1,010 research outputs found
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
First-principles study on the intermediate compounds of LiBH
We report the results of the first-principles calculation on the intermediate
compounds of LiBH. The stability of LiBH and LiBH has been examined with the ultrasoft pseudopotential method based on
the density functional theory. Theoretical prediction has suggested that
monoclinic LiBH is the most stable among the candidate
materials. We propose the following hydriding/dehydriding process of LiBH
via this intermediate compound : LiBHLiBH LiH HLiH B H. The hydrogen content and enthalpy of the first
reaction are estimated to be 10 mass% and 56 kJ/mol H, respectively, and
those of the second reaction are 4 mass% and 125 kJ/mol H. They are in good
agreement with experimental results of the thermal desorption spectra of
LiBH. Our calculation has predicted that the bending modes for the
-phonon frequencies of monoclinic LiBH are lower than
that of LiBH, while stretching modes are higher. These results are very
useful for the experimental search and identification of possible intermediate
compounds.Comment: 7 pages, 5 figures, submitted to PR
Lithium colloids and color center creation in electron-irradiated Li2NH observed by electron-spin resonance
4 pagesInternational audienceWe have irradiated Li2NH powder with MeV electrons at room temperature and investigated the introduced defects with electron spin resonance. CESR indicates the presence of nanosize metallic Li colloids seen as a Lorentzian line with a g = 2.0023 and a line width DeltaH = 50 microT. A second, broader, signal (DeltaH = 3 to 4 mT) appears superimposed upon the Li line at low T (Curie-type behavior) which exhibits complex T-dependence with a break near 180 K for its g-value and DeltaH. We are suggesting for the latter a vacancy-type defect in the NH-sublattice, with freezing of its H-component below 180 K. When heated both the Li colloids and the color centers anneal around 100 C probably due to hydrogen evolution and subsequent chemical degradation
Magnetization measurements on Li2Pd3B superconductor
Magnetization in DC magnetic fields and at different temperatures have been
measured on the Li2Pd3B compound. This material was recently found to show
superconductivity at 7-8K. Critical fields Hc1(0) and Hc2(0) have been
determined to be 135Oe and 4T, respectively. Critical current density, scaling
of the pinning force within the Kramer model and the irreversibility field data
are presented. Several superconductivity parameters were deduced: x(csi)=9.1
nm, l(lamda)=194nm and k=21. The material resembles other boride
superconductors from the investigated points of view.Comment: 10 pages, 5 figure
Complex transition metal hydrides incorporating ionic hydrogen: Synthesis and characterization of Na2Mg2FeH8 and Na2Mg2RuH8
A new class of quaternary complex transition metal hydrides (Na2Mg2TH8 (T = Fe, Ru)) have been synthesized and their structures determined by combined synchrotron radiation X-ray and powder neutron diffraction. The compounds can be considered as a link between ionic and complex hydrides in terms of incorporating independently coordinated ionic and covalent hydrogen. These novel isostructural complex transition metal hydrides crystallize in the orthorhombic space group Pbam, where the octahedral complex hydride anion is surrounded by a cubic array of four Mg2+ and four Na+ cations, forming distinct two-dimensional layers. An intriguing feature of these materials is the distorted octahedral coordination of the isolated Hâ anions by four Na+ and two Mg2+ cations, which form layers between the transition metal containing layers. The vibrational modes of the Hâ anions and complex hydride anion are independently observed for the first time in a quaternary complex transition metal hydride system by Raman and IR spectroscopy
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