- Cubic Fluorite-Type CaH2 with a Small Bandgap
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A cubic variant of CaH2 adopting a fluorite-type crystal structure was synthesized by cationic substitution with La or Y, yielding the first alkaline earth hydride-based with fluorite-type framework. The material has a bandgap of ~2.5 eV (greenish yellow in color), which is much smaller than that of orthorhombic PbCl2-type CaH2 (4.4 eV) and is, in fact, the smallest among alkaline or alkaline earth metal hydrides reported to date. Analysis of the density functional theory band structure of cubic-CaH2 indicates that its conduction band minimum is formed mainly by the interaction between the Ca 3d eg orbitals around the crystallographic cavity defined by cubes of H- ions. The use of such cavities in the creation of low-lying conduction band minima by semiconductors is extremely rare, and has similarities to inorganic electrides.
- Mizoguchi, Hiroshi,Park, Sangwon,Honda, Takashi,Ikeda, Kazutaka,Otomo, Toshiya,Hosono, Hideo
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- Electrochemical hydrogenation of Mg65Cu25Y 10 metallic glass
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Rapidly quenched ribbons of a Mg65Y10Cu25 metallic glass were electrochemically charged up to a maximum hydrogen content of about 3.7 wt.%. The hydrogen content was determined by hot extraction. The microstructure of different hydrogen-charged samples was investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The thermal behaviour was studied by differential scanning calorimetry (DSC) and thermal desorption analysis (TDA). Samples heated to selected temperatures were characterised by XRD. With increasing hydrogen content a change from a single-phase amorphous to a very fine nanocrystalline microstructure was observed, which is a consequence of hydride-forming reactions at room temperature. This strongly affects the thermal behaviour. With increasing fraction of nanocrystalline phases in the hydrogenated samples, grain growth processes are more pronounced than crystallisation of the residual amorphous phase for temperatures up to 623 K. Correspondingly, the fraction of nanocrystalline products of hydride-forming reactions, i.e. YH3, MgH2 and Cu2Mg, increases. Significant hydrogen desorption occurs at temperatures above 623 K and is mainly related to the reverse of those hydriding reactions.
- Savyak,Hirnyj,Bauer,Uhlemann,Eckert,Schultz,Gebert
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- Homoleptic tetrahydrometalate anions MH4-(M = Sc, Y, La). Matrix infrared spectra and DFT calculations
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Laser-ablated Sc, Y, and La atoms react with molecular hydrogen upon condensation in excess argon, neon, and deuterium to produce the metal dihydride molecules and dihydrogen complexes MH2 and (H2)MH2. The homoleptic tetrahydrometalate anions ScH4-, YH4-, and LaH4- are formed by electron capture and identified by isotopic substitution (D2, HD, and H2 + D2 mixtures). Doping with CCl4 to serve as an electron trap virtually eliminates the anion bands, and further supports the anion identifications. The observed vibrational frequencies are in agreement with the results of density functional theory calculations, which predict electron affinities in the 2.8-2.4 eV range for the (H2)ScH2, (H2)YH2, and (H2)LaH2 complexes, and indicate high stability for the MH4- (M = Sc, La, Y) anions and suggest the promise of synthesis on a larger scale for use as reducing agents.
- Wang, Xuefeng,Andrews, Lester
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p. 7610 - 7613
(2007/10/03)
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