Metamagnetic Behavior in a Quadruple Perovskite Oxide.

A cubic quadruple perovskite oxide CeMn3Cr4O12 has been synthesized under high-pressure and high-temperature conditions of 8 GPa and 1273 K. The X-ray absorption spectroscopy reveals that the Ce ions are in a trivalent state, as represented by the ionic model of Ce3+Mn3+3Cr3+4O12. The magnetic study demonstrates three independent antiferromagnetic transitions attributed to Ce (∼10 K), Mn (46 K), and Cr (133 K) ions. Furthermore, a magnetic field-induced antiferromagnetic-to-ferromagnetic (metamagnetic) transition of Ce3+ 4f moments is observed at low temperatures below 20 K, exhibiting a rare example of metamagnetism in the Ce3+-oxides. This finding represents that the 3d-electron magnetic sublattices play a role in the metamagnetism of 4f-electron magnetic moments, demonstrating a new aspect of the 3d-4f complex electron systems.

Dissimilarity measure of local structure in inorganic crystals using Wasserstein distance to search for novel phosphors.

To efficiently search for novel phosphors, we propose a dissimilarity measure of local structure using the Wasserstein distance. This simple and versatile method provides the quantitative dissimilarity of a local structure around a center ion. To calculate the Wasserstein distance, the local structures in crystals are numerically represented as a bag of interatomic distances. The Wasserstein distance is calculated for various ideal structures and local structures in known phosphors. The variation of the Wasserstein distance corresponds to the structural variation of the local structures, and the Wasserstein distance can quantitatively explain the dissimilarity of the local structures. The correlation between the Wasserstein distance and the full width at half maximum suggests that candidates for novel narrow-band phosphors can be identified by crystal structures that include local structures with small Wasserstein distances to local structures of known narrow-band phosphors. The quantitative dissimilarity using the Wasserstein distance is useful in the search of novel phosphors and expected to be applied in materials searches in other fields in which local structures play an important role.

Complementary evaluation of structure stability of perovskite oxides using bond-valence and density-functional-theory calculations.

Estimation of structure stability is an essential issue in materials design and synthesis. Global instability index (GII) based on bond-valence method is applied as a simple indication, while density functional theory calculation is adopted for accurate evaluation of formation energy. We compare the GII and total energy of typical ABO3-type perovskite oxides and rationalize their relationship, proposing that the criteria for empirically unstable structures (GII > 0.2 valence unit) correspond to the difference in total energy of 50-200 meV per formula unit.

Covalency Competition in the Quadruple Perovskite CdCu3Fe4O12.

Cadmium ions (Cd2+) are similar to calcium ions (Ca2+) in size, whereas the Cd2+ ions tend to form covalent bonds with the neighboring anions because of the high electronegativity. The covalent Cd-O bonds affect other metal-oxygen bonds, inducing drastic changes in crystal structures and electronic states. Herein, we demonstrate high-pressure synthesis, crystal structure, and properties of a new quadruple perovskite CdCu3Fe4O12. This compound exhibits an electronic phase transition accompanying a charge disproportionation of Fe ions without charge ordering below ∼200 K, unlike charge-disproportionation transition with rock-salt-type charge ordering for CaCu3Fe4O12. First-principle calculations and Mössbauer spectroscopy display that covalent Cd-O bonds effectively suppress the Fe-O bond covalency, resulting in an electronic state different from that of CaCu3Fe4O12. This finding proposes covalency competition among constituent metal ions dominating electronic states of complex metal oxides.

Unusual Selective Monitoring of N,N-Dimethylformamide in a Two-Dimensional Material Field-Effect Transistor.

N,N-Dimethylformamide (DMF) is an essential solvent in industries and pharmaceutics. Its market size range was estimated to be 2 billion U.S. dollars in 2022. Monitoring DMF in solution environments in real time is significant because of its toxicity. However, DMF is not a redox-active molecule; therefore, selective monitoring of DMF in solutions, especially in polar aqueous solutions, in real time is extremely difficult. In this paper, we propose a selective DMF sensor using a molybdenum disulfide (MoS2) field-effect transistor (FET). The sensor responds to DMF molecules but not to similar molecules of formamide, N,N-diethylformamide, and N,N-dimethylacetamide. The plausible atomic mechanism is the oxygen substitution sites on MoS2, on which the DMF molecule shows an exceptional orientation. The thin structure of MoS2-FET can be incorporated into a microfluidic chamber, which leads to DMF monitoring in real time by exchanging solutions subsequently. The designed device shows DMF monitoring in NaCl ionic solutions from 1 to 200 µL/mL. This work proposes the concept of selectively monitoring redox-inactive molecules based on the nonideal atomic affinity site on the surface of two-dimensional semiconductors.

Narrow-Band Emitting Phosphor Na2Cs2Sr(B9O15)2:Eu2+ Discovered from Local Structure Similarity with Sulfate Phosphor.

Narrow-band emitting phosphors are required to improve the performance of phosphor-converted light-emitting diodes. Here, we found a new narrow-band emitting phosphor Na2Cs2Sr(B9O15)2:Eu2+ using the local structure similarity with a known narrow-band emitting phosphor. In a 2D scatter plot of the structural similarity between the local structures, the Sr site in Na2Cs2Sr(B9O15)2 was located near the Ba site of the known narrow-band emitting sulfate phosphor BaSO4:Eu2+ with a distorted local structure. We synthesized Na2Cs2Sr(B9O15)2:Eu2+ and characterized the luminescence properties by microspectroscopy. Na2Cs2Sr(B9O15)2:Eu2+ showed a violet luminescence peaked at 417 nm, and the full-width at half-maximum was as narrow as 26 nm (1497 cm-1).

Highly active postspinel-structured catalysts for oxygen evolution reaction.

The rational design principle of highly active catalysts for the oxygen evolution reaction (OER) is desired because of its versatility for energy-conversion applications. Postspinel-structured oxides, CaB 2O4 (B = Cr3+, Mn3+, and Fe3+), have exhibited higher OER activities than nominally isoelectronic conventional counterparts of perovskite oxides LaBO3 and spinel oxides ZnB 2O4. Electrochemical impedance spectroscopy reveals that the higher OER activities for CaB 2O4 series are attributed to the lower charge-transfer resistances. A density-functional-theory calculation proposes a novel mechanism associated with lattice oxygen pairing with adsorbed oxygen, demonstrating the lowest theoretical OER overpotential than other mechanisms examined in this study. This finding proposes a structure-driven design of electrocatalysts associated with a novel OER mechanism.

Ultralarge Photoluminescence Enhancement of Monolayer Molybdenum Disulfide by Spontaneous Superacid Nanolayer Formation.

Due to the direct band gap nature, extensive studies have been conducted to improve the optical behavior in monolayer transition metal dichalcogenides (TMDCs) with a formula of MX2 (M = Mo, W; X = S, Se, Te). One of the strongest modulating agents of optical behavior is a molecular superacid treatment; however, the chemical event has not been unveiled. Also, the engineering protocol for keeping the treatment is immature. In this work, we systematically study the superacid treatment procedures on monolayer molybdenum disulfide (MoS2) and propose that the interaction, a hydrophilic interaction, between the superacid molecule and MoS2 surface would be critical. As a result of the interaction, the superacid molecules spontaneously form an acidic layer with the thickness of several nanometers on the surface. The power-dependent photoluminescence (PL) measurement indicates the edge of MoS2 flake is more effective and electronically modulated by the treatment. By understanding the superacid nanolayer formation by the treatment, we succeeded in maintaining the ultrastrong PL in the superacid-treated MoS2 for more than 30 days in the ambient air by encapsulation with transparent organic polymers. This study advances the understanding and designing applications of strong luminescent properties in the superacid-treated TMDCs and paves the way toward engineering exciton dynamics and an experimental platform for treating multibody states.

All-electron CI calculations of 3d transition-metal L(2,3) XANES using zeroth-order regular approximation for relativistic effects.

X-ray-absorption near-edge structures (XANES) at 3d transition-metal (TM) L(2,3) edges are computed using the all-electron configuration interaction (CI) method. Slater determinants for the CI calculations are composed of molecular orbitals obtained by density functional theory (DFT) calculations of model clusters. Relativistic effects are taken into account by the zeroth-order regular approximation (ZORA) using two-component wavefunctions. The theoretical spectra are found to be strongly dependent on the quality of the one-electron basis functions. On the other hand, a different choice of the exchange-correlation functionals for the DFT calculations does not exhibit visible changes in the spectral shape. Fine details of multiplet structures in the experimental TM L(2,3) XANES of MnO, FeO and CoO are well reproduced by the present calculations when the one-electron basis functions are properly selected. This is consistent with our previous report showing good agreement between theoretical and experimental TM L(2,3) XANES when four-component relativistic wavefunctions were used.

Multiplet calculations of L(2,3) x-ray absorption near-edge structures for 3d transition-metal compounds.

The purpose of this work is to compare the two different procedures to calculate the L(2,3) x-ray absorption spectra of transition-metal compounds: (1) the semi-empirical charge transfer multiplet (CTM) approach and (2) the ab initio configuration-interaction (CI) method based on molecular orbitals. We mainly focused on the difference in the treatment of ligand field effects and the charge transfer effects in the two methods. The reduction of multiplet interactions due to the solid state effects has been found by the ab initio CI approach. We have also found that the mixing between the original and the charge transferred configurations obtained by the ab initio CI approach is smaller than that obtained by the CTM approach, since charge transfer through the covalent bonding between metal and ligand atoms has been included by taking the molecular orbitals as the basis functions.

Basics and applications of ELNES calculations.

The electron energy loss near edge structures (ELNES) appearing in an electron energy loss spectrum obtained through transmission electron microscopy (TEM) have the potential to unravel atomic and electronic structures with sub-nano meter resolution. For this reason, TEM-ELNES has become one of the most powerful analytical methods in materials research. On the other hand, theoretical calculations are indispensable in interpreting the ELNES spectrum. Here, the basics and applications of one-particle, two-particle and multi-particle ELNES calculations are reviewed. A key point for the ELNES calculation is the proper introduction of the core-hole effect. Some applications of one-particle ELNES calculations to huge systems of more than 1000 atoms, and complex systems, such as liquids, are reported. In the two-particle calculations, the importance of the correct treatment of the excitonic interaction is demonstrated in calculating the low-energy ELNES, for example at the Li-K edge. In addition, an unusually strong excitonic interactions in the O-K edge of perovskite oxides is identified. The multi-particle calculations are necessary to reproduce the multiplet structures appearing at the transition metal L2,3-edges and rare-earth M4,5-edges. Applications to dilute magnetic semiconductors and Li-ion battery materials are presented. Furthermore, beyond the 'conventional' ELNES calculations, theoretical calculations of electron/X-ray magnetic circular dichroism (MCD) and the vibrational information in ELNES, are reported.

Bifunctional Oxygen Reaction Catalysis of Quadruple Manganese Perovskites.

Bifunctional electrocatalysts for oxygen evolution/reduction reaction (OER/ORR) are desirable for the development of energy conversion technologies. It is discovered that the manganese quadruple perovskites CaMn7 O12 and LaMn7 O12 show bifunctional catalysis in the OER/ORR. A possible origin of the high OER activity is the unique surface structure through corner-shared planar MnO4 and octahedral MnO6 units to promote direct OO bond formations.

First-principles multi-electron calculations for L(2,3) ELNES/XANES of 3d transition metal monoxides.

A computational method having quantitatively predictive performance of L(2,3) ELNES/XANES of 3d transition metal (TM) compounds without any empirical parameter is given. Calculations are made on three TM monoxides with rock salt structures, MnO, FeO and CoO using model clusters composed of a TM ion with six coordinating oxide ions. Multi-electron wavefunctions are expressed by a linear combination of Slater determinants made by fully relativistic molecular orbitals for TM-2p, 3d and O-2p. Relative intensity and positions of subpeaks are in excellent agreement to experimental spectra. Our analyses of multi-electron eigenstates found that these subpeaks originate not from different one electron configurations but entirely from the multiplet effects. The dependence of the branching ratio on the formal number of 3d electrons agrees with that of an empirical atomic multiplet theory reported in literature.

Electronic structure of lithium nickel oxides by electron energy loss spectroscopy.

The electronic structures of NiO, LiNiO2, and NiO2 are studied by the electron energy loss spectroscopy at Ni L(2,3), Ni M(2,3), and O K edges. The Ni L(2,3) edge spectra suggest that the formal charge of nickel is +2 in NiO, +3 with a low-spin state in LiNiO2, and +4 with a low-spin state in NiO2. This is well confirmed by first-principles calculations. The Ni M(2,3) edge spectra show similar chemical shifts to those of the Ni L(2,3) edge. Superposition of the Li K edge spectrum, however, hinders further analysis. Although the formal charge of oxygen is -2 in all the three phases, the O K edge spectra indicate a more remarkable difference in the electronic structure of the oxygen in NiO2 than that in either NiO or LiNiO2. The spectra suggest that lithium extraction from LiNiO2 reinforces the covalent bonding between the oxygen and nickel atoms and causes a notable reduction in electron density at the oxygen atoms.

Impact of local strain on Ti-L2,3 electron energy-loss near-edge structures of BaTiO3: a first-principles multiplet study.

Identification of local strains is crucial because the local strains largely influence the ferroelectric property of BaTiO3. The effects of local strains induced by external pressures on the Ti-L2,3 electron energy-loss near-edge structure (ELNES) of BaTiO3 were theoretically investigated using first-principles multiplet calculations. We revealed that the effects appear in the position of the spectral threshold, namely the spectrum shifts to lower and higher energy sides by the tensile and compressive pressures, respectively. We concluded that conventional ELNES observations can identify only large strains induced by -10 GPa, and 0.1 eV energy resolution is required to identify ±2% of strains.

Variation of Zr-L2,3 XANES in tetravalent zirconium oxides.

Zr-L2,3 XANESs of tetravalent zirconium oxides with different coordination numbers and local symmetries are systematically investigated by ab initio multiplet calculations using fully relativistic molecular spinors for model clusters. Experimental Zr-L2,3 XANESs are obtained for SrZrO3, m-ZrO2 (monoclinic) and t-ZrO2 (tetragonal). The theoretical spectra are in good agreement with the experimental data. The multiplet effects are found to play essential roles in determining the peak shape. The shapes of L3- and L2-edges are systematically different. The intensity ratios of the doublet peaks at both L3- and L2-edges are found to be sensitive to the coordination number of Zr. The ratio can therefore be used to estimate the coordination number of Zr in such oxides.

Theoretical ELNES using one-particle and multi-particle calculations.

One-, two-, and many-particle calculations for electron-energy-loss near-edge structures (ELNES) are reviewed. The most important point for the ELNES calculation is the proper introduction of the core-hole effect. By introducing the core-hole effect in a sufficiently large supercell, one-particle calculations are applicable to the ELNES of many edges. On the other hand, the two-particle interaction between the excited electron and the core-hole, namely the excitonic effect, is significant in the K edges of very light elements and the L(2,3) edges of Mg and Al. Many-particle interactions, including both electron-electron and electron-hole interactions, are indispensable for the L(2,3) edges of transition metals and the M(4,5) edges of lanthanides, namely white lines. In this review, we present the basics, methodologies, and some applications of one-, two-, and many-particle calculations. In addition, importance of momentum transfer vector in the ELNES calculations for comparison with the experiments is discussed.