Optical properties of MgH2 measured in situ in a novel gas cell for ellipsometry/spectrophotometry

The dielectric properties of alpha-MgH2 are investigated in the photon energy range between 1 and 6.5 eV. For this purpose, a novel sample configuration and experimental setup are developed that allow both optical transmission and ellipsometric measurements of a transparent thin film in equilibrium with hydrogen. We show that alpha-MgH2 is a transparent, colour neutral insulator with a band gap of 5.6 +/- 0.1 eV. It has an intrinsic transparency of about 80% over the whole visible spectrum. The dielectric function found in this work confirms very recent band structure calculations using the GW approximation by Alford and Chou [J.A. Alford and M.Y. Chou (unpublished)]. As Pd is used as a cap layer we report also the optical properties of PdHx thin films.Comment: REVTeX4, 15 pages, 12 figures, 5 table

Rising stars in energy research: 2022

Recognising the future leaders of Energy Research is fundamental to safeguarding tomorrow's driving force in innovation. This collection will showcase the high-quality work of internationally recognized researchers in the early stages of their careers. We aim to highlight research by leading scientists of the future across the entire breadth of Energy Research, and present advances in theory, experiment and methodology with applications to compelling problems

Origin of the large anharmonicity in the phonon modes of LiBH4

The dynamics and bonding of the complex hydride LiBH4 have been investigated by vibrational spectroscopy and density functional theory (DFT). The combination of infrared, Raman, and inelastic neutron-scattering (INS) spectroscopies on hydrided and deuterated samples reveals a complete picture of the dynamics of the BH4- ions as well as of the lattice. Particular emphasis is laid on a comparison between experiment and theory, revealing significant discrepancy between the two approaches for vibrations with high anharmonicity, which is related to large vibrational amplitudes. The latter is typical for librational modes in molecular crystals and pseudo-ionic crystals such as complex hydrides. The presented strategy for anharmonic frequency corrections might thus be generally applicable for this kind of materials. © 2013 Elsevier B.V

Gas-Solid Reaction of Carbon Dioxide with Alanates

An empirical study of the gas–solid reaction of carbon dioxide (CO₂) with alanates is presented. This investigation was triggered by reports of hazards related to the reaction of lithium aluminum hydride with carbon dioxide, together with the recent emergence of alanates as potential hydrogen storage materials. Furthermore, the reduction of CO₂ by hydrides is an alternative to the conventional CO₂ reduction employing hydrogen gas in combination with a catalyst. Experimentally this work was carried out by studying the decomposition of alanate samples in a carbon dioxide atmosphere with thermogravimetric and ex situ IR spectroscopic techniques. It is shown that alanates react with CO₂ at atmospheric pressure in two distinct temperature regions, yielding methane, hydrogen gas, and metal oxides as the major products. The experimental findings allowed us to postulate a mechanism for the complex metal hydride reduction of CO₂, involving alane as a highly reactive intermediate. It is suggested that nucleophilic attack of alane hydride ions onto the carbonyl carbon of CO₂ leads to sequential formation of aluminum formate and methoxy species which get converted into methane and metal oxides as the final products

Study of the hydride forming process of in-situ grown MgH2 thin films by activated reactive evaporation

MgHx thin films are grown by activated reactive evaporation in a Molecular Beam Epitaxy system fitted with an atomic hydrogen source. During deposition the electrical and optical properties are measured in-situ. The structural properties are determined ex-situ by Atomic Force Microscopy. These measurements confirm the growth of the MgH2 phase, however the presence of 10 vol.% of metallic Mg cannot be prevented. The metallic Mg grains cause an optical absorption edge at 2.0 eV, which has a completely different origin than the observed band gap of MgH2 at 5.6 eV The observed optical spectra can be modelled using an effective medium theory. The Mg hydride films are electrically insulating despite the presence of metallic Mg particles. Upon re-hydrogenation of a de-hydrogenated in-situ grown MgHx thin film, the absorption edge at 2.0 eV disappears and the resistivity decreases to values normally observed for ex-situ hydrogenated films. (C) 2008 Elsevier B.V. All rights reserved