Structure and Stability of Multivalent Metal Tetraborohydrides
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Structure and Stability of Multivalent Metal Tetraborohydrides Z. Łodziana,1,2 A. Borgschulte,1 R. Gremaud,1 A. Remhof,1 and A. Züttel,1 1 Empa Materials Science and Technology, Laboratory 138 (Hydrogen & Energy), CH-8600, Dübendorf, Switzerland 2 Institute of Nuclear Physics, PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland ABSTRACT Metal tetrahydroborates are interesting compounds as potential hydrogen storage materials because of the great gravimetric hydrogen density of up to 20 mass%. The thermodynamic stability of borohydrides of Al and Zr was analyzed by means of extensive density functional calculations. The solid phases of these compounds are formed by dispersive Van der Waals forces. These compounds are thermodynamically unstable at room temperature with respect to the decomposition into cation hydride, boron and hydrogen. Their stability isexplained by the formation of diborane as a necessary step in the decomposition path, pointing out to the kinetic factors that are important for the stability analysis of metal borohydrides. INTRODUCTION The metal tetrahydroborates (borohydrides) are compounds containing metal cations and BH4 groups. Recently, the tetrahydroborates have been intensively studied due to their high gravimetric hydrogen content, which makes them interesting potential hydrogen storage materials [1,2]. They have been shown to form stable compounds with alkali and alkaline earth elements, various transition metals, and with aluminum [3]. In general, for these materials the number of BH4 groups per metal atom reflects the valency of the cation. In the simplest picture, the tetrahydroborates can be thought of as ionic solids with metal cations and BH4 anions that are semi-covalently bound molecular units. This picture holds well for univalent (LiBH4, NaBH4, KBH4) and divalent (Mg(BH4)2, Ca(BH4)2) metal tetrahydroborates [4-8]. However, these compounds are very stable, and decompose above 500K that makes them difficult to use in practical applications. In general the stability of borohydrides containing metals at third or fourth oxidation state is lower than for respective compounds of metals with lower valency [3]. Unfortunately structural and thermodynamic properties of these compounds are relatively less known, even though they were intensively studied in the past in the gaseous form or in liquid solutions [3]. Among borohydrides containing metal cation with valency larger than two, Al(BH4)3 is a volatile liquid at ambient conditions (it decomposes spontaneously) [9]. The crystalline solid form of Zr(BH4)4 crystalizes in P43m symmetry that is stable below 302°C [10]. In this paper we study, by means of Density Functional Theory (DFT) calculations, thermodynamic stability of aluminum and zirconium borohydrides.
THEORY The present calculations were performed with periodic plane wave DFT approach. We used a plane-wave basis set and the projector-augmented wave (PAW) method, as implemented in the Vienna Ab initio Simulation Package (VASP) [11]. The non-linear core corrections were
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