Chemical Science Research Frontiers 2018 80 High-pressure synthesis of novel hydrides with high hydrogen densities – Li 3AlFeH 8 and LiAlFeH 6 Recent progress in synthesis techniques and theoretical calculations has enabled us to obtain novel hydrides with high hydrogen densities. Such hydrogen-rich materials are expected to be used as hydrogen storage materials. Some of them may show functionalities such as high-temperature superconductivity and fast ionic conductivity. We are synthesizing iron-containing complex hydrides because this class of materials consist of [FeH 6 ] 4– complex anions and tend to have high hydrogen densities, and iron is one of the most common metals. As example of iron-containing complex hydrides, we have already synthesized YLiFeH 6 and Li 4 FeH 6 . We are attempting to synthesize other iron- containing complex hydrides with higher hydrogen densities by incorporating H – anions into iron- containing complex hydrides. Since the [FeH 6 ] 4– complex anion is tetravalent and it is necessary to follow the 18-electron rule, possible combinations of countercations are limited to four types (Fig. 1(a)). When we try to synthesize a lightweight hydride, the combinations are severely limited. If we can incorporate H – anions into an iron complex hydride, the number of combinations of countercations will markedly increase as shown in Fig. 1(b). The purpose of the present study [1] is to synthesize the theoretically predicted Li 3 AlFeH 8 [2] using a high-pressure technique, which is a powerful technique for synthesizing novel hydrides. The starting material was a powder mixture of LiH, AlH 3 , and pure iron with a molar ratio of 3:1:1. The starting material was compacted into a small disk and was placed in a sample capsule made of boron nitride. The sample was pressurized to a target pressure ranging from 4- 7 GPa at room temperature and then heated 600 - 800°C. We changed the pressure and temperature to find the synthesis conditions with the aid of in situ measurements. Hydrogenation of the sample was achieved using a high-pressure cell developed by Fukai and Okuma [3]. A capsule made of NaCl was used as the hydrogen- sealing capsule. Hydrogen was evolved from a powder mixture of NaBH 4 and Ca(OH) 2 at around 400°C and confined in the hydrogen-sealing capsule. The sample was hydrogenated in a boron nitride capsule, in which hydrogen can permeate but other by-products from the internal hydrogen source cannot permeate. We observed the structural changes of the sample during the hydrogenation reaction in situ by synchrotron radiation X-ray diffraction (SR-XRD) measurement at SPring-8 BL14B1 . After the high- pressure treatment, the sample was quenched to room temperature and depressurized to ambient pressure. The recovered sample under ambient conditions was characterized by a conventional X-ray diffractometer. We searched for hydrogenation conditions of the powder mixture by in situ SR-XRD measurement. Figure 2 shows a series of X-ray diffraction profiles of the sample hydrogenated at 5 GPa and 600°C. New Bragg peaks were observed approximately 50 min after the sample was heated to 600°C in hydrogen fluid. The Bragg peaks were indexed by the unit cell of the theoretically predicted Li 3 AlFeH 8 , indicating that the powder mixture was hydrogenated to form Li 3 AlFeH 8 . The inset of Fig. 2 shows the time evolution of the peak intensity of the 011 Bragg peak from Li 3 AlFeH 8 . The increase in the peak intensity finished at around 300 min. Unreacted iron was still observed at this stage, indicating that a single phase of Li 3 AlFeH 8 cannot be obtained by a further hydrogenation reaction at 5 GPa and 600°C. The hydrogenated sample was recovered under ambient conditions. Figure 3(a) shows the powder X-ray diffraction profile of the sample and the calculated profile for theoretically predicted Li 3 AlFeH 8 Fig. 1. Schematics of charge neutrality in iron-containing complex hydrides (a) without H – anion incorporation and (b) with H – anion incorporation. Without H – anion incorporation M 1 + M 1 2+ M 1 2+ M 1 3+ M 2 2+ M 2 + M 2 + M 3 + M 3 + M 4 + M 1 + M 2 + M 2 + M 3 + M 3 + M 3 + M 5 + M 6 + M 4 + M 4 + With H – anion incorporation [FeH 6 ] 4 – [FeH 6 ] 4 – 2H – Tetravalent Hexavalent (a) (b) M 1 2+ M 1 2+ M 2 2+ M 3 2+ M 2 + + + + Research Frontiers 2018 81 (crystal structures shown in this article were drawn using the VESTA program [4]). The calculated profile reproduces the experimentally obtained one; we confirmed that the theoretically predicted Li 3 AlFeH 8 was synthesized. We searched for synthetic conditions where the single phase of Li 3 Al FeH 8 can be obtained. Unfortunately, we could not obtain a single phase of Li 3 AlFeH 8 up to 9 GPa and 900°C. Another novel hydride, LiAlFeH 6 , was found while optimizing the synthesis conditions of Li 3 AlFeH 8 . Figure 3(b) shows an X-ray diffraction profile of the recovered sample hydrogenated at 9 GPa and 900°C. The observed Bragg peaks were indexed by a hexagonal lattice. The chemical composition of the novel hydride was predicted to be LiAlFeH 6 based on the relationship between the constituent ions and the crystal structure volume reported by Sato et al. [5]. The crystal structure obtained by the first-principles calculations is shown in the inset of Fig. 3(b). The experimentally obtained X-ray diffraction profile was reproduced by the calculated profile for the theoretically predicted crystal structure. The theoretically predicted Li 3 AlFeH 8 was synthesized at 5 GPa. We did not obtain single-phase Li 3 AlFeH 8 ; however, another novel hydride, LiAlFeH 6 , was also synthesized while optimizing the reaction pressure-temperature conditions for Li 3 AlFeH 8 . We demonstrated that the combination of high- pressure synthesis and in situ SR-XRD is a powerful approach for obtaining novel hydrogen-rich materials. We are currently investigating the properties of the obtained hydrides. References [1] H. Saitoh, S. Takagi, T. Sato, Y. Iijima and S. Orimo: Int. J. Hydrogen Energy 42 (2017) 22489. [2] S. Takagi et al. : Appl. Phys. Lett. 104 (2014) 203901. [3] Y. Fukai and N. Okuma: Jpn. J. Appl. Phys. 32 (1993) L1256. [4] K. Momma and F. Izumi: J. Appl. Crystallogr. 41 (2008) 653. [5] T. Sato et al. : Sci. Rep. 6 (2016) 23592. Hiroyuki Saitoh National Institutes for Quantum and Radiological Science and Technology (QST) Email: saito.hiroyuki@qst.go.jp Fig. 2. (a) Series of synchrotron radiation powder X-ray diffraction profiles of the powder mixture hydrogenated at 5 GPa and 600°C. Bragg peaks with filled circles and Miller indices are from Li 3AlFeH 8. ( b) Time dependence of the integrated intensity of the 110 Bragg peak of Li 3 AlFeH 8 . Fig. 3. Powder X-ray diffraction profiles of recovered samples hydrogenated at (a) 5 GPa and 650°C and (b) 9 GPa and 900°C. The insets show the schematics of the crystal structures of (a) Li 3AlFeH 8 and (b) LiAlFeH 8 . Calculated profiles for the theoretically predicted crystal structures are shown in dashed lines. Bragg peaks with filled circles, filled squares, up-pointing triangles, and down-pointing triangles are from Li 3 AlFeH 8 , pure iron, LiAlFeH 6 , and an unknown phase, respectively. Intensity Lattice Spacing (Å) 020 011 120 11 – 1 111 031 2.5 2.75 3.0 3.25 3.5 4.0 4.5 0 100 200 300 400 Intensity of 020 peak Time (min) 100 min 200 min 300 min 5 GPa, 600°C (a) (b) a b c Intensity Fe unknown Li 3 AlFeH 8 Li 3 AlFeH 8 5 10 15 20 25 30 35 40 θ α 2 (deg . ) for MoK LiAlFeH 6 a [FeH 6 ] 4– H – Li + Al 3+ b c (a) (b) [FeH 6 ] 4– Li + Al 3+
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