi. The preferred sites for doped O atoms are the centers of Bi4 squares in the Bi square net. Several Bi 6p x/y bands of Y2 O2 Bi are raised in energy by oxygen doping because the 2p x/y orbitals of the doped oxygen make antibonding possible with the 6p x/y orbitals of surrounding Bi atoms. Consequently, the condition necessary for the "flat/steep" band model for superconductivity is satisfied in O-doped Y2 O2 Bi.

Superconductivity Induced by Oxygen Doping in Y2O2Bi

AbstractWhen doped with oxygen, the layered Y2O2Bi phase becomes a superconductor. This finding raises questions about the sites for doped oxygen, the mechanism of superconductivity, and practical guidelines for discovering new superconductors. We probed these questions in terms of first‐principles calculations for undoped and O‐doped Y2O2Bi. The preferred sites for doped O atoms are the centers of Bi4 squares in the Bi square net. Several Bi 6p x/y bands of Y2O2Bi are raised in energy by oxygen doping because the 2p x/y orbitals of the doped oxygen make antibonding possible with the 6p x/y orbitals of surrounding Bi atoms. Consequently, the condition necessary for the “flat/steep” band model for superconductivity is satisfied in O‐doped Y2O2Bi.

(Invited) "Structure-Property", "Composition-Property" and "Property-Property" Relations As Useful Tools for Understanding of Optical Materials Performance

Continuously growing demand for optical materials used in various applications (such as solid state lighting, lasers, sensors, scintillators etc) stimulates active research aimed at the development of new materials and optimization/improvement of the existing ones. Systematic comparative studies of large groups of isostructural compounds or isoelectronic impurities can help in uncovering different “structure-property”, “composition-property”, and “property-property” relations, which can lead to a deeper understanding of optical materials operation and can be used as reliable guidelines identifying directions for a smart search for new materials with better performance. In the present work we discuss our recent results of application of several different approaches (crystal field theory, density functional theory (DFT)-based computational techniques, configurational coordinate model etc) to various groups of materials – pyrochlores, perovskites, garnets, spinels, chalcopyrites, III-V and II-VI semiconductors, pure and doped with transition metal and rare earth ions. Firstly, an empirical model was developed to express the lattice constants of isostructural cubic and tetragonal compounds in terms of ionic radii and electronegativities of constituting ions [1]. The average deviation between the experimental values of the lattice constants and those estimated from the proposed theory is of the order of 1 % only. The model not only allows to fit the lattice constants of the considered crystals, but also gives an opportunity to predict the lattice constants of new compounds and analyze the stability criteria for existing materials. Secondly, DFT-based calculations were employed to explain peculiar features of the Cr4+ and Mn4+ spectra in Y2Ti2O7 and Y2Sn2O7 pyrochlores. In particular, it has been shown that the blue shift of the 3d-ions absorption peaks in Y2Sn2O7 in comparison with Y2Ti2O7 (in spite of larger interionic separations in the former case) can be explained by increased ligands charge in Y2Sn2O7 because of a weaker overlap of the oxygen orbitals with completely filled 4d10 orbitals of Sn4+ [2]. Thirdly, careful examination of variation of energies of the spin-forbidden transitions of transition metal ions (Mn4+, Cr3+, Ni2+) in various solids allowed to relate those energies with the covalent effects experienced by those ions. In this connection, a clear link between the degree of covalency of chemical bonds and spatial arrangements of crystal lattice ions was established. These findings can help in search for new efficient red phosphors based on the Mn4+ ions [3]. Fourthly, DFT-based calculations of the structural, optical, electronic properties of chalcopyrites and II-VI (III-V) semiconductors revealed important changes of the band gap character (direct or indirect) or enhanced absorptivity with composition, which has direct influence on perspectives of applications of these materials for solar cell materials and/or LED devices [4]. All obtained results are compared with the corresponding experimental data; agreement between both sets of data is discussed. Predictive potential of the presented calculations and possibility of applications of the developed models to other systems are highlighted.

Vacancy‐Induced Electronic Structure Variation of Acceptors and Correlation with Proton Conduction in Perovskite Oxides

AbstractIn most proton‐conducing perovskite oxides, the electrostatic attraction between negatively charged acceptor dopants and protonic defects having a positive charge is known to be a major cause of retardation of proton conduction, a phenomenon that is generally referred to as proton trapping. We experimentally show that proton trapping can be suppressed by clustering of positively charged oxygen vacancies to acceptors in BaZrO3−δ and BaCeO3−δ. In particular, to ensure the vacancy–acceptor association is effective against proton trapping, the valence electron density of acceptors should not significantly vary when the oxygen vacancies cluster, based on the weak hybridization between the valence d or p orbitals of acceptors and the 2p orbitals of oxygen.

Vacancy‐Induced Electronic Structure Variation of Acceptors and Correlation with Proton Conduction in Perovskite Oxides

In most proton-conducing perovskite oxides, the electrostatic attraction between negatively charged acceptor dopants and protonic defects having a positive charge is known to be a major cause of retardation of proton conduction, a phenomenon that is generally referred to as proton trapping. We experimentally show that proton trapping can be suppressed by clustering of positively charged oxygen vacancies to acceptors in BaZrO3-δ and BaCeO3-δ . In particular, to ensure the vacancy-acceptor association is effective against proton trapping, the valence electron density of acceptors should not significantly vary when the oxygen vacancies cluster, based on the weak hybridization between the valence d or p orbitals of acceptors and the 2p orbitals of oxygen.

Structural and Thermal Properties of BaTe2O6: Combined Variable-Temperature Synchrotron X-ray Diffraction, Raman Spectroscopy, and ab Initio Calculations.

Variable-temperature Raman spectroscopic and synchrotron X-ray diffraction studies were performed on BaTe2O6 (orthorhombic, space group: Cmcm), a mixed-valence tellurium compound with a layered structure, to understand structural stability and anharmonicity of phonons. The structural and vibrational studies indicate no phase transition in it over a wider range of temperature (20 to 853 K). The structure shows anisotropic expansion with coefficients of thermal expansion in the order αb ≫ αa > αc, which was attributed to the anisotropy in bonding and structure of BaTe2O6. Temperature evolution of Raman modes of BaTe2O6 indicated a smooth decreasing trend in mode frequencies with increasing temperature, while the full width at half-maximum (fwhm) of all modes systematically increases due to a rise in phonon scattering processes. With the use of our earlier reported isothermal mode Grüneisen parameters, thermal properties such as thermal expansion coefficient and molar specific heat are calculated. The pure anharmonic (explicit) and quasiharmonic (implicit) contribution to the total anharmonicity is delineated and compared. The temperature dependence of phonon mode frequencies and their fwhm values are analyzed by anharmonicity models, and the dominating anharmonic phonon scattering mechanism is concluded in BaTe2O6. In addition to the lattice modes, several external modes of TeOn (n = 5, 6) are found to be strongly anharmonic. The ab initio electronic structure calculations indicated BaTe2O6 is a direct band gap semiconductor with gap energy of ∼2.1 eV. Oxygen orbitals, namely, O-2p states in the valence band maximum and the sp-hybridized states in the conduction band minimum, are mainly involved in the electronic transitions. In addition a number of electronic transitions are predicted by the electronic structure calculations. Experimental photoluminescence results are adequately explained by the ab initio calculations. Further details of the structural and vibrational properties are explained in the manuscript.

Magnetic order and electronic properties of Li2Mn2(MoO4)3 material for lithium-ion batteries: ESR and magnetic susceptibility studies

We describe the application of electron spin resonance (ESR) and magnetic susceptibility methods to study the magnetic properties and valence state of transition metal ions in Li2Mn2(MoO4)3 polyanion compound previously studied for its cathode-active properties in lithium containing batteries. ESR measurements of Li2Mn2(MoO4)3 have shown the presence of Mn2+ ions in the octahedral environment of oxygen ions. It is found that the part of manganese ions occupy the anti-site positions in lithium sublattice. The absence of the ESR signal from molybdenum ions indicates that they are non-magnetic and adopt the 6+ valence state. Considerable overlapping between 3d orbitals of transition metal and 2p oxygen orbitals has been experimentally established. This leads to the indirect exchange interaction and antiferromagnetic ordering of manganese ions at 1.4 K.

Prominent role of oxygen in the multiferroicity ofDyMnO3andTbMnO3: A resonant soft x-ray scattering spectroscopy study

Oxygen is known to play an important role in the multiferroicity of rare earth manganites; however, how this role changes with rare earth elements is still not fully understood. To address this question, we have used resonant soft x-ray scattering spectroscopy to study the F-type (0,τ,0) diffraction peak from the antiferromagnetic order in DyMnO3 and TbMnO3. We focus on the measurements at O K edge of these two manganites, supplemented by the results at Mn L2 and Dy M5 edge of DyMnO3. We show that the electronic states of different elements are coupled more strongly in DyMnO3 than in TbMnO3, presumably due to the stronger lattice distortion and the tendency to develop E-type antiferromagnetism in the ferroelectric state that promote the orbital hybridization. We also show that the anomaly in the correlation length of (0,τ,0) peak in DyMnO3 signifies the exchange interaction between Mn and rare earth spins. Our findings reveal the prominent role of oxygen orbitals in the multiferroicity of rare earth manganites and the distinct energetics between them.

Perspective on the phase diagram of cuprate high-temperature superconductors.

Universal scaling laws can guide the understanding of new phenomena, and for cuprate high-temperature superconductivity the influential Uemura relation showed, early on, that the maximum critical temperature of superconductivity correlates with the density of the superfluid measured at low temperatures. Here we show that the charge content of the bonding orbitals of copper and oxygen in the ubiquitous CuO2 plane, measured with nuclear magnetic resonance, reproduces this scaling. The charge transfer of the nominal copper hole to planar oxygen sets the maximum critical temperature. A three-dimensional phase diagram in terms of the charge content at copper as well as oxygen is introduced, which has the different cuprate families sorted with respect to their maximum critical temperature. We suggest that the critical temperature could be raised substantially if one were able to synthesize materials that lead to an increased planar oxygen hole content at the expense of that of planar copper.

Characterizing the three-orbital Hubbard model with determinant quantum Monte Carlo

We characterize the three-orbital Hubbard model using state-of-the-art determinant quantum Monte Carlo (DQMC) simulations with parameters relevant to the cuprate high-temperature superconductors. The simulations find that doped holes preferentially reside on oxygen orbitals and that the ({\pi},{\pi}) antiferromagnetic ordering vector dominates in the vicinity of the undoped system, as known from experiments. The orbitally-resolved spectral functions agree well with photoemission spectroscopy studies and enable identification of orbital content in the bands. A comparison of DQMC results with exact diagonalization and cluster perturbation theory studies elucidates how these different numerical techniques complement one another to produce a more complete understanding of the model and the cuprates. Interestingly, our DQMC simulations predict a charge-transfer gap that is significantly smaller than the direct (optical) gap measured in experiment. Most likely, it corresponds to the indirect gap that has recently been suggested to be on the order of 0.8 eV, and demonstrates the subtlety in identifying charge gaps.

Emergence of charge order in a staggered loop-current phase of cuprate high-temperature superconductors

We study the emergence of charge ordered phases within a pi-loop current (piLC) model for the pseudogap based on a three-band model for underdoped cuprate superconductors. Loop currents and charge ordering are driven by distinct components of the short-range Coulomb interactions: loop currents result from the repulsion between nearest-neighbor copper and oxygen orbitals, while charge order results from repulsion between neighboring oxygen orbitals. We find that the leading piLC phase has an antiferromagnetic pattern similar to previously discovered staggered flux phases, and that it emerges abruptly at hole dopings p below the van Hove filling. Subsequent charge ordering tendencies in the piLC phase reveal that diagonal d-charge density waves (dCDW) are suppressed by the loop currents while axial order competes more weakly. In some cases we find a wide temperature range below the loop-current transition, over which the susceptibility towards an axial dCDW is large. In these cases, short-range axial charge order may be induced by doping-related disorder. A unique feature of the coexisting dCDW and piLC phases is the emergence of an incommensurate modulation of the loop currents. If the dCDW is biaxial (checkerboard) then the resulting incommensurate current pattern breaks all mirror and time-reversal symmetries, thereby allowing for a polar Kerr effect.

The transition from the open minimum to the ring minimum on the ground state and on the lowest excited state of like symmetry in ozone: A configuration interaction study.

The metastable ring structure of the ozone 1(1)A1 ground state, which theoretical calculations have shown to exist, has so far eluded experimental detection. An accurate prediction for the energy difference between this isomer and the lower open structure is therefore of interest, as is a prediction for the isomerization barrier between them, which results from interactions between the lowest two (1)A1 states. In the present work, valence correlated energies of the 1(1)A1 state and the 2(1)A1 state were calculated at the 1(1)A1 open minimum, the 1(1)A1 ring minimum, the transition state between these two minima, the minimum of the 2(1)A1 state, and the conical intersection between the two states. The geometries were determined at the full-valence multi-configuration self-consistent-field level. Configuration interaction (CI) expansions up to quadruple excitations were calculated with triple-zeta atomic basis sets. The CI expansions based on eight different reference configuration spaces were explored. To obtain some of the quadruple excitation energies, the method of Correlation Energy Extrapolation by Intrinsic Scaling was generalized to the simultaneous extrapolation for two states. This extrapolation method was shown to be very accurate. On the other hand, none of the CI expansions were found to have converged to millihartree (mh) accuracy at the quadruple excitation level. The data suggest that convergence to mh accuracy is probably attained at the sextuple excitation level. On the 1(1)A1 state, the present calculations yield the estimates of (ring minimum-open minimum) ∼45-50 mh and (transition state-open minimum) ∼85-90 mh. For the (2(1)A1-(1)A1) excitation energy, the estimate of ∼130-170 mh is found at the open minimum and 270-310 mh at the ring minimum. At the transition state, the difference (2(1)A1-(1)A1) is found to be between 1 and 10 mh. The geometry of the transition state on the 1(1)A1 surface and that of the minimum on the 2(1)A1 surface nearly coincide. More accurate predictions of the energy differences also require CI expansions to at least sextuple excitations with respect to the valence space. For every wave function considered, the omission of the correlations of the 2s oxygen orbitals, which is a widely used approximation, was found to cause errors of about ±10 mh with respect to the energy differences.

Multibandd−pmodel and self-doping in the electronic structure ofBa2IrO4

We introduce and investigate the multiband $d-p$ model describing a IrO$_4$ layer (such as realized in Ba$_2$IrO$_4$) where all $34$ orbitals per unit cell are partly occupied, i.e., $t_{2g}$ and $e_g$ orbitals at iridium and $2p$ orbitals at oxygen ions. The model takes into account anisotropic iridium-oxygen $d-p$ and oxygen-oxygen $p-p$ hopping processes, crystal-field splittings, spin-orbit coupling, and the on-site Coulomb interactions, both at iridium and at oxygen ions. We show that the predictions based on assumed idealized ionic configuration (with $n_0=5+4\times 6=29$ electrons per IrO$_4$ unit) do not explain well the independent \textit{ab initio} data and the experimental data for Ba$_2$IrO$_4$. Instead we find that the total electron density in the $d-p$ states is smaller, $n=29-x<n_0$ ($x>0$). When we fix $x=1$, the predictions for the $d-p$ model become more realistic and weakly insulating antiferromagnetic ground state with the moments lying within IrO$_2$ planes along (110) direction is found, in agreement with experiment and \textit{ab initio} data. We also show that: (i) holes delocalize over the oxygen orbitals and the electron density at iridium ions is enhanced, hence (ii) their $e_g$ orbitals are occupied by more than one electron and have to be included in the multiband $d-p$ model describing iridates.