## 3 Results
### Ti K-Edge X-ray Absorption Fine Structure (XAFS)
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Condition** & **Model** & **Phase** & **Symmetry** & **Shell** & \
\begin{tabular}{c} **Bond** \\ **Length** \\ \end{tabular} & **Debye-Waller** & **Edge Shift** &
\begin{tabular}{c} **Intrinsic** \\ **Loss** \\ \end{tabular} & **Reliability** \\ \cline{5-10}  & & & & \(r\)/Å & \(\sigma^{2}\)/Å\({}^{2}\) & \(\Delta E\)/eV & \(\sigma_{0}^{2}\) & \(R\)/\% \\ \hline \multirow{5}{*}{before des.} & A & Al-Ti & \(Fm\)/\(Sm\) & Al & 2.83 (5) & 0.023 (5) & \(-\)0.6 (20) & 0.61 & 17.8 \\ \cline{2-10}  & B & Al-Ti & \(Fm\)/\(Sm\) & Al & 2.83 (2) & 0.007 (4) & 0.2 (10) & 0.29 (6) & 4.9 \\ \cline{2-10}  & & Al-Ti & \(Fm\)/\(Sm\) & Al & 2.81 (1) & 0.0090 (8) & \(-\)1.3 (9) & 0.26 (35) & \\ \cline{2-10}  & & & & Na & 3.15 (5) & 0.05 (2) & & & \\ \cline{2-10}  & & Na(Al, Ti)H\({}_{4}\) & \(I_{41}\)/\(\alpha\) & & Al & 3.37 (5) & 0.007 (2) & \(-\)4.0 (36) & 0.23 (23) & 1.1 \\ \cline{2-10}  & & & & Na & 3.37 (5) & 0.05 (2) & & & \\ \hline \multirow{2}{*}{after re-abs.} & \multirow{2}{*}{-} & \multirow{2}{*}{Al-Ti} & \multirow{2}{*}{\(Fm\)/\(Sm\)} & \multirow{2}{*}{Al} & \multirow{2}{*}{2.780 (15)} & \multirow{2}{*}{0.0140 (14)} & \multirow{2}{*}{\(-\)0.8 (8)} & \multirow{2}{*}{0.61} & \multirow{2}{*}{2.2} \\ \cline{1-1} \cline{5-10}  & & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Fitting results for the radial distribution functions obtained from the EXAFS spectra of NaAlH\({}_{4}\)–0.02 TICl\({}_{3}\) before the desorption and after the re-absorption of hydrogen. Figure 6: Fitting curves for radial distribution functions obtained from EXAFS spectra of NaAlH\({}_{4}\)–0.02 TICl\({}_{3}\) before the hydrogen desorption. The samples before the desorption and after the re-absorption of hydrogen are mainly composed of NaAlH\({}_{4}\), and those after the desorption are composed of NaH and Al, as mentioned above. Figure 5a shows the XANES spectra. The profile after the desorption is different from that before the desorption but almost overlaps with that after the re-absorption. The absorption edges after the desorption and the re-absorption are shifted to the lower energy side (reduced) than that before the desorption, and the oxidation number is 2.0. Figure 5b shows the radial distribution functions before the desorption, after the desorption, and after the re-absorption of hydrogen. The profiles after the desorption and re-absorption almost coincide with each other, similar to the XANES spectra, and the amplitudes are increased and sharpened compared with those before the desorption. Since the first shell peaks in the radial distribution functions after the desorption and re-absorption are similar, it is believed that Ti existing in NaAlH\({}_{4}\) never returns after the decomposition. The local structure after the re-absorption was analyzed with the Al-Ti model and fixed \(S_{0}^{2}=0.61\). Figure 7 and Table 2 present the results. The fitting converged to a reliability factor of 2.2%. This Al-Ti alloy was already formed after the desorption because of the similarity of the XAFS profiles with that after the re-absorption. substitution for Al and no vacancy formation, followed by the model with Ti substitution for Al and one H vacancy formed. In the case of adding 3.125-25% Ti to the bulk NaAlH\({}_{4}\), the least unfavorable arrangement compared with pure NaAlH\({}_{4}\) is Ti replacing Al in the lattice. In this study, the ground state stability of Na(Al, Ti)H\({}_{4}\) was investigated using a similar method, based on the results of AXS and XAFS.