7 Quantum Beam Science Research Directorate, National Institutes for Quantum and Radiological Science and Technology, Sayo, Hyogo 679-5148, Japan; [EMAIL_ADDRESS] (C.N.) 8 Advanced Institute for Materials Research (WPI-AIMR), Hojuku University, Sendai, Miyagi 980-8577, Japan  
9 Cornergoneberace: ikaked@post-jparc.jpammonia borane, and metal-organic frameworks, which should meet the requirements of high storage capacity, appropriate thermodynamic properties, reversibility, and fast adsorption and desorption kinetics [1; 2]. Recently, complex hydrides have offered the possibility of designing a potential hydrogen storage system owing to their low weight and high hydrogen density. Complex hydrides are a group of materials that are a combination of hydrogen and group I and II salts of [AlH\({}_{4}\)]\({}^{-}\), [BH\({}_{4}\)]\({}^{-}\), and [NH\({}_{2}\)]\({}^{-}\), that is, alanates, borohydrides, and amides [3]. NaAlH\({}_{4}\) is a complex hydride [4]. Its crystal structure has been observed using single-crystal X-ray diffraction [5], and a space group of \(I\)4\({}_{1}\)/\(a\) with \(a\) = 5.0119(1) and \(c\) = 11.3147(5) A at 295 K has been determined for NaID\({}_{4}\) by powder neutron diffraction [6]. The Na atoms in NaAlD\({}_{4}\) are surrounded by eight D atoms from eight different [AlD\({}_{4}\)]\({}^{-}\) tetrahedra in the geometry of a distorted square antiprism. NaAlH\({}_{4}\) decomposes in the following three-step disproportionation process with a hydrogen desorption reaction [7]:  
\[\text{NaAlH}_{4}\ \rightleftarrows\ \frac{1}{3}\text{Na}_{3}\text{AlH}_{6}+ \frac{2}{3}\text{Al}+\text{H}_{2} \tag{1}\]  
\[\frac{1}{3}\text{Na}_{3}\text{AlH}_{6}\ \rightleftarrows\ \text{NaH}+\frac{1}{3} \text{Al}+\frac{1}{2}\text{H}_{2} \tag{2}\]  
\[\text{NaH}\ \rightarrow\ \text{Na}+\frac{1}{2}\text{H}_{2} \tag{3}\]  
Half of the hydrogen in NaAlH\({}_{4}\) (hydrogen-to-metal ratio; \(\Delta H/M\) = 1; 3.7 mass%) is attributed to Reaction (1) and a quarter (\(\Delta H/M\) = 0.5; 1.9 mass%) to Reaction (2). The melting point of NaAlH\({}_{4}\) is 454 K, and Na\({}_{3}\)AlH\({}_{6}\) decomposes at 525 K [8]. Reaction (3) (\(\Delta H/M\) = 0.5; 1.9 mass%) requires a temperature above 1000 K; Reactions (1) and (2) are important for hydrogen storage properties. Because all the reactions are sluggish, Reactions (1) and (2) are sometimes considered irreversible. [PERSON] and [PERSON] demonstrated that doping the complex metal hydride NaAlH\({}_{4}\) with a few mol% of \(\beta\)-TiCl\({}_{3}\) or Ti(OBu)\({}_{4}\) lowers the decomposition temperature, improves the kinetics, and, importantly, allows re-absorption of hydrogen for the decomposition products (recombination) [9]. In the system of NaAlH\({}_{4}\) with Ti compounds, the above two-step reaction proceeds below the melting points [10]. The subsequent hydrogen desorption behavior of TiCl\({}_{3}\)-doped NaAlH\({}_{4}\) seems to reproduce the first hydrogen desorption (\(\Delta H/M\) = 1.25; 4.6 mass%), except for the lower reversible hydrogen capacity (\(\Delta H/M\) = 1.0-1.1; 3.8-4.0 mass%) [10; 11]. Improvements in the kinetics of hydrogen desorption/absorption slowed with increasing amounts of added TiCl\({}_{3}\). However, an Arrhenius analysis of the hydrogen desorption indicated that there is a significant drop in the activation energy of the decomposition reaction, \(E_{a}\), upon the addition of TiCl\({}_{3}\) (\(-\)40% and \(-\)20% for NaAlH\({}_{4}\) and Na\({}_{3}\)AlH\({}_{6}\), respectively, with the addition of 0.9 mol% of TiCl\({}_{3}\)), which then remains approximately constant for higher doping levels [12]. Only the Arrhenius pre-exponential factor was TiCl\({}_{3}\)-concentration dependent. This suggests that the presence of Ti is a starting point for the recombination reaction of the undoped material. After the first report on Ti-doped NaAlH\({}_{4}\), other Ti-source materials have been explored as dopants. [PERSON] et al.