## 3 Results
### Ti K-Edge X-ray Absorption Fine Structure (XAFS)  
The oxidation state and local structure of Ti in NaAlH\({}_{4}\)-\(x\)TiCl\({}_{3}\) (\(x\) = 0.005, 0.010, 0.015, 0.020, 0.040, and 0.060) before the desorption and after the re-absorption of hydrogen  
Figure 4: Relationship between the relative intensity and the amount of Ti substituted at Al sites. The solid blue line represents the Ti amount at the Al site in NaAlH\({}_{4}\) with statistical error (dotted green lines). Figure 3: Relative intensity obtained from anomalous X-ray scattering measurement of NaAlH\({}_{4}\)–0.02 TiCl\({}_{3}\) before the desorption. Simulation curves represent the Ti-substituted Al, Ti-substituted Na, and nonsubstituted cases. were investigated by Ti K-edge X-ray absorption fine structure with reference to standard samples of Ti, TiO, Ti\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), TiH\({}_{2}\), TiCl\({}_{3}\), TiAl, Ti\({}_{3}\)Al, and TiAl\({}_{3}\). Figure 5a shows the X-ray absorption near-edge structure (XANES) profiles of NaAlH\({}_{4}\)-0.02 TiCl\({}_{3}\) before the desorption and Ti, TiAl\({}_{3}\), TiH\({}_{2}\), and TiCl\({}_{3}\). The XANES spectrum of Ti in NaAlH\({}_{4}\)-0.02 TiCl\({}_{3}\) before the desorption was different from that of any of the standard samples. The Ti oxidation number is in the range of 2.3-2.5, as listed in Table S1. Hence, it can be inferred that the chemical state and local structure of Ti in NaAlH\({}_{4}\)-0.02 TiCl\({}_{3}\) are independent of the amount of TiCl\({}_{3}\). The local structure around Ti in NaAlH\({}_{4}\)-0.02 TiCl\({}_{3}\) before the desorption was analyzed for radial distribution functions obtained from the EXAFS spectra (green line in Figure 5b) using the three cluster models shown in Table 2. Model A is a single phase of Al-Ti alloy with an intrinsic loss factor, \(S_{0}^{2}\) = 0.61, which was determined from the measurement of the Ti foil. In model B, the reduction factor was treated as a free parameter for the same Al-Ti alloy. Model C is composed of two phases of Al-Ti alloy and NaAlH\({}_{4}\) with substitution of Ti at the Al site, in which \(S_{0}^{2}\) = 0.61 \(\times\) (phase ratio) \(\times\) (normalization factor for the coordination number). Here, the presence ratio of Al-Ti and Na(Al, Ti)H\({}_{4}\) is 0.7:1.3 because the total amount of Ti is 0.02 and Ti in Na(Al, Ti)H\({}_{4}\) is 0.013 from the above AXS result. The TiH\({}_{2}\) path was negligible because it was not detected in the XANES and EXAFS spectra. Na(Al, Ti)H\({}_{4}\) has some paths, but isotropic expansion and contraction are assumed. Figure 6 shows the fitting analysis results. Model A is inappropriate, with a high reliability factor (17.8%). Model B is slightly improved, but \(S_{0}^{2}\) is too low compared with that of  
Figure 5: **(a)** XANES spectra and (**b**) radial distribution functions of NaAlH\({}_{4}\)–0.02 TiCl\({}_{3}\) before the desorption, after the desorption, and after the re-absorption of hydrogen. the standard sample, indicating lower Ti-Al coordination numbers for the Al-Ti phase than expected. Model C is the best with the lowest reliability factor (1.1%), and the fitting curve is clearly improved for bond lengths longer than \(r\) = 2.5 A. The bond lengths of Ti-Na and Ti-Al in Na(Al, Ti)H\({}_{4}\) were 10% shorter than those of the averaged structure of Rietveld refinements; however, the Debye-Waller factors (Na: 0.05(2), Al: 0.007(2) A\({}^{2}\)) were consistent with those of (Na: 0.0342(5), Al: 0.0157(3) A\({}^{2}\)).