Lattice dynamics of NaAlH$_4$ from high-temperature single-crystal Raman scattering: Evidence of highly stable AlH$_4^-$ anions

Polarized Raman scattering on single crystals of NaAlH$_4$ has been used to determine the symmetry properties and frequencies of the Raman-active vibrational modes over the temperature range from 300 to 425~K, i.e., up to the melting point $T_{\mathrm{melt}}$. Significant softening (by up to 6\,\%) is observed in the modes involving rigid translations of Na$^{+}$ cations and translations and librations of AlH$_4^{-}$. Surprisingly, the data indicate mode softening of less than 1.5\,\% for the Al-H stretching and Al-H bending modes of the AlH$_4^{-}$ anion. These results show that the AlH$_4^{-}$ anion remains a stable structural entity even near the melting point. The enhanced kinetics of absorption and desorption in Ti-doped NaAlH$_4$ powders is attributed to the effectiveness of Ti in promoting the break-up of the AlH$_4^{-}$ anions.