Second Moment Analysis of Sodium Aluminohexahydride (Na3AlH6) NMR

Author

Tim J. Ellis-Caleo, Washington University in St LouisFollow

Date Submitted

Spring 4-21-2013

Research Mentor and Department

Mark Conradi

Restricted/Unrestricted

Dissertation/Thesis

Abstract

Sodium aluminohexahydride (Na3AlH6) is a promising candidate for reversible hydrogen storage. Understanding the motion of individual atoms within the crystalline lattice is a key challenge of studying hydrogen storage candidates. This molecule is formed by sodium cations and an aluminohexahydride (AlH6-) anion cluster. We examined the relative widths of the lineshapes over the temperature range 23oC to 240oC and noticed significant narrowing, allowing us to conclude that some of the nuclei are moving. It is important to understand not only which nuclei are moving, but also how they are moving. In this case, second moment calculations can be performed to examine the effects of specific motions on the lineshapes. In particular, we looked at three distinct cases of movement: where the entire crystal is rigid, where the AlH6- clusters rotate isotropically on the 10-5 second time scale and finally where the sodium atoms diffuse rapidly through the lattice in addition to the isotropic rotation. We located all of the nuclei in the crystalline lattice and then designed computer software to perform the second moment calculations to approximate the linewidths. We were able to see that the hydrogen nuclei undergo rapid rotational motion as the experimental linewidths were much narrower than the calculated values for the rigid case. Also, we conclude that the sodium atoms are not diffusing as the hydrogen and aluminum lineshapes do not narrow to the degree predicted if the sodium atoms were diffusing. Our experimental lineshapes did match our calculated values well for the case of fast rotation of the hydrogen around the aluminum cluster without sodium diffusion, allowing us to conclude that this is the correct model.