Emergence of the isotropic Kitaev honeycomb latticeα-RuCl3and its magnetic properties.

We present a comprehensive investigation of the crystal and magnetic structures of the van der Waals antiferromagnetα-RuCl3using single crystal x-ray and neutron diffraction. The crystal structure at room temperature is a monoclinic (C2/m). However, with decreasing temperature, a remarkable first-order structural phase transition is observed, leading to the emergence of a rhombohedral (R3-) structure characterized by three-fold rotational symmetry forming an isotropic honeycomb lattice. On further cooling, a zigzag-type antiferromagnetic order develops belowTN=6∼6.6K. The critical exponent of the magnetic order parameter was determined to beß=0.11(1), which is close to the two-dimensional Ising model. Additionally, the angular dependence of the magnetic critical field of the zigzag antiferromagnetic order for the polarized ferromagnetic phase reveals a six-fold rotational symmetry within theab-plane. These findingsreflect the symmetry associated with the Ising-like bond-dependent Kitaev spin interactions and underscore the universality of the Kitaev interaction-dominated antiferromagnetic system.

Rapid FEV1/FVC Decline Is Related With Incidence of Obstructive Lung Disease and Mortality in General Population.

BACKGROUND: Forced expiratory volume in 1 second (FEV1)/forced vital capacity (FVC) naturally decreases with age; however, an excessive decline may be related with increased morbidity and mortality. This study aimed to evaluate the FEV1/FVC decline rate in the Korean general population and to identify whether rapid FEV1/FVC decline is a risk factor for obstructive lung disease (OLD) and all-cause and respiratory mortality. METHODS: We evaluated individuals aged 40-69 years who underwent baseline and biannual follow-up spirometric assessments for up to 18 years, excluding those with airflow limitations at baseline. Based on the quartiles of the annual FEV1/FVC decline rate, the most negative FEV1/FVC change (1st quartile of annual FEV1/FVC decline rate) was classified as rapid FEV1/FVC decline. We investigated the risk of progression to OLD and all-cause and respiratory mortality in individuals with rapid FEV1/FVC decline. RESULTS: The annual FEV1/FVC decline rate in the eligible 7,768 patients was 0.32 percentage point/year. The incidence rate of OLD was significantly higher in patients with rapid FEV1/FVC decline than in those with non-rapid FEV1/FVC decline (adjusted incidence rate, 2.119; 95% confidence interval [CI], 1.932-2.324). Rapid FEV1/FVC decline was an independent risk factor for all-cause mortality (adjusted hazard [HR], 1.374; 95% CI, 1.105-1.709) and respiratory mortality (adjusted HR, 1.353; 95% CI, 1.089-1.680). CONCLUSION: The annual FEV1/FVC decline rate was 0.32%p in the general population in Korea. The incidence rate of OLD and the hazards of all-cause and respiratory mortality were increased in rapid FEV1/FVC decliners.

Systemic corticosteroid use and cardiovascular risk in patients hospitalized for pneumonia.

BACKGROUND: Limited data are available concerning cardiovascular risk with respect to adjunctive corticosteroid use in patients with pneumonia. We aimed to assess the associations between systemic corticosteroid use and the occurrence of major adverse cardiovascular events (MACEs) in patients hospitalized for pneumonia. METHODS: Among study participants enrolled via surveillance for severe acute respiratory infection from July 2016 to January 2017, the clinical course of patients with pneumonia was retrospectively investigated until December 2019. We evaluated the occurrence of in-hospital and after-discharge MACEs according to steroid use during hospitalization. RESULTS: Of the 424 patients hospitalized for pneumonia, 118 (28.8%) received systemic corticosteroids during hospitalization. The most common reason for steroid use was acute exacerbation of chronic lung disease (75.4%). Systemic steroid use was significantly associated with an increased risk of in-hospital MACEs; it was not associated with after-discharge MACEs. The risk of in-hospital MACEs was significantly greater in patients with more comorbidities, more severe pneumonia, and a higher inflammatory marker level; moreover, it was positively associated with duration and cumulative dose of steroid treatment. CONCLUSION: Systemic corticosteroid use was associated with an increased risk of in-hospital MACEs in patients hospitalized for pneumonia.

Different treatment response to systemic corticosteroids according to white blood cell counts in severe COVID-19 patients.

BACKGROUND: Limited data are available in COVID-19 patients on the prediction of treatment response to systemic corticosteroid therapy based on systemic inflammatory markers. There is a concern whether the response to systemic corticosteroid is different according to white blood cell (WBC) counts in COVID-19 patients. We aimed to assess whether WBC count is related with the clinical outcomes after treatment with systemic corticosteroids in severe COVID-19. METHODS: We conducted a retrospective cohort study and analysed the patients hospitalised for severe COVID-19 and received systemic corticosteroids between July 2020 and June 2021. The primary endpoint was to compare the composite poor outcome of mechanical ventilation, extracorporeal membrane oxygenation, and mortality among the patients with different WBC counts. RESULTS: Of the 585 COVID-19 patients who required oxygen supplementation and systemic corticosteroids, 145 (24.8%) belonged to the leukopoenia group, 375 (64.1%) belonged to the normal WBC group, and 65 (11.1%) belonged to the leukocytosis group. In Kaplan-Meier curve, the composite poor outcome was significantly reduced in leukopoenia group compared to leukocytosis group (log-rank p-value < 0.001). In the multivariable Cox regression analysis, leukopoenia group was significantly associated with a lower risk of the composite poor outcome compared to normal WBC group (adjusted hazard ratio [aHR] = 0.32, 95% CI 0.14-0.76, p-value = 0.009) and leukocytosis group (aHR = 0.30, 95% CI = 0.12-0.78, p-value = 0.013). There was no significant difference in aHR for composite poor outcome between leukocytosis and normal WBC group. CONCLUSION: Leukopoenia may be related with a better response to systemic corticosteroid therapy in COVID-19 patients requiring oxygen supplementation.KEY MESSAGESIn severe COVID-19 treated with systemic corticosteroids, patients with leukopoenia showed a lower hazard for composite poor outcome compared to patients with normal white blood cell counts or leukocytosis.Leukopoenia may be a potential biomarker for better response to systemic corticosteroid therapy in COVID-19 pneumonia.

Triangular Magnet Emergent from Noncentrosymmetric Sr0.94Mn0.86Te1.14O6 Single Crystals.

We report the successful growth of high-quality single crystals of Sr0.94Mn0.86Te1.14O6 (SMTO) using a self-flux method. The structural, electronic, and magnetic properties of SMTO are investigated by neutron powder diffraction (NPD), single-crystal X-ray diffraction (SCXRD), thermodynamic, and nuclear magnetic resonance techniques in conjunction with density functional theory calculations. NPD unambiguously determined octahedral (trigonal antiprismatic) coordination for all cations with the chiral space group P312 (no. 149), which is further confirmed by SCXRD data. The Mn and Te elements occupy distinct Wyckoff sites, and minor anti-site defects were observed in both sites. X-ray photoelectron spectroscopy reveals the existence of mixed valence states of Mn in SMTO. The magnetic susceptibility and specific heat data evidence a weak antiferromagnetic order at TN = 6.6 K. The estimated Curie-Weiss temperature θCW = -21 K indicates antiferromagnetic interaction between Mn ions. Furthermore, both the magnetic entropy and the 125Te nuclear spin-lattice relaxation rate showcase that short-range spin correlations persist well above the Néel temperature. Our work demonstrates that Sr0.94(2)Mn0.86(3)Te1.14(3)O6 single crystals realize a noncentrosymmetric triangular antiferromagnet.

Open-Framework Iron Fluoride Phosphates Based on Chain, Trinuclear, and Tetranuclear Chain FeIII Building Units: Crystal Structures and Magnetic Properties.

We report the synthesis and characterization of a series of new open-framework iron fluoride-fluorophosphates based on linear, trinuclear, and tetranuclear chain FeIII building units. KFe2(PO3F)2F3 (I) consists of {Fe2(O3F)2F2}10- zigzag chains interconnected by P(O/F)4 tetrahedra forming a three-dimensional (3D) open framework. K2Fe(PO2.5F1.5)2F2 (II) is built up by {Fe(PO2.5F1.5)2F2}2- chains separated by K+ cation layers. The framework for K3Fe3(PO4)(PO3F)2F5 (III) contains two-dimensional {Fe3O4F4(PO3F)2}2- sheets, which are built from trimeric Fe-octahedra insulated by PO3F tetrahedra. The macroanionic framework of K3Fe4(PO4)2F9 (IV) comprises linear {Fe4O8F9}10- chains consisting of tetranuclear magnetic clusters of [Fe4O8F9]10- formed via corner-sharing fluorine atoms decorated with PO4 groups. The magnetic characterization of three iron fluorophosphates reveals diversified magnetism: S = 5/2 spin chains for I, antiferromagnetically coupled triangular Fe units for III, and coupled tetrahedral S = 5/2 spin chains for IV. IV shows strong geometric frustration thanks to its spin motifs of corner-shared tetrahedral clusters.

Bosonic spinons in anisotropic triangular antiferromagnets.

Anisotropic triangular antiferromagnets can host two primary spin excitations, namely, spinons and triplons. Here, we utilize polarization-resolved Raman spectroscopy to assess the statistics and dynamics of spinons in Ca3ReO5Cl2. We observe a magnetic Raman continuum consisting of one- and two-pair spinon-antispinon excitations as well as triplon excitations. The twofold rotational symmetry of the spinon and triplon excitations are distinct from magnons. The strong thermal evolution of spinon scattering is compatible with the bosonic spinon scenario. The quasilinear spinon hardening with decreasing temperature is envisaged as the ordering of one-dimensional topological defects. This discovery will enable a fundamental understanding of novel phenomena induced by lowering spatial dimensionality in quantum spin systems.

Extrinsic Surface Magnetic Anisotropy Contribution in Pt/Y3Fe5O12 Interface in Longitudinal Spin Seebeck Effect by Graphene Interlayer.

A recent study found that magnetization curves for Y3Fe5O12 (YIG) slab and thick films (>20 µm thick) differed from bulk system curves by their longitudinal spin Seebeck effect in a Pt/YIG bilayer system. The deviation was due to intrinsic YIG surface magnetic anisotropy, which is difficult to adopt extrinsic surface magnetic anisotropy even when in contact with other materials on the YIG surface. This study experimentally demonstrates evidence for extrinsic YIG surface magnetic anisotropy when in contact with a diamagnetic graphene interlayer by observing the spin Seebeck effect, directly proving intrinsic YIG surface magnetic anisotropy interruption. We show the Pt/YIG bilayer system graphene interlayer role using large area single and multilayered graphenes using the longitudinal spin Seebeck effect at room temperature, and address the presence of surface magnetic anisotropy due to magnetic proximity between graphene and YIG layer. These findings suggest a promising route to understand new physics of spin Seebeck effect in spin transport.

Variable Structures and One-Dimensional-Canting Antiferromagetism Found in Fluoride Selenates, A2Mn(SeO4)F3 (A = K, Rb, Cs).

Three new alkali-metal manganese fluoride selenates, A2Mn(SeO4)F3 (A = K, Rb, Cs), were prepared through hydrothermal redox reactions. The products consisted of one-dimensional polymeric anionic ∞[Mn(SeO4)F3]2- chains, where the A+ cations are connected by O and/or F atoms to form blocks with two-dimensional layers. A2Mn(SeO4)F3 (A= Rb, Cs) is isostructural with the monoclinic space group P21/c, while K2Mn(SeO4)F3 crystallizes in the orthorhombic space group Pbcn. A2Mn(SeO4)F3 (A = K, Rb, Cs) forms spin chains of Mn3+ with different Mn-F-Mn bridges, which showed canting antiferromagnetic behaviors. Single-crystal magnetic measurements revealed that the magnetic moments of the Mn ions were more canted for larger alkali-metal compounds in an antiferromagnetically ordered state.

Cycloidal Magnetic Order Promoted by Labile Mixed Anionic Paths in M2(SeO3)F2 (M = Mn2+, Ni2+).

Two M2(SeO3)F2 fluoro-selenites (M = Mn2+, Ni2+) have been synthesized using optimized hydrothermal reactions. Their 3D framework consists of 1D-[MO2F2]4-chains of edge-sharing octahedra with a rare topology of alternating O-O and F-F µ2 bridges. The interchain corner-sharing connections are assisted by the mixed O vs F anionic nature and develop a complex set of M-X-M superexchanges as calculated by LDA+U. Their interplay induces prominent in-chain antiferromagnetic frustration, while the interchain exchanges are responsible for the cycloidal magnetic structure observed below TN ≈ 21.5 K in the Ni2+ case. For comparison the Mn2+ compound develops a nearly collinear spin (canted) ordering below TN ≈ 26 K with ferromagnetic chain units.

Role of Ferromagnetic Monolayer WSe2 Flakes in the Pt/Y3Fe5O12 Bilayer Structure in the Longitudinal Spin Seebeck Effect.

The spin Seebeck effect (SSE) has attracted renewed interest as a promising phenomenon for energy harvesting systems. A noteworthy effort has been devoted to improving the SSE voltage by inserting ultrathin magnetic layers including Fe70Cu30 interlayers in Pt/Y3Fe5O12 (Pt/YIG) systems with increased spin-mixing conductance at the interfaces. Nevertheless, the responsible underlying physics associated with the role of the interlayer in Pt/YIG systems in the SSE is still unknown. In this paper, we demonstrate that with a monolayer tungsten diselenide (ML WSe2) interlayer in the Pt/YIG bilayer system, the longitudinal SSE (LSSE) voltage is significantly increased by the increased spin accumulation in the Pt layer; the spin fluctuation in ML WSe2 amplifies the spin current transmission because the in-plane-aligned WSe2 spins are coupled to thermally pumped spins under nonequilibrium magnetization conditions in the LSSE configuration at room temperature. The thermopower (VLSSE/ΔT) improves by 323% with respect to the value of the reference Pt/YIG bilayer sample in the LSSE at room temperature. In addition, the induced ferromagnetic properties of the ML WSe2 flakes on YIG increase the LSSE voltage (VLSSE) of the sample; the ferromagnetic properties are a result of the improved magnetic moment density in the ML WSe2 flakes and their two-dimensional (2D) ML nature in the LSSE under nonequilibrium magnetization conditions. The results can extend the application range of the materials in energy harvesting and provide important information on the physics of the LSSE with a transition metal dichalcogenide intermediate layer in spin transport.

Enhanced Spin Seebeck Thermopower in Pt/Holey MoS2/Y3Fe5O12 Hybrid Structure.

We first observed the spin-to-charge conversion due to both the inverse Rashba-Edelstein effect (IREE) and inverse spin-Hall effect in a holey multilayer molybdenum disulfide (MoS2) intermediate layer in a Pt/YIG structure via LSSE measurements under nonequilibrium magnetization. We found an enhancement of approximately 238%, 307%, and 290% in the longitudinal spin Seebeck effect (LSSE) voltage, spin-to-charge current, and thermoelectric (TE) power factor, respectively, compared with the monolayer MoS2 interlayer in a Pt/YIG structure. Such an enhancement in the LSSE performance of Pt/holey MoS2/YIG can be explained by the improvement of spin accumulation in the Pt layer by induced spin fluctuation as well as increased additional spin-to-charge conversion due to in-plane IREE. Our findings represent a significant achievement in the understanding of spin transport in atomically thin MoS2 interlayers and pave the way toward large-area TE energy-harvesting devices in two-dimensional transition metal dichalcogenide materials.

Magnon bound states versus anyonic Majorana excitations in the Kitaev honeycomb magnet α-RuCl3.

The pure Kitaev honeycomb model harbors a quantum spin liquid in zero magnetic fields, while applying finite magnetic fields induces a topological spin liquid with non-Abelian anyonic excitations. This latter phase has been much sought after in Kitaev candidate materials, such as α-RuCl3. Currently, two competing scenarios exist for the intermediate field phase of this compound (B = 7 - 10 T), based on experimental as well as theoretical results: (i) conventional multiparticle magnetic excitations of integer quantum number vs. (ii) Majorana fermionic excitations of possibly non-Abelian nature with a fractional quantum number. To discriminate between these scenarios a detailed investigation of excitations over a wide field-temperature phase diagram is essential. Here, we present Raman spectroscopic data revealing low-energy quasiparticles emerging out of a continuum of fractionalized excitations at intermediate fields, which are contrasted by conventional spin-wave excitations. The temperature evolution of these quasiparticles suggests the formation of bound states out of fractionalized excitations.

Transition-Metal Monofluorophosphate Ba2M2(PO3F)F6 (M = Mn, Co, and Ni): Varied One-Dimensional Transition-Metal Chains and Antiferromagnetism.

Four transition-metal monofluorophosphates, with a chemical formula of Ba2M2(PO3F)F6 (M = Mn, Co, Ni, and Cu), have been synthesized hydrothermally using phosphoric and fluorophosphoric acids. The structures of Ba2M2(PO3F)F6 were intensively investigated by single-crystal and powder X-ray diffraction. Their networks exhibited three-dimensional frameworks formed by cis-MO2F4 octahedra and PO3F tetrahedra. Ba2M2(PO3F)F6 (M = Mn, Co, and Cu) are isostructural, crystallizing in the monoclinic space group P21/c, while Ba2Ni2(PO3F)F6 adopted an unpredetermined structure, crystallizing in the monoclinic space group P21/n. The different arrangements of the same cis-MO2F4 moieties and PO3F groups within one-dimensional chains lead to different frameworks in this late-first-row transition-metal series. By a comparison of normalized bond lengths with their normal counterparts, the more elongated octahedral cis-MO2F4 units in Ba2Cu2(PO3F)F6 reveal the presence of static Jahn-Teller distortion, uncovering the puzzling reason for the exceptional structure of Ba2Ni2(PO3F)F6 in the series. Ba2M2(PO3F)F6 (M = Mn, Co, and Ni) forms spin chains of M2+ made of coupled dimers or tetramers, showing dominant antiferromagnetic behavior.

Randomly Hopping Majorana Fermions in the Diluted Kitaev System α-Ru_{0.8}Ir_{0.2}Cl_{3}.

dc and ac magnetic susceptibility, magnetization, specific heat, and Raman scattering measurements are combined to probe low-lying spin excitations in α-Ru_{1-x}Ir_{x}Cl_{3} (x≈0.2), which realizes a disordered spin liquid. At intermediate energies (ℏω>3 meV), Raman spectroscopy evidences linearly ω-dependent Majorana-like excitations, obeying Fermi statistics. This points to robustness of a Kitaev paramagnetic state under spin vacancies. At low energies below 3 meV, we observe power-law dependences and quantum-critical-like scalings of the thermodynamic quantities, implying the presence of a weakly divergent low-energy density of states. This scaling phenomenology is interpreted in terms of the random hoppings of Majorana fermions. Our results demonstrate an emergent hierarchy of spin excitations in a diluted Kitaev honeycomb system subject to spin vacancies and bond randomness.

Lone-pair self-containment in pyritohedron-shaped closed cavities: optimized hydrothermal synthesis, structure, magnetism and lattice thermal conductivity of Co15F2(TeO3)14.

A new oxofluoride Co15F2(TeO3)14 has been prepared by optimized hydrothermal synthesis involving a complex mineralization process. The crystal structure consists of a three-dimensional network of CoO5(O,F) octahedra, distorted CoO5 square pyramids, TeO3 trigonal pyramids and grossly distorted TeO3+3 octahedra, which are linked by sharing corners and edges. The Te(iv) lone pairs are accommodated within novel pyritohedron-shaped [(TeO3)14]28- units. This special framework provides a much larger free space that allows Te atoms to vibrate with a large amplitude, which leads to extremely low lattice thermal conductivity. Magnetic susceptibility data for Co15F2(TeO3)14 show antiferromagnetic ordering below 9.6 K with a substantial orbital component to the effective magnetic moment. An S = 3/2 honeycomb-like spin network was carefully analyzed by experimental techniques and first principles calculations.

Raman spectroscopic diagnostic of quantum spin liquids.

Quantum spin liquids are outstanding examples of highly quantum entangled phases of matter and serve as a testbed to gauge central concepts of strongly correlated materials. Enormous research efforts in the past few decades have brought an in-depth understanding of these novel phases, although their conundrums have not yet been solved completely. In this review, we give an overview of the three different classes of spin-liquid materials: (i) a one-dimensional spin chain system KCuF3, (ii) a kagome antiferromagnet ZnCu3(OH)6Cl2, and (iii) a Kitaev honeycomb material [Formula: see text]-RuCl3. The emphasis is on demonstrating the success of the Raman scattering technique for probing fractionalized excitations in the aforementioned spin-liquid compounds, complementing a well-established neutron scattering method. Irrespective of dimensionality, spin topology, and spin-exchange type, the three materials share several common features in the spectral shape and temperature dependence of magnetic excitations, which can be taken as Raman spectroscopic fingerprints of quantum spin liquids.

Exotic Low-Energy Excitations Emergent in the Random Kitaev Magnet Cu_{2}IrO_{3}.

We report on magnetization M(H), dc and ac magnetic susceptibility χ(T), specific heat C_{m}(T) and muon spin relaxation (µSR) measurements of the Kitaev honeycomb iridate Cu_{2}IrO_{3} with quenched disorder. In spite of the chemical disorders, we find no indication of spin glass down to 260 mK from the C_{m}(T) and µSR data. Furthermore, a persistent spin dynamics observed by the zero-field muon spin relaxation evidences an absence of static magnetism. The remarkable observation is a scaling relation of χ[H,T] and M[H,T] in H/T with the scaling exponent α=0.26-0.28, expected from bond randomness. However, C_{m}[H,T]/T disobeys the predicted universal scaling law, pointing towards the presence of additional low-lying excitations on the background of bond-disordered spin liquid. Our results signify a many-faceted impact of quenched disorder in a Kitaev spin system due to its peculiar bond character.

Variable Dimensionality, Valence, and Magnetism in Fluoride-Rich Iron Phosphates Ba xFe x(PO4)F y (1 ≤ x ≤ 3, 2 ≤ y ≤ 12).

We report the synthesis and characterization of three fluoride-rich barium iron phosphates Ba xFe x(PO4)F y (1 ≤ x ≤ 3, 2 ≤ y ≤ 12), which exhibited abundant structural chemistry, exhibiting diverse frameworks and connecting modes between [FeO nF6- n] m- octahedra surrounding Fe2+ or Fe3+ ions. BaFe(PO4)F2 (I) consisted of two-dimensional [Fe(PO4)F2]2- sheets built from linear ∞[Fe2O6F4]10- moieties formed by fluorine corner-sharing FeO4F2 and FeO2F4 octahedra with linking PO4 tetrahedra. Mixed-valence Ba2Fe2(PO4)F6 (II) possessed a three-dimensional framework containing Fe4O6F12 tetramers formed by the edge-sharing oxygen or fluorine atoms of cis-FeF4O2 octahedra. Ba3Fe3(PO4)F12 (III) contained one-dimensional columns of ∞[Fe3(PO4)F12]6- infinite sections built from cis-FeF4O2 and FeF5O octahedra and tetrahedral PO4 linkers. The magnetic characterization of Ba xFe x(PO4)F y unveiled diverse magnetism: an S = 5/2 spin chain for (I), a weak ferrimagnet or canted antiferromagnet for (II) thanks to the presence of distinct Fe2+ and Fe3+ sites identified by Mössbauer spectroscopy, and coupled spin-trimers for (III).

Sub-gap optical response in the Kitaev spin-liquid candidate α-RuCl3.

We report detailed optical experiments on the layered compound α-RuCl3 focusing on the THz and sub-gap optical response across the structural phase transition from the monoclinic high-temperature to the rhombohedral low-temperature structure, where the stacking sequence of the molecular layers is changed. This type of phase transition is characteristic for a variety of tri-halides crystallizing in a layered honeycomb-type structure and so far is unique, as the low-temperature phase exhibits the higher symmetry. One motivation is to unravel the microscopic nature of THz and spin-orbital excitations via a study of temperature and symmetry-induced changes. The optical studies are complemented by thermal expansion experiments. We document a number of highly unusual findings: A characteristic two-step hysteresis of the structural phase transition, accompanied by a dramatic change of the reflectivity. A complex dielectric loss spectrum in the THz regime, which could indicate remnants of Kitaev physics. Orbital excitations, which cannot be explained based on recent models, and an electronic excitation, which appears in a narrow temperature range just across the structural phase transition. Despite significant symmetry changes across the monoclinic to rhombohedral phase transition and a change of the stacking sequence, phonon eigenfrequencies and the majority of spin-orbital excitations are not strongly influenced. Obviously, the symmetry of a single molecular layer determines the eigenfrequencies of most of these excitations. Only one mode at THz frequencies, which becomes suppressed in the high-temperature monoclinic phase and one phonon mode experience changes in symmetry and stacking. Finally, from this combined terahertz, far- and mid-infrared study we try to shed some light on the so far unsolved low energy (<1 eV) electronic structure of the ruthenium 4d 5 electrons in α-RuCl3.