Structure and hydrogen dynamics of pure and Ti-doped sodium alanate

We have studied the structure, energetics, and dynamics of pure and Ti-doped sodium alanate $\left(\mathrm{Na}\mathrm{Al}{\mathrm{H}}_4\right)$, focusing on the possibility of substitutional Ti doping in the bulk. Our ab initio calculations reproduce well the measured neutron inelastic scattering spectrum, which exhibits surprisingly strong and sharp two-phonon features. The calculations also reveal that substitutional Ti doping is energetically possible, and imply that Ti prefers to substitute for Na and is a powerful hydrogen attractor that facilitates multiple Al-H bond breaking. Our results hint at ways of improving the hydrogen dynamics and storage capacity of the alanates.

Figure 1

(Color online) Measured (top) and calculated (bottom) NIS spectra of $\mathrm{Na}\mathrm{Al}{\mathrm{H}}_4$. The calculated 1 and $1+2$ phonon contributions are shown. The structure of $\mathrm{Na}\mathrm{Al}{\mathrm{H}}_4$ is shown in the inset; grey tetrahedra represent $\mathrm{Al}{\mathrm{H}}_4$ units. The measurements were performed at $8\mathrm{K}$.

Figure 2

(Color online) Calculated one-phonon NIS spectra for (a) $\mathrm{Na}\mathrm{Al}{\mathrm{H}}_4$ and for a single $\mathrm{M}{\mathrm{H}}_4$ tetrahedron for various cases: (b) pure alanate, (c) $\mathrm{Ti}\to \mathrm{Na}$, and (d) $\mathrm{Ti}\to \mathrm{Al}$.

Figure 3

(Color online) Energy along the path from $\mathrm{Ti}\to \mathrm{Na}$ to $\mathrm{Ti}\to \mathrm{Na}\left(\mathrm{H}\right)$ structures (see text). Insets show the local structure and bond distances at the two minima. The zero of energy is arbitrary.