Multiemission of Ce3+ from a Single Crystallographic Site Induced by Disordering of Ions.

White-light emission with a single activator is an attractive function of phosphors. In this work, we investigated the photoluminescence properties of Ca5.7Y1.3Si7O16.7N3.3, which is a compound denoted as Ca4+xY3-xSi7O15+xN5-x discovered by our group, with Ce-activation using optical measurements and density functional theory (DFT) calculation. Samples showed a tunable emission from purple to white under ultraviolet (UV) light. In this compound, Ca and Y as well as anions are distributed disorderly, and Ca/Y ions occupy two crystallographically distinct sites; those sites are possible sites for Ce substitution. DFT calculation and structural refinement revealed that the tunable emission was generated by Ce at the crystallographically equivalent site but with distinct local structures caused by the disordering of cations and anions. As far as we know, this is the first report about a white-light-emitting phosphor with only Ce activation.

Effect of the oxygen vacancy electronic state on Ni migration in Li0.5(Ni0.8Mn0.1Co0.1)O2 cathode material.

Cation migration coupled with oxygen vacancy formation is known to drive the layered to disordered spinel/rock-salt phase transformation in the high-Ni layered oxide cathodes of Li-ion batteries. However, the effect of different electronic states of oxygen vacancies on the cation migration still remains elusive. Here, we investigate Ni migration in delithiated Ni-rich Li0.5Ni0.8Mn0.1Co0.1O2 (hence Li0.5NMC811) in the presence of neutral and charged oxygen vacancies by means of first-principles density functional theory (DFT) calculations coupled with the nudged elastic band (NEB) method. We find that oxygen vacancies with neutral or +2 charge favor the Ni migration to Li tetrahedral and/or octahedral sites, both thermodynamically and kinetically. As for the case of +1 charged oxygen vacancies, while they thermodynamicaly favor the Ni migration to the Li site, the relatively high migration barrier suggests that they kinetically prohibit the Ni migration. Our results suggest that controlling the formation of oxygen vacancies is the key to enhancing the Ni-rich NMC structural stability in particular in their charged states.

Locality error free effective core potentials for 3d transition metal elements developed for the diffusion Monte Carlo method.

Pseudopotential locality errors have hampered the applications of the diffusion Monte Carlo (DMC) method in materials containing transition metals, in particular oxides. We have developed locality error free effective core potentials, pseudo-Hamiltonians, for transition metals ranging from Cr to Zn. We have modified a procedure published by some of us in Bennett et al. [J. Chem. Theory Comput. 18, 828 (2022)]. We carefully optimized our pseudo-Hamiltonians and achieved transferability errors comparable to the best semilocal pseudopotentials used with DMC but without incurring in locality errors. Our pseudo-Hamiltonian set (named OPH23) bears the potential to significantly improve the accuracy of many-body-first-principles calculations in fundamental science research of complex materials involving transition metals.

Evolutionary Algorithm Directed Synthesis of Mixed Anion Compounds LaF2 X (X=Br, I) and LaFI2.

Mixed-anion compounds have attracted growing attentions, but their synthesis is challenging, making a rational search desirable. Here, we explored LaF3 -LaX3 (X=Cl, Br, I) system using ab initio structure searches based on evolutionary algorithms, and predicted LaF2 X and LaFX2 (X=Br, I), which are respectively isostructural with LaHBr2 and YH2 I, consisting of layered La-F blocks with single and double ordered honeycomb lattices, separated by van der Waals gaps. We successfully synthesized these compounds: LaF2 Br and LaFI2 crystallize in the predicted structure, while LaF2 I is similar to the predicted one but with different layer stacking. LaF2 I exhibits fluoride ion conductivity comparable to that of non-doped LaF3 , and has the potential to show better ionic conductivity upon appropriate doping, given the theoretically lower diffusion energy barrier and the presence of soft iodine anions. This study shows the structure prediction using evolutionary algorithms will accelerate the discovery of mixed-anion compounds in future, in particular those with an ordered anion arrangement.

Shry: Application of Canonical Augmentation to the Atomic Substitution Problem.

A common approach for studying a solid solution or disordered system within a periodic ab initio framework is to create a supercell in which certain amounts of target elements are substituted with other elements. The key to generating supercells is determining how to eliminate symmetry-equivalent structures from many substitution patterns. Although the total number of substitutions is on the order of trillions, only symmetry-inequivalent atomic substitution patterns need to be identified, and their number is far smaller than the total. Our developed Python software package, which is called Shry (Suite for High-throughput generation of models with atomic substitutions implemented by Python), allows the selection of only symmetry-inequivalent structures from the vast number of candidates based on the canonical augmentation algorithm. Shry is implemented in Python 3 and uses the CIF format as the standard for both reading and writing the reference and generated sets of substituted structures. Shry can be integrated into another Python program as a module or can be used as a stand-alone program. The implementation was verified through a comparison with other codes with the same functionality, based on the total numbers of symmetry-inequivalent structures, and also on the equivalencies of the output structures themselves. The provided crystal structure data used for the verification are expected to be useful for benchmarking other codes and also developing new algorithms in the future.

Diffusion Monte Carlo evaluation of disiloxane linearisation barrier.

The disiloxane molecule is a prime example of silicate compounds containing the Si-O-Si bridge. The molecule is of significant interest within the field of quantum chemistry, owing to the difficulty in theoretically predicting its properties. Herein, the linearisation barrier of disiloxane is investigated using a fixed-node diffusion Monte Carlo (FNDMC) approach, which is one of the most reliable ab initio methods in accounting for the electronic correlation. Calculations utilizing the density functional theory (DFT) and the coupled cluster method with single and double substitutions, including noniterative triples (CCSD(T)) are carried out alongside FNDMC for comparison. It is concluded that FNDMC successfully predicts the disiloxane linearisation barrier and does not depend on the completeness of the basis-set as much as DFT or CCSD(T), thus establishing its suitability.

Peculiar Atomic Bond Nature in Platinum Monatomic Chains.

Metal atomic chains have been reported to change their electronic or magnetic properties by slight mechanical stimulus. However, the mechanical response has been veiled because of lack of information on the bond nature. Here, we clarify the bond nature in platinum (Pt) monatomic chains by our in situ transmission electron microscope method. The stiffness is measured with sub-N/m precision by quartz length-extension resonator. The bond stiffnesses at the middle of the chain and at the connection to the base are estimated to be 25 and 23 N/m, respectively, which are higher than the bulk counterpart. Interestingly, the bond length of 0.25 nm is found to be elastically stretched to 0.31 nm, corresponding to a 24% strain. Such peculiar bond nature could be explained by a novel concept of "string tension". This study is a milestone that will significantly change the way we think about atomic bonds in one-dimension.

Synthesis, Electronic Structure, and Physical Properties of Layered Oxypnictides Sr2ScCrAsO3 and Ba3Sc2Cr2As2O5.

New CrAs-based layered mixed-anion compounds Sr2ScCrAsO3 (SrScO-21113) and Ba3Sc2Cr2As2O5 (BaScO-32225) were synthesized, and their electronic structures and physical properties were investigated. The structures of these compounds comprise stacking of the anti-fluorite CrAs layer and perovskite-like SrScO or BaScO layers. The lattice constants of these compounds are relatively longer than those of the related compounds, such as BaCr2As2, owing to the insertion of a large perovskite blocking layer of SrScO/BaScO. While there are variations in the crystal structure of this system, such as 21113 and 32225, their chemical stability calculated by the first-principles calculations indicated that SrScO-21113 is energetically favorable compared to SrScO-32225. The formation energies of BaScO-32225 and BaScO-21113 are close to each other; in the experiment, while there was an indication of BaScO-21113 formation, only BaScO-32225 was formed as a single phase because of the low chemical stability of BaScO-21113. The partial density of states indicates that the majority of states are obtained from the 3d4-electrons of the Cr element hybridized modestly with p electrons at the Fermi energy. The magnetic properties of these compounds were paramagnetic, and they were different from related compounds, such as BaCr2As2, probably because of their long a-axis lengths. The temperature dependences of the electrical resistivities of both samples were in good agreement with the electronic band structure calculations. The variety of structures in the series of compounds with a CrAs layer results in different physical properties, and further development of new compounds may bring novel functionalities, such as superconductivity.

Two-Dimensional Perovskite Oxynitride K2 LaTa2 O6 N with an H+ /K+ Exchangeability in Aqueous Solution Forming a Stable Photocatalyst for Visible-Light H2 Evolution.

Undoped layered oxynitrides have not been considered as promising H2 -evolution photocatalysts because of the low chemical stability of oxynitrides in aqueous solution. Here, we demonstrate the synthesis of a new layered perovskite oxynitride, K2 LaTa2 O6 N, as an exceptional example of a water-tolerant photocatalyst for H2 evolution under visible light. The material underwent in-situ H+ /K+ exchange in aqueous solution while keeping its visible-light-absorption capability. Protonated K2 LaTa2 O6 N, modified with an Ir cocatalyst, exhibited excellent catalytic activity toward H2 evolution in the presence of I- as an electron donor and under visible light; the activity was six times higher than Pt/ZrO2 /TaON, one of the best-performing oxynitride photocatalysts for H2 evolution. Overall water splitting was also achieved using the Ir-loaded, protonated K2 LaTa2 O6 N in combination with Cs-modified Pt/WO3 as an O2 evolution photocatalyst in the presence of an I3 - /I- shuttle redox couple.

A new ab initio modeling scheme for the ion self-diffusion coefficient applied to the ε-Cu3Sn phase of the Cu-Sn alloy.

We present a new scheme for modeling of the ion self-diffusion coefficient. Our scheme broadens the applicable scope of the 'ab initio + modeling' approach, which combines modeling of the self-diffusion coefficient with ab initio predictions. Essential concepts in our scheme are 'domain division' and 'coarse-graining' of the diffusion network, according to calculated barrier energies. With the former concept, the diffusion network is divided into a few types of simple disjunct domains. Their networks are further simplified with the latter idea that groups some ion sites and regards them as just a single site. We applied this scheme to Cu diffusion in the ε-Cu3Sn phase of the Cu-Sn alloy and succeeded in reproducing experimental diffusion coefficients in a wide range of temperature.

Method for the Calculation of the Hamaker Constants of Organic Materials by the Lifshitz Macroscopic Approach with Density Functional Theory.

The Hamaker constants, which are coefficients providing quantitative information on intermolecular forces, were calculated for a number of different materials according to the Lifshitz theory via simple DFT calculations without any experimental measurements being performed. The physical properties (polarizability, dipole moment, molecular volume, and vibrational frequency) of organic molecules were calculated using the B3LYP density functional and the aug-cc-pVDZ basis set. Values for the Hamaker constants were obtained using the approximation of the Lorentz-Lorenz equation and Onsager's equation with these properties. It was found that, in the case of "nonassociative" materials, such as hydrocarbons, ethers, ketones, aldehydes, carboxylic acids, esters, nitriles, and hydrosilanes, and halides, the calculated Hamaker constants were similar in value to their experimentally determined counterparts. Moreover, with this calculation method, it is easy to create the molecular model and the CPU time can be shortened.

High-Pressure Synthesis of A2 NiO2 Ag2 Se2 (A=Sr, Ba) with a High-Spin Ni2+ in Square-Planar Coordination.

Square-planar coordinate Ni2+ ions in oxides are exclusively limited to a low-spin state (S=0) owing to extensive crystal field splitting. Layered oxychalcogenides A2 NiII O2 Ag2 Se2 (A=Sr, Ba) with the S=1 NiO2 square lattice are now reported. The structural analysis revealed that the Ni2+ ion is under-bonded by a significant tensile strain from neighboring Ag2 Se2 layers, leading to the reduction in crystal field splitting. Ba2 NiO2 Ag2 Se2 exhibits a G-type spin order at 130 K, indicating fairly strong in-plane interactions. The high-pressure synthesis employed here possibly assists the expansion of NiO2 square lattice by taking the advantage of the difference in compressibility in oxide and selenide layers.

A Stable, Narrow-Gap Oxyfluoride Photocatalyst for Visible-Light Hydrogen Evolution and Carbon Dioxide Reduction.

Mixed anion compounds such as oxynitrides and oxychalcogenides are recognized as potential candidates of visible-light-driven photocatalysts since, as compared with oxygen 2p orbitals, p orbitals of less electronegative anion (e.g., N3-, S2-) can form a valence band that has more negative potential. In this regard, oxyfluorides appear unsuitable because of the higher electronegativity of fluorine. Here we show an exceptional case, an anion-ordered pyrochlore oxyfluoride Pb2Ti2O5.4F1.2 that has a small band gap (ca. 2.4 eV). With suitable modification of Pb2Ti2O5.4F1.2 by promoters such as platinum nanoparticles and a binuclear ruthenium(II) complex, Pb2Ti2O5.4F1.2 worked as a stable photocatalyst for visible-light-driven H2 evolution and CO2 reduction. Density functional theory calculations have revealed that the unprecedented visible-light-response of Pb2Ti2O5.4F1.2 arises from strong interaction between Pb-6s and O-2p orbitals, which is enabled by a short Pb-O bond in the pyrochlore lattice due to the fluorine substitution.

Stabilization Mechanism of the Tetragonal Structure in a Hydrothermally Synthesized BaTiO3 Nanocrystal.

Higher OH concentration is identified in tetragonal barium titanate (BaTiO3) nanorods synthesized by a hydrothermal method with a 10 vol % ethylene glycol solvent (Inada, M.; et al. Ceram. Int. 2015, 41, 5581-5587). This is apparently inconsistent with the known fact that higher OH concentration in the conventional hydrothermal synthesis makes pseudocubic BaTiO3 nanocrystals more stable than the tetragonal one. To understand where and how the introduced OH anions are located and behave in the nanocrystals, we applied ab initio analysis to several possible microscopic geometries of OH locations, confirming the relative stability of the tetragonal distortion over the pseudocubic one because of the preference of trans-type configurations of OH anions. We also performed Fourier transform infrared and X-ray diffraction analysis, all being consistent with the microscopic picture established by the ab initio geometrical optimizations.

Undoped Layered Perovskite Oxynitride Li2 LaTa2 O6 N for Photocatalytic CO2 Reduction with Visible Light.

Oxynitrides are promising visible-light-responsive photocatalysts, but their structures are almost confined with three-dimensional (3D) structures such as perovskites. A phase-pure Li2 LaTa2 O6 N with a layered perovskite structure was successfully prepared by thermal ammonolysis of a lithium-rich oxide precursor. Li2 LaTa2 O6 N exhibited high crystallinity and visible-light absorption up to 500 nm. As opposed to well-known 3D oxynitride perovskites, Li2 LaTa2 O6 N supported by a binuclear RuII complex was capable of stably and selectively converting CO2 into formate under visible light (λ>400 nm). Transient absorption spectroscopy indicated that, as compared to 3D oxynitrides, Li2 LaTa2 O6 N possesses a lower density of mid-gap states that work as recombination centers of photogenerated electron/hole pairs, but a higher density of reactive electrons, which is responsible for the higher photocatalytic performance of this layered oxynitride.

Valence Band Engineering of Layered Bismuth Oxyhalides toward Stable Visible-Light Water Splitting: Madelung Site Potential Analysis.

A layered oxychloride Bi4NbO8Cl is a visible-light responsive catalyst for water splitting, with its remarkable stability ascribed to the highly dispersive O-2p orbitals in the valence band, the origin of which, however, remains unclear. Here, we systematically investigate four series of layered bismuth oxyhalides, BiOX (X = Cl, Br, I), Bi4NbO8X (X = Cl, Br), Bi2GdO4X (X = Cl, Br), and SrBiO2X (X = Cl, Br, I), and found that Madelung site potentials of anions capture essential features of the valence band structures of these materials. The oxide anion in fluorite-like blocks (e.g., [Bi2O2] slab in Bi4NbO8Cl) is responsible for the upward shift of the valence band, and the degree of electrostatic destabilization changes depending on building layers and their stacking sequence. This study suggests that the Madelung analysis enables a prediction and design of the valence band structures of bismuth and other layered oxyhalides and is applicable even to a compound where DFT calculation is difficult to perform.

Investigation into Structural Phase Transitions in Layered Titanium-Oxypnictides by a Computational Phonon Analysis.

We applied ab initio phonon analysis to layered titanium-oxypnictides, Na2Ti2Pn2O (Pn = As and Sb), and found a clear contrast between the cases with lighter/heavier pnictogen in comparison with experiments. The result completely explains the experimental structure at low temperature, C2/m for Pn = As, within the conventional charge density wave, while there arise discrepancies when the pnictogen gets heavier. Our phonon calculation using the GGA-PBE functional predicts that a Cmce polymorph is more stable than the experimentally observed one (Cmcm) for Pn = Sb. On the basis of further quantitative analysis, we suggest the possibility that the GGA-PBE functional does not properly reproduce the electron correlation effects for Pn = Sb, and this could be the reason for the present discrepancy.

Bayesian molecular design with a chemical language model.

The aim of computational molecular design is the identification of promising hypothetical molecules with a predefined set of desired properties. We address the issue of accelerating the material discovery with state-of-the-art machine learning techniques. The method involves two different types of prediction; the forward and backward predictions. The objective of the forward prediction is to create a set of machine learning models on various properties of a given molecule. Inverting the trained forward models through Bayes' law, we derive a posterior distribution for the backward prediction, which is conditioned by a desired property requirement. Exploring high-probability regions of the posterior with a sequential Monte Carlo technique, molecules that exhibit the desired properties can computationally be created. One major difficulty in the computational creation of molecules is the exclusion of the occurrence of chemically unfavorable structures. To circumvent this issue, we derive a chemical language model that acquires commonly occurring patterns of chemical fragments through natural language processing of ASCII strings of existing compounds, which follow the SMILES chemical language notation. In the backward prediction, the trained language model is used to refine chemical strings such that the properties of the resulting structures fall within the desired property region while chemically unfavorable structures are successfully removed. The present method is demonstrated through the design of small organic molecules with the property requirements on HOMO-LUMO gap and internal energy. The R package iqspr is available at the CRAN repository.

GPGPU for orbital function evaluation with a new updating scheme.

We have accelerated an ab initio quantum Monte Carlo electronic structure calculation using general purpose computing on graphical processing units (GPGPU). The part of the code causing the bottleneck for extended systems is replaced by Compute Unified Device Architecture-GPGPU subroutine kernels which build up spline basis set expansions of electronic orbital functions at each Monte Carlo step. We have achieved a speedup of a factor of 30 for the bottleneck for a simulation of solid TiO(2) with 1536 electrons. To improve the performance with GPGPU we propose a new updating scheme for Monte Carlo sampling, quasi-simultaneous updating, which is intermediate between configuration-by-configuration updating and the widely used particle-by-particle updating. The error in the energy due to by the single precision treatment and the new updating scheme is found to be within the required accuracy of ~10(-3) hartree per primitive cell.

Excitons and biexcitons in symmetric electron-hole bilayers.

Symmetric electron-hole bilayer systems have been studied at zero temperature using the diffusion quantum Monte Carlo method. A flexible trial wave function is used that can describe fluid, excitonic, and biexcitonic phases. We calculate condensate fractions and pair correlation functions for a large number of densities r(s) and layer separations d. At small d we find a one-component fluid phase, an excitonic fluid phase, and a biexcitonic fluid phase, and the transitions among them appear to be continuous. At d=0, excitons appear to survive down to about r(s)=0.5 a.u., and biexcitons form at r(s)>2.5 a.u.