Kazutoshi Miwa

Experimental studies on intermediate compound of LiBH4

The formation condition of an intermediate compound of LiBH4 during the partial dehydriding reaction and its local atomistic structure have been experimentally investigated. LiBH4 changes into an intermediate compound accompanying the release of approximately 11mass% of hydrogen at 700–730K. The Raman spectra indicate that the B–H bending and stretching modes of the compound appear at lower and higher frequencies, respectively, as compared to those of LiBH4. These features are consistent with the theoretical calculation on the monoclinic Li2B12H12, consisting of Li+ and [B12H12]2− ions, as a possible intermediate compound of LiBH4.

First-principles study on the stability of intermediate compounds of LiBH(4)

Note: Times Cited: 110 Reference EPFL-ARTICLE-206017doi:10.1103/PhysRevB.74.075110View record in Web of Science URL: ://WOS:000240238800042 Record created on 2015-03-03, modified on 2017-05-12

Formation of an intermediate compound with a B12H12 cluster: experimental and theoretical studies on magnesium borohydride Mg(BH4)2.

Experimental and theoretical studies on Mg(BH4)2 were carried out from the viewpoint of the formation of the intermediate compound MgB12H12 with B12H12 cluster. The full dehydriding and partial rehydriding reactions of Mg(BH4)2 occurred according to the following multistep reaction: Mg(BH4)2 -->1/6MgB12H12 + 5/6MgH2 + 13/6H2 <--> MgH2 + 2B + 3H2 <--> Mg + 2B + 4H2. The dehydriding reaction of Mg(BH4)2 starts at approximately 520 K, and 14.4 mass% of hydrogen is released upon heating to 800 K. Furthermore, 6.1 mass% of hydrogen can be rehydrided through the formation of MgB12H12. The mechanism for the formation of MgB12H12 under the present rehydriding condition is also discussed.

First-principles studies of complex hydride YMn2H6 and its synthesis from metal hydride YMn2H4.5

First-principles calculations were performed for a complex hydride YMn2H6 to investigate its electronic structure and thermodynamic stability. The results indicated that an Y atom and one of two Mn atoms were ionized as Y3+ and Mn2+, respectively, and another Mn atom bound covalently to H atoms to form a [MnH6]5− complex anion. Based on the enthalpy change of −65 kJ/mol estimated from the calculation, we experimentally verified a possible low-pressure synthesis of YMn2H6 from a metal hydride YMn2H4.5. X-ray diffractometry confirmed the formation of YMn2H6 after hydrogenation below 5 MPa, much lower than the previously reported value of 170 MPa.

True Boundary for the Formation of Homoleptic Transition-Metal Hydride Complexes†

Despite many exploratory studies over the past several decades, the presently known transition metals that form homoleptic transition-metal hydride complexes are limited to the Groups 7-12. Here we present evidence for the formation of Mg3 CrH8 , containing the first Group 6 hydride complex [CrH7 ](5-) . Our theoretical calculations reveal that pentagonal-bipyramidal H coordination allows the formation of σ-bonds between H and Cr. The results are strongly supported by neutron diffraction and IR spectroscopic measurements. Given that the Group 3-5 elements favor ionic/metallic bonding with H, along with the current results, the true boundary for the formation of homoleptic transition-metal hydride complexes should be between Group 5 and 6. As the H coordination number generally tends to increase with decreasing atomic number of transition metals, the revised boundary suggests high potential for further discovery of hydrogen-rich materials that are of both technological and fundamental interest.

Enhanced tunability of thermodynamic stability of complex hydrides by the incorporation of H– anions

First-principles calculations were employed to investigate hypothetical complex hydrides (M,M′)4FeH8 (M = Na, Li; M′=Mg, Zn, Y, Al). Besides complex anion [FeH6]4–, these materials contain two H– anions, which raise the total anionic charge state from tetravalent to hexavalent, and thereby significantly increasing the number of combinations of countercations. We have determined that similar to complex hydrides (M,M′)2FeH6 containing only [FeH6]4–, the thermodynamic stability is tuned by the average cation electronegativity. Thus, the chemical flexibility provided by incorporating H– enhances the tunability of thermodynamic stability, which will be beneficial in obtaining optimal stability for hydrogen storage materials.

Formation of an Fe–H complex anion in YFe2: adjustment of imbalanced charge by using additional Li as an electron donor

The novel complex hydride YLiFeH6 with the Fe–H complex anion ([FeH6]4−) was synthesized by hydrogenation of YFe2 together with additional Li. Li adjusts the original imbalanced charge between Y3+ and [FeH6]4− by donating an electron to convert into Li+ during the hydrogenation, resulting in the formation of the well charge-balanced state of Y3+Li+[FeH6]4−.

Theoretical investigation of Fe substitution for Mn in complex hydride YMn2H6

Complex hydride YMn2H6 with a space group Fm3¯m has two sorts of Mn, in which one presents as a divalent cation Mn2+ on the 8c site and the other forms a complex anion [MnH6]5− on the 4a site. We investigate the stabilities of Fe substitutions for a part of Mn atoms on either the site using first-principles calculations. Our results reveal that an Fe atom prefers the 4a occupation, and the origin of this site preference can be understood by the balance of two competing energy gains brought by the 3d bands for either the 4a or 8c site substituted Fe.

Formation of novel transition metal hydride complexes with ninefold hydrogen coordination

Ninefold coordination of hydrogen is very rare, and has been observed in two different hydride complexes comprising rhenium and technetium. Herein, based on a theoretical/experimental approach, we present evidence for the formation of ninefold H- coordination hydride complexes of molybdenum ([MoH9]3−), tungsten ([WH9]3−), niobium ([NbH9]4−) and tantalum ([TaH9]4−) in novel complex transition-metal hydrides, Li5MoH11, Li5WH11, Li6NbH11 and Li6TaH11, respectively. All of the synthesized materials are insulated with band gaps of approximately 4 eV, but contain a sufficient amount of hydrogen to cause the H 1s-derived states to reach the Fermi level. Such hydrogen-rich materials might be of interest for high-critical-temperature superconductivity if the gaps close under compression. Furthermore, the hydride complexes exhibit significant rotational motions associated with anharmonic librations at room temperature, which are often discussed in relation to the translational diffusion of cations in alkali-metal dodecahydro-closo-dodecaborates and strongly point to the emergence of a fast lithium conduction even at room temperature.

Li-ion diffusion in Li intercalated graphite C6Li and C12Li probed by mu+SR

In order to study a diffusive behavior of Li+ in Li intercalated graphites, we have measured muon spin relaxation (μ+SR) spectra for C6Li and C12Li synthesized with an electrochemical reaction between Li and graphite in a Li-ion battery. For both compounds, it was found that Li+ ions start to diffuse above 230 K and the diffusive behavior obeys a thermal activation process. The activation energy (Ea) for C6Li is obtained as 270(5) meV, while Ea = 170(20) meV for C12Li. Assuming a jump diffusion of Li+ in the Li layer of C6Li and C12Li, a self-diffusion coefficient DLi at 310 K was estimated as 7.6(3) × 10-11 (cm2 s-1) in C6Li and 14.6(4) × 10-11 (cm2 s-1) in C12Li.