## 4 Discussion  
The neutron and X-ray diffraction profiles indicate that NaCl and Al form by doping TiCl\({}_{3}\) with NaAlH\({}_{4}\), according to Equation (4). No Ti-related phase was observed in the diffraction patterns immediately after the addition, whereas Al-Ti alloys with a shoulder on the low scattering angle side of the Al diffraction peak were detected after hydrogen desorption/absorption cycles. This implies the formation of Al-Ti alloys with an fcc structure. However, to detect trace elements, the AXS method is more reliable than typical diffraction measurements because of its elemental selectivity. In this study, the AXS measurements revealed that 1.3% of Ti was substituted at the Al site in NaAlH\({}_{4}\) immediately after the addition of TiCl\({}_{3}\). So far, substitution of NaAlH\({}_{4}\) with Ti has been suggested, but it has not been applied because the lattice constants do not change before and after the addition. Additionally, Ti K-edge XAFS supports that Ti occupies the Al site in NaAlH\({}_{4}\) before the hydrogen desorption and that Ti forms Al-Ti alloy. Furthermore, the neutron and X-ray diffraction refinement results suggest the substitution, but the lattice parameter  
Figure 8: Variations of the enthalpy of formation, \(\Delta H_{\rm F}\), and the heat of substitution, \(\Delta H_{\rm S}\), as a function of the Ti content in doped NaAlH\({}_{4}\). change was negligible, as in previous reports. The local structure change with shortening of the bond lengths of Ti-Na and Ti-Al derived from the EXAFS analysis may induce a lower symmetry of the crystal structure. Isolated and dispersed nanoparticles of Al-Ti are expected to precipitate because the alloy was not detected in the neutron and X-ray diffraction profiles, and long-range correlation was not present in the EXAFS spectra. The Ti K-edge EXAFS spectrum after the hydrogen desorption was reproduced with an fcc Al-Ti alloy. This Al-Ti phase originates from the substitution of Ti at the Al site in NaAlH\({}_{4}\). Based on the XANES spectra, the oxidation number of Ti was changed from 2.4 to 2.0 through the hydrogen desorption. The similarity in the XANES and EXAFS spectra after the desorption and after the re-absorption of hydrogen shows that Ti was not substituted into recombined NaAlH\({}_{4}\) again and that the formation of Al-Ti is irreversible. Regarding the cyclic properties of NaAlH\({}_{4}\) with Ti dopants, the hydrogen amount after the second time is approximately 20% less than that of the first, independent of the additive species. This indicates that parts of Al and Ti in NaAlH\({}_{4}\) are consumed to irreversibly form Al-Ti alloys. The hydrogenation-decomposition-desorption-recombination (HDDR) process is known as microstructure control technology [56]. This disproportionation reaction under a hydrogen atmosphere is sometimes utilized to obtain very fine crystalline grains or metastable phases. A similar reaction may occur in the first desorption/absorption reaction of the NaAlH\({}_{4}\)-TiCl\({}_{3}\) system to prepare highly dispersed Al-Ti nanoparticles working as a catalyst for the subsequent hydrogen desorption/re-absorption. DFT calculations revealed that the NaAlH\({}_{4}\) substituted by the amount of Ti in the AXS result is metastable. Because their configurational entropies are low (-0.019 to -0.33 kJ/mol atom for \(x\) = 0.0078-0.0156), the stability of a single phase of the doped NaAlH\({}_{4}\) is not expected. However, the actual doping reaction includes the formation of other stable phases of NaCl and Al-Ti, as expressed in Equation (4). Gibbs energy calculations above the room temperature have shown the possibility that a high driving force with the formation of NaCl proceeds the addition reaction of TiCl\({}_{3}\) in NaAlH\({}_{4}\)[24]. On the other hand, Ti does not return to NaAlH\({}_{4}\) without the energy gain of the NaCl formation because the single phase of the doped NaAlH\({}_{4}\) is unstable.