Hidekazu Ikeno

To simulate the luminescence emission spectra of Ce ³⁺ ‐activated phosphors, we propose a method based on scalar‐relativistic density functional theory with corrections for on‐site Coulomb interactions, and first‐principles molecular dynamics (FPMD) calculations, accounting for atomic thermal motion beyond harmonic oscillations. The FPMD calculations at finite temperature were performed with the excited‐state (ES) electron configuration corresponding to the Ce 4f ⁰ 5d ¹ state. Subsequently, the transition energies and probabilities of 4f ⁰ 5d ¹ →4f ¹ 5d ⁰ transitions in each FPMD snapshot were calculated using the N −1 electron configuration in which the highest occupied level in the ground‐state (GS) configuration is vacated. This is justified because applying Janak's theorem to a flat‐band system allows each Kohn‐Sham orbital's energy to be regarded as the total energy of an ES generated by an excitation from the highest level. The time‐averaged spectrum was calculated by accumulating emission lines at the transition energies with amplitudes proportional to the probabilities. The spectra calculated for Ce ³⁺ ‐activated yttrium aluminum garnet exhibit shapes and positions that resemble those measured at temperatures above 300 K. The peak energies and full widths at half maximum at 300 K for the series of Ce ³⁺ ‐activated phosphors correlate with the experimental values, demonstrating the potential for predicting emission spectra of novel phosphors.

The La–Co co-substituted magnetoplumbite-type (M-type) ferrites AFe₁₂O₁₉ (A = Ca, Sr and Ba, ion sizes Ca²⁺ < Sr²⁺ < Ba²⁺) with Co compositions around 0.2 have been subjected to ⁵⁹Co-NMR. The results show that Co occupies the 4f₁, 2a and 12k sites, and that the smaller the A ion, the more Co tends to occupy the 4f₁ minority spin site, which is effective in enhancing both uniaxial anisotropy and magnetisation. First-principles total energy calculations based on density functional theory (DFT) of undoped AFe₁₂O₁₉ and a supercell ( 2 × 2 × 1 of the unit cell) in which 1/96 of Fe³⁺ is replaced by Co²⁺ were performed to predict the stable structure and Co occupancy sites. The results show that regardless of A, Co is most stable when it occupies the 4f₁ site, followed by the 2a and 12k sites with energy differences on the order of 100 meV, and Co practically does not occupy the 2b and 4f₂ sites. As the A ion becomes smaller, the energy difference when Co occupies each Fe site tends to increase, and the Co occupancy of the 4f₁ site also increases. The site selectivity of Co can be roughly explained as a result of the difference in uniaxial strain along the c-axis associated with the difference in A. However, the influence of the A ion differs between the R and S blocks and the local strain also has a secondary effect on the Co distribution. Based on these results, the guidelines for improving the performance (anisotropy and magnetisation) of La–Co co-substituted M-type ferrite magnets with a limited amount of Co can be summarised as follows: It is effective to select as small A ions as possible and to post-anneal at low temperature or cool slowly to concentrate Co at the 4f₁ site in tetrahedral coordination.

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.

Machine learning in conjunction with validation experiments uncovers new Eu²⁺-activated phosphor materials with a designed green-color luminescence.

Postspinel-structured oxides, CaB₂O₄ (B = Cr³⁺, Mn³⁺, and Fe³⁺), have exhibited systematically higher catalytic activities in the oxygen evolution reaction (OER) than nominally conventional counterparts of perovskite LaBO₃ and spinel ZnB₂O₄.

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.

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.

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. Graphical abstract

We propose a new algorithm for numerical evaluation of x-ray absorption spectra based on the Green's function formalism in this paper. The method stands on the reduced-shifted conjugate-gradient (RSCG) method, which enables us to obtain the Green's functions with different energy-shifts simultaneously. The seed switching technique is combined into the RSCG method to improve the numerical accuracy and robustness of the algorithm, which we refer to the RSCG-SS method. The RSCG-SS method is applied for calculations of x-ray absorption spectra at the L_2,3-edges of transition metals in simple oxides as a benchmark. The theoretical spectra obtained by the new algorithms are identical to those obtained by the full-diagonalization method following the Fermi's golden rule. The RSCG-SS method can also be applied for the calculation of spectra with a sizable Hamiltonian matrix (∼300,000) with reasonable computational costs.