Accurate screened exchange band structures for the transition metal monoxides MnO, FeO, CoO and NiO

Evolutionary Algorithm-Based Crystal Structure Prediction of CuxZnyOz Ternary Oxides

Binary zinc(II) oxide (ZnO) and copper(II) oxide (CuO) are used in a number of applications, including optoelectronic and semiconductor applications. However, no crystal structures have been reported for ternary Cu-Zn-O oxides. In that context, we investigated the structural characteristics and thermodynamics of CuxZnyOz ternary oxides to map their experimental feasibility. We combined evolutionary crystal structure prediction and quantum chemical methods to investigate potential CuxZnyOz ternary oxides. The USPEX algorithm and density functional theory were used to screen over 4000 crystal structures with different stoichiometries. When comparing compositions with non-magnetic CuI ions, magnetic CuII ions, and mixed CuI-CuII compositions, the magnetic Cu2Zn2O4 system is thermodynamically the most favorable. At ambient pressures, the thermodynamically most favorable ternary crystal structure is still 2.8 kJ/mol per atom higher in Gibbs free energy compared to experimentally known binary phases. The results suggest that thermodynamics of the hypothetical CuxZnyOz ternary oxides should also be evaluated at high pressures. The predicted ternary materials are indirect band gap semiconductors.

Density Functional Studies on the Atomistic Structure and Properties of Iron Oxides: A Parametric Study

With the aim to find the best simulation routine to accurately predict the ground-state structures and properties of iron oxides (hematite, magnetite, and wustite) using density functional theory (DFT) with Hubbard-U correction, a significant amount of DFT calculations were conducted to investigate the influence of various simulation parameters (energy cutoff, K-point, U value, magnetization setting, smearing value, etc.) and pseudopotentials on the structures and properties of iron oxides. With optimized simulation parameters, the obtained equation of state, lattice constant, bulk moduli, and band gap is much closer to the experimental values compared with previous studies. Due to the strong coupling between the 2p orbital of O and the 3d orbital of Fe, it was found that Hubbard-U correction obviously improved the results for all three kinds of iron oxides including magnetite which has not yet been tested with U correction before, but the U value should be different for different oxides (3 ev, 4 ev, 4 ev for hematite, magnetite, and wustite, respectively). Two kinds of spin magnetism settings for FeO are considered, which should be chosen according to different calculation purposes. The detailed relationship between the parameter settings and the atomic structures and properties were analyzed, and the general principles for future DFT calculation of iron oxides were provided.

Spin splitting in monoperiodic systems described by magnetic line groups

In this paper we report the classification of all the 81 magnetic line group families into seven spin splitting prototypes, in analogy to the similar classification previously reported for the 1651 magnetic space groups, 528 magnetic layer groups, and 394 magnetic rod groups. According to this classification, electrically induced (Pekar-Rashba) spin splitting is possible in the antiferromagnetic structures described by magnetic line groups of type I (no anti-unitary operations) and III, both in the presence and in the absence of the space inversion operation. As a specific example, a group theoretical analysis of spin splitting in CoO (8, 8) nanotube is carried out and its predictions are confirmed byab initiodensity functional theory calculations.

Structural Properties and Magnetic Ground States of 100 Binary d-Metal Oxides Studied by Hybrid Density Functional Methods

d-metal oxides play a crucial role in numerous technological applications and show a great variety of magnetic properties. We have systematically investigated the structural properties, magnetic ground states, and fundamental electronic properties of 100 binary d-metal oxides using hybrid density functional methods and localized basis sets composed of Gaussian-type functions. The calculated properties are compared with experimental information in all cases where experimental data are available. The used PBE0 hybrid density functional method describes the structural properties of the studied d-metal oxides well, except in the case of molecular oxides with weak intermolecular forces between the molecular units. Empirical D3 dispersion correction does not improve the structural description of the molecular oxides. We provide a database of optimized geometries and magnetic ground states to facilitate future studies on the more complex properties of the binary d-metal oxides.

Magnetic Structure of CoO

The paper reports evidence that the multi-spin-axis magnetic structure proposed in 1964 by van Laar is realized in antiferromagnetic CoO. This tetragonal spin arrangement produces both the strong tetragonal and the weaker monoclinic distortion experimentally observed in this material. The monoclinic distortion is proposed to be a “monoclinic-like” distortion of the array of the oxygen atoms, comparable with the rhombohedral-like distortion of the oxygen atoms recently proposed to be present in NiO and MnO. The monoclinic-like distortion has no influence on the tetragonal magnetic structure, which is generated by a special nonadiabatic atomic-like motion of the electrons near the Fermi level. It is argued that it is this atomic-like motion that qualifies CoO to be a Mott insulator.

Metal oxide adsorption on fullerene C60 and its potential for adsorption of pollutant gases; density functional theory studies

Combinations of fullerene and metal oxides (MOx) are interesting, not only because they display the individual properties of fullerene and of MOx nanoparticles, but they may also exhibit synergetic properties that are advantageous for gas sensing applications. In the present work, the adsorption of some different MOxs on fullerene C₆₀, and also the NO₂ and CO sensing properties of these complexes, have been theoretically studied. All quantum mechanical computations have been carried out using Gaussian G09, employing the DFT method at the B97D/6-311G(d,p) level. NBO theory has been used for analysis of the charge transfers during gas adsorption. The chemical nature of the newly formed bonds in the studied complexes and their relative strength have been analysed using AIM2000 software. The results show that MOx/C₆₀ complexes are much stronger adsorbents for NO₂ and CO than C₆₀ is. It is also expected that these complexes have more optical and electrical sensitivity in the selectivity towards gases, including NO₂ and CO.

Combinations of C₆₀ and metal oxides (MOx) are interesting, not only because they display the individual properties of C₆₀ and of MOx nanoparticles, but they may also exhibit synergetic properties that are advantageous for gas sensing applications.

Non-equilibrium metal oxides via reconversion chemistry in lithium-ion batteries

Binary metal oxides are attractive anode materials for lithium-ion batteries. Despite sustained effort into nanomaterials synthesis and understanding the initial discharge mechanism, the fundamental chemistry underpinning the charge and subsequent cycles-thus the reversible capacity-remains poorly understood. Here, we use in operando X-ray pair distribution function analysis combining with our recently developed analytical approach employing Metropolis Monte Carlo simulations and non-negative matrix factorisation to study the charge reaction thermodynamics of a series of Fe- and Mn-oxides. As opposed to the commonly believed conversion chemistry forming rocksalt FeO and MnO, we reveal the two oxide series topotactically transform into non-native body-centred cubic FeO and zincblende MnO via displacement-like reactions whose kinetics are governed by the mobility differences between displaced species. These renewed mechanistic insights suggest avenues for the future design of metal oxide materials as well as new material synthesis routes using electrochemically-assisted methods.

Magnetic fluctuations and the spin-orbit interaction in Mott insulating CoO

Motivated by the presence of an unquenched orbital angular momentum in CoO, a team at Chalk River, including a recently hired research officer Roger Cowley, performed the first inelastic neutron scattering experiments on the classic Mott insulator [Sakuraiet al1968Phys. Rev.167510]. Despite identifying two magnon modes at the zone boundary, the team was unable to parameterise the low energy magnetic excitation spectrum belowTNusing conventional pseudo-bosonic approaches, instead achieving only qualitative agreement. It would not be for another 40 years that Roger, now at Oxford and motivated by the discovery of the high-Tccuprate superconductors [Bednorz and Muller 1986Z. Phys. B64189], would make another attempt at the parameterisation of the magnetic excitation spectrum that had previously alluded him at the start of his career. Upon his return to CoO, Roger found a system embroiled in controversy, with some of its most fundamental parameters still remaining undetermined. Faced with such a formidable task, Roger performed a series of inelastic neutron scattering experiments in the early 2010s on both CoO and a magnetically dilute structural analogue Mg0.97Co0.03O. These experiments would prove instrumental in the determination of both single-ion [Cowleyet al2013Phys. Rev. B88205117] and cooperative magnetic parameters [Sarteet al2018Phys. Rev. B98024415] for CoO. Both these sets of parameters would eventually be used in a spin-orbit exciton model [Sarteet al2019Phys. Rev. B100075143], developed by his longtime friend and collaborator Bill Buyers, to successfully parameterise the complex spectrum that both measured at Chalk River almost 50 years prior. The story of CoO is of one that has come full circle, one filled with both spectacular failures and intermittent, yet profound, little victories.

Band Gap in Magnetic Insulators from a Charge Transition Level Approach

The theoretical description of the electronic structure of magnetic insulators and, in particular, of transition-metal oxides (TMOs), MnO, FeO, CoO, NiO, and CuO, poses several problems due to their highly correlated nature. Particularly challenging is the determination of the band gap. The most widely used approach is based on density functional theory (DFT) Kohn-Sham energy levels using self-interaction-corrected functionals (such as hybrid functionals). Here, we present a different approach based on the assumption that the band gap in some TMOs can have a partial Mott-Hubbard character and can be defined as the energy associated with the process Mm+(3dn) + Mm+(3dn) → M(m+1)+(3dn-1) + M(m-1)+(3dn+1). The band gap is thus associated with the removal (ionization potential, I) and addition (electron affinity, A) of one electron to an ion of the lattice. In fact, due to the hybridization of metal with ligand orbitals, these energy contributions are not purely atomic in nature. I and A can be computed accurately using the charge transition level (CTL) scheme. This procedure is based on the calculation of energy levels of charged states and goes beyond the approximations inherent to the Kohn-Sham (KS) approach. The novel and relevant aspect of this work is the extension of CTLs from the domain of point defects to a bulk property such as the band gap. The results show that the calculation based on CTLs provides band gaps in better agreement with experiments than the KS approach, with direct insight into the nature of the gap in these complex systems.

Structural Distortion Stabilizing the Antiferromagnetic and Insulating Ground State of NiO

We report evidence that the experimentally observed small deformation of antiferromagnetic NiO modifies the symmetry of the crystal in such a way that the antiferromagnetic state becomes an eigenstate of the electronic Hamiltonian. This deformation closely resembles a rhombohedral contraction, but does not possess the perfect symmetry of a trigonal (rhombohedral) space group. We determine the monoclinic base centered magnetic space group of the antiferromagnetic structure within the deformed crystal which is strongly influenced by the time-inversion symmetry of the Hamiltonian. The antiferromagnetic state is evidently stabilized by a nonadiabatic atomic-like motion of the electrons near the Fermi level. This atomic-like motion is characterized by the symmetry of the Bloch functions near the Fermi level and provides in NiO a perfect basis for a Mott insulator in the antiferromagnetic phase.

Evolutionary Algorithm-Based Crystal Structure Prediction for Copper(I) Fluoride

Despite numerous experimental studies since 1824, the binary copper(I) fluoride remains unknown. A crystal structure prediction has been carried out for CuF using the USPEX evolutionary algorithm and a dispersion-corrected hybrid density functional method. In total about 5000 hypothetical structures were investigated. The energetics of the predicted structures were also counter-checked with local second-order Møller-Plesset perturbation theory. Herein 39 new hypothetical copper(I) fluoride structures are reported that are lower in energy compared to the previously predicted cinnabar-type structure. Cuprophilic Cu-Cu interactions are present in all the low-energy structures, leading to ordered Cu substructures such as helical or zig-zag-type Cu-Cu motifs. The lowest-energy structure adopts a trigonal crystal structure with space group P3₁ 21. From an electronic point of view, the predicted CuF modification is a semiconductor with an indirect band gap of 2.3 eV.

Crystal Structure Prediction of Magnetic Transition-Metal Oxides by Using Evolutionary Algorithm and Hybrid DFT Methods

Although numerous crystal structures have been successfully predicted by using currently available computational techniques, prediction of strongly correlated systems such as transition-metal oxides remains a challenge. To overcome this problem, we have interfaced evolutionary algorithm-based USPEX method with the CRYSTAL code, enabling the use of Gaussian-type localized atomic basis sets and hybrid density functional (DFT) methods for the prediction of crystal structures. We report successful crystal structure predictions of several transition-metal oxides (NiO, CoO, α-Fe2O3, V2O3, and CuO) with correct atomic magnetic moments, spin configurations, and structures by using the USPEX method in combination with the CRYSTAL code and Perdew-Burke-Ernzerhof (PBE0) hybrid functional. Our benchmarking results demonstrate that USPEX + hybrid DFT is a suitable combination to reliably predict the magnetic structures of strongly correlated materials.

Importance of the Kinetic Energy Density for Band Gap Calculations in Solids with Density Functional Theory

Recently, exchange-correlation potentials in density functional theory were developed with the goal of providing improved band gaps in solids. Among them, the semilocal potentials are particularly interesting for large systems since they lead to calculations that are much faster than with hybrid functionals or methods like GW. We present an exhaustive comparison of semilocal exchange-correlation potentials for band gap calculations on a large test set of solids, and particular attention is paid to the potential HLE16 proposed by Verma and Truhlar. It is shown that the most accurate potential is the modified Becke-Johnson potential, which, most noticeably, is much more accurate than all other semilocal potentials for strongly correlated systems. This can be attributed to its additional dependence on the kinetic energy density. It is also shown that the modified Becke-Johnson potential is at least as accurate as the hybrid functionals and more reliable for solids with large band gaps.

Predicting Band Gaps with Hybrid Density Functionals

We compare the ability of four popular hybrid density functionals (B3LYP, B3PW91, HSE, and PBE0) for predicting band gaps of semiconductors and insulators over a large benchmark set using a consistent methodology. We observe no significant statistical difference in their overall performance, although the screened hybrid HSE is more accurate for typical semiconductors. HSE can improve its accuracy for large band gap materials-without affecting that of semiconductors-by including a larger portion of Hartree-Fock exchange in its short-range. Given that screened hybrids are computationally much less expensive than their global counterparts, we conclude that they are a better option for the black box prediction of band gaps.