Electronic correlations and intrinsic magnetism of interstitial quasi-atomic states in Li8Au electride.

We investigate the electronic sub-system of a recently designed Li8Au superconducting electride to reveal its many-body correlated nature and magnetic properties. Using maximally localized Wannier functions (MLWFs) to describe the interstitial anion electron (IAE) states, it was found that these states are partially occupied with a population of 1.5e- and have negligible hybridization with the almost completely filled p-Au states. The averaged interaction screened Hubbard parameter U for quasi-atomic IAE states evaluated by the constrained random-phase approximation (CRPA) method is 2 eV, comparable to the width of the electride band suggesting moderate electronic correlations. Using dynamical mean field theory (DMFT) approach we found that IAEs in Li8Au electride behave as magnetic centers and possess their own well localised magnetic moments of 0.5µB per quasi-atomic IAE. The obtained results deepen the understanding of the significance of many-body effects in the IAE subsystem of electronic states and reveal the mechanism for the formation of intrinsic magnetic moments on IAEs, which behave like ferromagnetic quasi-atoms in the Li8Au electride. Overall, the observed correlation effects in Li8Au emphasize their importance in materials with excess electrons confined in cavities.

First-principles definition of ionicity and covalency in molecules and solids.

The notions of ionicity and covalency of chemical bonds, effective atomic charges, and decomposition of the cohesive energy into ionic and covalent terms are fundamental yet elusive. For example, different approaches give different values of atomic charges. Pursuing the goal of formulating a universal approach based on firm physical grounds (first-principles or non-empirical), we develop a formalism based on Wannier functions with atomic orbital symmetry and capable of defining these notions and giving numerically robust results that are in excellent agreement with traditional chemical thinking. Unexpectedly, in diamond-like boron phosphide (BP), we find charges of +0.68 on phosphorus and -0.68 on boron atoms, and this anomaly is explained by the Zintl-Klemm nature of this compound. We present a simple model that includes energies of the highest occupied cationic and lowest unoccupied anionic atomic orbitals, coordination numbers, and strength of interatomic orbital overlap. This model captures the essential physics of bonding and accurately reproduces all our results, including anomalous BP.

Exploring correlation effects and volume collapse during electride dimensionality change in Ca2N.

We investigate the role of interstitial electronic states in the metal-to-semiconductor transition and the origin of the volume collapse in Ca2N during the pressure-induced phase transitions accompanied by changes of electride subspace dimensionality. Our findings highlight the importance of correlation effects in the electride subsystem as an essential component of the complex phase transformation mechanism. By employing a simplified model that incorporates the distortion of the local environment surrounding the interstitial quasi-atom (ISQ) which emerges under pressure and solving this model by Dynamical Mean Field Theory (DMFT), we successfully reproduced the evolution between the metallic and semiconducting phases and captured the remarkable volume collapse. Central to this observation is a significant enhancement of the localization of excess electrons and the emergence of antiferromagnetic pairing among them, leading to a spin-state transition with a notable reduction in the magnetic moment on the interstitial states.

Localization Mechanism of Interstitial Electronic States in Electride Mayenite.

Electrides contain interstitial electrons with the states that are spatially separated from the crystal framework states and form a detached electronic subsystem. In mayenite [Ca12Al14O32]2+(e-)2 interstitial electrons form a unique charge network where localization and delocalization coexist, pointing to the importance of investigating the many-body nature of electride states. Using density functional theory and dynamical mean-field theory, we show a tendency toward electron localization and antiferromagnetic pairing, which leads to the formation of an experimentally observed peak under the Fermi level. The effect is associated with strong hybridization between interstitial electronic states, which removes the degeneracy and leads to the formation of a singlet state on a bonding molecular orbital as well as with the Coulomb interaction between interstitial electrons. Our work provides a fundamental understanding of the localization mechanism of interstitial electrons in mayenite and proposes a new approach for a proper description of the electronic subsystem of mayenite and other electrides.

Importance of the many-body effects on the structural properties of the novel iron oxide Fe2O.

The importance of many-body effects on the electronic and magnetic properties and stability of different structural phases was studied in novel iron oxide Fe2O. It was found that while Hubbard repulsion hardly affects the electronic spectrum of this material (m*/m ≈ 1.2), it strongly changes its phase diagram, shifting critical pressures of structural transitions to much lower values. Moreover, the P3̄m1 structure previously obtained in the density functional theory (DFT) becomes energetically unstable if many-body effects are taken into consideration. It is shown that these changes are due to magnetic moment fluctuations in the DFT+DMFT (method which combines density functional theory and dynamical mean-field theory) approach, which strongly modify the phase diagram of Fe2O.

Weak Coulomb correlations stabilize the electride high-pressure phase of elemental calcium.

Theoretical studies using the state-of-the-art density functional theory and dynamicalmean-field theory (DFT + DMFT) method show that weak electronic correlation effects are crucial for reproducing the experimentally observed pressure-induced phase transitions of calcium from ß-tin to Cmmm and then to the simple cubic structure. The formation of an electride state in calcium leads to the emergence of partially filled and localized electronic states under compression. The electride state was described using a basis containing molecular orbitals centered on the interstitial site and Ca-d states. We investigate the influence of Coulomb correlations on the structural properties of elemental Ca, noting that approaches based on the Hartree-Fock method (DFT + U or hybrid functional schemes) are poorly suited for describing correlated metals. We find that only the DFT + DMFT method reproduces the correct sequence of high-pressure phase transitions of Ca at low temperatures.

Influence of Molecular Orbitals on Magnetic Properties of [x].

Recent discoveries of various novel iron oxides and hydrides, which become stable at very high pressure and temperature, are extremely important for geoscience. In this paper, we report the results of an investigation on the electronic structure and magnetic properties of the hydride FeO 2 H x , using density functional theory plus dynamical mean-field theory (DFT+DMFT) calculations. An increase in the hydrogen concentration resulted in the destruction of dimeric oxygen pairs and, hence, a specific band structure of FeO 2 with strongly hybridized Fe- t 2 g -O- p z anti-bonding molecular orbitals, which led to a metallic state with the Fe ions at nearly 3+. Increasing the H concentration resulted in effective mass enhancement growth which indicated an increase in the magnetic moment localization. The calculated static momentum-resolved spin susceptibility demonstrated that an incommensurate antiferromagnetic (AFM) order was expected for FeO 2 , whereas strong ferromagnetic (FM) fluctuations were observed for FeO 2 H.

Electronic correlations in uranium hydride UH5 under pressure.

We report results of calculations based on density functional theory and dynamical mean-field theory for the electronic structure of uranium hydride UH5 under pressure, a compound of the uranium-based hydride family some members of which have been predicted to be superconducting. The effective electronic mass enhancement m*/m ∼ 1.4 indicates that the Coulomb correlations have a moderate strength. However, the topology of the Fermi surface changes strongly at the influence of the correlation effects: one hourglass-like pocket running along the Γ-A direction splits into two elliptical pockets centered at the A point. This result shows the possibility of an unconventional pairing mechanism for uranium hydrides in addition to the electron-phonon pairing that was studied in previous investigations.

Charge and spin degrees of freedom in strongly correlated systems: Mott states opposite Hund's metals.

A correlated metallic state can arise as a result of the presence either strong charge or strong spin fluctuations. In the first case, as was shown first in (2004 Phys. Today 57 53) for the Hubbard model on the Bethe lattice, the system is a correlated metallic state close to the Mott-insulator state if the ratio of the value of the Coulomb interaction parameter U and the band width W is [Formula: see text]. The later case exist if [Formula: see text] and Hund's exchange parameter [Formula: see text]. In both cases narrowing of the bands near the Fermi level and renormalization of the effective electron mass is observed, although the mechanism for realizing this state will be fundamentally different. We performed the electronic structure calculations of the paramagnetic phase [Formula: see text]-iron which is a typical Hund's metal. We showed that the statistical distribution of charge between possible electronic d-configurations has a very weak dependence on the exchange interaction and is specific for metals. At the same time, the distribution of statistical weights between different spin configurations fundamentally changes with the inclusion of J. If we neglect Hund's interaction by setting J = 0, the contributions from the low-spin configurations for all possible charge states dominate. The exchange interaction causes a redistribution of probability in favor of high-spin multiplets, leading to the formation of a larger local moment. We also performed calculations for the two-bands half-filled model. By varying the values of the Coulomb and Hund's exchange interaction parameters, we reproduced the region of the phase diagram of the model in which the system undergoes a transition from the Mott-insulator state to the Hund's metal. This transition is accompanied by a change in the statistical probability distribution of possible multiple configurations. In the region corresponding to the Hund's metal state, a change of J leads to the effect of weights redistribution similar that we observe in [Formula: see text]-iron.

Unexpected 3+ valence of iron in FeO2, a geologically important material lying "in between" oxides and peroxides.

Recent discovery of the pyrite FeO2, which can be an important ingredient of the Earth's lower mantle and which in particular may serve as an extra source of water in the Earth's interior, opens new perspectives for geophysics and geochemistry, but this is also an extremely interesting material from physical point of view. We found that in contrast to naive expectations Fe is nearly 3+ in this material, which strongly affects its magnetic properties and makes it qualitatively different from well known sulfide analogue - FeS2. Doping, which is most likely to occur in the Earth's mantle, makes FeO2 much more magnetic. In addition we show that unique electronic structure places FeO2 "in between" the usual dioxides and peroxides making this system interesting both for physics and solid state chemistry.