Ab initio study of magnesium alanate, Mg(AlH4)2

Magnesium alanate Mg(AlH4)2 has recently raised interest as a potential material for hydrogen storage. We apply ab initio calculations to characterize structural, electronic and energetic properties of Mg(AlH4)2. Density functional theory calculations within the generalized gradient approximation (GGA) are used to optimize the geometry and obtain the electronic structure. The latter is also studied by quasi-particle calculations at the GW level. Mg(AlH4)2 is a large band gap insulator with a fundamental band gap of 6.5 eV. The hydrogen atoms are bonded in AlH4 complexes, whose states dominate both the valence and the conduction bands. On the basis of total energies, the formation enthalpy of Mg(AlH4)2 with respect to bulk magnesium, bulk aluminum and hydrogen gas is 0.17 eV/H2 (at T = 0). Including corrections due to the zero point vibrations of the hydrogen atoms this number decreases to 0.10 eV/H2. The enthalpy of the dehydrogenation reaction Mg(AlH4)2 -> MgH2 +2Al+3H2(g) is close to zero, which impairs the potential usefulness of magnesium alanate as a hydrogen storage material.Comment: 5 pages, 3 figure

The high-pressure phase of boron, {\gamma}-B28: disputes and conclusions of 5 years after discovery

{\gamma}-B28 is a recently established high-pressure phase of boron. Its structure consists of icosahedral B12 clusters and B2 dumbbells in a NaCl-type arrangement (B2){\delta}+(B12){\delta}- and displays a significant charge transfer {\delta}~0.5- 0.6. The discovery of this phase proved essential for the understanding and construction of the phase diagram of boron. {\gamma}-B28 was first experimentally obtained as a pure boron allotrope in early 2004 and its structure was discovered in 2006. This paper reviews recent results and in particular deals with the contentious issues related to the equation of state, hardness, putative isostructural phase transformation at ~40 GPa, and debates on the nature of chemical bonding in this phase. Our analysis confirms that (a) calculations based on density functional theory give an accurate description of its equation of state, (b) the reported isostructural phase transformation in {\gamma}-B28 is an artifact rather than a fact, (c) the best estimate of hardness of this phase is 50 GPa, (d) chemical bonding in this phase has a significant degree of ionicity. Apart from presenting an overview of previous results within a consistent view grounded in experiment, thermodynamics and quantum mechanics, we present new results on Bader charges in {\gamma}-B28 using different levels of quantum-mechanical theory (GGA, exact exchange, and HSE06 hybrid functional), and show that the earlier conclusion about significant degree of partial ionicity in this phase is very robust

First-principles calculations of the crystal structure, electronic structure, and thermodynamic stability of Be(BH4)(2)

Contains fulltext : 72422.pdf (publisher's version ) (Open Access

Electronic Structure & Formation Enthalpy of Alanates & Boronates

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Complex Hydrides. A collection of Density Functionale studies

Contains fulltext : 45154_comphy.pdf (publisher's version ) (Open Access)Radboud University Nijmegen, Faculteit NWI, 28 augustus 2008Promotor : Groot, R.A. de184 p

A first-principles study of the electronic structure and stability of Be(BH4)2

Contains fulltext : 34891.pdf (preprint version ) (Closed access)Alanates and boranates are studied intensively because of their potential use as hydrogen storage materials. In this paper we present a first-principles study of the electronic structure and the energetics of beryllium boranate, Be(BH4)2. From total energy calculations we show that - in contrast to the other boranates and alanates - hydrogen desorption directly to the elements is likely, and is at least competitive with desorption to the elemental hydride (BeH2). The formation enthalpy of Be(BH4)2 is only -0.12 eV/H2 (at T=0K). This low value can be rationalized by the participation of all atoms in the covalent bonding, in contrast to the ionic bonding observed in other boranates. From calculations of thermodynamic properties at finite temperature we estimate a decomposition temperature of 162 K at a pressure of 1 bar.907 p

Prediction strategies in a TV recommender system - Method and experiments

Predicting the interests of a user in information is an important process in personalized information systems. In this paper, we present a way to create prediction engines that allow prediction techniques to be easily combined into prediction strategies. Prediction strategies choose one or a combination of prediction techniques at the moment a prediction is required, taking into account the most up -to-date knowledge about the current user, other users, the information and the system itself. Results of two experiments show that prediction strategies improve both the accuracy and stability of prediction engines. One of these experiments involves a TV recommender system. This paper describes the method of prediction strategies, how they have been applied in the TV recommender system and results of the experiment in detail

Case-based reasoning as a prediction strategy for hybrid recommender systems

Hybrid recommender systems are capable of providing better rec-ommendations than non-hybrid ones. Our approach to hybrid recommenders is the use of prediction strategies that determine which prediction technique(s) should be used at the moment an actual prediction is required. In this paper, we determine whether case-based reasoning can provide more accurate prediction strategies than rule-based predictions strategies created manually by experts. Experiments show that case-based reasoning can indeed be used to create pre-diction strategies; it can even increase the accuracy of the recommender in sys-tems where the accuracy of the used prediction techniques is highly spread