Density functional study of electronic,bonding, and vibrational properties of Ca (NH < inf > 2 < /inf > BH < inf > 3 < /inf > ) < inf > 2 < /inf >

Abstract

We present structural, electronic, bonding and vibrational properties of new type hydrogen storage material calcium amidoborane Ca(NH 2BH 3) 2 by first principles density functional theory using plane wave pseudopotential method. The calculated ground state properties are in good agreement with experiments. The computed Bulk modulus of Ca(NH 2BH 3) 2 is found to be 28.7 GPa which is slightly higher than that of NH 3BH 3 indicating that the material is hard over NH 3BH 3. From the band structure calculations, the compound is found to be a direct band gap insulator with a band gap of 3.27 eV at the Γ point. The calculated bandstructure shows that the top of the valance band is from the p states of N and the bottom of the conduction band is from d states of Ca. The Mulliken bond populations, Born effective charges and charge density distributions are used to analyze the bonding nature of the compound. It is found that the N-H and B-H bonds are covalent in nature. Further we also compared the phonon density of states and vibrational frequencies of Ca(NH 2BH 3) 2 with NH 3BH 32. The study reveals that in both the cases the heavier mass atoms Ca, N, B are involved in the low frequency vibrations whereas the higher frequency vibrations are from H atoms. It is also observed that the vibrational frequencies of B-H bonds are soft in Ca(NH 2BH 3) 2 when compared to NH 3BH 3 and thereby concluded that Ca(NH 2BH 3) 2 is a potential hydrogen storage material for fuel cell applications when compared to NH 3BH 3. Copyright © 2012 Wiley Periodicals, Inc.