Electron Localization Induced by Disordered Anions in an Organic Conductor.

We report on a new organic conductor κ″-(ET)2Cu[N(CN)2]Br (κ″-Br), which is the first polymorph of an organic superconductor κ-(ET)2Cu[N(CN)2]Br (κ-Br), where ET denotes bis(ethylenedithio)tetrathiafulvalene. κ″-Br has a similar κ-type arrangement of ET molecules to κ-Br, but, in contrast to the orthorhombic κ-Br, which has ordered polyanion chains, presents a monoclinic crystal structure with disordered polymeric anion chains. To elucidate the electronic state of κ″-Br, we performed band calculations as well as transport, magnetic, and optical measurements. The calculated band dispersion, magnitude of electron correlation, and room-temperature optical conductivity spectra of κ″-Br were comparable to those of κ-Br. Despite these similarities, the κ″-Br salt exhibited a semiconducting behavior. The electron spin resonance and Raman spectroscopies indicated that there is neither magnetic nor charge order in κ″-Br, suggesting the occurrence of Anderson localization due to disordered anion layers.

Molybdenum-Ruthenium-Carbon Solid-Solution Alloy Nanoparticles: Can They Be Pseudo-Technetium Carbide?

Technetium (Tc), atomic number 43, is an element that humans cannot freely use even in the 21st century because Tc is radioactive and has no stable isotope. In this report, we present molybdenum-ruthenium-carbon solid-solution alloy (MoxRu1-xCy) nanoparticles (NPs) that are expected to have an electronic structure similar to that of technetium carbide (TcCy). MoxRu1-xCy NPs were synthesized by annealing under a helium/hydrogen atmosphere following thermal decomposition of metal precursors. The obtained NPs had a solid-solution structure in the whole composition range. MoxRu1-xCy with a cubic structure (down to 30 atom % Mo in the metal ratio) showed a superconducting state, and the transition temperature (Tc) increased with increasing Mo composition. The continuous change in Tc across that of TcCy indicates the continuous control of the electronic structure by solid-solution alloying, leading to pseudo-TcCy. Density functional theory calculations indicated that the synthesized Mo0.53Ru0.47C0.41 has a similar electronic structure to TcC0.41.

High-Performance All-Solid-State Proton Rectifier Using a Heterogeneous Membrane Composed of Coordination Polymer and Layered Double Hydroxide.

Rational control of unidirectional proton transport is highly challenging, primarily owing to the difficulty in introducing an asymmetric factor into proton conducting media. In this study, free-standing membranes of a proton-conducting two-dimensional porous coordination polymer, Cu2 (CuTCPP) (H2 TCPP: 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin) and a hydroxide ion-conducting layered double hydroxide, Mg-Al-LDH(NO3 ), were combined to generate a pH gradient in the conducting media. The current-voltage measurements revealed that the heterogeneous membrane exhibits a significant unidirectional proton transport with a proton rectification ratio exceeding 200 under 90 % relative humidity in the initial voltage scan. This value is the highest among the reported all-solid-state proton rectifiers. The high designability of both components with well-defined structures, which is in contrast to the organic polymers used so far, provides a new avenue for developing and understanding the proton-rectifying behavior in the solid state.

Rational Construction of Molecular Electron-Conducting Nanowires Encapsulated in a Proton-Conducting Matrix in a Charge Transfer Salt.

Insulated molecular wires have gained significant attention owing to their potential contribution in the fields of nanoelectronics and low-dimensional chemistry/physics. Based on molecular charge transfer salts, we demonstrate, for the first time, the rational construction of molecular electron-conducting wires encapsulated in a proton-conducting matrix, which possibly paves the way to ionoelectronics. As expected from the molecular structure of the newly designed complex anion (i.e., propeller-shaped structure with hydrogen-bonding sites at four edges), a three-dimensional hydrogen-bonded framework was constructed within the crystal, which contains a one-dimensional array of an electron donor, tetrathiafulvalene (TTF). From the single-crystal crystallographic and spectroscopic studies, it was clarified that the nonstoichiometric deprotonation of anions and partial oxidation of TTFs occur, whereas the anion is electronically inert. Moderate conductivities of electron and proton were confirmed by dc and ac conductivity measurements. In addition, the electronic isolation of TTF wires was confirmed by the magnetic susceptibility data.

Systematic Tuning of the Magnetic Properties in Mixed-Metal MOF-74.

Recently, mixed-metal metal-organic frameworks (MOFs) have been attracting much attention in various fields. In this study, we have systematically investigated the magnetic properties of CoxNi1-x-MOF-74 [Co2xNi2(1-x)(dhtp), where H4dhtp = 2,5-dihydroxyterephthalic acid] with two different kinds of metals (Co and Ni) across the composition range 0 ≤ x ≤ 1. Bimetallic CoxNi1-x-MOF-74 (x = 0.752, 0.458, and 0.233) were successfully synthesized and confirmed to have homogeneous metal distributions. Weak ferromagnetic (canted antiferromagnetic) behavior was exhibited, while homometallic Co-MOF-74 and Ni-MOF-74 are antiferromagnetic. We also investigated the effects of C2H4 sorption on the magnetic properties and found that C2H4-adsorbed Co0.5Ni0.5-MOF-74 exhibited a change in the interchain magnetic interaction.

An Approach to an Ideal Molecule-Based Mixed Conductor with Comparable Proton and Electron Conductivity.

We synthesized a molecule-based proton-electron mixed conductor (PEMC), a Pt(III) dithiolate complex with 1,4-naphthoquinone skeletons. The π-planar Pt complex involves a π-stacking column, which is connected by one-dimensional hydrogen bonding chains composed of water molecules. The room-temperature (RT) proton conductivity is 8.0 × 10-5 S cm-1 under ambient conditions, which is >2 orders of magnitude higher than that of the isomorphous Ni complex (7.2 × 10-7 S cm-1). The smaller activation energy (0.23 eV) compared to that of the Ni complex (0.42 eV) possibly originates from the less dense water, which promotes the reorientational dynamics, in the Pt complex with an expanded lattice, namely, negative chemical pressure upon substitution of Ni with the larger Pt. In addition, the Pt complex shows a relatively high RT electronic conductivity of 1.0 × 10-3 S cm-1 caused by the π-columns, approaching an ideal PEMC with comparable proton and electron conduction.

Ni@onion-like carbon and Co@amorphous carbon: control of carbon structures by metal ion species in MOFs.

We first report the facile synthesis of metal-carbon composites consisting of metal nanoparticles (NPs) and different types of carbon species: onion-like and amorphous carbon, Ni@onion-like carbon and Co@amorphous carbon. By simply changing the metal species in an isostructural metal-organic framework, thermal decompositions of MOF-74 directly afforded different types of metal NPs and carbon composites, which exhibited good electrical conductivity. In particular, the Ni@onion-like carbon, having a well-ordered carbon structure, had high electrical conductivity (σ = 5.3 Ω-1 cm-1 at 295 K), explained by a modified model of the Efros-Shklovskii variable range hopping.

Heavy Hydrogen Doping into ZnO and the H/D Isotope Effect.

Hydrogen (H) can drastically change the physical properties of solids by the doping of host materials with minimum perturbation to the lattice because of its small size, quantum nature, and a variety of charged states from -1 (hydride, H-) to +1 (proton, H+). While the H-doping amount is limited under equilibrium conditions, H2+ ion irradiation at low temperature is a promising method for introducing a large amount of hydrogen into any material. Although the application of this method offers the potential for exploring unforeseen fascinating properties, the effects of nonequilibrium H doping at very low temperature below 10 K are largely underexplored and are not well understood. In this article, we report heavy H (D) doping into ZnO films by H2+ (D2+) irradiation at 7 K, which resulted in metallic conductivity and an isotope effect on the conductivity at 7 K. The H/D isotope effect is attributable to metastable H (D) trapping sites generated by the effect of irradiation. The isotope effect is decreased at low acceleration voltage. Furthermore, the subsequent thermal excursion induces a large irreversible decrease in resistivity, indicating the migration of H (D) from metastable trapping sites upon heating. This work provides a new strategy to control the physical properties of materials and to investigate the H (D) migration occurring with increasing temperature after excess H doping at very low temperature.

Surface morphology-induced spin-crossover-inactive high-spin state in a coordination framework.

Here we report a surface morphology-induced spin state control in ultrathin films of a spin-crossover (SCO) material. The surface microstructure of film domains exhibited selectivity, to stabilize the SCO-active high-spin (HS) or SCO-inactive high-spin (HS2) states. To date, the latter has only been confirmed in the bulk counterpart at gigapascal pressure.

Crystal Size Effect on the Spin-Crossover Behavior of {Fe(py)2[Pt(CN)4]} (py = Pyridine) Monitored by Raman Spectroscopy.

The spin-crossover phenomenon in nanomaterials has been the subject of exploratory investigations for stimuli-responsive switching properties at nanoscale. Using variable-temperature Raman spectroscopy, we investigated changes in the temperature-driven spin-transition property of {Fe(py)2[Pt(CN)4]} (py = pyridine) induced by a size reduction from a bulk polycrystalline powder to an ultrathin film (crystallite size, 15 nm). When the crystallite size was reduced, the spin-transition temperature shifted lower and the spin-transition curves became less steep. In the thin-film form, the residual high-spin fraction was found to be about 20%.

Triangular-Lattice Organic Mott Insulator with a Disorder-Free Polyanion.

We report the synthesis, crystal and band structures, and transport properties of organic conductor κ-(ET)2Cu[Au(CN)2]Cl [ET = bis(ethylenedithio)tetrathiafulvalene], which has a triangular spin-lattice (S = 1/2) composed of (ET)2•+ dimers and polyanions with no disorder. The anisotropy of triangular lattice t'/t = 1.19 and physical properties indicate that this material is the first ET-based quantum-spin-liquid candidate having a nearly regular triangular lattice with a disorder-free anion.

Bis(ethylenedithio)tetrathiafulvalene Cation Radical Salts Composed of Nonuniform Silver(I) Complex Polyanions.

Rational control of the molecular arrangement in solids has been the subject of intense research for many years. In particular, the structural control of bis(ethylenedithio)tetrathiafulvalene (ET) radical cations has attracted special interest because of the primary effect on the electronic properties of the salts. In this study, we obtained the first ET cation radical salts formed with nonuniform silver(I) complex polyanions, which involve multiple kinds of openings in the anionic layer, by an electrocrystallization method. θ-(ET)2Ag2(CN)[N(CN)2]2 (1) with a θ-type ET packing motif contains double helical chains composed of AgN(CN)2, whereas α″-(ET)2Ag2(CN)(SCN)2 (2) with an α″-type ET packing motif contains zigzag ladders composed of AgSCN. Both silver(I)-based tube-like assemblies are connected to each other by a cyano group, affording nonuniform polyanionic structures. Although both salts show semiconducting behavior, there is a distinct difference in their spin geometry, with an S = 1/2 Heisenberg antiferromagnetic square lattice in 1, which is associated with charge disproportionation or dynamical charge fluctuation in the ET layers, and an S = 1/2 Heisenberg anisotropic triangular lattice in 2, which results in spin frustration in the ET layers. The ability of the nonuniform polymeric structures in the anionic layers to act as templates for various arrangements of ET radical cations is demonstrated.

Conducting Coronene Cation Radical Salt Containing Magnetic Metal Ions.

Coronene is the smallest homologue of benzene and is the smallest fragment of graphene among 6-fold symmetric polycyclic aromatic hydrocarbons. In this study, we obtained the first coronene cation radical solid containing magnetic counterions by an electrochemical method. Coronene monocations in the 1:1 salt, (coronene•+)(FeBr4-), assemble in a stacking manner via π-π interactions, which lead to a rather high room-temperature conductivity of 0.6 S cm-1. The salt shows semiconducting behavior as expected from the calculated band structure, and activation energies were estimated to be 0.25 eV at T ≥ 220 K and 0.18 eV at T ≤ 220 K. The magnetic susceptibility follows the Curie-Weiss law down to about 30 K, with a Curie constant (4.47 emu K mol-1) expected for S = 5/2 spins of iron(III) ions and a high Weiss temperature (-32.2 K). Upon further cooling, the salt exhibits a susceptibility kink at 16.2 K followed by the loss of a significant fraction of the susceptibility due to long-range antiferromagnetic ordering. Theoretical calculations predicted that the indirect π-d magnetic exchange interaction through C-H···Br hydrogen bonds is equal to Jπd = -3.10 K. Although the absolute value is lower than that of the direct d-d magnetic exchange interaction between the FeBr4- anions (Jdd = -13.35 K), it is evident that the π-d interactions play a certain role in determining the magnetic behavior. Considering that an isomorphous salt, (coronene•+)(GaBr4-) involving a nonmagnetic counterpart GaBr4-, exhibits singlet-triplet magnetic behavior with a spin gap of 1.44 × 103 K, it is most likely that in (coronene)(FeBr4) the nonmagnetic π-electrons serve as mediators of the magnetic ordering of d-spins through the π-d interactions.

Partial Substitution of Ag(I) for Cu(I) in Quantum Spin Liquid κ-(ET)2Cu2(CN)3, Where ET Is Bis(ethylenedithio)tetrathiafulvalene.

Three mixed crystals, κ-(ET)2Ag2 xCu2(1- x)(CN)3 [ET is bis(ethylenedithio)tetrathiafulvalene; 0.24 < x < 0.71] with a κ-type packing motif of face-to-face ET dimers, were obtained by electrocrystallization. Regardless of the composition, each ET dimer fits into a hexagonal anionic opening (i.e., key-on-hole packing) similar to its parent spin liquid candidate, κ-(ET)2Cu2(CN)3. X-ray diffraction and energy dispersive spectroscopy analyses revealed that Cu and Ag atoms are statistically disordered with a fairly homogeneous distribution in a crystal. A structural variation depending on x is responsible for the change in the calculated band parameters related to intermolecular interactions, electron correlations, and frustrations. A salt with nearly equimolar amounts of Ag and Cu ( x = 0.49) is semiconductive at ambient pressure and undergoes a Mott transition upon application of hydrostatic pressure. Along with the positive pressure dependence of the transition temperature, the temperature-independent amplitude of magnetic torque at low temperatures suggests that the insulating phase is a quantum spin liquid. Further application of pressure results in the appearance of a superconducting phase. Contrary to those of the parent salts, κ-(ET)2Cu2(CN)3 and κ-(ET)2Ag2(CN)3, the transition temperature increases as the pressure increases and eventually reaches 4.5 K at 1.65 GPa.

Heavy interstitial hydrogen doping into SrTiO3.

We achieve the introduction of an extremly large amount of interstitial hydrogen into strontium titanate (SrTiO3) by low-temperature hydrogen ion beam irradiation. The in situ transport measurements reveal an unprecedented thermal hysteresis of the resistivity.

The Electronic State of Hydrogen in the α†Phase of the Hydrogen-Storage Material PdH(D)x : Does a Chemical Bond Between Palladium and Hydrogen Exist?

The palladium-hydrogen system is one of the most famous hydrogen-storage systems. Although there has been much research on ß-phase PdH(D)x , we comprehensively investigated the nature of the interaction between Pd and H(D) in α-phase PdH(D)x (x<0.03 at 303 K), and revealed the existence of Pd-H(D) chemical bond for the first time, by various in situ experimental techniques and first-principles theoretical calculations. The lattice expansion, magnetic susceptibility, and electrical resistivity all provide evidence. In situ solid-state 1 H and 2 H NMR spectroscopy and first-principles theoretical calculations revealed that a Pd-H(D) chemical bond exists in the α phase, but the bonding character of the Pd-H(D) bond in the α phase is quite different from that in the ß phase; the nature of the Pd-H(D) bond in the α phase is a localized covalent bond whereas that in the ß phase is a metallic bond.

Discovery of Hexagonal Structured Pd-B Nanocrystals.

We report on hexagonal close-packed (hcp) palladium (Pd)-boron (B) nanocrystals (NCs) by heavy B doping into face-centered cubic (fcc) Pd NCs. Scanning transmission electron microscopy-electron energy loss spectroscopy and synchrotron powder X-ray diffraction measurements demonstrated that the B atoms are homogeneously distributed inside the hcp Pd lattice. The large paramagnetic susceptibility of Pd is significantly suppressed in Pd-B NCs in good agreement with the reduction of density of states at Fermi energy suggested by X-ray absorption near-edge structure and theoretical calculations.

The Room-Temperature Superionic Conductivity of Silver Iodide Nanoparticles under Pressure.

We performed variable-temperature synchrotron powder X-ray diffraction measurements and impedance spectroscopy under pressure for silver iodide (AgI) nanoparticles with a diameter of 11 nm. The superionic conducting α-phase of AgI nanoparticles was successfully stabilized down to at least 20 °C by applying a pressure of 0.18 GPa, whereas the transition temperature was 147 °C in bulk AgI at ambient pressure. To our knowledge, this is the first example of the α-phase of AgI existing stably at room temperature.

Mixed-Valence Nickel Bis(azamacrocycle) Compounds with Ghost-Leg-type Sheets.

The fabrication of so-called ghost-leg sheets and their electronic properties is reported. This unique sheet structure is composed of one-dimensional mixed-valence nickel chains, which are linked with one another by bis(azamacrocycle) ligands. They are also topologically unique NiII /NiIII mixed-valence complexes, as confirmed by X-ray and optical measurements. Moreover, their magnetic susceptibilities indicated two-dimensional antiferromagnetic behavior following the Fisher 1D chain model with interchain interactions, where spins on NiIII sites mutually interact antiferromagnetically in the sheets.

An Electrically Conductive Single-Component Donor-Acceptor-Donor Aggregate with Hydrogen-Bonding Lattice.

An electrically conductive D-A-D aggregate composed of a single component was first constructed by use of a protonated bimetal dithiolate (complex 1H2). The crystal structure of complex 1H2 has one-dimensional (1-D) π-stacking columns where the D and A moieties are placed in a segregated-stacking manner. In addition, these segregated-stacking 1-D columns are stabilized by hydrogen bonds. The result of a theoretical band calculation suggests that a conduction pathway forms along these 1-D columns. The transport property of complex 1H2 is semiconducting (Ea = 0.29 eV, ρrt = 9.1 × 104 Ω cm) at ambient pressure; however, the resistivity becomes much lower upon applying high pressure up to 8.8 GPa (Ea = 0.13 eV, ρrt = 6.2 × 10 Ω cm at 8.8 GPa). The pressure dependence of structural and optical changes indicates that the enhancement of conductivity is attributed to not only an increase of π-π overlapping but also a unique pressure-induced intramolecular charge transfer from D to A moieties in this D-A-D aggregate.