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Abstract Molecular hydrogen (H2) ortho‐para conversion (O/P conversion) proceeds slowly at low temperatures accompanying a heat release. Thus, catalysts for accelerating this conversion rate are highly demanded in terms of the storage and utilization of liquid H2. The catalysts for this purpose are experimentally screened by examining a broad range of materials covering magnetic, non‐magnetic, metallic, and nonmetallic oxides. The primary conclusions obtained are summarized below. (1) active materials are required to be non‐metallic and to bear the cations with ionic radii smaller than the bond length of H2. (2) Metallic materials have almost no activity irrespective of with or without magnetism (3) The activity of materials belonging to (1) is largely enhanced when the constituting cation has a magnetic moment. In addition, there is a class of materials for which the activity is distinctly enhanced just upon substitution by the foreign ions.

Abstract As a next‐generation photovoltaic device, perovskite solar cells are rapidly emerging. Nevertheless, both solution and device stability pose challenges for commercialization due to chemical degradation caused by internal and external factors. Especially, the decomposition of iodoplumbate in a perovskite solution hinders the long‐term use of perovskite solutions. Moreover, the synthesis of stable perovskites at low temperature is important for stable devices and wide applications (flexible devices and high reproducibility). Herein, the critical composition of perovskite is found to obtain high stabilities of both iodoplumbate and perovskite crystals by utilizing CsPbBr3 and FAPbI3, exhibiting high device performance and long‐term solution storage. The novel composition of CsPbBr3‐alloyed FAPbI3 not only crystallizes under annealing‐free conditions but also demonstrates excellent iodoplumbate stability for 100 days (∼3000 h) without any degradation. Furthermore, high device stabilities are achieved over 2000 and 3000 h under extreme conditions of A.M. 1.5 and 85°C/85% relative humidity, respectively. Overall, the device exhibited a high power conversion efficiency of 23.4%, and furthermore, CsPbBr3‐alloyed FAPbI3 was devoted to widen the applications in both flexible and carbon‐electrode devices, thereby addressing both scientific depths and potential commercial materials.

Abstract The tunability of reaction pathways is required for exploring efficient and low cost catalysts for ammonia synthesis. There is an obstacle by the limitations arising from scaling relation for this purpose. Here, we demonstrate that the alkali earth imides (AeNH) combined with transition metal (TM = Fe, Co and Ni) catalysts can overcome this difficulty by utilizing functionalities arising from concerted role of active defects on the support surface and loaded transition metals. These catalysts enable ammonia production through multiple reaction pathways. The reaction rate of Co/SrNH is as high as 1686.7 mmol·gCo −1·h−1 and the TOFs reaches above 500 h−1 at 400 °C and 0.9 MPa, outperforming other reported Co-based catalysts as well as the benchmark Cs-Ru/MgO catalyst and industrial wüstite-based Fe catalyst under the same reaction conditions. Experimental and theoretical results show that the synergistic effect of nitrogen affinity of 3d TMs and in-situ formed NH2− vacancy of alkali earth imides regulate the reaction pathways of the ammonia production, resulting in distinct catalytic performance different from 3d TMs. It was thus demonstrated that the appropriate combination of metal and support is essential for controlling the reaction pathway and realizing highly active and low cost catalysts for ammonia synthesis.

Abstract High energy‐conversion efficiency (ZT) of thermoelectric materials has been achieved in heavy metal chalcogenides, but the use of toxic Pb or Te is an obstacle for wide applications of thermoelectricity. Here, high ZT is demonstrated in toxic‐element free Ba3BO (B = Si and Ge) with inverse‐perovskite structure. The negatively charged B ion contributes to hole transport with long carrier life time, and their highly dispersive bands with multiple valley degeneracy realize both high p‐type electronic conductivity and high Seebeck coefficient, resulting in high power factor (PF). In addition, extremely low lattice thermal conductivities (κlat) 1.0–0.4 W m−1 K−1 at T = 300–600 K are observed in Ba3BO. Highly distorted O–Ba6 octahedral framework with weak ionic bonds between Ba with large mass and O provides low phonon velocities and strong phonon scattering in Ba3BO. As a consequence of high PF and low κlat, Ba3SiO (Ba3GeO) exhibits rather high ZT = 0.16–0.84 (0.35–0.65) at T = 300–623 K (300–523 K). Finally, based on first‐principles carrier and phonon transport calculations, maximum ZT is predicted to be 2.14 for Ba3SiO and 1.21 for Ba3GeO at T = 600 K by optimizing hole concentration. Present results propose that inverse‐perovskites would be a new platform of environmentally‐benign high‐ZT thermoelectric materials.

ABSTRACTAs an important part of water splitting, the oxygen evolution reaction (OER) requires efficient, low-cost, and stable catalysts to overcome its sluggish kinetic barrier. In this study, based on previously reported OER catalyst materials of boron monosulfide mixed with graphene (r-BS+G), nickel foam (NF) is introduced as a supporting material for an r-BS+G electrocatalyst. The resulting r-BS+G-NF exhibits a very low overpotential at 10 (245 mV), 100 (308 mV), and 500 (405 mV) mA cm−2, with a low Tafel slope (56 mV dec−1). In addition, r-BS+G-NF exhibits high durability and can maintain high activity for more than 100 h at 100 mA cm−2. This is in sharp contrast to the catalyst without graphene (r-BS+NF), which shows lower durability. The results suggest that the unique morphology of the NF provides a large electrochemically active area and exposes more active sites on the surface of the prepared electrocatalyst, while the flexible graphene sheets play an important role as a support for effectively combining r-BS and NF. Consequently, the self-supporting structure can improve the OER performance as well as stability. Therefore, this study provides a promising strategy for use as an efficient and stable OER catalyst at high current densities.

Boron-containing materials have experimentally and theoretically been shown to be promising materials for CO2 capture and conversion. Here, hydrogen-deficient 2D hydrogen boride sheets are shown to physisorb CO2 and promote conversion to methane and ethane.

Abstract By breaking the restrictions on traditional alloying strategy, the high entropy concept has promoted the exploration of the central area of phase space, thus broadening the horizon of alloy exploitation. This review highlights the marriage of the high entropy concept and van der Waals systems to form a new family of materials category, namely the high entropy van der Waals materials (HEX, HE = high entropy, X = anion clusters) and describes the current issues and next challenges. The design strategy for HEX has integrated the local feature (e.g., composition, spin, and valence states) of structural units in high entropy materials and the holistic degrees of freedom (e.g., stacking, twisting, and intercalating species) in van der Waals materials, and is successfully used for the discovery of high entropy dichalcogenides, phosphorus tri‐chalcogenides, halogens, and MXene. The rich combination and random distribution of the multiple metallic constituents on the nearly regular 2D lattice give rise to a flexible platform to study the correlation features behind a range of selected physical properties, e.g., superconductivity, magnetism, and metal–insulator transition. The deliberate design of structural units and their stacking configuration can also create novel catalysts to enhance their performance in a bunch of chemical reactions.

Nanoscale boron-based materials are promising for electronic devices, and it is important to understand their chemical stability. Here, hydrogen boride nanosheets are stable against hydrolysis in water, which ab initio calculations attribute to the charge state of boron and the boron-boron bond network.

Abstract Tin mono‐selenide (SnSe) exhibits the world record of thermoelectric conversion efficiency ZT in the single crystal form, but the performance of polycrystalline SnSe is restricted by low electronic conductivity (σ) and high thermal conductivity (κ), compared to those of the single crystal. Here an effective strategy to achieve high σ and low κ simultaneously is reported on p‐type polycrystalline SnSe with isovalent Te ion substitution. The nonequilibrium Sn(Se1−xTex) solid solution bulks with x up to 0.4 are synthesized by the two‐step process composed of high‐temperature solid‐state reaction and rapid thermal quenching. The Te ion substitution in SnSe realizes high σ due to the 103‐times increase in hole carrier concentration and effectively reduced lattice κ less than one‐third at room temperature. The large‐size Te ion in Sn(Se1−xTex) forms weak SnTe bonds, leading to the high‐density formation of hole‐donating Sn vacancies and the reduced phonon frequency and enhanced phonon scattering. This result—doping of large‐size ions beyond the equilibrium limit—proposes a new idea for carrier doping and controlling thermal properties to enhance the ZT of polycrystalline SnSe.

Clathrate-like or caged compounds have attracted great interest owing to their structural flexibility, as well as their fertile physical properties. Here, we report the pressure-induced reemergence of superconductivity in BaIr2Ge7 and Ba3Ir4Ge16, two new caged superconductors with two-dimensional building blocks of cage structures. After suppression of the ambient-pressure superconducting (SC-I) states, new superconducting (SC-II) states emerge unexpectedly, with Tc increased to a maximum of 4.4 and 4.0 K for BaIr2Ge7 and Ba3Ir4Ge16, respectively. Combined with high-pressure synchrotron x-ray diffraction and Raman measurements, we propose that the reemergence of superconductivity in these caged superconductors can be ascribed to a pressure-induced phonon softening linked to cage shrinkage.

Abstract Metal halide perovskites (MHPs) are plausible candidates for practical p‐type semiconductors. However, in thin film transistor (TFT) applications, both 2D PEA2SnI4 and 3D FASnI3 MHPs have different drawbacks. In 2D MHP, the TFT mobility is seriously reduced by grain‐boundary issues, whereas 3D MHP has an uncontrollably high hole density, which results in quite a large threshold voltage (Vth). To overcome these problems, a new concept based on a 2D–3D core–shell structure is herein proposed. In the proposed structure, a 3D MHP core is fully isolated by a 2D MHP, providing two desirable effects as follows. (i) Vth can be independently controlled by the 2D component, and (ii) the grain‐boundary resistance is significantly improved by the 2D/3D interface. Moreover, SnF2 additives are used, and they facilitate the formation of the 2D/3D core–shell structure. Consequently, a high‐performance p‐type Sn‐based MHP TFT with a field‐effect mobility of ≈25 cm2 V−1 s−1 is obtained. The voltage gain of a complementary metal oxide semiconductor (CMOS) inverter comprising an n‐channel InGaZnOx TFT and a p‐channel Sn‐MHP TFT is ≈200 V/V at VDD = 20 V. Overall, the proposed 2D/3D core–shell structure is expected to provide a new route for obtaining high‐performance MHP TFTs.

Ammonia synthesis via the Haber–Bosch process typically takes place at an elevated temperature in order to achieve a reasonable rate. Here the authors report on a CaFH solid solution with low activation energy for catalytic ammonia synthesis at lower temperatures.

Ferromagnetic quasi-atomic behavior of interstitial anionic electrons (IAEs) in practical electrides is yet to be discovered experimentally. Here, the authors reveal that IAEs in two-dimensional electride [Gd2C]²+⋅2e- behave as magnetic elements with their own magnetic moment.

Stabilize the active metal single atoms under harsh conditions is critical for the development of single atom catalysts. Here the authors report a nanoporous crystal, 12CaO·7Al2O3, that can firmly stabilize Pt single atoms in its surface cavities for efficient catalytic hydrogenation of nitroarenes.

In Suzuki coupling reactions, excellent catalytic performance require negatively charged Pd species and the avoidance of metal leaching or clustering. Here the authors implanted Pd sites into an intermetallic electride, Y3Pd2, which serves as an efficient and stable catalyst for Suzuki coupling reactions.

Abstract Introducing artificial strain in epitaxial thin films is an effective strategy to alter electronic structures of transition metal oxides (TMOs) and to induce novel phenomena and functionalities not realized in bulk crystals. This study reports a breaking of the conventional trade‐off relation in thermopower (S)–conductivity (σ) and demonstrates a 2 orders of magnitude enhancement of power factor (PF) in compressively strained LaTiO3 (LTO) films. By varying substrates and reducing film thickness down to 4 nm, the out‐of‐plane to the in‐plane lattice parameter ratio is controlled from 0.992 (tensile strain) to 1.034 (compressive strain). This tuning induces the electronic structure change from a Mott insulator to a metal and leads to a 103‐fold increase in σ up to 2920 S cm−1. Concomitantly, the sign of S inverts from positive to negative, and both σ and S increase and break the trade‐off relation between them in the n‐type region. As a result, the PF (=S2σ) is significantly enhanced to 300 µW m−1K−2, which is 102 times larger than that of bulk LTO. Present results propose epitaxial strain as a means to finely tune strongly correlated TMOs close to their Mott transition, and thus to harness the hidden large thermoelectric PF.

Over the last half century, layered materials have been at the forefront of materials science, spearheading the discovery of new phenomena and functionalities. Certain layered materials are known to possess electronic states unassociated with any of the constituent atoms, having a large proportion of their probability amplitude in the space between the layers. Usually, such a nucleus‐free interlayer state has energy above the Fermi level and is unoccupied. However, the energy decreases when cations are intercalated and may cross the Fermi level, as in the case of C6Ca, a superconductor with a Tc of 11.5 K. A major thrust to the research of interlayer electrons comes with the discovery of layered electrides, which are alternating stacks of positively charged ionic layers and negatively charged sheets of electrons in the interlayer space. When intercalation compounds and layered electrides are thinned down to the atomic scale, the interlayer states survive as surface states floating over the surface. This review provides a unified overview of the two classes of materials hosting interlayer floating electrons near the Fermi level, intercalation compounds and layered electrides, and their properties, including high electron mobility, low work function, ultralow interlayer friction, superconductivity, and plasmonic properties.

Hydride ions are promising for energy storage since they are abundant, lightweight, and highly mobile, but ionic conductivity should be improved. Here the authors achieve fast hydride ion conductivity in a mixed-anion compound by tuning oxygen content.

Two-dimensional materials are promising for electrochemical storage and conversion, but are somewhat limited in composition. Here the authors use a computational strategy to predict the existence of a layered boride material, which they synthesize and demonstrate prospective for use as an anode material.

Transparent silica glass is an indispensable material for today's information technology society as an ultra-low loss optical fiber. Recently, further low-loss optical fibers are demanded due to rapidly expanding information. A small amount of fluorine doping is known to reduce the Rayleigh scattering further by reducing the density fluctuations of silica glass, even though the fluorine is an impurity for the pristine silica glass. This study devoted to answer the riddle of F-doping and an accurate force-matching potential optimized by machine-learning enables us to understand how fluorine remedies the disorders in silica glass. It was found that fluorine mainly replaces an oxygen in tetrahedral SiO4 unit, but also forms five-fold Si, such as SiO4F and SiO3F2 units. The former disrupts silica network, while the latter attenuates the network rigidity by loosening the five Si-O(F) bonds. Since the two local effects of fluorine spreads through the glass network, F-doping prompts silica glass to relax further even at temperature lower than the glass transition (fictive) temperature. Consequently, fluorine minimizes the density fluctuation (thus, Rayleigh scattering) of silica glass at around 1 wt%, although fluorine itself is an impurity for silica glass.

Summary: The charge states of elements dictate the behavior of electrons and phonons in a lattice, either directly or indirectly. Here, we report the discovery of an anomalous charge state evolution in the superconducting M3Al2C (M = Mo, W) system, where electron doping can be achieved through “oxidation.” Specifically, with the continuous removal of electron donor (Al) from the structure, we found an electron doping effect in the negatively charged transition metals. Over a certain threshold, the charge state of transition metals goes through a sudden reversion from negative to positive, which leads to a subsequent structure collapse. Concomitantly, the previous robust superconducting transition temperatures (Tcs) can be flexibly modulated. Detailed analysis reveals the origin of the superconductivity and the intimate relationship between the charge state and the electron-phonon coupling constant. The peculiar charge state in M3Al2C plays an important role in both its structure and superconductivity.

Low-dimensional lead-free luminescent halides have emerged as highly promising phosphors for white-light emission. Recently, we reported a broadband blue-emitting copper(I) iodide-based material, Cs3Cu2I5, with a high photoluminescence quantum yield (PLQY) (∼90%) and a zero-dimensional nature, providing significant dimensionality for the photoactive site. However, this material is insufficient as a white-light emitter owing to the deficient yellow emission. In this paper, we report a novel yellow luminescent phosphor, CsCu2I3, with a 1D structure for the photoactive site. This material exhibits a broadband emission centered at ∼560 nm with a PLQY of ∼8%. We demonstrate a thin film with white-light emission that can be fabricated using one-step spin-coating of a mixed precursor solution of 1D CsCu2I3 (yellow) and 0D Cs3Cu2I5 (blue).

We investigate the ultrafast lattice dynamics in BaFe_{2}As_{2} via time-resolved x-ray diffraction measurements using an x-ray free-electron laser at SPring-8 Angstrom Compact free-electron LAser (SACLA). The profile of the Bragg peaks substantially changes after strong photoexcitation, where the c axis length of the lattice initially contracts, followed by expansion. These ultrafast lattice contractions and expansions significantly depend on the pump fluence, and it is suggested that a strong normal stress is applied along the c axis. To obtain a phenomenological understanding of the observed lattice dynamics, we performed calculations based on a two-temperature model and an ultrafast thermoelasticity model. Our results showed that a steep gradient of the electron temperature induced by photoexcitation generates a blast force, which acts as a strong normal stress. We propose that a temporal stress can be applied by a strong optical pump, and this scheme can be another route for the application of uniaxial stress to superconductors.

In 2012, we reported that C12A7 electride (C12A7: e−) significantly promotes the catalytic activity of Ru nanoparticles for ammonia synthesis through the electron donation from the C12A7: e− with a low work function (2.4 eV) to Ru [...]

We report the single crystal growth and characterization of the highest Tc iron-based superconductor SmFeAsO1−xHx. Some sub-millimeter-sized crystals were grown using the mixture flux of Na3As + 3NaH + As at 3.0 GPa and 1473 K. The chemical composition analyses confirmed 10% substitution of hydrogen for the oxygen site (x = 0.10), however, the structural analyses suggested that the obtained crystal forms a multi-domain structure. By using the FIB technique we fabricated the single domain SmFeAsO0.9H0.10 crystal with the Tc of 42 K, and revealed the metallic conduction in in-plane (ρab), while semiconducting in the out-of-plane (ρc). From the in-plane Hall coefficient measurements, we confirmed that the dominant carrier of SmFeAsO0.9H0.10 crystal is an electron, and the hydride ion occupied at the site of the oxygen ion effectively supplies a carrier electron per iron following the equation: O2− → H− + e−.

Intermetallics: Superconductivity coexists with electride behavior Coexistence of electride behavior and superconductivity is observed in a hexagonal phase of a Nb5Ir3 intermetallic with tunable electronic properties by introducing foreign atoms. A team led by Yaoqing Zhang and Hideo Hosono from Japan Science and Technology Agency and Tokyo Institute of Technology report a new hexagonal phase in the phase diagram of Nb–Ir binary intermetallics with interesting interplay of superconductivity and electride state. The electride state is formed by electrons detaching from the atoms but localizing in a one-dimensional channel space. This electride becomes a superconductor below a transition temperature of about 9.4 K, which could be enhanced to 10.5 K upon filling interstitial cavities with foreign oxygens. The results suggest a general rule governing the formation of electrides and form the basis for novel stable functional electrides.

The transparency of oxide semiconductors is a significant feature that enables the fabrication of fully transparent electronics. Unfortunately, practical transparent electronics using amorphous oxide semiconductors (AOSs) have not yet been realized, owing to significant photo-instabilities of these materials. Previous studies have revealed that the photo-instability can be attributed to sub-gap states (SGSs) near the valence-band maximum (VBM). Thus, it is inferred that the energy difference between the SGSs and the conduction-band minimum must be widened sufficiently in order to make it fully transparent over the entire visible-light region. In this work, we examined the electronic structures of a variety of AOSs and found that their ionization potentials vary greatly, depending upon the specific metal cations. This finding enabled us to increase the optical bandgap by modifying the VBM levels, resulting in a high mobility of 9 cm2/Vs and an ultra-wide bandgap of 3.8 eV for amorphous Zn–Ga–O (a-ZGO). We show that a-ZGO thin-film transistors exhibit no negative-bias illumination-stress instability with no passivation and no light-shielding layer.

LaFeAsO1-xHx is a prototypical iron-based superconductor. However, its phase diagram presents unique properties compared to conventional iron-based superconductors: (1) the superconducting phase is sandwiched between two antiferromagnetic (AF) phases appearing in lightly (x ≤ 0.05) and heavily (0.49 ≤ x) H-doped regimes. (2) The AF phase (0.49 ≤ x) is strongly suppressed when applying pressure. We measured the relaxation time T1 at 75As sites for x=0.6 under pressures up to 3.7 GPa to investigate the behavior near the quantum critical point (QCP). The relaxation rate divided by temperature (T) 1/T1T exhibited a peak at the AF transition temperature (TN) followed by Curie-Weiss behavior at low temperatures below TN.. The result demonstrates that the gapped excitation originating from the spin-density-wave state and the gapless excitation characteristic to the QCP coexist near the pressure-induced QCP.

In some kind of three-dimensional higher-order topological insulators, bulk and surface electronic states are gapped, while there appear gapless hinge states protected by spatial symmetry. Here we show by ab initio calculations that the La apatite electride is a higher-order topological crystalline insulator. It is a one-dimensional electride, in which the one-dimensional interstitial hollows along the c axis support anionic electrons, and the electronic states in these one-dimensional channels are well approximated by the one-dimensional Su-Schrieffer-Heeger model. When the crystal is cleaved into a hexagonal prism, the 120^{∘} hinges support gapless hinge states, with their filling quantized to be 2/3. This quantization of the filling comes from a topological origin. We find that the quantized value of the filling depends on the fundamental blocks that constitute the crystal. The apatite consists of the triangular blocks, which is crucial for giving nontrivial fractional charge at the hinge.

Nitride semiconductors are attractive for various applications. Here a computational screening study identifies a number of possible ternary nitrides as promising candidates, including a phase that the authors synthesize for the first time via a high-pressure route.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Preparation of ordered CoPt on Si substrates is significant for expanding future applications of spintronic devices. In this study, ordered CoPt alloys including the L 1 _0 phase with a maximum coercivity of 2.1 kOe are formed in electron-beam-deposited 11.4 nm thick Pt/Co bilayer thin films on Si/SiO _2 substrates via interdiffusion during rapid thermal annealing (RTA). The effects of RTA temperature on the magnetic properties, crystal structures, cross-sectional elemental profiles, and surface morphologies of the films are analyzed by vibrating sample magnetometer (VSM), grazing incidence x-ray diffraction (GI-XRD), energy-dispersive x-ray spectroscopy (EDX), and scanning electron microscope (SEM), respectively. For the as-deposited film, polycrystalline Pt was confirmed by uniform Debye–Scherrer rings of Pt. At 200 °C, interdiffusion between Co and Pt atoms in the film started to be observed by EDX elemental maps, and at 300 °C, alloying of Co and Pt atoms was confirmed by diffraction peaks corresponding to A 1-disordered CoPt. At 400 °C, the in-plane coercivity of the film began to increase. At 700 °C, ordered CoPt alloys were confirmed by superlattice diffraction peaks. At 800 °C, a graded film containing L 1 _0 -ordered CoPt was found to be formed and a maximum coercivity of 2.1 kOe was observed by VSM, where the easy axis of magnetization was oriented along the in-plane direction. At 900 °C, deformation of the ordered CoPt alloys was observed by GI-XRD, and the grain size of the film reached a maximum.

Electric dipole engineering is now an emerging technology for high electron-mobility transistors, ferroelectric random access memory and multiferroic devices etc. Although various studies to provide insight into dipole moment behaviour, such as phase transition, order and disorder states, have been reported, macroscopic spontaneous polarization has been mainly discussed so far. Here, visualization of the electric dipole arrangement in layered ferroelectrics Bi2SiO5 by means of combined analysis of maximum entropy charge density and electrostatic potential distribution analysis based on synchrotron radiation X-ray powder diffraction data is reported. It was found that the hierarchical dipole orders, the weak-ferroelectric and ferroelectric configurations, were observed in the Bi2O2 and the SiO3 layers, respectively, and the ferrielectric configuration was realised by the interlayer interaction. This discovery provides a new method to visualize the local polarization in ferroelectric materials.

A prototypical electron-doped iron-based superconductor $LaFeAsO_{1-x}H_x$ undergoes an antiferromagnetic (AF) phase for $x \geq 0.49$. We performed NMR measurements on $LaFeAsO_{0.4}H_{0.6}$ at 3.7 GPa to investigate the magnetic properties in the vicinity of a pressure-induced QCP. The linewidth of $~^1H$-NMR spectra broadens at low temperatures below 30 K, suggesting that the ordered spin moments remain at 3.7 GPa. The coexistence of gapped and gapless spin excitations was confirmed in the ordered state from the relaxation time $T_1$ of $~^{75}As$. The pressure-induced QCP is estimated to be 4.1 GPa from the pressure dependence of the gapped excitation.

We propose a light-emitting thin film using an amorphous oxide semiconductor (AOS) because AOS has low defect density even fabricated at room temperature. Eu-doped amorphous In-Ga-Zn-O thin films fabricated at room temperature emitted intense red emission at 614 nm. It is achieved by precise control of oxygen pressure so as to suppress oxygen-deficiency/excess-related defects and free carriers. An electronic structure model is proposed, suggesting that non-radiative process is enhanced mainly by defects near the excited states. AOS would be a promising host for a thin film phosphor applicable to flexible displays as well as to light-emitting transistors.

Polycrystalline SnS thin films were fabricated by a H2S-free process combing pulsed laser deposition at room temperature and post-deposition thermal annealing in Ar. Thermal annealing improved the crystalline quality of the SnS films and the best films were obtained by 400 °C annealing. The obtained SnS films exhibited p-type conduction with the highest Hall mobility of 28 cm2/(V ⋅ s) and the carrier densities of 1.5 × 1015 – 1.8 × 1016 cm−3. The SnS TFT exhibited p-type operation with a field effect mobility and an on-off drain current ratio of 0.4 cm2/(V ⋅ s) and 20, respectively.

Recent discoveries of topological phases realized in electronic states in solids have revealed an important role of topology, which ubiquitously appears in various materials in nature. Many well-known materials have turned out to be topological materials, and this new viewpoint of topology has opened a new horizon in material science. In this paper, we find that electrides are suitable for achieving various topological phases, including topological insulating and topological semimetal phases. In the electrides, in which electrons serve as anions, the bands occupied by the anionic electrons lie near the Fermi level, because the anionic electrons are weakly bound by the lattice. This property of the electrides is favorable for achieving band inversions needed for topological phases, and, thus, the electrides are prone to topological phases. From such a point of view, we find many topological electrides, Y_{2}C [nodal-line semimetal (NLS)], Sc_{2}C (insulator with a π Zak phase), Sr_{2}Bi (NLS), HfBr (quantum spin Hall system), and LaBr (quantum anomalous Hall insulator), by using ab initio calculation. The close relationship between the electrides and the topological materials is useful in material science in both fields.

This review shows the highlights of a 4-year-long research project supported by the Japanese Government to explore new superconducting materials and relevant functional materials. The project found several tens of new superconductors by examining ∼1000 materials, each of which was chosen by Japanese experts with a background in solid state chemistry. This review summarizes the major achievements of the project in newly found superconducting materials, and the fabrication wires and tapes of iron-based superconductors; it incorporates a list of ∼700 unsuccessful materials examined for superconductivity in the project. In addition, described are new functional materials and functionalities discovered during the project.

New 3d transition metal-based mixed-pnictide compounds, LaTM2AsN (TM = Fe, Co, and Ni) are synthesized by solid state reactions under a high pressure of 2.5 GPa. These compounds crystallize with an orthorhombic structure (space group Cmcm) containing four formula units per unit cell. The crystal structure consists of an anisotropic network of TMAs3N tetrahedra sharing As-As edges along the in-plane ac direction and N corners along the b-direction, forming a TM honeycomb lattice with a boat-shape conformation bridged by TM-N-TM linear bonds. The temperature dependences of the electrical resistivity and magnetic susceptibility indicate that these crystals are itinerant antiferromagnets exhibiting parasitic ferromagnetism with transition temperatures of 560, 260, and 410 K for TM = Fe, Co, and Ni, respectively. These compounds are expected to be parent materials for new superconductors.

Inspired by the recent demonstration that Ca_{2}N is a two-dimensional electride in which electrons near the Fermi level float between ionic layers, we have conducted a systematic search for 2D electrides by an approach combining crystal structure databases and ab initio electronic structure calculations. Starting from a set of working hypotheses, we have identified six 2D metallic electrides, of which one is nonmagnetic and five are nearly ferromagnetic ferrimagnets with a distinct spin texture. The latter five materials are the first magnetic electrides reported at ambient pressure.

The effect of electron doping by trivalent charge state rare-earth-ion (RE = La, Ce, Pr and Nd) substitutions on the superconductivity in BaFe _2 As _2 was examined using epitaxial films. Each of the RE substitutions suppressed the resistivity anomaly associated with magnetic/structural phase transitions, leading to resistivity drops and superconductivity transitions. Bulk superconductivity was observed at the maximum onset critical temperature ( T _c ^onset ) of 22.4 K for La doping and 13.4 K for Ce doping, while only broad resistivity drops were observed at 6.2 K for Pr doping and 5.8 K for Nd doping although zero resistivity and the distinct Meissner effect were not observed at least down to 2 K. The decrease in T _c ^onset with increasing the number of RE 4f electrons cannot be explained in terms of the crystalline quality or crystallographic structure parameters of BaFe _2 As _2 films. It was clarified, based on resistivity–temperature analyses, that magnetic scattering became increasingly significant in the above order of RE dopants. The negative magnetoresistance was enhanced by Ce and Pr doping, implying that the decrease in T _c originates from magnetic pair breaking by interaction of the localized 4f orbitals in the RE dopants with the itinerant Fe 3d orbitals.

While most ceramics are composed of ubiquitous elements (the ten most abundant elements within the Earth's crust), many advanced materials are based on rare elements. A 'rare-element crisis' is approaching owing to the imbalance between the limited supply of rare elements and the increasing demand. Therefore, we propose a 'ubiquitous element strategy' for materials research, which aims to apply abundant elements in a variety of innovative applications. Creation of innovative oxide materials and devices based on conventional ceramics is one specific challenge. This review describes the concept of ubiquitous element strategy and gives some highlights of our recent research on the synthesis of electronic, thermionic and structural materials using ubiquitous elements.