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.