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International audience; Fe4Nb2O9 and Co4Nb2O9 are antiferromagnetic compounds belonging to the M4M′2O9 series (M = Mn, Fe, Co; M′ = Nb, Ta), whose structure (P[3 with combining macron]c1) derives from the Cr2O3 corundum one. By mixing cobalt and iron in a 1 : 1 ratio, an Fe2Co2Nb2O9 oxide has been synthesized. Its structural, magnetic and electric characterization studies confirm that its unit cell volume (V = 331.19(1) Å3), Néel temperature (TN = 58 K) and dielectric permittivity are intermediate between those of the Fe4Nb2O9 and Co4Nb2O9 end members. The neutron diffraction study evidences an Fe:Co random occupation of the (4d) Wyckoff sites. Its antiferromagnetic structure, refined in the C2/c′ magnetic space group, shows magnetic moments in the planes of the honeycomb (HC) layers of the M4M′2O9 structure, and stacked along c, as for Fe4Nb2O9 and Co4Nb2O9. However, the magnetic field induced magnetization M(H) of Fe2Co2Nb2O9 below TN differs strongly from both end members, exhibiting a sublinear dependence. A similar rounded shape of the electric polarization P(H) curve is evidenced, in marked contrast to the linear M(H) and P(H) behaviours observed below the spin-flop magnetic field for this class of linear magnetoelectric materials. As a result, Fe2Co2Nb2O9 exhibits larger P values and steeper P responses to H at low H than both Fe4Nb2O9 and Co4Nb2O9 end members. This result is explained by a microscopic model based on the Fe and Co mixed occupation. Substitutional disorder locally allows for stronger magnetoelectric couplings and its effect does not cancel on average. This chemical substitution effect opens a new route to enhance the magnetoelectric response of honeycomb antiferromagnets.

The simple ABO3 and A-site-ordered AA′3B4O12 perovskites represent two types of classical perovskite functional materials. There are well-known simple perovskites with ferroelectric properties, while there is still no report of ferroelectricity due to symmetry breaking transition in A-site-ordered quadruple perovskites. Here we report the high pressure synthesis of an A-site-ordered perovskite PbHg3Ti4O12, the only known quadruple perovskite that transforms from high-temperature centrosymmetric paraelectric phase to low-temperature non-centrosymmetric ferroelectric phase. The coordination chemistry of Hg2+ is changed from square planar as in typical A-site-ordered quadruple perovskite to a rare stereo type with 8 ligands in PbHg3Ti4O12. Thus PbHg3Ti4O12 appears to be a combinatory link from simple ABO3 perovskites to A-site-ordered AA′3Ti4O12 perovskites, sharing both displacive ferroelectricity with former and structure coordination with latter. This is the only example so far showing ferroelectricity due to symmetry breaking phase transition in AA′3B4O12-type A-site-ordered perovskites, and opens a direction to search for ferroelectric materials.

Proceedings of the National Academy of Sciences of the United States of America 117(48), 30220 - 30227 (2020). doi:10.1073/pnas.2005701117, Using inelastic X-ray scattering beyond the dipole limit and hard X-ray photoelectron spectroscopy we establish the dual nature of the U 5f electrons in UM$_2$Si$_2$ (M = Pd, Ni, Ru, Fe), regardless of their degree of delocalization. We have observed that the compounds have in common a local atomic-like state that is well described by the U 5f$^2$ configuration with the $Γ^{(1)}_1$ and $Γ_2$ quasi-doublet symmetry. The amount of the U 5f$^3$ configuration, however, varies considerably across the UM$_2$Si$_2$ series, indicating an increase of U 5f itineracy in going from M = Pd to Ni to Ru and to the Fe compound. The identified electronic states explain the formation of the very large ordered magnetic moments in UPd$_2$Si$_2$ and UNi$_2$Si$_2$, the availability of orbital degrees of freedom needed for the hidden order in URu2Si2 to occur, as well as the appearance of Pauli paramagnetism in UFe$_2$Si$_2$. A unified and systematic picture of the UM$_2$Si$_2$ compounds may now be drawn, thereby providing suggestions for additional experiments to induce hidden order and/or superconductivity in U compounds with the tetragonal body-centered ThCr$_2$Si$_2$ structure., Published by National Acad. of Sciences, Washington, DC

A new oxide, LaMn3Co2Mn2O12, was synthesized under high-pressure (7 GPa) and high-temperature (1423 K) conditions. The compound crystallizes in an AA′3B4O12-type quadruple perovskite structure with...

An ilmenite-like monoclinic phase of HgMnO3 with space group P21/c was prepared using high-pressure and high-temperature methods at 18 GPa and 1473 K. The MnO6 octahedra form a two-dimensional (2D) network in the bc plane, leading to a long-range antiferromagnetic ordering with a low Neel temperature of TN ∼ 32 K. As the synthesis pressure increases to 20 GPa, a new perovskite-like rhombohedral phase with space group R3c was found to occur. The rhombohedral phase exhibits a three-dimensional (3D) network for the MnO6 octahedra, giving rise to an antiferromagnetic ordering at TN ∼ 60 K. X-ray absorption spectroscopy confirms the invariable Mn4+ charge state in these two polymorphic phases, in agreement with the Curie-Weiss and bond valence sum analysis. HgMnO3 provides an interesting example to study the magnetic properties from 2D to 3D by varying synthesis pressure.

Half metals, in which one spin channel is conducting while the other is insulating with an energy gap, are theoretically considered to comprise 100% spin-polarized conducting electrons, and thus have promising applications in high-efficiency magnetic sensors, computer memory, magnetic recording, and so on. However, for practical applications, a high Curie temperature combined with a wide spin energy gap and large magnetization is required. Realizing such a high-performance combination is a key challenge. Herein, a novel A- and B-site ordered quadruple perovskite oxide LaCu

International audience; Synthetic crystals of the ferberite FeWO 4 have been grown by the chemical transport reaction starting from a polycrystalline sample of this phase. Magnetic susceptibility measurements showed an antiferromagnetic Neél temperature of T N = 75 K. The anisotropy in the magnetic susceptibility can be ascribed to the magnetocrystalline anisotropy of the Fe 2+ ion, the oxidation state of which was confirmed by X-ray absorption spectroscopy. While X-ray photoemission analysis indicated that all W ions are in the expected 6+ charge state, the dielectric permittivity of the FeWO 4 crystals was found to be leaky, hindering changes at T N to be detected. Subsequent thermoelectric power measurements suggested the presence of about 1.5% Fe 3+. X-ray diffraction experiments confirmed the basic crystal structure of the wolframite type and revealed some structural disorder in the 1% range. Transmission electron microscopy allowed us to unveil the occurrence of stacking faults attributed to the similarity of the atomic environment of the Fe and W species.

Semiconducting substances form one of the most important families of functional materials. However, semiconductors containing only metals are very rare. The chemical mechanisms behind their ground‐state properties are only partially understood. Our investigations have rather unexpectedly revealed the semiconducting behaviour (band gap of 190 meV) for the intermetallic compound Be5Pt formed at a very low valence‐electron count. Quantum‐chemical analysis shows strong charge transfer from Be to Pt and reveals a three‐dimensional entity of vertex‐condensed empty Be4 tetrahedrons with multi‐atomic cluster bonds interpenetrated by the framework of Pt‐filled vertex‐condensed Be4 tetrahedrons with two‐atomic polar Be−Pt bonds. The combination of strong Coulomb interactions with relativistic effects results in a band gap., Progressive metals: Be5Pt is a rare representative of semiconductors that are formed by metallic elements only. The gap in the band structure appears due to both relativistic effects and a unique chemical‐bonding situation with polar two‐atom and multi‐atom Pt–Be interactions.

The development of cost-effective, non-noble metal electrocatalysts for the oxygen evolution reaction (OER) is of paramount importance for sustainable technologies. Efficient screening strategies for electrocatalysts can greatly increase the commercialization speed of these advanced technologies. Here, ternary phase diagrams with large-scale tuning and designated-scale tuning strategies are applied for the first time to provide a new method for screening perovskite oxide-based electrocatalysts for OERs. Specifically, the family of double perovskites (SrFeCoNiMoO, 0 ≤ x, y ≤ 1.5) was utilized to understand the role of transition metals in perovskite oxides. Ternary phase diagrams can facilitate a rapid screening process, provide a straightforward relationship between phase structures and catalytic activities, and help to confirm the effects of various combinations of transition metals on the OER activity. The Fe-Co system (SrFeCoMoO) improves the catalytic activities, as demonstrated by the reduced Tafel slope and enhanced stability, while the Fe-Ni system (SrFeNiMoO) improves the surface kinetic properties of the OER, as demonstrated by its reduced overpotential. Significantly, the Co, Ni, and Fe ternary phase systems can serve as the synergistic coactive sites (SrFeCoNiMoO) to catalyze the OER, resulting in an improved overall OER performance. This systematic study not only demonstrates a new strategy to allow the rapid screening of double perovskite OER catalysts based on large-scale tuning and designated-scale tuning strategies but, more importantly, also provides an insightful understanding of the use of multitransition metal-based double perovskites for catalysis of the OER.

Controlling ferroic orders (ferroelectricity, ferromagnetism and ferroelasticity) by optical methods is a significant challenge due to the large mismatch in energy scales between the order parameter coupling strengths and the incident photons. Here, we demonstrate an approach to manipulate multiple ferroic orders in an epitaxial mixed-phase BiFeO3 thin film at ambient temperature via laser illumination. Phase-field simulations indicate that a light-driven flexoelectric effect allows the targeted formation of ordered domains. We also achieved precise sequential laser writing and erasure of different domain patterns, which demonstrates a deterministic optical control of multiferroicity at room temperature. As ferroic orders directly influence susceptibility and conductivity in complex materials, our results not only shed light on the optical control of multiple functionalities, but also suggest possible developments for optoelectronics and related applications.

The electronic and magnetic structure of orthorhombic (o-) YMn0.5Fe0.5O3 (YMFO) epitaxial film (space group Pbnm) deposited on the YAlO3 (010) substrate have been investigated using linear polarization-dependent x-ray absorption spectroscopy and magnetization measurements. The magnetic-ordering temperature of around 280 K is observed in o-YMFO film. The x-ray absorption spectra at the Fe-K and the Mn-K edges indicate the existence of anisotropic crystal field in o-YMFO film, with the longest and shortest Fe–O and Mn–O bonds tend to align with the crystallographic b- and a-axis, respectively, whereas the medium Fe(Mn)–O bond is aligned with the c-axis. The experimental x-ray magnetic linear dichroism at the Fe-L2 edges demonstrate an unusual single antiferromagnetic axis of Fe3+ ions below magnetic-ordering temperature, while a multi antiferromagnetic axis is generally observed in o-RFeO3 (R = rare earth) thin films. Our configuration-interaction cluster calculations also reveal that the single antiferromagnetic axis of the Fe sublattice is aligned with the b-axis in o-YMFO film, whereas it is directed along the a-axis in the bulk o-YFeO3.

The new fully rock-salt ordered Ir5+ double perovskite SrLaCuIrO6 was synthesized, and its structural, magnetic and electronic properties were investigated. The compound crystallizes tetragonally (space group I4/m). The Cu2+ cation leads to strong Jahn-Teller distortion of the CuO6 octahedra. The compound shows semiconducting behavior with an estimated band gap of approximately 0.2 eV. At around 10 K a magnetic transition is observed.

If different active sites in a catalyst have optimal binding to different reaction intermediates and short reaction paths among them, they may work cooperatively to enhance the oxygen evolution reaction (OER) activity. Based on this design principle, in this study, we start with a B-site ordered double perovskite Sr2FeMoO6−δ with poor OER activity as the host material to fulfill the requirement of a short pathway, and then, replace Mo with Ni and Fe with Co to optimize the synergistic interplay of the multi-active sites. Replacing Mo with Ni indeed dramatically enhances the OER activity and structural/operating stability. Further improvement in OER performance is realized by partial substitution of Fe with Co, leading to the development of a material with the nominal composition of Sr2Fe0.8Co0.2Mo0.65Ni0.35O6−δ, which outperforms the noble metal oxide IrO2 and is better than most of the electrocatalysts developed based on a single descriptor, such as Ba0.5Sr0.5Co0.8Fe0.2O3−δ (eg occupancy close to unity), PrBaCo2O5+δ (O 2p-band center relative to the Fermi level), and La0.5Sr0.5CoO3−δ (charge-transfer energy) in many aspects. As a universal method, combined structural and compositional tuning to create a cooperative effect among different active sites for intermediate adsorption and reaction in an ordered structure may provide a new way for the design of superior electrocatalysts for various applications.

We report the synthesis of a quasi-one-dimensional (1D) iron selenide ${\mathrm{Ba}}_{9}{\mathrm{Fe}}_{3}{\mathrm{Se}}_{15}$. It was synthesized at high temperature and high pressure of 5.5 GPa and systematically studied via structural, magnetic, and transport measurements at both ambient and high pressure. ${\mathrm{Ba}}_{9}{\mathrm{Fe}}_{3}{\mathrm{Se}}_{15}$ crystallizes in a monoclinic structure and consists of face-sharing $\mathrm{Fe}{\mathrm{Se}}_{6}$ octahedral chains running in $c$-direction. It shows a sizable unquenched orbital moment and undergoes a ferromagnetic-like phase transition at 14 K. At ambient pressure, ${\mathrm{Ba}}_{9}{\mathrm{Fe}}_{3}{\mathrm{Se}}_{15}$ exhibits an insulating behavior with a band gap of \ensuremath{\sim}460 meV. Under high pressure, a complete metallization occurs at \ensuremath{\sim}29 GPa, which is accompanied by a spin state crossover from high-spin to low-spin state.

We report on the synthesis and physical properties of cm-sized CoGeO3 single crystals grown in a high pressure mirror furnace at pressures of 80 bar. Direction dependent magnetic susceptibility measurements on our single crystals reveal highly anisotropic magnetic properties that we attribute to the impact of strong single ion anisotropy appearing in this system with TN∼33.5 K. Furthermore, we observe effective magnetic moments that are exceeding the spin only values of the Co ions, which reveals the presence of sizable orbital moments in CoGeO3.

Using soft x-ray absorption spectroscopy we were able to determine unambiguously the charge and spin states of the transition metal ions in stoichiometric $\mathrm{Y}{\mathrm{Cu}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{12}$ and $\mathrm{Ca}{\mathrm{Cu}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{12}$. The trivalent and low-spin nature of both the Cu and Co ions in $\mathrm{Y}{\mathrm{Cu}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{12}$ makes this correlated system to be effectively a nonmagnetic band semiconductor. The substitution of Y by Ca produces formally tetravalent Co ions but the doped holes are primarily on the oxygen ligands. Concerning the spin degrees of freedom, the trivalent Co ions in $\mathrm{Y}{\mathrm{Cu}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{12}$ remain low spin upon the Y-Ca substitution, very much unlike the ${\mathrm{La}}_{1\text{\ensuremath{-}}x}{\mathrm{Sr}}_{x}\mathrm{Co}{\mathrm{O}}_{3}$ system. The tetravalent Co ions in $\mathrm{Ca}{\mathrm{Cu}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{12}$ are interestingly also in the low-spin state, which then explains the good electrical conductivity of $\mathrm{Ca}{\mathrm{Cu}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{12}$ since charge exchange between neighboring ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}$ ions will not be hampered by the spin-blockade mechanism that otherwise would be in effect if the ${\mathrm{Co}}^{4+}$ and ${\mathrm{Co}}^{3+}$ spin quantum numbers were to differ by more than one-half. We infer that the stability of the Co low-spin state is related to the very short Co-O bond lengths.

The hunt for an ideal topological insulator, where the Dirac point is situated in a desirable energetic position and the bulk remains insulating, has motivated experiments on band structure engineering in these materials. To achieve this, ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ and ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ are commonly combined in ternary compounds or, less frequently, in heterostructures. Here we report on the growth of ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}/{\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ heterostructures by means of molecular-beam epitaxy. Using angle-resolved photoelectron spectroscopy, we are able to differentiate between the shift of the chemical potential and the changes in the electronic structure, causing the lift-off of the Dirac point away from the bulk valence band when varying the ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ adlayer thickness. Our paper demonstrates that the important modulation of the surface states takes place for the very first ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ layers, while thicker adlayers only cause a gradual change of the bulk states and a rigid shift of the chemical potential. Furthermore, we observe the occurrence of diffusion between the ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ layers and conclude that a growth at room temperature, followed by annealing, maintains an acceptable crystalline quality while substantially reducing the interdiffusion.

ReO${}_{3}$ is a remarkable transition metal oxide in that it has the highest conductivity of all oxides, comparable even with that of silver. Using state of the art bulk-sensitive angle-resolved photoelectron spectroscopy, the authors are able to observe clear dispersions of the Re 5$d$ and O 2$p$ derived bands as well as the momentum splitting of the Fermi surface due to the Re 5$d$ spin-orbit interaction. The experimental results are compared quantitatively to density functional theory band structure calculations, thereby providing a deeper understanding of the material class of the 5$d$ oxides.

PbMO3 (M = 3d transition metals) family shows systematic variations in charge distribution and intriguing physical properties due to its delicate energy balance between Pb 6s and transition metal 3d orbitals. However, the detailed structure and physical properties of PbFeO3 remain unclear. Herein, we reveal that PbFeO3 crystallizes into an unusual 2ap × 6ap × 2ap orthorhombic perovskite super unit cell with space group Cmcm. The distinctive crystal construction and valence distribution of Pb2+0.5Pb4+0.5FeO3 lead to a long range charge ordering of the -A-B-B- type of the layers with two different oxidation states of Pb (Pb2+ and Pb4+) in them. A weak ferromagnetic transition with canted antiferromagnetic spins along the a-axis is found to occur at 600 K. In addition, decreasing the temperature causes a spin reorientation transition towards a collinear antiferromagnetic structure with spin moments along the b-axis near 418 K. Our theoretical investigations reveal that the peculiar charge ordering of Pb generates two Fe3+ magnetic sublattices with competing anisotropic energies, giving rise to the spin reorientation at such a high critical temperature., PbFeO3 is part of a family of lead based perovskites with many intriguing properties; however, difficulties in synthesis have hampered investigation. Here, the authors present a detailed study of the structure of PbFeO3 observing unique charge ordering and spin orientation among the constituent ions.

Made available in DSpace on 2021-06-25T10:51:07Z (GMT). No. of bitstreams: 0 Previous issue date: 2021-01-01 New materials discovery is the driving force for progress in solid state physics and chemistry. Here we solve the crystal structure and comprehensively study physical properties of ZnVSb in the polycrystalline form. Synchrotron x-ray diffraction reveals that the compound attains a layered ZrSiS-type structure (P4/nmm, a = 4.09021(2) Å, c = 6.42027(4) Å). The unit cell is composed of a 2D vanadium network separated by Zn-Sb blocks that are slightly distorted from the ideal cubic arrangement. A considerable amount of vacancies were observed on the vanadium and antimony positions; the experimental composition is ZnV0.91Sb0.96. Low-temperature x-ray diffraction shows very subtle discontinuity in the lattice parameters around 175 K. Bonding V-V distance is below the critical separation of 2.97 Å known from the literature, which allows for V-V orbital overlap and subsequent metallic conductivity. From ab initio calculations, we found that ZnVSb is a pseudogap material with an expected dominant vanadium contribution to the density of states at the Fermi level. The energy difference between the antiferromagnetic and nonordered magnetic configurations is rather small (0.34 eV/f.u.). X-ray photoelectron spectroscopy fully corroborates the results of the band structure calculations. Magnetic susceptibility uncovered that, in ZnVSb, itinerant charge carriers coexist with a small, localized magnetic moment of ca. 0.25 μB. The itinerant electrons arise from the ordered part of the vanadium lattice. Fractional localization, in turn, was attributed to V atoms neighboring vacancies, which hinder full V-V orbital overlap. Furthermore, the susceptibility and electrical resistivity showed a large hysteresis between 120 K and 160 K. The effect is not sensitive to magnetic fields up to 9 T. Curie-Weiss fitting revealed that the amount of itinerant charge carriers in ZnVSb drops with decreasing temperature below 160 K, which is accompanied by a concurrent rise in the localized magnetic moment. This observation together with the overall shape of the hysteresis in the resistivity allows for suggesting a plausible origin of the effect as a charge-transfer metal-insulator transition. Ab initio phonon calculations show the formation of a collective phonon mode at 2.8 THz (134 K). The experimental heat capacity reflected this feature by a boson peak with Einstein temperature of 116 K. Analysis of the heat capacity with both an ab initio perspective and Debye-Einstein model revealed a sizable anharmonic contribution to heat capacity, in line with disordered nature of the material. Further investigation of the electron and phonon properties for ZnVSb is likely to provide valuable insight into the relation between structural disorder and the physical properties of transition-metal-bearing compounds. Physics Department Colorado School of Mines Institute of Low Temperature and Structure Research Polish Academy of Sciences University of Illinois at Urbana-Champaign Instituto de Física Teórica São Paulo State University (UNESP) University of California Santa Barbara Max Planck Institute for Chemical Physics of Solids Department of Chemistry and Biochemistry The Ohio State University National Synchrotron Radiation Research Center (NSRRC) Instituto de Física Teórica São Paulo State University (UNESP)

A quadruple perovskite oxide $\mathrm{Ca}{\mathrm{Cu}}_{3}{\mathrm{Co}}_{2}{\mathrm{Re}}_{2}{\mathrm{O}}_{12}$ was synthesized by high-pressure annealing. This compound crystallizes in an $A$- and $B$-site ordered quadruple perovskite structure with space group $Pn\text{\ensuremath{-}}3$. The charge combination is determined to be $\mathrm{Ca}{{\mathrm{Cu}}^{2+}}_{3}{{\mathrm{Co}}^{2+}}_{2}{{\mathrm{Re}}^{6+}}_{2}{\mathrm{O}}_{12}$ by bond valence sum analysis and x-ray absorption spectroscopy. In contrast to other isostructural $A{\mathrm{Cu}}_{3}{B}_{2}{B}_{2}^{\ensuremath{'}}{O}_{12}$ compounds with a single magnetic transition, a long-range antiferromagnetic phase transition originating from the ${A}^{\ensuremath{'}}$-site ${\mathrm{Cu}}^{2+}$ sublattice is found to occur at ${T}_{N}\ensuremath{\approx}28\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Subsequently, the spin coupling between the $B$-site ${\mathrm{Co}}^{2+}$ and ${B}^{\ensuremath{'}}$-site ${\mathrm{Re}}^{6+}$ ions contributes to a ferrimagnetic transition around ${T}_{\mathrm{C}}\ensuremath{\approx}20\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Strong electrical insulating behavior is identified by optical measurement with an energy gap of approximately 3.75 eV. The mechanisms of the spin interactions are discussed in detail.

Zeolites are so well known in real world applications and after decades of scientific study that they hardly need any intro-duction: their importance in chemistry cannot be overemphasized. Here we add to the remarkable properties that they dis-play by reporting our discovery that the simplest zeolite, sodalite, when doped with Cr3+ in the \b{eta}-cage, is a frustrated magnet. Soft X-ray absorption spectroscopy and magnetic measurements reveal that the Cr present is Cr(III). Cr(III), with its isotropic 3d3 valence electron configuration, is well-known as the basis for many geometrically frustrated magnets, but it is especially surprising that a material like the Ca8Al12Cr2O29 zeolite is a frustrated magnet. This finding illustrates the value of exploring the properties of even well-known materials families.

Physical review / X 11(1), 011002 (2021). doi:10.1103/PhysRevX.11.011002, Herein we show that nonresonant inelastic x-ray scattering involving an $s$ core level is a powerful spectroscopic method to characterize the excited states of transition metal compounds. The spherical charge distribution of the $s$ core hole allows the orientational dependence of the intensities of the various spectral features to produce a spatial charge image of the associated multiplet states in a straightforward manner, thereby facilitating the identification of their orbital character. In addition, the $s$ core hole does not add an extra orbital angular momentum component to the multiplet structure so that the well-established Sugano-Tanabe-Kamimura diagrams can be used for the analysis of the spectra. For $\alpha$-MnS we observe the spherical charge density corresponding to its high-spin $3d^5$ ($^6A_1$) ground state configuration and we were able to selectively image its excited states and identify them as $t_{2g}$ ($^5T_2$) and $e_g$ ($^5E$) with an energy splitting $10Dq$ of 0.78 eV., Published by APS, College Park, Md.

The double perovskite oxide ${\mathrm{Sr}}_{2}\mathrm{CrOs}{\mathrm{O}}_{6}$ with a $3{d}^{3}\text{\ensuremath{-}}5{d}^{3}$ configuration exhibits very high-${T}_{C}$ ferrimagnetism (\ensuremath{\sim}725 K) at the end point of half-metallicity. Many substitution studies have been conducted theoretically and experimentally over the last two decades to shed more light on the open issue of how the $3{d}^{3}\text{\ensuremath{-}}5{d}^{3}$ configuration generates the high-${T}_{C}$ ferrimagnetic state and to accelerate development toward applications. We have succeeded in synthesizing a solid solution of ${\mathrm{Sr}}_{2}{\mathrm{Cr}}_{1--x}{\mathrm{Ni}}_{x}\mathrm{Os}{\mathrm{O}}_{6}$ under high-pressure and high-temperature conditions. ${\mathrm{Sr}}_{2}{\mathrm{Cr}}_{0.5}{\mathrm{Ni}}_{0.5}\mathrm{Os}{\mathrm{O}}_{6}$ exhibits magnetization sixfold greater ($\ensuremath{\sim}1.2\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$/formula unit at 5 K) than that of ${\mathrm{Sr}}_{2}\mathrm{CrOs}{\mathrm{O}}_{6}$. This enhancement is preserved even at room temperature. X-ray absorption spectroscopy revealed that the electronic configuration is ${\mathrm{Sr}}_{2}{({{\mathrm{Cr}}^{3+}}_{2/3}{{\mathrm{Cr}}^{6+}}_{1/3})}_{0.5}{{\mathrm{Ni}}^{2+}}_{0.5}{\mathrm{Os}}^{5+}{\mathrm{O}}_{6}$, indicating that the valence state of Os does not change from the host state $[{\mathrm{Os}}^{5+}(5{d}^{3})]$. Instead, nonmagnetic ${\mathrm{Cr}}^{6+}(3{d}^{0})$ is partly generated among coexisting ${\mathrm{Cr}}^{3+}(3{d}^{3})$. X-ray magnetic circular dichroism measurements showed that the Os ions are antiferromagnetically coupled to the Cr and ferromagnetically to the Ni. The replacement of antiferromagnetic Cr by ferromagnetic Ni explains the increase of the net magnetism in this ferrimagnetic system. We infer that the strong antiferromagnetic exchange interaction of the $3{d}^{3}\text{\ensuremath{-}}5{d}^{3}$ configuration associated with the ${\mathrm{Cr}}^{3+}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}{\mathrm{Os}}^{5+}$ bond still accounts for the robust high-${T}_{C}$ ferrimagnetism of the Ni-substituted series. We deduce from the experiments that the ferromagnetic exchange interaction of the $3{d}^{8}\text{\ensuremath{-}}5{d}^{3}$ configuration of the ${\mathrm{Ni}}^{2+}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}{\mathrm{Os}}^{5+}$ is stronger than that of the $3{d}^{8}\text{\ensuremath{-}}3{d}^{3}$ configuration of the ${\mathrm{Ni}}^{2+}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}{\mathrm{Cr}}^{3+}$, suggesting that the larger 5d orbital of the Os allows for a stronger virtual hopping from the Ni than the smaller $3d$ orbital of the Cr. The present results can help to further develop practical materials and to resolve open issues concerning the relative strengths of the various exchange interactions.

We have successfully grown centimeter-sized layered $$\textit{R}\mathrm{SrNiO}_{4}$$ R SrNiO 4 single crystals under high oxygen pressures of 120–150 bar by the floating zone technique. This enabled us to perform neutron scattering experiments where we observe close to quarter-integer magnetic peaks below $$\sim 77~\mathrm{K}$$ ∼ 77 K that are accompanied by steep upwards dispersing spin excitations. Within the high-frequency Ni–O bond stretching phonon dispersion, a softening at the propagation vector for a checkerboard modulation can be observed. We were able to simulate the magnetic excitation spectra using a model that includes two essential ingredients, namely checkerboard charge disproportionation and nano phase separation. The results thus suggest that charge disproportionation is preferred instead of a Jahn–Teller distortion even for this layered $$\mathrm{Ni}^{3+}$$ Ni 3 + system.

The crystal-field ground state wave function of CeCu$_2$Si$_2$ has been investigated with linear polarized $M$-edge x-ray absorption spectroscopy from 250mK to 250K, thus covering the superconducting ($T_{\text{c}}$=0.6K), the Kondo ($T_{\text{K}}$$\approx$20K) as well as the Curie-Weiss regime. The comparison with full-multiplet calculations shows that the temperature dependence of the experimental linear dichroism is well explained with a $\Gamma_7^{(1)}$ crystal-field ground-state and the thermal population of excited states at around 30meV. The crystal-field scheme does not change throughout the entire temperature range thus making the scenario of orbital switching unlikely. Spectroscopic evidence for the presence of the Ce 4$f^0$ configuration in the ground state is consistent with the possibility for a multi-orbital character of the ground state. We estimate from the Kondo temperature and crystal-field splitting energies that several percents of the higher lying $\Gamma_6$ state and $\Gamma_7^{(2)}$ crystal-field states are mixed into the primarily $\Gamma_7^{(1)}$ ground state. This estimate is also supported by re-normalized band-structure calculations that uses the experimentally determined crystal-field scheme., Comment: 8 pages, 5 figures

The simple ABO3 and A-site-ordered AA′3B4O12 perovskites represent two types of the most classical perovskite-based functional materials. While there are well-known simple perovskites with ferroelectric properties, so far there is no report of ferrolectricity due to symmetry breaking transition in A-site-ordered quadruple perovskites, AA′3B4O12. Here we report the synthesis at high pressure and temperature of a new A-site-ordered perovskite, PbHg3Ti4O12. Remarkably, PbHg3Ti4O12 is the only known quadruple perovskite that transforms from a high-temperature centrosymmetric (Im-3), paraelectric phase to a low-temperature, non-centrosymmetric (Imm2) ferroelectric phase. Moreover, the average ionic radius of A-site cations for PbHg3Ti4O12 is large~ 1.1 Å and the tolerance factor t is about 0.88. Surprisingly the coordination chemistry of Hg2+ is changed from the usual square planar as in typical A-site-ordered quadruple perovskite to a rare stereo type with 8 ligands in PbHg3Ti4O12 driven via pressures. Thus PbHg3Ti4O12 appears to be a combinatory link from simple ATiO3 perovskite to AA′3B4O12 type A-site-ordered perovskites, sharing both displacive ferroelectricity with the former and the structure coordination with the latter. This is the first example of ferroelectricity due to a symmetry breaking phase transition in AA′3B4O12-type A-site-ordered perovskite, and opens a new direction to search for ferroelectric materials in combinatory perovskites.

The simple ABO3 and A-site-ordered AA′3B4O12 perovskites represent two types of classical perovskite functional materials. There are well-known simple perovskites with ferroelectric properties, while there is still no report of ferroelectricity due to symmetry breaking transition in A-site-ordered quadruple perovskites. Here we report the high pressure synthesis of an A-site-ordered perovskite PbHg3Ti4O12, the only known quadruple perovskite that transforms from high-temperature centrosymmetric paraelectric phase to low-temperature non-centrosymmetric ferroelectric phase. The coordination chemistry of Hg2+ is changed from square planar as in typical A-site-ordered quadruple perovskite to a rare stereo type with 8 ligands in PbHg3Ti4O12. Thus PbHg3Ti4O12 appears to be a combinatory link from simple ABO3 perovskites to A-site-ordered AA′3Ti4O12 perovskites, sharing both displacive ferroelectricity with former and structure coordination with latter. This is the only example so far showing ferroelectricity due to symmetry breaking phase transition in AA′3B4O12-type A-site-ordered perovskites, and opens a direction to search for ferroelectric materials., There are few reports of ferroelectricity due to symmetry breaking transition in A-site-ordered quadruple perovskites. Here, the authors find one with phase transition from a high-temperature centrosymmetric paraelectric phase to a low-temperature non-centrosymmetric ferroelectric phase in a high pressure synthesized compound.

A new oxide, LaMn

The Mn3WO6 oxide, prepared by a solid-state reaction at ambient pressure, is found to crystallize at room temperature in the centrosymmetric R3 trigonal structure of Mg3TeO6. By X-ray absorption s...

Polycrystalline samples of $\mathrm{B}{\mathrm{a}}_{3}\mathrm{NiO}{\mathrm{s}}_{2}{\mathrm{O}}_{9}$ were synthesized at ambient pressure (AP) and high pressure (HP) conditions, respectively. Both samples are electrically semiconducting. The AP $\mathrm{B}{\mathrm{a}}_{3}\mathrm{NiO}{\mathrm{s}}_{2}{\mathrm{O}}_{9}$ crystallizes in the $6H$ perovskite structure with space group $P{6}_{3}/mmc$, consisting of face-sharing $\mathrm{O}{\mathrm{s}}_{2}{\mathrm{O}}_{9}$ dimer units and corner-sharing $\mathrm{Ni}{\mathrm{O}}_{6}$ octahedra. Magnetic measurements indicated that AP $\mathrm{B}{\mathrm{a}}_{3}\mathrm{NiO}{\mathrm{s}}_{2}{\mathrm{O}}_{9}$ is antiferromagnetically ordered below 130 K. HP $\mathrm{B}{\mathrm{a}}_{3}\mathrm{NiO}{\mathrm{s}}_{2}{\mathrm{O}}_{9}$ crystallizes in the $6H$ perovskite structure too, but the face-sharing octahedral sites appear to be occupied by both $\mathrm{N}{\mathrm{i}}^{2+}$ and $\mathrm{O}{\mathrm{s}}^{5+}$ ions, whereas the corner-sharing site is occupied exclusively by $\mathrm{O}{\mathrm{s}}^{5+}$. HP $\mathrm{B}{\mathrm{a}}_{3}\mathrm{NiO}{\mathrm{s}}_{2}{\mathrm{O}}_{9}$ undergoes a high-temperature ($\ensuremath{\approx}\phantom{\rule{0.16em}{0ex}}400\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) weak ferromagnetic transition, which is much different from the antiferromagnetism of the AP phase. The long-range magnetic order of HP $\mathrm{B}{\mathrm{a}}_{3}\mathrm{NiO}{\mathrm{s}}_{2}{\mathrm{O}}_{9}$ was confirmed by neutron powder diffraction. X-ray magnetic circular dichroism analysis supported ferromagnetic coupling between Os and Ni moments which leads to a spin arrangement, where the ferromagnetic moments mainly arise from $\mathrm{N}{\mathrm{i}}^{2+}$ ions.

We report our study about the growth and characterization of $\mathrm{B}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{3}$ thin films on top of ${\mathrm{Y}}_{3}\mathrm{F}{\mathrm{e}}_{5}{\mathrm{O}}_{12}$(111), $\mathrm{T}{\mathrm{m}}_{3}\mathrm{F}{\mathrm{e}}_{5}{\mathrm{O}}_{12}$(111), $\mathrm{F}{\mathrm{e}}_{3}{\mathrm{O}}_{4}$(111), and $\mathrm{F}{\mathrm{e}}_{3}{\mathrm{O}}_{4}$(100) single-crystal substrates. Using molecular-beam epitaxy, we were able to prepare the topological insulator/ferromagnetic insulator heterostructures with no or minimal chemical reaction at the interface. We observed the anomalous Hall effect on these heterostructures and also a suppression of the weak antilocalization in the magnetoresistance, indicating a topological surface-state gap opening induced by the magnetic proximity effect. However, we did not observe any obvious x-ray magnetic circular dichroism (XMCD) on the Te ${M}_{45}$ edges. The results suggest that the ferromagnetism induced by the magnetic proximity effect via van der Waals bonding in $\mathrm{B}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{3}$ is too weak to be detected by XMCD, but still can be observed by electrical transport measurements. This is in fact not inconsistent with reported density-functional calculations on the size of the gap opening.

We have investigated the electronic structure of iridates in the double perovskite crystal structure containing either Ir$^{4+}$ or Ir$^{5+}$ using hard x-ray photoelectron spectroscopy. The experimental valence band spectra can be well reproduced using tight binding calculations including only the Ir $5d$, O $2p$ and O $2s$ orbitals with parameters based on the downfolding of the density-functional band structure results. We found that regardless of the A and B cations, the A$_2$BIrO$_6$ iridates have essentially zero O $2p$ to Ir $5d$ charge transfer energies. Hence, double perovskite iridates turn out to be extremely covalent systems with the consequence being that the magnetic exchange interactions become very long-ranged, thereby hampering the materialization of the long-sought Kitaev physics. Nevertheless, it still would be possible to realize a spin-liquid system using the iridates with a proper tuning of the various competing exchange interactions., 9 pages, 7 figures, submitted to Physical Review B

The ability to determine the electronic structure of catalysts during electrochemical reactions is highly important for identification of the active sites and the reaction mechanism. Here we successfully applied soft X-ray spectroscopy to follow in operando the valence and spin state of the Co ions in Li2Co2O4 under oxygen evolution reaction (OER) conditions. We have observed that a substantial fraction of the Co ions undergo a voltage-dependent and time-dependent valence state transition from Co3+ to Co4+ accompanied by spontaneous delithiation, whereas the edge-shared Co–O network and spin state of the Co ions remain unchanged. Density functional theory calculations indicate that the highly oxidized Co4+ site, rather than the Co3+ site or the oxygen vacancy site, is mainly responsible for the high OER activity., Determining catalyst electronic structures during electrochemical reactions is crucial to understand mechanisms. Here authors perform in operando soft X-ray spectroscopy on a cobalt oxide catalyst during O2 evolution and observe voltage and time-dependent valence state transitions.

An ilmenite-like monoclinic phase of HgMnO

The development of high‐density magnetic recording media is limited by superparamagnetism in very small ferromagnetic crystals. Hard magnetic materials with strong perpendicular anisotropy offer stability and high recording density. To overcome the difficulty of writing media with a large coercivity, heat‐assisted magnetic recording was developed, rapidly heating the media to the Curie temperature Tc before writing, followed by rapid cooling. Requirements are a suitable Tc, coupled with anisotropic thermal conductivity and hard magnetic properties. Here, Rh2CoSb is introduced as a new hard magnet with potential for thin‐film magnetic recording. A magnetocrystalline anisotropy of 3.6 MJ m-3 is combined with a saturation magnetization of μ0>7sub>Ms = 0.52 T at 2 K (2.2 MJ m-3 and 0.44 T at room temperature). The magnetic hardness parameter of 3.7 at room temperature is the highest observed for any rare‐earth‐free hard magnet. The anisotropy is related to an unquenched orbital moment of 0.42 μB on Co, which is hybridized with neighboring Rh atoms with a large spin–orbit interaction. Moreover, the pronounced temperature dependence of the anisotropy that follows from its Tc of 450 K, together with a thermal conductivity of 20 W m-1 K-1, make Rh2CoSb a candidate for the development of heat‐assisted writing with a recording density in excess of 10 Tb in.-2.