T2g Orbitals Research Articles

Antipolar 2D Metallicity with Tunable Valence Wx+ (5 ≤ x ≤ 5.6) in the Layered Monophosphate Tungsten Bronzes [Ba(PO4)2]WmO3m-3.

The newly discovered series of layered monophosphate tungsten bronzes (L-MPTB) [Ba(PO4)2]WmO3m-3 consist of m-layer-thick slabs of WO6 octahedra separated by barium-phosphate spacers. They display a 2D metallic behavior confined in the central part of the perovskite slabs. Here, we report the missing m = 2 member of this series, containing the rather uncommon W5+ oxidation state. We have analyzed its structure-property relationships in relation to the other members of the L-MPTB family. In particular, we have determined its crystal structure by means of single-crystal X-ray and electron diffraction and investigated its physical properties from resistivity, Seebeck-coefficient and heat-capacity measurements combined with first-principles calculations. All the L-MPTB compounds show metallic behavior down to 1.8 K without any clear charge-density-wave (CDW) order. The m = 2 member, however, displays an increased influence of the spacer that translates into anisotropic negative thermal expansion, reversed thermopower and reversed crystal-field splitting of the tungsten t2g orbitals. Our analysis of the full [Ba(PO4)2]WmO3m-3 series reveals a systematic and significant W off-centering in their octahedral coordination. We identify the resulting anti-polar character of these W displacements as the crucial aspect behind the 2D metallicity of these systems: It leads to the presence of bound charges whose screening determines the distribution of mobile charges, tending to accumulate at the center of the [WmO3-m] block. We argue that this mechanism is analogous to enhanced conductivity observed for charged domain walls in ferroelectrics, thus providing a general design rule to promote 2D metallicity in layered systems.

Interpretation of complex optical properties and optical transitions of epitaxial LaMnO3 thin films

An in-depth analysis of the optical properties of epitaxial (001) oriented LaMnO3films grown on SrTiO3and LaAlO3single crystal substrates is performed by spectroscopic ellipsometry to determine the complex dielectric function (ε = ε1 + iε2) spectra from 0.12 to 5.89 eV. Density Functional Theory and many body perturbation theory within the G0W0 and the Bethe-Salpeter Equation approximations, are also employed to generate theoretical spectra in ε. Critical point energies (CPs) from 0.91 to 1.49 eV are interpreted as energy separation between eg and t2g orbitals. CPs from 3.31 to 3.53 eV and 3.75 to 5.23 are interpreted as Mn d exchange splitting and strong charge transfer transitions, respectively. Direct and indirect bandgaps from Tauc-plots are identified in the range of 0.100–0.594 eV. Anisotropy presents in LaMnO3 on LaAlO3 is attributed to substantial distortion in the out-of-plane epitaxial strain compared to LaMnO3 on SrTiO3. These findings provide a comprehensive understanding of the origins and characteristics of features observed in optical properties of epitaxial LaMnO3, offering insights for development of optical and electronic applications based on these materials.

Exploring magnetic space groups of [formula omitted] MXene and its connection to vibrational and electronic properties

The vibrational, electronic, and magnetic properties of two-dimensional Cr2N MXene were investigated. Two crystal cells (hexagonal and monoclinic) were considered with their respective magnetic space groups. In the absence of experimental data to fine-tune the Ueff (Hubbard correction), we utilized cell parameters and magnetic moment as a reference window, derived from meta-GGA calculations performed with the SCAN functional. A value of Ueff (1.25 eV) was determined, which does not overestimate the lattice parameters and magnetic moment values. Phonon scattering was calculated, and the vibrational modes were indexed. According to the density of states, the observed splitting in the eg and t2g orbitals, and the crystal field analysis, we deduce that chromium in the MXene Cr2N predominantly adopts an octahedral coordination environment, a combination of octahedral and tetrahedral coordination results in the splitting of the d orbitals. Finally, using Monte Carlo simulation, the critical temperature (Tc) for each space group with different functionals was obtained.

Giant Tunability of Rashba Splitting at Cation-Exchanged Polar Oxide Interfaces by Selective Orbital Hybridization.

The 2D electron gas (2DEG) at oxide interfaces exhibits extraordinary properties, such as 2D superconductivity and ferromagnetism, coupled to strongly correlated electrons in narrow d-bands. In particular, 2DEGs in KTaO3 (KTO) with 5d t2g orbitals exhibit larger atomic spin-orbit coupling and crystal-facet-dependent superconductivity absent for 3d 2DEGs in SrTiO3 (STO). Herein, by tracing the interfacial chemistry, weak anti-localization magneto-transport behavior, and electronic structures of (001), (110), and (111) KTO 2DEGs, unambiguously cation exchange across KTO interfaces is discovered. Therefore, the origin of the 2DEGs at KTO-based interfaces is dramatically different from the electronicreconstruction observed at STO interfaces. More importantly, as the interface polarization grows with the higher order planes in the KTO case, the Rashba spin splitting becomes maximal for the superconducting (111) interfaces approximately twice that of the (001) interface. The larger Rashba spin splitting couples strongly to the asymmetric chiral texture of the orbital angular moment, and results mainly from the enhanced inter-orbital hopping of the t2g bands and more localized wave functions. This finding has profound implications for the search for topological superconductors, as well as the realization of efficient spin-charge interconversion for low-power spin-orbitronics based on (110) and (111) KTO interfaces.

Ligand‐to‐Metal Charge Transfer Motivated the Whole‐Voltage‐Range Anionic Redox in P2‐Type Layered Oxide Cathodes

AbstractAnionic redox in layered transition metal oxides (TMOs) cathodes presents significant opportunities for achieving high energy density batteries. However, the O 2p non‐bonding state can only undergo redox at high voltages and is often accompanied by severe structural distortion. In this study, the whole‐voltage‐range anionic redox has successfully been achieved in Metal‐organic frameworks (MOF) derived chemically homogeneous P2‐Na0.67Ni0.2Mg0.2Mn0.6O2 (NNMMO). Through a systematic analysis of the orbital combinations of TMs and O, it is found that the Mg‐induced O 2p non‐bonding state as an electron donor, forming strong π‐type interactions between Ni 3d spin‐down t2g orbitals. The resulting π‐configurational oxygen occupies an anti‐bonding state just below the Fermi level, exhibiting high oxidation activity and stability. Additionally, the high covalency between Mn and O increases the strength of the Mn–O bond, mitigating the structural aberration of NNMMO. As a result, NNMMO demonstrates a high capacity of 216 mAh g−1 at 0.1 C and stable battery performance, with a capacity retention of ≈90% after 500 cycles at 2 C. This π‐type anionic redox and intrinsic competition mechanism present an alternative strategy for achieving advanced cathode materials for sodium‐ion batteries.

Composition-driven Mott transition within SrTi 1−x V x O3

The last few decades has seen the rapid growth of interest in the bulk perovskite-type transition metal oxides SrVO3 and SrTiO3. The electronic configuration of these perovskites differs by one electron associated to the transition metal species which gives rise to the drastically different electronic properties. Therefore, it is natural to look into how the electronic structure transitions between these bulk structures by using doping. Measurements of the substitutional doped SrTi 1−x V x O3 shows an metal–insulator transition (MIT) as a function of doping. By using supercell density functional theory with dynamical mean field theory (DFT+DMFT), we show that the MIT is indeed the result of the combination of local electron correlation effects (Mott physics) within the t2g orbitals and the atomic site configuration of the transition metals which may indicate dependence on site disorder. SrTi 1−x V x O3 may be an ideal candidate for benchmarking cutting-edge Mott–Anderson models of real systems. We show that applying an effective external perturbation on SrTi 1−x V x O3 can switch the system between the insulating and metallic phase, meaning this is a bulk system with the potential use in Mott electronic devices.

Activation of MnO6 Units via an Interfacial Electric Field: Electron Injection into Mn t2g for Rapid and Stable Sodium Ion Storage in CeO2/MnOx.

Manganese-based oxides (MnOx) suffer from sluggish charge diffusion kinetics and poor cycling stability in sodium ion storage. Herein, an interfacial electric field (IEF) in CeO2/MnOx is constructed to obtain high electronic/ionic conductivity and structural stability of MnOx. The as-designed CeO2/MnOx exhibits a remarkable capacity of 397 F g-1 and favorable cyclic stability with 92.13% capacity retention after 10,000 cycles. Soft X-ray absorption spectroscopy and partial density of states results reveal that the electrons are substantially injected into the Mn t2g orbitals driven by the formed IEF. Correspondingly, the MnO6 units in MnOx are effectively activated, endowing the CeO2/MnOx with fast charge transfer kinetics and high sodium ion storage capacity. Moreover, In situRaman verifies a remarkably increased structural stability of CeO2/MnOx, which is attributed to the enhanced Mn─O bond strength and efficiently stabilized MnO6 units. Mechanism studies show that the downshift of Mn 3d-band center dramatically increases the Mn 3d-O 2p orbitals overlap, thus inhibiting the Jahn-Teller (J-T) distortion of MnOx during sodium ion insertion/extraction. This work develops an advanced strategy to achieve both fast and sustainable sodium ion storage in metal oxides-based energy materials.

Giant magnetic anisotropy energy and Os–Ir coupling driven variations in the physical properties of Y[formula omitted]NiIrO[formula omitted

Spintronics is a prosperous domain that comprises colossal applications executing both the charge and spin of the electrons. So double perovskite oxides (DPO) having unusual magnetism are very relevant for these objectives. Therefore, we systematically examine the effect of strong coupling between Ir and Os 5d orbitals on the structural stabilities, electronic structure, and magnetic properties of the Y2NiIrO6 DPO using ab-initio calculations by replacing Os with one of the Ir ion (Os@Ir). We predict that the antiferromagnetic spin–orbit coupling between half-filled Ni 3d and partially-filled Ir 5d in the pristine structure, results in a ferrimagnetic (FiM) Mott-insulating phase due to unusual Jeff=12 state of Ir+4 having an energy band gap (Eg) of 0.43 eV. The calculated partial spin moments (ms) on the Ni+2 and Ir+4 ions are 1.68 and −0.49μB with electronic configurations of t2g3↑t2g3↓eg2↑eg0↓ (S = 1) and t2g3↑t2g2↓eg0 (S =12), respectively. Furthermore, our estimated Curie temperature (TC) of 198 K employing the Heisenberg model is near to the experimentally examined value of 192 K. The easy magnetic axis yields to be the b-axis with a giant magnetocrystalline anisotropy energy (MAE) constant of 1.7×108 erg/cm3, which is the origin of a huge coercive field of 1.1 T. In the case of the Os@Ir motif, Eg closed where the filled Ir t2g and partially filled Os t2g orbitals are mainly responsible for conductivity. Surprisingly, the ms on the Ir ion almost becomes zero (+0.01μB), turns out to be in a +3 state with t2g3↑t2g3↓eg0 (S = 0) despite +4 along with 1.68/−1.55μB on the Ni+2/Os+5. Finally, MAE value and TC in Os-doped structure increase due to large structural distortions as compared to pristine one which makes the system technologically important.

Metallic ruthenium ilmenites: First-principles study of MgRuO3 and CdRuO3

Ilmenites ABO3 provide a platform for electron correlation and magnetism on alternatively stacked honeycomb layers of edge-sharing AO6 or BO6 octahedra. When A and B are 3d transition metals, strong electron correlation makes the systems Mott insulators showing various magnetic properties, while when B is Ir with 5d electrons, competition between electron correlation and spin–orbit coupling realizes a spin–orbital coupled Mott insulator as a potential candidate for quantum spin liquids. Here, we theoretically investigate intermediate 4d ilmenites, ARuO3 with A = Mg and Cd, which have recently been synthesized and shown to be metallic, unlike the 3d and 5d cases. By using first-principles calculations, we optimize the lattice structures and obtain the electronic band structures. We show that MgRuO3 exhibits strong dimerization on RuO6 honeycomb layers, leading to the formation of bonding and anti-bonding bands for one of the three t2g orbitals; the lattice symmetry is lowered from R3̄ to P1̄, and the Fermi surfaces are composed of the other two t2g orbitals. In contrast, we find that CdRuO3 has a lattice structure close to R3̄, and all three t2g orbitals contribute almost equally to the Fermi surfaces. A comparison of our results with other Ru honeycomb materials, such as Li2RuO3, indicates that the metallic ruthenium ilmenites stand on a subtle balance among electron correlation, spin–orbit coupling, and electron–phonon coupling.

Emergence of exotic electronic and magnetic phases upon electron filling in Na2BO3 (B = Ta, Ir, Pt, and Tl): a first-principles study.

Competition between spin-orbit interaction and electron correlations can stabilize a variety of non-trivial electronic and magnetic ground states. Using density functional theory calculations, here we show that different exotic electronic and magnetic ground states can be obtained by electron filling of the B-site cation in the Na2BO3 family of compounds (B = Ta, Ir, Pt and Tl). Electron filling leads to a Peierls insulator state with a direct band gap to j = 1/2 spin-orbit assisted Mott-insulator to band insulator and then to negative charge-transfer half-metal transition. The magnetic ground state also undergoes a transition from a non-magnetic state to a zigzag antiferromagnetic state, a re-entrant non-magnetic state and finally to a ferromagnetic state. The electron localization function shows a ladder type dimerization or Peierls instability in Na2TaO3. Maximally localized Wannier function calculations reveal delocalization of electrons through the eg orbitals, which form a π bond, and localization of electrons through the t2g orbitals, which form a σ bond, between the neighbouring tantalum ions. Na2TlO3 shows Stoner or band ferromagnetism due to the localized moments with up-spin on oxygen ligands created by the negative charge-transfer character, interacting via the down-spin itinerant electrons of the Tl 5d-O 2p hybridized band. These findings are significant for practical applications; for instance the direct band gap insulator Na2TaO3 shows potential for utilisation in solar cells, while Na2TlO3, which exhibits ferromagnetic half metallicity, holds promise for spintronic device applications.

P–d coupling: prerequisite for band-like doping levels in metal oxides

The existence of strong p–d coupling between O/N-pπ and metal t2g orbitals is the prerequisite for inducing band-like N doping levels.

Multiorder and structural mechanism of the LiRh<sub>2</sub>O<sub>4</sub> tetragonal phase formation

The results of the group-theoretical analysis of phase transitions in the LiRh2O4 are presented. It is found that the critical irreducible representation inducing phase transitions and multiorder in this substance is the eight-dimensional representation \(~{{k}_{{11}}}{{{\text{\tau }}}_{5}} + {{k}_{{10}}}{{{\text{\tau }}}_{2}}\). It is shown that the multiorder and the structural mechanism of formation of the tetragonal phase of lithium rhodonite are related to the displacements of oxygen atoms, the tilts of the [RhO]6 octahedra, and the ordering of the rhodium t2g orbitals.

Tuning orbital-selective phase transitions in a two-dimensional Hund’s correlated system

Hund’s rule coupling (J) has attracted much attention recently for its role in the description of the novel quantum phases of multi-orbital materials. Depending on the orbital occupancy, J can lead to various intriguing phases. However, experimental confirmation of the orbital occupancy dependency has been difficult as controlling the orbital degrees of freedom normally accompanies chemical inhomogeneities. Here, we demonstrate a method to investigate the role of orbital occupancy in J related phenomena without inducing inhomogeneities. By growing SrRuO3 monolayers on various substrates with symmetry-preserving interlayers, we gradually tune the crystal field splitting and thus the orbital degeneracy of the Ru t2g orbitals. It effectively varies the orbital occupancies of two-dimensional (2D) ruthenates. Via in-situ angle-resolved photoemission spectroscopy, we observe a progressive metal-insulator transition (MIT). It is found that the MIT occurs with orbital differentiation: concurrent opening of a band insulating gap in the dxy band and a Mott gap in the dxz/yz bands. Our study provides an effective experimental method for investigation of orbital-selective phenomena in multi-orbital materials.

Independent Tuning of the pKa or the E1/2 in a Family of Ruthenium Pyridine-Imidazole Complexes.

Two series of RuII(acac)2(py-imH) complexes have been prepared, one with changes to the acac ligands and the other with substitutions to the imidazole. The proton-coupled electron transfer (PCET) thermochemistry of the complexes has been studied in acetonitrile, revealing that the acac substitutions almost exclusively affect the redox potentials of the complex (|ΔE1/2| ≫ |ΔpKa|·0.059 V) while the changes to the imidazole primarily affect its acidity (|ΔpKa|·0.059 V ≫ |ΔE1/2|). This decoupling is supported by DFT calculations, which show that the acac substitutions primarily affect the Ru-centered t2g orbitals, while changes to the py-imH ligand primarily affect the ligand-centered π orbitals. More broadly, the decoupling stems from the physical separation of the electron and proton within the complex and highlights a clear design strategy to separately tune the redox and acid/base properties of H atom donor/acceptor molecules.

Polarons and Charge Transfer in FeCr2O4 Chromite Treated by the DFT + U Method

The electronic structure of chromite (FeCr2O4 spinel) is described and the orbital ordering, band gap, and charge transfer are analyzed consistently in the framework of density functional theory taking into account strong electron correlations (DFT + U method). It is shown that the top of the chromite valence band in this model is formed by the ordered t2g orbitals of iron atoms located at tetrahedral sites, and the formation of hole polarons occurs involving just these orbitals. The nonadiabatic activation barrier determining the hole polaron transport is considered. The results of calculations of the band gap and activation energy are compared to the available experimental data.

Discovery of the High‐Entropy Carbide Ceramic Topological Superconductor Candidate (Ti0.2Zr0.2Nb0.2Hf0.2Ta0.2)C

AbstractHigh‐entropy ceramics (HECs) are solid solutions of inorganic compounds with one or more Wyckoff sites shared by equal or near‐equal atomic ratios of multi‐principal elements. Material design and property tailoring possibilities emerge from this new class of materials. Herein, the discovery of superconductivity 2.35 K and topological properties in the (Ti0.2Zr0.2Nb0.2Hf0.2Ta0.2)C high‐entropy carbide ceramic (HECC) is reported, which is not observed before in any of the investigated HECC. Density functional theory calculations show that six type‐II Dirac points exist in (Ti0.2Zr0.2Nb0.2Hf0.2Ta0.2)C, which mainly contributed from the t2g orbitals of transition metals and the p orbitals of C. Due to the stability of the structure, robust superconductivity (SC) under pressure in this HEC superconductor is also observed. This study expands the physical properties of HECs, which may become a new material platform for SC research, especially for studying the coupling between SC and topological physics.

Coherent detection of hidden spin-lattice coupling in a van der Waals antiferromagnet.

Strong interactions between different degrees of freedom lead to exotic phases of matter with complex order parameters and emergent collective excitations. Conventional techniques, such as scattering and transport, probe the amplitudes of these excitations, but they are typically insensitive to phase. Therefore, novel methods with phase sensitivity are required to understand ground states with phase modulations and interactions that couple to the phase of collective modes. Here, by performing phase-resolved coherent phonon spectroscopy (CPS), we reveal a hidden spin-lattice coupling in a vdW antiferromagnet FePS3 that eluded other phase-insensitive conventional probes, such as Raman and X-ray scattering. With comparative analysis and analytical calculations, we directly show that the magnetic order in FePS3 selectively couples to the trigonal distortions through partially filled t2g orbitals. This magnetoelastic coupling is linear in magnetic order and lattice parameters, rendering these distortions inaccessible to inelastic scattering techniques. Our results not only capture the elusive spin-lattice coupling in FePS3 but also establish phase-resolved CPS as a tool to investigate hidden interactions.

Nanosecond Metal-to-Ligand Charge-Transfer State in an Fe(II) Chromophore: Lifetime Enhancement via Nested Potentials.

Examples of Fe complexes with long-lived (≥1 ns) charge-transfer states are limited to pseudo-octahedral geometries with strong σ-donor chelates. Alternative strategies based on varying both coordination motifs and ligand donicity are highly desirable. Reported herein is an air-stable, tetragonal FeII complex, Fe(HMTI)(CN)2 (HMTI = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene), with a 1.25 ns metal-to-ligand charge-transfer (MLCT) lifetime. The structure has been determined, and the photophysical properties have been examined in a variety of solvents. The HMTI ligand is highly π-acidic due to low-lying π*(C═N), which enhances ΔFe via stabilizing t2g orbitals. The inflexible geometry of the macrocycle results in short Fe-N bonds, and density functional theory calculations show that this rigidity results in an unusual set of nested potential energy surfaces. Moreover, the lifetime and energy of the MLCT state depends strongly on the solvent environment. This dependence is caused by modulation of the axial ligand-field strength by Lewis acid-base interactions between the solvent and the cyano ligands. This work represents the first example of a long-lived charge transfer state in an FeII macrocyclic species.

The d Orbital Multi Pattern Occupancy in a Partially Filled d Shell: The KFeF3 Perovskite as a Test Case

The occupancy of the d shell in KFeF3 is t2g4eg2, with five α and one β electrons. The Jahn-Teller lift of degeneracy in the t2g sub-shell produces a tetragonal relaxation of the unit cell (4.09 vs. 4.22 Å, B3LYP result) not observed experimentally. In order to understand the origin of this apparent contradiction, we explored, with a 2 × 2 × 2 supercell (40 atoms per cell), all possible local structures in which contiguous Fe atoms have a different occupancy of the t2g orbitals with the minority spin electron. A total of 6561 configurations (with occupancies from (8,0,0) to (3,2,2) of the 3 t2g orbitals of the 8 Fe atoms) have been explored, with energies in many cases lower (by up to 1550 μEh per 2 Fe atoms) than the one of the fully ordered case, both for the ferromagnetic and the anti-ferromagnetic solutions. The results confirm that the orientation of the β d electron of Fe influences the electrostatics (more efficient relative orientation of the Fe quadrupoles of the d shell) of the system, but not the magnetic interactions. Three hybrid functionals, B3LYP, PBE0, and HSE06, provide very similar results.

Role of Local Structural Distortions on the Origin of j = 1/2 Pseudo-Spin State in Sodium Iridate

Na2IrO3 (NIO) is known to be a spin–orbit (SO) driven j = 1/2 pseudo-spin Mott–Hubbard (M–H) insulator. The mixing of j = 1/2 pseudo-spin derived from t2g orbitals with the eg orbitals has not been probed yet from the viewpoint of local structural distortions. Using a combination of theoretical calculations and X-ray spectroscopy, we show that the energetics in the vicinity of Fermi level (EF) is governed by SO interactions, electron correlation, and local octahedral distortions. The j = 3/2 and 1/2 pseudo-spin states have an admixture of both t2g and eg characters due to a local structural distortion. Reduction of the local octahedral symmetry also enables Ir 5d–O 2p hybridization around the EF resulting in an M–H insulator with enhanced charge-transfer character. The possibility of the Slater insulator phase is also ruled out by a combination of the absence of room-temperature density of states in valence band spectra, calculated moments, and temperature-dependent magnetization measurements.