Structure and stability of complex metal hydrides - theoretical approach
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N1.7.1
Mater. Res. Soc. Symp. N 1.7 (2004)
Structure and stability of complex metal hydrides – theoretical approach Zbigniew Łodziana1,2 and Tejs Vegge1,3 1) Center for Atomic-scale Materials Physics and Department of Physics, DTU, Building 307, DK-2800 Lyngby, Denmark 2) IFJ-PAN, ul. Radzikowskiego 152, Kraków, Poland 3) Materials Research Department, Risoe National Laboratory, DK-4000 Roskilde, Denmark. Abstract In this paper systematic approach to study the structural stability of the complex hydride, LiBH4 is presented. The ground state energies of various symmetry structures are determined by means of Density Functional Theory. Simulated annealing method is used to confirm if ground state structures represent real energy minima. The vibrational spectrum and temperature dependence of the free energy of the structures with the lowest energies is determined. Calculated Raman active modes for three symmetries are presented. We show that at high temperatures LiBH4 possesses monoclinic symmetry and some of the low energy structures are unstable with respect to atomic vibrations. Our studies point to the necessity of calculation of the phonon spectra for complex metal hydrides that contain covalently bounded hydrogen. Introduction Limited energy resources, green house effects and a global demand for security of energy supply have motivated large research programs into a possible hydrogen economy. However, several technological problems still exist; the most prominent being a safe, reliable and commercially competitive method to store hydrogen for use in the transport sector [1]. Solid-state hydrogen storage in metal hydrides holds potential, but as of now, the reversible storage capacity is either too low or the kinetics too slow for most commercial applications. A related class of materials, the so-called complex hydrides, were recently found to display large reversible storage capacity, as illustrated in titanium doped NaAlH4 by Bogdanovic and Schwickardi [2], or for LiBH4 by Züttel et al.[3]. Only a few examples of reversible hydrogen storage in complex metal hydrides are known because synthesis and experimental characterisation of these materials is difficult. For example low scattering cross-section of hydrogen in diffraction experiments makes difficult determination of hydride structure. It was shown recently that theoretical studies could properly describe not only the ground state properties of the complex hydrides but their free energy and structure at finite temperatures. Using a combination of several ab initio methods we can now determine the Helmholtz free energy of complex hydrides, and thus investigate the structural stability at finite temperatures - an essential aspect due to large entropic contributions. These methods were recently used to predict the presence of a new monoclinic high temperature phase in LiBH4 [4] and if combined with, e.g., theoretical methods to locate the rate-limiting step in hydrogen-metal interactions [5] it is potentially possible to design new complex storage materials wit
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