B NMR spectroscopy has been employed to identify the reaction intermediates and products formed in the amorphous phase during the thermal hydrogen desorption of metal tetrahydroborates (borohydrides) LiBH 4 , Mg(BH 4 ) 2 , LiSc(BH 4 ) 4 , and the mixed Ca(AlH 4 ) 2 -LiBH 4 system. The 11 B magic angle spinning (MAS) and cross polarization magic angle spinning (CPMAS) spectral features of the amorphous intermediate species closely coincide with those of a model compound, closo-borane K 2 B 12 H 12 that contains the [B 12 H 12 ] 2anion. The presence of [B 12 H 12 ] 2in the partially decomposed borohydrides was further confirmed by high-resolution solution 11 B and 1 H NMR spectra after dissolution of the intermediate desorption powders in water. The formation of the closo-borane structure is observed as a major intermediate species in all of the metal borohydride systems we have examined.

Storing molecular hydrogen in porous media is one of the promising avenues for mobile hydrogen storage. In order to achieve technologically relevant levels of gravimetric density, the density of adsorbed H2 must be increased beyond levels attained for typical high surface area carbons. Here, we demonstrate a strong correlation between exposed and coordinatively unsaturated metal centers and enhanced hydrogen surface density in many framework structures. We show that the MOF-74 framework structure with open Zn(2+) sites displays the highest surface density for physisorbed hydrogen in framework structures. Isotherm and neutron scattering methods are used to elucidate the strength of the guest-host interactions and atomic-scale bonding of hydrogen in this material. As a metric with which to compare adsorption density with other materials, we define a surface packing density and model the strength of the H(2-)surface interaction required to decrease the H(2)-H(2) distance and to estimate the largest possible surface packing density based on surface physisorption methods.

The oxychalcogenides A2F2Fe2OQ2 (A = Sr, Ba; Q = S, Se), which contain Fe2O square planar layers of the anti-CuO2 type, were predicted using a modular assembly of layered secondary building units and subsequently synthesized. The physical properties of these compounds were characterized using magnetic susceptibility, electrical resistivity, specific heat, (57)Fe Mossbauer, and powder neutron diffraction measurements and also by estimating their exchange interactions on the basis of first-principles density functional theory electronic structure calculations. These compounds are magnetic semiconductors that undergo a long-range antiferromagnetic ordering below 83.6-106.2 K, and their magnetic properties are well-described by a two-dimensional Ising model. The dominant antiferromagnetic spin exchange interaction between S = 2 Fe(2+) ions occurs through corner-sharing Fe-O-Fe bridges. Moreover, the calculated spin exchange interactions show that the A2F2Fe2OQ2 (A = Sr, Ba; Q = S, Se) compounds represent a rare example of a frustrated antiferromagnetic checkerboard lattice.

At 77 K, hydrogen surface excess sorption of carbon aerogels scales with BET surface area up to 2550 m2/g, yielding up to 5 wt % gravimetric density. The surface area dependence for the aerogel with a surface area of 3200 m2/g is somewhat weaker, with a gravimetric density of 5.3 wt %.

The structure of the fully ordered α-Na(3)Ti(2)(PO(4))(3) NASICON compound was elucidated using high-quality single-crystal data. The cation/vacancy distribution forms a homogeneous 3D arrangement and could represent the absolute cationic ordering available in the full Na(3)M(2)(PO(4))(3) series, as verified for M = Fe. For M = Ti, the reversible α → γ transition was observed at 85 °C, leading to the standard disordered R ̅3c γ model. Through JPDF analysis, the most probable Na(+) zigzag M(2)-M(1) diffusion scheme was directly deduced using our accurate crystallographic data.

Inorganic compounds made up of low-dimensional ferromagnetic (FM) units display fascinating properties and provide a rich opportunity to investigate FM ground states, fieldinduced transitions, [1] and magnetization steps. [2][3][4] Even the spin-valve effect, realized in multilayer thin films, is found in the magnetic metal Ca 3 Ru 2 O 7[5] and its Cr-doped analogue [6] in which FM double-perovskite layers are antiferromagnetically coupled. [7] The inorganic compound Cr 2 Si 2 Te 6 , also consisting of FM layers, [8a,b] turns out to be a unique example of a bulk 2D FM Ising system. [8c] It is a great synthetic challenge to discover new magnetic transition-metal oxides made up of FM layers. In searching for such materials, a rational approach rather than by a blind exploration of chemical systems should be used. In general, a transition-metal cation at a coordinate site with three-fold or higher rotational symmetry can lead to uniaxial magnetism if its d electron count and spin state are such that there occurs an unevenly-filled degenerate level, as found for high-spin Fe 2+ ions at linear-coordinate sites [8a] and high-spin Co 3+ and Co 2+ ions at trigonal-prismatic sites. [8b,c] In principle, high-spin Fe 2+ O 6 octahedra can support uniaxial magnetism as long as they possess three-fold rotational symmetry. It can be imagined that isolated FM layers form from such FeO 6 octahedra by edge-sharing because the Fe-O-Fe angle will be close to 908 so that the nearest-neighbor spin exchange would be FM.Our guided search for such a magnetic system led to the synthesis of BaFe 2 (PO 4 ) 2 that turns out to be the first oxide 2D Ising ferromagnet. It consists of FM honeycomb layers of edge-sharing FeO 6 octahedra containing high-spin Fe 2+ ions. Such FeO 6 octahedra showing uniaxial magnetism are expected to be susceptible to Jahn-Teller (JT) instability. [9] Indeed, on cooling, BaFe 2 (PO 4 ) 2 undergoes a rare re-entrant structural transition owing to the competition between uniaxial magnetism and the JT distortion.The three main 2D triangular lattices consisting of edgesharing MO 6 octahedra are shown in Figure 1. Starting from the [MO 2 ] triangular lattice, the ordering of one quarter of the M vacancies leads to the [M 3/4 O 2 ] KagomØ lattice, and that of one third of the M vacancies to the [M 2/3 O 2 ] honeycomb lattice. They all possess similar intralayer spin exchange paths in different ratios (Figure 1). The superexchange (SE) path J 1 consists of two M-O-M bridges with a M-O-M angle of about 908, while the super-superexchange (SSE) paths J 2 and J 3 involve M À O···O À M bridges. For M = Fe, it is expected that the 908 SE J 1 path is ferromagnetic (FM) for high-spin (HS) Fe 2+ (d 6 , S = 2) but antiferromagnetic (AFM) for HS Fe 3+ (d 5 , S = 5/2) cations, according to dominant direct t 2g -t 2g overlaps. [10] Results of a literature search for such magnetic iron oxides with pertinent 2D or pseudo 2D lattices are summarized in Table 1, [11][12][13] which indicates that the Fe 2+ ions is the key cation lea...

Octahedral molecular sieves (OMS) are built of transition metal-oxygen octahedra that delimit sub-nanoscale cavities. Compared to other microporous solids, OMS exhibit larger versatility in properties, provided by various redox states and magnetic behaviors of transition metals. Hence, OMS offer opportunities in electrochemical energy harnessing devices, including batteries, electrochemical capacitors and electrochromic systems, provided two conditions are met: fast exchange of ions in the micropores and stability upon exchange. Here we unveil a novel OMS hexagonal polymorph of tungsten oxide called h’-WO3, built of (WO6)6 tunnel cavities. h’-WO3 is prepared by a one-step soft chemistry aqueous route leading to the hydrogen bronze h’-H0.07WO3. Gentle heating results in h’-WO3 with framework retention. The material exhibits an unusual combination of 1-dimensional crystal structure and 2-dimensional nanostructure that enhances and fastens proton (de)insertion for stable electrochromic devices. This discovery paves the way to a new family of mixed valence functional materials with tunable behaviors.

A lithium salt of anionic scandium tetraborohydride complex, LiSc(BH4)4, was studied both experimentally and theoretically as a potential hydrogen storage medium. Ball milling mixtures of LiBH4 and ScCl3 produced LiCl and a unique crystalline hydride, which has been unequivocally identified via multinuclear solid-state nuclear magnetic resonance (NMR) to be LiSc(BH4)4. Under the present reaction conditions, there was no evidence for the formation of binary Sc(BH4)3. These observations are in agreement with our first-principles calculations of the relative stabilities of these phases. A tetragonal structure in space group I4̅ (#82) is predicted to be the lowest energy state for LiSc(BH4)4, which does not correspond to structures obtained to date on the crystalline ternary borohydride phases made by ball milling. Perhaps reaction conditions are resulting in formation of other polymorphs, which should be investigated in future studies via neutron scattering on deuterides. Hydrogen desorption while heating these Li−Sc−B−H materials up to 400 °C yielded only amorphous phases (besides the virtually unchanged LiCl) that were determined by NMR to be primarily ScB2 and [B12H12]−2 anion containing (e.g., Li2B12H12) along with residual LiBH4. Reaction of a desorbed LiSc(BH4)4 + 4LiCl mixture (from 4LiBH4/ScCl3 sample) with hydrogen gas at ∼70 bar resulted only in an increase in the contents of Li2B12H12 and LiBH4. Full reversibility to reform the LiSc(BH4)4 was not found. Overall, the Li−Sc−B−H system is not a favorable candidate for hydrogen storage applications.