Band Alignment of Oxides by Learnable Structural-Descriptor-Aided Neural Network and Transfer Learning.

The band alignment of semiconductors, insulators, and dielectrics is relevant to diverse material properties and device structures utilizing their surfaces and interfaces. In particular, the ionization potential and electron affinity are fundamental quantities that describe surface-dependent band-edge positions with respect to the vacuum level. Their accurate and systematic determination, however, demands elaborate experiments or simulations for well-characterized surfaces. Here, we report machine learning for the band alignment of nonmetallic oxides using a high-throughput first-principles calculation data set containing about 3000 oxide surfaces. Our neural network accurately predicts the band positions for relaxed surfaces of binary oxides simply by using the information on bulk structures and surface termination planes. Moreover, we extend the model to naturally include multiple-cation effects and transfer it to ternary oxides. The present approach enables the band alignment of a vast number of solid surfaces, thereby opening the way to a systematic understanding and materials screening.

Emergence of Dynamically-Disordered Phases During Fast Oxygen Deintercalation Reaction of Layered Perovskite.

Determination of a reaction pathway is an important issue for the optimization of reactions. However, reactions in solid-state compounds have remained poorly understood because of their complexity and technical limitations. Here, using state-of-the-art high-speed time-resolved synchrotron X-ray techniques, the topochemical solid-gas reduction mechanisms in layered perovskite Sr3 Fe2 O7- δ (from δ ∼ 0.4 to δ = 1.0), which is promising for an environmental catalyst material is revealed. Pristine Sr3 Fe2 O7- δ shows a gradual single-phase structural evolution during reduction, indicating that the reaction continuously proceeds through thermodynamically stable phases. In contrast, a nonequilibrium dynamically-disordered phase emerges a few seconds before a first-order transition during the reduction of a Pd-loaded sample. This drastic change in the reaction pathway can be explained by a change in the rate-determining step. The synchrotron X-ray technique can be applied to various solid-gas reactions and provides an opportunity for gaining a better understanding and optimizing reactions in solid-state compounds.

Hole-Doping to a Cu(I)-Based Semiconductor with an Isovalent Cation: Utilizing a Complex Defect as a Shallow Acceptor.

p-Type doping in Cu(I)-based semiconductors is pivotal for solar cell photoabsorbers and hole transport materials to improve the device performance. Impurity doping is a fundamental technology to overcome the intrinsic limits of hole concentration controlled by native defects. Here, we report that alkali metal impurities are prominent p-type dopants for the Cu(I)-based cation-deficient hole conductors. When the size mismatch with Cu+ in the host lattice is increased, these isovalent impurities are preferentially located at interstitial positions to interact with the constituent Cu cations, forming stable impurity-defect complexes. We demonstrate that the Cs impurity in γ-CuI semiconductors enhances hole concentration controllability for single crystals and thin films in the range of 1013-1019 cm-3. First-principles calculations indicate that the Cs impurity forms impurity-defect complexes that act as shallow acceptors leading to the increased p-type conductivity. This isovalent doping provides an approach for controlled doping into cation-deficient semiconductors through an interaction of impurities with native defects.

Tuning the optical band gap and electrical properties of NiO thin films by nitrogen doping: a joint experimental and theoretical study.

A joint experimental and theoretical study is presented to reveal the influence of nitrogen doping on the optical and electrical properties of NiO thin films. Nitrogen addition can significantly enhance the subgap absorption. The molecular state of nitrogen (N2) has been identified in these doped thin films by electron energy loss spectroscopy.

Switchable Electric Dipole from Polaron Localization in Dielectric Crystals.

Ferroelectricity in crystals is associated with the displacement of ions or rotations of polar units. Here we consider the dipole created by donor doping (D^{+}) and the corresponding bound polaron (e^{-}). A dipole of 6.15 Debye is predicted, from Berry phase analysis, in the Ruddlesden-Popper phase of Sr_{3}Ti_{2}O_{7}. A characteristic double-well potential is formed, which persists for high doping densities. The effective Hubbard U interaction can vary the defect state from metallic, a two-dimensional polaron, through to a zero-dimensional polaron. The ferroelectriclike behavior reported here is localized and distinct from conventional spontaneous lattice polarization.

Data-Mining Element Charges in Inorganic Materials.

Oxidation states are well-established in chemical science teaching and research. We data-mine more than 168 000 crystallographic reports to find an optimal allocation of oxidation states to each element. In doing so, we uncover discrepancies between textbook chemistry and reported charge states observed in materials. We go on to show how the oxidation states we recommend can significantly facilitate materials discovery and the heuristic design of novel inorganic compounds.

Spin-Glass Magnetic Properties of A-Site Columnar-Ordered Quadruple Perovskites Y2MnGa(Mn4-xGax)O12 with 0 ≤ x ≤ 3.

Y2MnGa(Mn4-xGax)O12 solid solutions were synthesized at high pressure of ∼6 GPa and high temperature of ∼1570 K for the 0 ≤ x ≤ 3 compositional range. Synchrotron X-ray and neutron powder diffraction were used to study the crystal structures and cation distributions. These solutions adopt the parent structure of the A-site columnar-ordered quadruple perovskite family with space group P42/nmc (No. 137). They have lattice parameters of a = 7.36095 Å and c = 7.753 84 Å (x = 0), a = 7.361 68 Å and c = 7.716 16 Å (x = 1), a = 7.360 34 Å and c = 7.67142 Å (x = 2), and a = 7.363 93 Å and c = 7.616 85 Å (x = 3) at room temperature. The x = 0 sample has a cation distribution of [Y3+2]A[Mn3+]A'[Ga3+0.68Mn2+0.32]A″[Mn3.68Ga0.32]BO12 with a preferred localization of Ga3+ in the tetrahedral A″ site and with a small amount of Ga3+ in the octahedral B site. A complete triple A-site order, [Y3+2]A[Mn3+]A'[Ga3+]A″[Mn3+4-xGa3+x]BO12, is realized for x ≥ 1. All samples demonstrate spin-glass-like magnetic properties, and the absence of a long-range magnetic order at the ground state at 1.5 K was confirmed by neutron diffraction for the x = 1 sample. First-principles calculations indicated the spin-glass-like magnetic ordering is derived from the Ga substitution to the B sites and gave evidence that the ideal cation distribution could produce robust ferromagnetism in this family of perovskites.

Effect of MnO2 Crystal Structure on Aerobic Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid.

Aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) as a bioplastics monomer is efficiently promoted by a simple system based on a nonprecious-metal catalyst of MnO2 and NaHCO3. Kinetic studies indicate that the oxidation of 5-formyl-2-furancarboxylic acid (FFCA) to FDCA is the slowest step for the aerobic oxidation of HMF to FDCA over activated MnO2. We demonstrate through combined computational and experimental studies that HMF oxidation to FDCA is largely dependent on the MnO2 crystal structure. Density functional theory (DFT) calculations reveal that vacancy formation energies at the planar oxygen sites in α- and γ-MnO2 are higher than those at the bent oxygen sites. ß- and λ-MnO2 consist of only planar and bent oxygen sites, respectively, with lower vacancy formation energies. Consequently, ß- and λ-MnO2 are likely to be good candidates as oxidation catalysts. On the other hand, experimental studies reveal that the reaction rates per surface area for the slowest step (FFCA oxidation to FDCA) decrease in the order of ß-MnO2 > λ-MnO2 > γ-MnO2 ≈ α-MnO2 > δ-MnO2 > ε-MnO2; the catalytic activity of ß-MnO2 exceeds that of the previously reported activated MnO2 by three times. The order is in good agreement not only with the DFT calculation results, but also with the reduction rates per surface area determined by the H2-temperature-programmed reduction measurements for MnO2 catalysts. The successful synthesis of high-surface-area ß-MnO2 significantly improves the catalytic activity for the aerobic oxidation of HMF to FDCA.

Direct Observation of Magnetization Reversal by Electric Field at Room Temperature in Co-Substituted Bismuth Ferrite Thin Film.

Using the electric field to manipulate the magnetization of materials is a potential way of making low-power-consumption nonvolatile magnetic memory devices. Despite concentrated effort in the last 15 years on magnetic multilayers and magnetoelectric multiferroic thin films, there has been no report on the reversal of out-of-plane magnetization by an electric field at room temperature without the aid of an electric current. Here, we report direct observation of out-of-plane magnetization reversal at room temperature by magnetic force microscopy after electric polarization switching of cobalt-substituted bismuth ferrite thin film grown on (110)o-oriented GdScO3 substrate. A striped pattern of ferroelectric and weakly ferromagnetic domains was preserved after reversal of the out-of-plane electric polarization.

Heterogeneously Catalyzed Aerobic Oxidation of Sulfides with a BaRuO3 Nanoperovskite.

A rhombohedral BaRuO3 nanoperovskite, which was synthesized by the sol-gel method using malic acid, could act as an efficient heterogeneous catalyst for the selective oxidation of various aromatic and aliphatic sulfides with molecular oxygen as the sole oxidant. BaRuO3 showed much higher catalytic activities than other catalysts, including ruthenium-based perovskite oxides, under mild reaction conditions. The catalyst could be recovered by simple filtration and reused several times without obvious loss of its high catalytic performance. The catalyst effect, 18O-labeling experiments, and kinetic and mechanistic studies showed that substrate oxidation proceeds with oxygen species caused by the solid. The crystal structure of ruthenium-based oxides is crucial to control the nature of the oxygen atoms and significantly affects their oxygen transfer reactivity. Density functional theory calculations revealed that the face-sharing octahedra in BaRuO3 likely are possible active sites in the present oxidation in sharp contrast to the corner-sharing octahedra in SrRuO3, CaRuO3, and RuO2. The superior oxygen transfer ability of BaRuO3 is also applicable to the quantitative conversion of dibenzothiophene into the corresponding sulfone and gram-scale oxidation of 4-methoxy thioanisole, in which 1.20 g (71% yield) of the analytically pure sulfoxide could be isolated.

High-Mobility p-Type and n-Type Copper Nitride Semiconductors by Direct Nitriding Synthesis and In Silico Doping Design.

Thin-film photovoltaics (PV) have emerged as a technology that can meet the growing demands for efficient and low-cost large-scale cells. However, the photoabsorbers currently in use contain expensive or toxic elements, and the difficulty in bipolar doping, particularly in a device structure, requires elaborate optimization of the heterostructures for improving the efficiency. This study shows that bipolar doping with high hole and electron mobilities in copper nitride (Cu3 N), composed solely of earth-abundant and environmentally benign elements, is readily available through a novel gaseous direct nitriding reaction applicable to uniform and large-area deposition. A high-quality undoped Cu3 N film is essentially an n-type semiconductor, while p-type conductivity is realized by interstitial fluorine doping, as predicted using density functional theory calculations and directly proven by atomically resolved imaging. The synthetic methodology for high-quality p-type and n-type films paves the way for the application of Cu3 N as an alternative absorber in thin-film PV.

Complex Structural Behavior of BiMn7O12 Quadruple Perovskite.

Structural properties of a quadruple perovskite BiMn7O12 were investigated by laboratory and synchrotron X-ray powder diffraction between 10 and 650 K, single-crystal X-ray diffraction at room temperature, differential scanning calorimetry (DSC), second-harmonic generation, and first-principles calculations. Three structural transitions were found. Above T1 = 608 K, BiMn7O12 crystallizes in a parent cubic structure with space group Im3̅. Between 460 and 608 K, BiMn7O12 adopts a monoclinic symmetry with pseudo-orthorhombic metrics (denoted as I2/m(o)), and orbital order appears below T1. Below T2 = 460 K, BiMn7O12 is likely to exhibit a transition to space group Im. Finally, below about T3 = 290 K, a triclinic distortion takes place to space group P1. Structural analyses of BiMn7O12 are very challenging because of severe twinning in single crystals and anisotropic broadening and diffuse scattering in powder. First-principles calculations confirm that noncentrosymmetric structures are more stable than centrosymmetric ones. The energy difference between the Im and P1 models is very small, and this fact can explain why the Im to P1 transition is very gradual, and there are no DSC anomalies associated with this transition. The structural behavior of BiMn7O12 is in striking contrast with that of LaMn7O12 and could be caused by effects of the Bi3+ lone electron pair.

Perovskite-Type InCoO3 with Low-Spin Co3+: Effect of In-O Covalency on Structural Stabilization in Comparison with Rare-Earth Series.

Perovskite rare-earth cobaltites ACoO3 (A = Sc, Y, La-Lu) have been of enduring interest for decades due to their unusual structural and physical properties associated with the spin-state transitions of low-spin Co3+ ions. Herein, we have synthesized a non-rare-earth perovskite cobaltite, InCoO3, at 15 GPa and 1400 °C and investigated its crystal structure and magnetic ground state. Under the same high-pressure and high-temperature conditions, we also prepared a perovskite-type ScCoO3 with an improved cation stoichiometry in comparison to that in a previous study, where synthesis at 6 GPa and 1297 °C yielded a perovskite cobaltite with cation mixing on the A-site, (Sc0.95Co0.05)CoO3. The two perovskite phases have nearly stoichiometric cation compositions, crystallizing in the orthorhombic Pnma space group. In the present investigation, comprehensive studies on newly developed and well-known Pnma ACoO3 perovskites (A = In, Sc, Y, Pr-Lu) show that InCoO3 does not fulfill the general evolution of crystal metrics with A-site cation size, indicating that InCoO3 and rare-earth counterparts have different chemistry for stabilizing the Pnma structures. Detailed structural analyses combined with first-principles calculations reveal that the origin of the anomaly for InCoO3 is ascribed to the A-site cation displacements that accompany octahedral tilts; despite the highly tilted CoO6 network, the In-O covalency makes In3+ ions reluctant to move from their ideal cubic-symmetry position, leading to less orthorhombic distortion than would be expected from electrostatic/ionic size mismatch effects. Magnetic studies demonstrate that InCoO3 and ScCoO3 are diamagnetic with a low-spin state of Co3+ below 300 K, in contrast to the case of (Sc0.95Co0.05)CoO3, where the high-spin Co3+ ions on the A-site generate a large paramagnetic moment. The present work extends the accessible composition range of the low-spin orthocobaltite series and thus should help to establish a more comprehensive understanding of the structure-property relation.

Chemical and Lattice Stability of the Tin Sulfides.

The tin sulfides represent a materials platform for earth-abundant semiconductor technologies. We present a first-principles study of the five known and proposed phases of SnS together with SnS2 and Sn2S3. Lattice-dynamics techniques are used to evaluate the dynamical stability and temperature-dependent thermodynamic free energy, and we also consider the effect of dispersion forces on the energetics. The recently identified π-cubic phase of SnS is found to be metastable with respect to the well-known orthorhombic Pnma/Cmcm equilibrium. The Cmcm phase is a low-lying saddle point between Pnma local minima on the potential-energy surface and is observed as an average structure at high temperatures. Bulk rocksalt and zincblende phases are found to be dynamically unstable, and we show that whereas rocksalt SnS can potentially be stabilized under a reduction of the lattice constant the hypothetical zincblende phase proposed in several previous studies is extremely unlikely to form. We also investigate the stability of Sn2S3 with respect to SnS and SnS2 and find that both dispersion forces and vibrational contributions to the free energy are required to explain its experimentally observed resistance to decomposition.

Lattice dynamics of the tin sulphides SnS2, SnS and Sn2S3: vibrational spectra and thermal transport.

We present an in-depth first-principles study of the lattice dynamics of the tin sulphides SnS2, Pnma and π-cubic SnS and Sn2S3. An analysis of the harmonic phonon dispersion and vibrational density of states reveals phonon bandgaps between low- and high-frequency modes consisting of Sn and S motion, respectively, and evidences a bond-strength hierarchy in the low-dimensional SnS2, Pnma SnS and Sn2S3 crystals. We model and perform a complete characterisation of the infrared and Raman spectra, including temperature-dependent anharmonic linewidths calculated using many-body perturbation theory. We illustrate how vibrational spectroscopy could be used to identify and characterise phase impurities in tin sulphide samples. The spectral linewidths are used to model the thermal transport, and the calculations indicate that the low-dimensional Sn2S3 has a very low lattice thermal conductivity, potentially giving it superior performance to SnS as a candidate thermoelectric material.

Discovery of a Novel Sn(II)-Based Oxide ß-SnMoO4 for Daylight-Driven Photocatalysis.

Daylight-driven photocatalysts have attracted much attention in the context of "green" technology. Although various active materials have been reported and their applications are rapidly increasing, many are discovered after enormous experimental efforts. Herein the discovery of a novel oxide photocatalyst, ß-SnMoO4, is demonstrated via a rational search of 3483 known and hypothetical compounds with various compositions and structures over the whole range of SnO-MO q/2 (M: Ti, Zr, and Hf (q = 4); V, Nb, and Ta (q = 5); Cr, Mo, and W (q = 6)) pseudobinary systems. Screening using thermodynamic stability, band gap, and band-edge positions by density functional theory calculations identifies ß-SnMoO4 as a potential target. Then a low temperature route is used to successfully synthesize the novel crystal, which is confirmed by X-ray powder diffraction and Mössbauer spectroscopy. ß-SnMoO4 is active for the photocatalytic decomposition of a methylene blue solution under daylight and its activity is comparable to a known photocatalyst, ß-SnWO4.

Discovery of earth-abundant nitride semiconductors by computational screening and high-pressure synthesis.

Nitride semiconductors are attractive because they can be environmentally benign, comprised of abundant elements and possess favourable electronic properties. However, those currently commercialized are mostly limited to gallium nitride and its alloys, despite the rich composition space of nitrides. Here we report the screening of ternary zinc nitride semiconductors using first-principles calculations of electronic structure, stability and dopability. This approach identifies as-yet-unreported CaZn2N2 that has earth-abundant components, smaller carrier effective masses than gallium nitride and a tunable direct bandgap suited for light emission and harvesting. High-pressure synthesis realizes this phase, verifying the predicted crystal structure and band-edge red photoluminescence. In total, we propose 21 promising systems, including Ca2ZnN2, Ba2ZnN2 and Zn2PN3, which have not been reported as semiconductors previously. Given the variety in bandgaps of the identified compounds, the present study expands the potential suitability of nitride semiconductors for a broader range of electronic, optoelectronic and photovoltaic applications.

LiNbO3-Type Oxide (Tl(1-x)Sc(x))ScO3: High-Pressure Synthesis, Crystal Structure, and Electronic Properties.

We investigate the synthesis of a thallium scandate, TlScO3, under high-pressure (6-7.7 GPa) and high-temperature (1373-1773 K) conditions. At 6 GPa, a LiNbO3-type phase appears in a narrow temperature range and in mixtures with other phases. At 7.7 GPa and 1673 K, a new LiNbO3-type oxide is found with a composition of (Tl(1-x)Sc(x))ScO3 and x ≈ 0.26 as determined by structural analysis from X-ray powder diffraction data. It crystallizes in space group R3c (No. 161) with lattice parameters of a = 5.50283(7) Å and c = 14.4606(2) Å. It is stable at least up to 800 K at ambient pressure. The point-charge model gives an electric polarization of 60 µC/cm(2). First-principles calculations show that centrosymmetric ilmenite-type and polar LiNbO3-type structures of stoichiometric TlScO3 have almost the same lowest energy, and the next stable structure is a GdFeO3-type perovskite structure.

Intercalation and Push-Out Process with Spinel-to-Rocksalt Transition on Mg Insertion into Spinel Oxides in Magnesium Batteries.

On the basis of the similarity between spinel and rocksalt structures, it is shown that some spinel oxides (e.g., MgCo2O4, etc) can be cathode materials for Mg rechargeable batteries around 150 °C. The Mg insertion into spinel lattices occurs via "intercalation and push-out" process to form a rocksalt phase in the spinel mother phase. For example, by utilizing the valence change from Co(III) to Co(II) in MgCo2O4, Mg insertion occurs at a considerably high potential of about 2.9 V vs. Mg2+/Mg, and similarly it occurs around 2.3 V vs. Mg2+/Mg with the valence change from Mn(III) to Mn(II) in MgMn2O4, being comparable to the ab initio calculation. The feasibility of Mg insertion would depend on the phase stability of the counterpart rocksalt XO of MgO in Mg2X2O4 or MgX3O4 (X = Co, Fe, Mn, and Cr). In addition, the normal spinel MgMn2O4 and MgCr2O4 can be demagnesiated to some extent owing to the robust host structure of Mg1-xX2O4, where the Mg extraction/insertion potentials for MgMn2O4 and MgCr2O4 are both about 3.4 V vs. Mg2+/Mg. Especially, the former "intercalation and push-out" process would provide a safe and stable design of cathode materials for polyvalent cations.