First-principles Study on the Vibration Modes and Electronic Structure of Alkali and Alkaline-earth Amides and Alanates

Light alkaline and alkaline-earth metal hydrides such as amides $M$(NH$_2$)$_n$ and alanates $M$(AlH$_4$)$_n$ ($M$=K, Na, Li, Ca, and Mg) have attracted a growing interest as reversible hydrogen storage materials recently because of their innately high hydrogen contents. [1, 2] We study the electronic structure of the amides and alanates with different cations, focusing on the role of cation states from first-principles calculations based on the all-electron FLAPW method. Calculated breathing stretch vibration modes for these compounds are compared with measured infrared and Raman spectra. In the amides, we find a significant tendency such that the breathing stretch vibration frequencies and the structural parameters of NH$_2$ vary in accordance with the ionization energy of cation, which may be explained by the strength in hybridization between cation orbitals and molecular orbitals of (NH$_2$)$^-$. We elucidate the microscopic mechanism of correlations between the breathing stretch vibration frequencies of N-H and structural parameters by analyzing the calculated electronic structure from a view point of the molecular-orbitals. A similar tendency in the alanates is also discussed. [1] P. Chen, Z. Xiong, J. Luo, J. Lin and K.L. Tan, Nature \textbf{420}, 302 (2002). [2] B. Bogdanovi and M. Schwickardi, J. Alloys Compd. \textbf{253-254}, 1 (1997).