Controlling the thermoelectric properties of organo-metallic coordination polymers through backbone geometry.

Poly(nickel-benzene-1,2,4,5-tetrakis(thiolate)) (Ni-btt), an organometallic coordination polymer (OMCP) characterized by the coordination between benzene-1,2,4,5-tetrakis(thiolate) (btt) and Ni2+ ions, has been recognized as a promising p-type thermoelectric material. In this study, we employed a constitutional isomer based on benzene-1,2,3,4-tetrakis(thiolate) (ibtt) to generate the corresponding isomeric polymer, poly(nickel-benzene-1,2,3,4-tetrakis(thiolate)) (Ni-ibtt). Comparative analysis of Ni-ibtt and Ni-btt reveals several common infrared (IR) and Raman features attributed to their similar square-planar nickel-sulfur (Ni-S) coordination. Nevertheless, these two polymer isomers exhibit substantially different backbone geometries. Ni-btt possesses a linear backbone, whereas Ni-ibtt exhibits a more undulating, zig-zag-like structure. Consequently, Ni-ibtt demonstrates slightly higher solubility and an increased bandgap in comparison to Ni-btt. The most noteworthy dissimilarity, however, manifests in their thermoelectric properties. While Ni-btt exhibits p-type behavior, Ni-ibtt demonstrates n-type carrier characteristics. This intriguing divergence prompted further investigation into the influence of OMCP backbone geometry on the electronic structure and, particularly, the thermoelectric properties of these materials.

Inverse-Perovskite Ba3 BO (B = Si and Ge) as a High Performance Environmentally Benign Thermoelectric Material with Low Lattice Thermal Conductivity.

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 Ba3 BO (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 Ba3 BO. 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 Ba3 BO. As a consequence of high PF and low κlat , Ba3 SiO (Ba3 GeO) 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 Ba3 SiO and 1.21 for Ba3 GeO 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.

Electronic and crystal structures ofLnFeAsO1-xHx(Ln= La, Sm) studied by x-ray absorption spectroscopy, x-ray emission spectroscopy, and x-ray diffraction: II pressure dependence.

We examine electronic and crystal structures of iron-based superconductorsLnFeAsO1-xHx(Ln= La, Sm) under pressure by means of x-ray absorption spectroscopy (XAS), x-ray emission spectroscopy (XES), and x-ray diffraction. In LaFeAsO the pre-edge peak on high-resolution XAS at the Fe-Kabsorption edge gains in intensity on the application of pressure up to 5.7 GPa and it saturates in the higher pressure region. We found integrated-absolute difference values on XES forLn= La, corresponding to a spin state, decline on the application of pressure, and then it is minimized when theTcapproaches the maximum at around 5 GPa. In contrast, such the optimum value was not detected forLn= Sm. We reveal that the superconductivity is closely related to the lower spin state forLn= La unlike Sm case. We observed that As height from the Fe basal plane and As-Fe-As angle on the FeAs4tetrahedron forLn= La deviate from the optimum values of the regular tetrahedron in superconducting (SC) phase, which has been widely accepted structural guide to SC thus far. In contrast, the structural parameters were held near the optimum values up to ∼15 GPa forLn= Sm.

Electronic and crystal structures ofLnFeAsO1-xHx(Ln= La, Sm) studied by x-ray absorption spectroscopy, x-ray emission spectroscopy, and x-ray diffraction (part I: carrier-doping dependence).

A carrier doping by a hydrogen substitution in LaFeAsO1-xHxis known to cause two superconducting (SC) domes with the magnetic order at both end sides of the doping. In contrast, SmFeAsO1-xHxhas a similar phase diagram but shows single SC dome. Here, we investigated the electronic and crystal structures for iron oxynitrideLnFeAsO1-xHx(Ln= La, Sm) with the range ofx= 0-0.5 by using x-ray absorption spectroscopy, x-ray emission spectroscopy, and x-ray diffraction. For both compounds, we observed that the pre-edge peaks of x-ray absorption spectra near the Fe-Kedge were reduced in intensity on doping. The character arises from the weaker As-Fe hybridization with the longer As-Fe distance in the higher doped region. We can reproduce the spectra near the Fe-Kedge according to the Anderson impurity model with realistic valence structures using the local-density approximation (LDA) plus dynamical mean-field theory (DMFT). ForLn= Sm, the integrated-absolute difference (IAD) analysis from x-ray Fe-Kßemission spectra increases significantly. This is attributed to the enhancement of magnetic moment of Fe 3delectrons stemming from the localized picture in the higher doped region. A theoretical simulation implementing the self-consistent vertex-correction method reveals that the single dome superconducting phase forLn= Sm arises from a better nesting condition in comparison withLn= La.

Ship-in-a-Bottle Synthesis of High Concentration of N2 Molecules in a Cage-Structured Electride.

We report the formation of neutral nitrogen molecules in the cages of [Ca12Al14O32]2+ (C12A7) framework compensated by extra-framework anions. NH3 treatment of C12A7 electride (C12A7:e-) at 800 °C leads to the formation of N2 and NH2- species in the C12A7 cages. N2 and NHx species in the cages are identified using the Raman spectroscopy of 14NH3 and 15NH3-treated C12A7:e-. The concentration of H and N in the C12A7 cages after NH3 treatment is ∼1021 cm-3. We propose a two-step mechanism, supported by density functional theory (DFT) modeling, of N2 incorporation into the C12A7 cages, i.e., incorporation of NH2- formed from decomposition of NH3 at C12A7:e- surface followed by the NH2- species reacting to form N2 molecules. Encapsulation of neutral molecules, as opposed to negatively charged species reported in C12A7 previously, offers new opportunities for trapping and storing gaseous substances in nanoporous materials.

Synthesis and Photoluminescence Properties of Rare-Earth-Activated Sr3-xAxAlO4H (A = Ca, Ba; x = 0, 1): New Members of Aluminate Oxyhydrides.

A series of aluminate-based oxyhydrides, Sr3-xAxAlO4H (A = Ca, Ba; x = 0, 1), has been synthesized by high-temperature reaction of oxide and hydride precursors under a H2 atmosphere. Their crystal structures determined via X-ray and neutron powder diffraction are isostructural with tetragonal Sr3AlO4F (space group I4/mcm), consisting of (Sr1-x/3Ax/3)2H layers and isolated AlO4 tetrahedra. Rietveld refinement based on the diffraction patterns and bond-valence-sum analysis show that Ba preferentially occupies the 10-coordinated Sr1 sites, while Ca strongly prefers to occupy the 8-coordinated Sr2 sites. Luminescence owing to the 4f-5d transition of Eu2+ or Ce3+ was observed from Eu- and Ce-doped samples, Sr3-x-yAxByAlO4H (A = Ca, Ba; B = Eu, Ce; x = 0, 1, y = 0.02), under excitation of near-ultraviolet light. Compared with its fluoride analogue, Sr3AlO4H:Ce3+ shows red shifts of both the excitation and emission bands, which is consistent with the reported hydride-based phosphors and can be explained by the covalency of the hydride ligands. The observed luminescence spectra can be decomposed into two sets of sub-bands corresponding to Ce3+ centers occupying Sr1 and Sr2 sites with distinctly different Stokes shifts (1.27 and 0.54 eV, respectively), as suggested by the results of constrained density functional theory (cDFT). The cDFT results also suggest that the large shift for Ce3+ at Sr1 is induced by large distortion of the coordinated structure with shortening of the H-Ce bond in the excited state. The current findings expand the class of oxyhydride materials and show the potential of hydride-based phosphors for optical applications.

Toward 2D Magnets in the (MnBi2 Te4 )(Bi2 Te3 )n Bulk Crystal.

2D magnets and their engineered magnetic heterostructures are intriguing materials for both fundamental physics and application prospects. On the basis of the recently discovered intrinsic magnetic topological insulators (MnBi2 Te4 )(Bi2 Te3 )n , here, a new type of magnet, in which the magnetic layers are separated by a large number of non-magnetic layers and become magnetically independent, is proposed. This magnet is named as a single-layer magnet, regarding the vanishing interlayer exchange coupling. Theoretical calculations and magnetization measurements indicate that, the decoupling of the magnetic layers starts to emerge from n = 2 and 3, as revealed by a unique slow-relaxation behavior below a ferromagnetic-type transition at Tc = 12-14 K. Magnetization data analysis shows that the proposed new magnetic states have a strong uniaxial anisotropy along the c-axis, forming an Ising-type magnetic structure, where Tc is the ordering temperature for each magnetic layer. The characteristic slow relaxation, which exists only along the c-axis but is absent along the ab plane, can be ascribed to interlayer coherent spin rotation and/or intralayer domain wall movement. The present results will stimulate further theoretical and experimental investigations for the prototypical magnetic structures, and their combination with the topological surface states may lead to exotic physical properties.

Low-Temperature Synthesis of Perovskite Oxynitride-Hydrides as Ammonia Synthesis Catalysts.

Mixed anionic materials such as oxyhydrides and oxynitrides have recently attracted significant attention due to their unique properties, such as fast hydride ion conduction, enhanced ferroelectrics, and catalytic activity. However, high temperature (≥800 °C) and/or complicated processes are required for the synthesis of these compounds. Here we report that a novel perovskite oxynitride-hydride, BaCeO3-xNyHz, can be directly synthesized by the reaction of CeO2 with Ba(NH2)2 at low temperatures (300-600 °C). BaCeO3-xNyHz, with and without transition metal nanoparticles, functions as an efficient catalyst for ammonia synthesis through the lattice N3- and H- ion-mediated Mars-van Krevelen mechanism, while ammonia synthesis occurs over conventional catalysts through a Langmuir-Hinshelwood mechanism with high energy barriers (85-121 kJ mol-1). As a consequence, the unique reaction mechanism leads to enhancement of the activity of BaCeO3-based catalysts by a factor of 8-218 and lowers the activation energy (46-62 kJ mol-1) for ammonia synthesis. Furthermore, isotopic experiments reveal that this catalyst shifts the rate-determining step for ammonia synthesis from N2 dissociation to N-H bond formation.

Nephelauxetic effect of the hydride ligand in Sr2LiSiO4H as a host material for rare-earth-activated phosphors.

Strontium lithium orthosilicate hydride Sr2LiSiO4H was synthesized by the reaction of Sr2SiO4 with LiH at 700 °C in a H2 rich atmosphere. Rietveld refinement of the neutron powder diffraction pattern revealed that Sr2LiSiO4H is isostructural to Sr2LiSiO4F (space group P21/m) and its channel-like structure preferentially accommodates H- ions over F- ions. In addition, Sr2LiSiO4H is stable in air and its Eu2+-doped analog exhibits yellow photoluminescence with an emission band at 544 nm and a broad excitation band ranging from 250 to 450 nm. These bands were observed in the longer wavelength region when compared with those displayed by Sr2LiSiO4F:Eu2+. The red shift, which is induced by H- substitution, is consistent with the constrained density functional theory calculations, predicting the photo-excitation and emission energies of 4f-5d transitions. The present study reports the synthesis of stable oxyhydrides acting as phosphor hosts for rare earth ions. The phosphor hosts exhibit large nephelauxetic effects owing to the presence of H- ligands.

Improved color uniformity in white light-emitting diodes using newly developed phosphors.

We report novel white light-emitting diode (WLED) devices that improve emission color uniformity. The WLEDs consist of a violet chip and a mixed-phosphor layer of three phosphors previously developed by us. It is found that each phosphor does not reabsorb the luminescence from the other phosphors; consequently, the emission color of the WLEDs does not get affected by the mounted quantity of phosphors and/or the variation in chip emission wavelength. Furthermore, an encapsulated WLED with a hemispherical dome-shaped mixed-phosphor layer enables an area to be irradiated with uniform color, producing an excellent color rendering index and improved luminous flux because of the reduced inelastic scattering loss in the phosphor layer.

Phase transition in CaFeAsH: bridging 1111 and 122 iron-based superconductors.

Iron-based superconductors can be categorized into two types of parent compounds by considering the nature of their temperature-induced phase transitions; namely, first order transitions for 122- and 11-type compounds and second-order transitions for 1111-type compounds. This work examines the structural and magnetic transitions (ST and MT) of CaFeAsH by specific heat, X-ray diffraction, neutron diffraction, and electrical resistivity measurements. Heat capacity measurements revealed a second-order phase transition that accompanies an apparent single peak at 96 K. However, a clear ST from the tetragonal to orthorhombic phase and an MT from the paramagnetic to the antiferromagnetic phase were detected. The structural (Ts) and Néel temperatures (TN) were respectively determined to be 95(2) and 96 K by X-ray and neutron diffraction and resistivity measurements. This small temperature difference, Ts-TN, was attributed to strong magnetic coupling in the inter-layer direction owing to CaFeAsH having the shortest lattice constant c among parent 1111-type iron arsenides. Considering that a first-order transition takes place in 11- and 122-type compounds with a short inter-layer distance, we conclude that the nature of the ST and MT in CaFeAsH is intermediate in character, between the second-order transition for 1111-type compounds and the first-order transition for other 11- and 122-type compounds.

Rattling of Oxygen Ions in a Sub-Nanometer-Sized Cage Converts Terahertz Radiation to Visible Light.

A simple and robust approach to visualization of continuous wave terahertz (CW-THz) light would open up opportunities to couple physical phenomena that occur at fundamentally different energy scales. Here we demonstrate how nanoscale cages of Ca12Al14O33 crystal enable conversion of CW-THz radiation to visible light. These crystallographic cages are partially occupied with weakly bonded oxygen ions and give rise to a narrow conduction band that can be populated with localized, yet mobile electrons. CW-THz light excites a nearly stand-alone rattling motion of the encaged oxygen species, which promotes electron transfer from them to the neighboring vacant cages. When the power of CW-THz light reaches tens of watts, the coupling between forced rattling in the confined space, electronic excitation and ionization of oxygen species, and corresponding recombination processes result in emission of bright visible light.

Nanocomposite Phosphor Consisting of CaI2:Eu2+ Single Nanocrystals Embedded in Crystalline SiO2.

High luminescence efficiency is obtained in halide- and chalcogenide-based phosphors, but they are impractical because of their poor chemical durability. Here we report a halide-based nanocomposite phosphor with excellent luminescence efficiency and sufficient durability for practical use. Our approach was to disperse luminescent single nanocrystals of CaI2:Eu2+ in a chemically stable, translucent crystalline SiO2 matrix. Using this approach, we successfully prepared a nanocomposite phosphor by means of self-organization through a simple solid-state reaction. Single nanocrystals of 6H polytype (thr notation) CaI2:Eu2+ with diameters of about 50 nm could be generated not only in a SiO2 amorphous powder but also in a SiO2 glass plate. The nanocomposite phosphor formed upon solidification of molten CaI2 left behind in the crystalline SiO2 that formed from the amorphous SiO2 under the influence of a CaI2 flux effect. The resulting nanocomposite phosphor emitted brilliant blue luminescence with an internal quantum efficiency up to 98% upon 407 nm violet excitation. We used cathodoluminescence microscopy, scanning transmission electron microscopy, and Rietveld refinement of the X-ray diffraction patterns to confirm that the blue luminescence was generated only by the CaI2:Eu2+ single nanocrystals. The phosphor was chemically durable because the luminescence sites were embedded in the crystalline SiO2 matrix. The phosphor is suitable for use in near-ultraviolet light-emitting diodes. The concept for this nanocomposite phosphor can be expected to be effective for improvements in the practicality of poorly durable materials such as halides and chalcogenides.

The Unique Electronic Structure of Mg2 Si: Shaping the Conduction Bands of Semiconductors with Multicenter Bonding.

The electronic structures of the antifluorite-type compound Mg2 Si is described in which a sublattice of short cation-cation contacts creates a very low conduction band minimum. Since Mg2 Si shows n-type conductivity without intentional carrier doping, the present result indicates that the cage defined by the cations plays critical roles in carrier transport similar to those of inorganic electrides, such as 12 CaO⋅7 Al2 O3 :e- and Ca2 N. A distinct difference in the location of conduction band minimum between Mg2 Si and the isostructural phase Na2 S is explained in terms of factors such as the differing interaction strengths of the Si/S 3s orbitals with the cation levels, with the more core-like character of the S 3s leading to a relatively low conduction band energy at the Γ point. Based on these results and previous research on electrides, approaches can be devised to control the energy levels of cation sublattices in semiconductors.

Large-moment antiferromagnetic order in overdoped high-Tc superconductor 154SmFeAsO1-x D x.

In iron-based superconductors, high critical temperature (Tc) superconductivity over 50 K has only been accomplished in electron-doped hREFeAsO (hRE is heavy rare earth (RE) element). Although hREFeAsO has the highest bulk Tc (58 K), progress in understanding its physical properties has been relatively slow due to difficulties in achieving high-concentration electron doping and carrying out neutron experiments. Here, we present a systematic neutron powder diffraction study of 154SmFeAsO1-x D x , and the discovery of a long-range antiferromagnetic ordering with x ≥ 0.56 (AFM2) accompanying a structural transition from tetragonal to orthorhombic. Surprisingly, the Fe magnetic moment in AFM2 reaches a magnitude of 2.73 µB/Fe, which is the largest in all nondoped iron pnictides and chalcogenides. Theoretical calculations suggest that the AFM2 phase originates in kinetic frustration of the Fe-3dxy orbital, in which the nearest-neighbor hopping parameter becomes zero. The unique phase diagram, i.e., highest-Tc superconducting phase adjacent to the strongly correlated phase in electron-overdoped regime, yields important clues to the unconventional origins of superconductivity.

Ferrimagnetic Cage Framework in Ca12Fe10Si4O32Cl6.

The positively charged cage framework of the natural mineral mayenite, which enables various species with negative charge to be stabilized, is one of the key structures to provide the new functionalities exploited in applications. Here we report the structural and magnetic properties of recently found eltyubyuite, Ca12Fe10Si4O32Cl6, which is the first compound bearing a transition metal oxide as a main constituent in the mayenite-type structure. From neutron powder diffraction measurements at T = 20 K and the low temperature Mössbauer measurement, we determined the magnetic structure of eltyubyuite to be a ferrimagnet with oppositely aligned magnetic moments of +3.17(3) and -3.05(8) µB in two tetrahedral Fe sites with different oxygen ligands, all bridging oxygens or mixed bridging and nonbridging oxygens. As far as is known, this result is likely to be a first example showing ferrimagnetism stemming from only tetrahedral Fe3+ ions. The reduced magnetic moment per Fe3+ and the resultant small net moment per unit cell of 22 µB at µ0H = 5 T and T = 15 K are attributed to strong covalency in much shorter Fe-O bonds in the FeO4 tetrahedra.

Pressure effect on iron-based superconductor LaFeAsO1-xHx: Peculiar response of 1111-type structure.

A systematic study of the crystal structure of a layered iron oxypnictide LaFeAsO1-xHx as a function of pressure was performed using synchrotron X-ray diffraction. This compound exhibits a unique phase diagram of two superconducting phases and two parent phases. We established that the As-Fe-As angle of the FeAs4 tetrahedron widens on the application of pressure due to the interspace between the layers being nearly infilled by the large La and As atoms. Such rarely observed behaviour in iron pnictides implies that the FeAs4 coordination deviates from the regular tetrahedron in the present systems. This breaks a widely accepted structural guide that the superconductivity favours the regular tetrahedron, albeit the superconducting transition temperature (Tc) increases from 18 K at ambient pressure to 52 K at 6 GPa for x = 0.2. In the phase diagram, the second parent phase at x ~ 0.5 is suppressed by pressure as low as ~1.5 GPa in contrast to the first parent phase at x ~ 0, which is robust against pressure. We suggest that certain spin-fluctuation from the second parent phase is strongly related to high-Tc under pressure.

Hydride-Based Electride Material, LnH2 (Ln = La, Ce, or Y).

In view of the strong electron-donating nature of H(-) and extensive vacancy formation in metals by hydrogen insertion, a series of LnH2+x (Ln = La, Ce, or Y) compounds with fluorite-type structures were verified to be the first hydride-based electride, where itinerant electrons populating the cage are surrounded by H(-) anions. The electron transfer into the cage probably originates from Ln-cage covalent interaction. To the best of our knowledge, anion-rich electrides are extremely rare, and a key requirement for their formation is that the cage site is not occupied by lone pair electrons of the adjacent ions. In the case of LnH2, the cage site is surrounded by eight H(-) anions with isotopic electronic character caused by the lack of mixing of H p-orbital character. Notably, Ru-loaded LnH2+x electride powders synthesized by hydrogen embrittlement (Ln = La or Ce) were found to work as efficient catalysts for ammonia synthesis at ambient pressure, without showing serious signs of hydrogen poisoning. There are several possible origins of the observed high catalytic activity in the hydride promotors: the small work function of LnH2+x derived from the covalent interaction between Ln cation and the H(-) σ donor, and the formation of Ln nitride during catalytic reaction.

Correction: Essential role of hydride ion in ruthenium-based ammonia synthesis catalysts.

[This corrects the article DOI: 10.1039/C6SC00767H.].

Essential role of hydride ion in ruthenium-based ammonia synthesis catalysts.

The efficient reduction of atmospheric nitrogen to ammonia under low pressure and temperature conditions has been a challenge in meeting the rapidly increasing demand for fertilizers and hydrogen storage. Here, we report that Ca2N:e-, a two-dimensional electride, combined with ruthenium nanoparticles (Ru/Ca2N:e-) exhibits efficient and stable catalytic activity down to 200 °C. This catalytic performance is due to [Ca2N]+·e1-x-H x- formed by a reversible reaction of an anionic electron with hydrogen (Ca2N:e- + xH ↔ [Ca2N]+·e1-x-H x-) during ammonia synthesis. The simplest hydride, CaH2, with Ru also exhibits catalytic performance comparable to Ru/Ca2N:e-. The resultant electrons in these hydrides have a low work function of 2.3 eV, which facilitates the cleavage of N2 molecules. The smooth reversible exchangeability between anionic electrons and H- ions in hydrides at low temperatures suppresses hydrogen poisoning of the Ru surfaces. The present work demonstrates the high potential of metal hydrides as efficient promoters for low-temperature ammonia synthesis.