66 research outputs found
Electronic structure and optical properties of lightweight metal hydrides
We study the electronic structures and dielectric functions of the simple
hydrides LiH, NaH, MgH2 and AlH3, and the complex hydrides Li3AlH6, Na3AlH6,
LiAlH4, NaAlH4 and Mg(AlH4)2, using first principles density functional theory
and GW calculations. All these compounds are large gap insulators with GW
single particle band gaps varying from 3.5 eV in AlH3 to 6.5 eV in the MAlH4
compounds. The valence bands are dominated by the hydrogen atoms, whereas the
conduction bands have mixed contributions from the hydrogens and the metal
cations. The electronic structure of the aluminium compounds is determined
mainly by aluminium hydride complexes and their mutual interactions. Despite
considerable differences between the band structures and the band gaps of the
various compounds, their optical responses are qualitatively similar. In most
of the spectra the optical absorption rises sharply above 6 eV and has a strong
peak around 8 eV. The quantitative differences in the optical spectra are
interpreted in terms of the structure and the electronic structure of the
compounds.Comment: 13 pages, 10 figure
Electronic Structure of the Complex Hydride NaAlH4
Density functional calculations of the electronic structure of the complex
hydride NaAlH4 and the reference systems NaH and AlH3 are reported. We find a
substantially ionic electronic structure for NaAlH4, which emphasizes the
importance of solid state effects in this material. The relaxed hydrogen
positions in NaAlH4 are in good agreement with recent experiment. The
electronic structure of AlH3 is also ionic. Implications for the binding of
complex hydrides are discussed.Comment: 4 pages, 5 figure
Comparative Investigation of MgMnSiO<sub>4</sub> and Olivine-Type MgMnSiS<sub>4</sub> as Cathode Materials for Mg Batteries
The
identification of potential cathode materials is necessary for the
development of a new magnesium-based battery technology. Most attempts
focus on oxide and sulfide materials, which in general suffer, respectively,
of poor Mg mobility and low intercalation voltage. New chemistries
should be explored. In this work, we investigate the basic electrode
characteristics of olivine-type thiosilicates MgMSiS<sub>4</sub> (M
= Fe, Mn) with the double challenge of (1) raising the low intercalation
voltage of transition metal sulfides and (2) improving the poor Mg
diffusion of the oxosilicate counterparts MgMSiO<sub>4</sub> (M =
Fe, Mn). Density functional theory (DFT) calculations corroborate
both expectations. The calculated average Mg deintercalation voltage
for MgMnSiS<sub>4</sub> (2.31 V) is above that of virtual MgMnS<sub>2</sub> compounds (around 1.8 V), accounting for the inductive effect
of the Si<sup>4+</sup> ion on the transition metal. The calculated
energy barriers for Mg diffusion in Mg<sub><i>x</i></sub>MnSiO<sub>4</sub> are 0.75 eV at <i>x</i> ∼ 1 and
higher than 1.1 eV at <i>x</i> ∼ 0 and <i>x</i> = 0.5. The energy barriers decrease to 0.68 and 0.76 eV in Mg<sub><i>x</i></sub>MnSiS<sub>4</sub> (<i>x</i> ∼
0, 1), thanks to the more covalent Mn–S bond (compared to the
Mn–O bond) that renders less oxidized Mn ions, therefore favoring
Mg<sup>2+</sup> mobility. Although these results are promising, more
work is needed to ensure the potential application of thiosilicates
as cathode materials for Mg batteries
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