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 superconductorsLnFeAsO_1-xH_x(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 theT_capproaches 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 LaFeAsO_1-xH_xis known to cause two superconducting (SC) domes with the magnetic order at both end sides of the doping. In contrast, SmFeAsO_1-xH_xhas a similar phase diagram but shows single SC dome. Here, we investigated the electronic and crystal structures for iron oxynitrideLnFeAsO_1-xH_x(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-Kβemission 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 NH^2- species in the C12A7 cages. N₂ and NH_x 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 NH^2- formed from decomposition of NH₃ at C12A7:e^- surface followed by the NH^2- 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, Sr_3-xA_xAlO₄H (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 (Sr_1-x/3A_x/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, Sr_3-x-yA_xB_yAlO₄H (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 T_c = 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 T_c 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, BaCeO_3-xN_yH_z, can be directly synthesized by the reaction of CeO₂ with Ba(NH₂)₂ at low temperatures (300-600 °C). BaCeO_3-xN_yH_z, with and without transition metal nanoparticles, functions as an efficient catalyst for ammonia synthesis through the lattice N^3- 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.

Nuclear magnetic resonance on $LaFeAsO_{0.4}H_{0.6}$ at 3.7 GPa

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.   Edited by: A. Goñi, A. Cantarero, J. S. Reparaz

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

Strong nephelauxetic effect on Eu²⁺ ion in Sr₂LiSiO₄H: enhancement of Eu 5d centroid shift by hydride ligand coordination.

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 (T_s) and Néel temperatures (T_N) were respectively determined to be 95(2) and 96 K by X-ray and neutron diffraction and resistivity measurements. This small temperature difference, T_s-T_N, 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.

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.

A novel red-emitting K2Ca(PO4)F:Eu2+ phosphor with a large Stokes shift

We report a K₂CaPO₄F:Eu²⁺ phosphor with a new crystal structure. This phosphor has a large Stokes shift and converts near-ultraviolet light to red luminescence without absorption of other visible light. The mechanism was elucidated by applying a constrained density functional theory to the solved crystal structure.

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.

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) μ_B 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 μ_B at μ₀H = 5 T and T = 15 K are attributed to strong covalency in much shorter Fe-O bonds in the FeO₄ tetrahedra.

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

Correction for ‘Essential role of hydride ion in ruthenium-based ammonia synthesis catalysts’ by Masaaki Kitano et al., Chem. Sci., 2016, 7, 4036-4043.

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

Ruthenium-loaded metal hydrides with hydrogen vacancies function as efficient catalysts for ammonia synthesis under low temperature and low pressure conditions.

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]⁺·e_1–x^–H_x^– formed by a reversible reaction of an anionic electron with hydrogen (Ca₂N:e^– + xH ↔ [Ca₂N]⁺·e_1–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.

Hydrogen-Substituted Superconductors SmFeAsO(1-x)Hx Misidentified As Oxygen-Deficient SmFeAsO(1-x)

We investigated the preferred electron dopants at the oxygen sites of 1111-type SmFeAsO by changing the atmospheres around the precursor with the composition of Sm:Fe:As:O = 1:1:1:1 - x in high-pressure synthesis. Under H2O and H2 atmospheres, hydrogens derived from H2O or H2 molecules were introduced into the oxygen sites as a hydride ion, and SmFeAsO(1-x)Hx was obtained. However, when the H2O and H2 sources were removed from the synthetic process, nearly stoichiometric SmFeAsO was obtained and the maximum amount of oxygen vacancies introduced remained x = 0.05(4). Density functional theory calculations indicated that substitution of hydrogen in the form of H(-) is more stable than the formation of an oxygen vacancy at the oxygen site of SmFeAsO. These results strongly imply that oxygen-deficient SmFeAsO(1-x) reported previously is SmFeAsO(1-x)Hx with hydride ion incorporated unintentionally during high-pressure synthesis.

Superconductivity in room-temperature stable electride and high-pressure phases of alkali metals

S-band metals such as alkali and alkaline earth metals do not undergo a superconducting transition (SCT) at ambient pressure, but their high-pressure phases do. By contrast, room-temperature stable electride [Ca(24)Al(28)O(64)](4+)⋅4e(-) (C12A7:e(-)) in which anionic electrons in the crystallographic sub-nanometer-size cages have high s-character exhibits SCT at 0.2-0.4 K at ambient pressure. In this paper, we report that crystal and electronic structures of C12A7:e(-) are close to those of the high-pressure superconducting phase of alkali and alkaline earth metals and the SCT of both materials is induced when electron nature at Fermi energy (EF) switches from s- to sd-hybridized state.

Comparison of impurity effect between two Tc domes in LaFeAsO(1-x)H(x)

Hydrogen-doped LaFeAsO(1-x)H(x) system has a unique 2-dome structure. The effects of Zn impurities on the first and second superconducting domes in LaFe(1-y)Zn(y)AsO(1-x)H(x) (x = 0.10 for the first superconducting phase (SC1) and 0.35 for the second superconducting phase (SC2)) are examined by electrical resistivity, Hall effect and magnetization measurements. Substitution with Zn strongly suppresses the critical temperature (Tc) and behaves as a nonmagnetic impurity. The suppression rates are: 17(4) K/% (SC1) and 6.9(5) K/% (SC2). It was considered that Zn impurities induced Anderson localization, which triggered the disappearance of the superconductivity, but no decisive conclusions on the dominant pairing symmetry for each dome were obtained.

Narrow bandgap in β-BaZn₂As₂ and its chemical origins

β-BaZn2As2 is known to be a p-type semiconductor with the layered crystal structure similar to that of LaZnAsO, leading to the expectation that β-BaZn2As2 and LaZnAsO have similar bandgaps; however, the bandgap of β-BaZn2As2 (previously reported value ~0.2 eV) is 1 order of magnitude smaller than that of LaZnAsO (1.5 eV). In this paper, the reliable bandgap value of β-BaZn2As2 is determined to be 0.23 eV from the intrinsic region of the temperature dependence of electrical conductivity. The origins of this narrow bandgap are discussed based on the chemical bonding nature probed by 6 keV hard X-ray photoemission spectroscopy, hybrid density functional calculations, and the ligand theory. One origin is the direct As-As hybridization between adjacent [ZnAs] layers, which leads to a secondary splitting of As 4p levels and raises the valence band maximum. The other is that the nonbonding Ba 5d(x(2)-y(2)) orbitals form an unexpectedly deep conduction band minimum (CBM) in β-BaZn2As2 although the CBM of LaZnAsO is formed mainly of Zn 4s. These two origins provide a quantitative explanation for the bandgap difference between β-BaZn2As2 and LaZnAsO.

Model of the electronic structure of electron-doped iron-based superconductors: evidence for enhanced spin fluctuations by diagonal electron hopping

We present a theoretical understanding of the superconducting phase diagram of the electron-doped iron pnictides. We show that, besides the Fermi surface nesting, a peculiar motion of electrons, where the next nearest neighbor (diagonal) hoppings between iron sites dominate over the nearest neighbor ones, plays an important role in the enhancement of the spin fluctuation and thus superconductivity. In the highest T(c) materials, the crossover between the Fermi surface nesting and this "prioritized diagonal motion" regime occurs smoothly with doping, while in relatively low T(c) materials, the two regimes are separated and therefore results in a double dome T(c) phase diagram.

Magnetic structure and electromagnetic properties of LnCrAsO with a ZrCuSiAs-type structure (Ln = La, Ce, Pr, and Nd)

We report the synthesis, structure, and electromagnetic properties of Cr-based layered oxyarsenides LnCrAsO (Ln = La, Ce, Pr, and Nd) with a ZrCuSiAs-type structure. All LnCrAsO samples showed metallic electronic conduction. Electron doping in LaCrAsO by Mn-substitution for the Cr sites gave rise to a metal-insulator transition. Analysis of powder neutron diffraction data revealed that LaCrAsO had G-type antiferromagnetic (AFM) ordering, i.e., a checkerboard-type AFM ordering in the CrAs plane and antiparallel spin coupling between the adjacent CrAs planes, at 300 K with a large spin moment of 1.57 μB along the c axis. The magnetic susceptibility of LaCrAsO was very small (on the order of 10(-3) emu/mol) and showed a broad hump at ∼550 K. First-principles density functional theory calculations of LaCrAsO explained its crystal structure and metallic nature well, but could not replicate the antiparallel spin coupling between the CrAs layers. The electronic structure of LaCrAsO is discussed with regard to those of related compounds LaFeAsO and LaMnAsO.

Detection of antiferromagnetic ordering in heavily doped LaFeAsO(1-x)H(x) pnictide superconductors using nuclear-magnetic-resonance techniques

We studied double superconducting (SC) domes in LaFeAsO(1-x)H(x) by using 75As and 1H nuclear-magnetic-resonance techniques and unexpectedly discovered that a new antiferromagnetic (AF) phase follows the double SC domes on further H doping, forming a symmetric alignment of AF and SC phases in the electronic phase diagram. We demonstrated that the new AF ordering originates from the nesting between electron pockets, unlike the nesting between electron and hole pockets, as seen in the majority of undoped pnictides. The new AF ordering is derived from the features common to high-Tc pnictides; however, it has not been reported so far for other high-Tc pnictides because of their poor electron doping capability.

T(c) maximum in solid solution of pyrite IrSe2-RhSe2 induced by destabilization of anion dimers

We have established a well-defined dome-shaped T(c) curve in Ir(0.94-x)Rh(x)Se(2) superconductors. The maximum T(c)(onset) of 9.6 K was obtained at x = 0.36, at which the Se-Se separation in the dimer anion is the longest. Simultaneously, the destabilization of Se-Se dimers accompanied by partial electron transfer from the Ir/Rh to the chalcogenide ions resulted in the emergence of optimal T(c).

Superconductivity in defective pyrite-type iridium chalcogenides Ir(x)Ch2 Ch = Se and Te

We report superconductivity in defective pyrite-type iridium chalcogenides Ir(x)Ch2 (Ch = Se and Te). Maximum values of T(c) of 6.4 K for Ir(0.91)Se(2) and 4.7 K for Ir(0.93)Te(2) were observed. It was found that Ir(0.75)Ch(2) (Ir(3)Ch(8)) is close to the boundary between metallic and insulating states and Ir(x)Ch(2) systems undergo nonmetal to metal transitions as x increases. On the basis of density functional theory calculations and the observed large variation in the Ch-Ch distance with x, we suggest that Ir(0.75)Ch(2) (Ir(3)Ch(8)) is the parent compound for the present superconductors.

Determination of the local structure of a cage with an oxygen ion in Ca12Al14O33

The crystal structure of mayenite (12CaO·7Al2O3) has been investigated by single-crystal synchrotron diffraction with high resolution and accuracy, using a four-circle diffractometer equipped with an avalanche photodiode detector (APD) detector installed at PF14A in Tsukuba, Japan. Analysis revealed random displacements of ions by the electrostatic force of the O2−ion (O3) clathrated in two out of 12 cages. O3 ions are located at general positions close to the \bar 4 site at the centre of each cage. The difference-density map revealed two large peaks corresponding to displaced Ca ions. The positive ions close to O3 are displaced and one-to-one correspondence was found between one of the four equivalent O3 ions and the displaced ions. When an O3 ion is present in the cage the Al ion at the 16cposition moves 0.946 (3) Å toward the O3 ion. One of the Al—O bonds is broken and a new Al—O3 bond is created. The result is an AlO4tetrahedron that is quite deformed. The three O1 ions and the O2 ion of the destroyed AlO4tetrahedron are forcibly displaced. O1 and O2 have two and one displaced ions, respectively. The local structure of the cage occupied by one of the four O3 ions was determined by calculating the electrostatic potential and electric field in the deformed cage, although the position of one of the displaced O1 ions was not clearly identified.

Coexistence of light and heavy carriers associated with superconductivity and antiferromagnetism in CeNi0.8Bi2 with a Bi square net

We found that the ZrCuSiAs-type crystal CeNi(0.8)Bi(2) with a layered structure composed of alternate stacking of [CeNi(x)Bi(1)](δ+) and Bi(2)(δ-) exhibits a superconductive transition at ∼4  K. The conductivities, magnetic susceptibilities, and heat capacities measurements indicate the presence of two types of carriers with notable different masses, i.e., a light electron responsible for superconductivity and a heavy electron interacting with the Ce 4f electron. This observation suggests that 6p electrons of Bi(2) forming the square net and electrons in CeNi(x)Bi(1) layers primarily correspond to the light and heavy electrons, respectively.

Magnetic lattice dynamics of the oxygen-free FeAs pnictides: how sensitive are phonons to magnetic ordering?

To shed light on the role of magnetism on the superconducting mechanism of the oxygen-free FeAs pnictides, we investigate the effect of magnetic ordering on phonon dynamics in the low-temperature orthorhombic parent compounds, which present a spin density wave. The study covers both the 122 (AFe(2)As(2); A = Ca, Sr, Ba) and 1111 (AFeAsF; A = Ca, Sr) phases. We extend our recent work on the Ca (122 and 1111) and Ba (122) cases by treating, computationally and experimentally, the 122 and 1111 Sr compounds. The effect of magnetic ordering is investigated through detailed non-magnetic and magnetic lattice dynamical calculations. The comparison of the experimental and calculated phonon spectra shows that the magnetic interactions/ordering have to be included in order to reproduce well the measured density of states. This highlights a spin-correlated phonon behavior which is more pronounced than the apparently weak electron-phonon coupling estimated in these materials. Furthermore, there is no noticeable difference between phonon spectra of the 122 Ba and Sr, whereas there are substantial differences when comparing these to CaFe(2)As(2) originating from different aspects of structure and bonding.

Insular superconductivity in a Co-doped iron pnictide CaFe1-xCoxAsF

The presence of a macroscopic phase separation between the superconducting and magnetic phases in CaFe1-xCoxAsF is demonstrated by muon spin rotation measurements conducted across their phase boundaries (x=0.05-0.15). The magnetic phase tends to retain the high transition temperature (Tm>Tc), while Co doping induces strong randomness. The volumetric fraction of the superconducting phase is nearly proportional to the Co content x with a constant superfluid density. These observations suggest the formation of superconducting "islands" (or domains) associated with Co ions in the Fe2As2 layers, indicating a very short coherence length.

Metallic state in a lime-alumina compound with nanoporous structure

We report a metallic state in a nanostructured porous crystal 12CaO x 7Al2O3 by incorporating electrons in the inherent subnanometer-sized cages, in which a three-dimensionally closely packed cage structure acts as an electronic conduction path. High-density electron doping ( approximately 2 x 10(21) cm(-3)), which was achieved by a thermal treatment in Ti metal vapor at approximately 1100 degrees C, induces homogenization of the cage geometry to a symmetric state, resulting in an insulator-metal transition with a sharp enhancement of the electron drift mobility from approximately 0.1 to 4 cm(2) V(-1) s(-1). The results provide an approach for the realization of electroactive functions in materials composed only of environmentally benign elements by utilizing the appropriate nanostructures.

Nanoporous Crystal 12CaO·7Al2O3: A Playground for Studies of Ultraviolet Optical Absorption of Negative Ions

A novel nanoporous material 12CaO.7Al2O3 (C12A7) offers a possibility of incorporating large concentrations (>1021 cm-3) of a wide range of extraframework anions inside its nanopores. We have investigated, both experimentally and theoretically, optical absorption associated with several types of such anions, including F-, OH-, O-, O2-, O2-, and O22-, and assigned their optical absorption bands. It is demonstrated that the chemical identity and concentration of extraframework anions can be controlled by an appropriate treatment of "as grown" C12A7. We also show that the position of the adsorption edge is, in turn, determined by the chemical identity of the extraframework species and can be varied in the range of approximately 4-6 eV. We suggest that C12A7 is a unique host material, which can be used as a playground for studying negatively charged species that are unstable in other environments.

Elucidation of Codoping Effects on the Solubility Enhancement of Er3+ in SiO2 Glass: Striking Difference between Al and P Codoping

The codoping effect mechanism of Al and P on the solubility enhancement of Er(3+) ion in SiO(2) glass was clarified by electron spin-echo envelope modulation spectroscopy. It turned out that doped P ions preferentially coordinate to the Er(3+) ion to form a "solvation shell structure", and the environment is similar to that in phosphate glass, while doped Al ions do not form such a selective solvation structure, taking octahedral coordination. This striking difference indicates that the primary roles of the P-doping and the Al-doping are attributed to "enthalpy of mixing" and to "entropy of mixing", respectively.

Hydride Ion as Photoelectron Donor in Microporous Crystal

Atomic hydrogen (H0) and trapped electrons generated by UV illumination (lambda approximately 330 nm) at 4 K were observed using electron paramagnetic resonance (EPR) in a 12CaO.7Al2O3 (C12A7) crystal heated in a hydrogen atmosphere. The concentration ratio of generated H0 to the electrons encaged in the subnanometer-sized cages of C12A7 (F+ centers) is almost 1:1, providing direct evidence that a hydride ion, H-, accommodated in the cage by the heat treatment was dissociated to a pair of an H0 and an electron by a UV photon: H- --> H0 + e- (F+). After annealing at 300 K, H0 was completely annihilated, while approximately 60% of the trapped electrons survived. The remaining electrons can hop between neighboring cages and give electrical conductivity to C12A7. The hyperfine splitting of the EPR spectrum of H0 in C12A7 (48.6 mT) is 4% smaller than that of the neutral hydrogen atom (50.6 mT), implying that H0 is trapped at the interstitial sites among the cages.