Chemical Science Research Frontiers 2022 90 Spin tube oxide obtained by topochemical dehydration Dehydration is an essential reaction for maintaining biological functions such as maintaining a constant body temperature and is also involved in other simple organic reactions such as the synthesis of ethers from alcohols [1]. In the case of inorganic materials, dehydration reactions can be seen, for example, in the processes of molding and baking clay during which our ancestors made earthenware and pottery. The Hayabusa2 spacecraft, which succeeded in its mission in 2020, has shown that the near-Earth asteroid Ryugu has an unexpectedly low water content resulting from its surface material being composed of carbonaceous chondrites that have undergone heating and dehydration [2]. Oxides, which represent inorganic materials, exhibit various properties that make them function as magnets, batteries, catalysts, and superconductors among others. Interestingly, functionalities of oxide materials can be achieved through the successful control of oxygen content. For example, in the high- T c oxide superconductor YBa 2 Cu 3 O 7– x , the transition temperature of superconductivity can be tuned by adjusting the oxygen content x [3]. The main method of controlling the oxygen content of oxides has been to adjust the gas atmosphere during synthesis at high temperatures. Recently, however, it has been reported that low-temperature topochemical reactions using metal hydrides such as CaH 2 are powerful methods to remove oxygen ions from oxides during reduction reactions [4]. In this study, we have succeeded in obtaining a new material, SrCoO 2 , by electrochemical insertion of a proton (H + ) into the oxide SrCoO 2.5 , followed by heating and a dehydration reaction ( Fig. 1(a)) [5]. Here, the cobalt valence is reduced from trivalence in the starting material SrCoO 2.5 to divalence in the reactant SrCoO 2 , indicating that this method of combining protonation and dehydration is a powerful reduction reaction process. Note that SrCoO 2 cannot be obtained by the reduction reaction using metal hydrides, which have been known as the strongest reducing agent. We h a v e p e r f o r m e d s o f t X - r a y a b s o r p t i o n spectroscopy (sXAS) experiments at SPring-8 BL25SU to investigate the local structure and valence state of SrCoO 2.5 and materials obtained by our developed new reduction reaction method using protonation and dehydration. Figure 1(b) shows the sXAS spectra around the Co L -edge for the samples before and after protonation and after subsequent dehydration in a vacuum. The electrochemical protonation of SrCoO 2.5 results in HSrCoO 2.5 , indicating that the Co L -edge shifts to a lower energy accompanied by the reduction from Co 3+ to Co 2+ . On the other hand, after HSrCoO 2.5 is dehydrated at a high temperature, the position of the Co L -edge does not change, indicating that the divalent state Co 2+ is retained during the dehydration reaction. However, the spectral shape changed significantly, indicating a significant change in the local structure around Co cations after dehydration. That is, the spectrum of HSrCoO 2.5 before dehydration contains a mixture of spectra corresponding to both octahedral and tetrahedral coordination around Co cations, but after heating and dehydration, the former spectrum Fig. 1. (a) New reduction process using the combination of electrochemical protonation and subsequent dehydration discovered in this study. (b – c) Experimental results of sXAS measurements performed at SPring-8 BL25SU. (a) SrCoO 2.5 HSrCoO 2.5 SrCoO 2 SrCoO 2 (SrCoO 2 ) (HSrCoO 2.5 ) (SrCoO 2.5 ) H + H 2 O dehydration protonation (b) Energy (eV) Energy (eV) 770 780 790 800 775 785 795 805 Intensity (arb. units) Intensity (arb. units) Before protonation After protonation After protonation and dehydration 778.7 778.8 780.1 776 778 780 782 784 Distorted a’ b’ c’ d’ e’ e d c b a Tetrahedron Tetrahedron Octahedron Square planar (c) Research Frontiers 2022 91 corresponding to octahedral coordination disappears and the only that corresponding to the distorted tetrahedral coordination is observed ( Fig. 1(c)). This result was also confirmed by the results of in situ sXAS experiments (Fig. 2). This means that a large number of oxygen ions can be removed from HSrCoO 2.5 at about 300℃ during the dehydration reaction (HSrCoO 2.5 → SrCoO 2 + 0.5H 2 O). SrCoO 2 obtained through this reaction is a new material, and its crystal structure was also determined by scanning transmission electron microscopy (STEM) experiments and complemental theoretical calculations (Fig. 3(a)). The feature of this new reduction reaction method is the existence of a protonated intermediate HSrCoO 2.5 ( Fig. 1(a) center), in which the CoO 6 octahedron is distorted upon protonation and destabilized under local strain. It is therefore considered that stable water (H 2 O) can be removed along with oxygen ions from the crystal lattice of HSrCoO 2.5 by heating and dehydration, resulting in SrCoO 2 ( Fig. 1(a) right). Thus far, in perovskite-related AB O 2 ( A , alkaline-earth metal ions or rare-earth metal ions; B , transition metal ions) compounds, only those with iron (Fe), copper (Cu), and nickel (Ni) at the B site are known to exist. Therefore, it is interesting and important to note that a new variation of cobalt Co was obtained in this study by a new reduction process in which the oxide is protonated followed by a dehydration reaction. The insertion of H + into oxide thin films using the ionic liquid gating technique is a recent subject of research worldwide. However, there has been no research on the concept of using an electrochemically protonated material as a precursor for a dehydration reaction to synthesize a new material with low oxygen content, as in this study. Therefore, this study is the first demonstration that the dehydration reaction after protonation is a powerful method of reduction reaction to remove many oxygen ions from an oxide. In the future, it is expected that this reducing reaction will be used to develop new oxide functional materials. For example, it will be interesting to apply this method to powder samples and to develop reduced oxides with new structures in combination with the substrate strain arising from the thin film growth. In terms of applications, the potential utilization in the development of hydrogen storage materials holds promise. Moreover, SrCoO 2 obtained in this study has a unique crystal structure called a four-leg spin tube (Fig. 1(a) right and Fig. 3(b)). This structure closely resembles the “spin ladder structure” that has attracted attention in the field of condensed matter physics since the discovery of high- T c cuprate superconductors. Therefore, it is expected to be a new platform for the emergence of novel quantum magnetism and superconductivity, and it will be interesting to see the future experimental and theoretical development of this research. Hiroshi Takatsu *, Hao-Bo Li † and Hiroshi Kageyama Graduate School of Engineering, Kyoto University *Email: takatsu@scl.kyoto-u.ac.jp † Present address: SANKEN, Osaka University References [1] A. D. McNaught and A. Wilkinson: Compendium of Chemical Terminology, 2nd ed. (Blackwell Science: Oxford, UK, 1997). [2] T. Yokoyama et al .: Science 379 (2022) 6634. [3] R. J. Cava et al .: Phys. Rev. B 36 (1987) 5719. [4] e.g., M. Hayward et al .: J. Am. Chem. Soc. 121 (1999) 8843. [5] H.-B. Li, S. Kobayashi, C. Zhong, M. Namba, Y. Cao, D. Kato, Y. Kotani, Q. Lin, M. Wu, W.-H. Wang, M. Kobayashi, K. Fujita, C. Tassel, T. Terashima, A. Kuwabara, Y. Kobayashi, H. Takatsu and H. Kageyama: J. Am. Chem. Soc. 143 (2021) 17517. Fig. 2. In situ sXAS spectra of the Co L -edge of HSrCoO 2.5 during heating in a vacuum. Arrows indicate the characteristic peaks of the CoO 6 octahedron, which disappear at high temperatures in a vacuum. Fig. 3. (a) High-angle annular dark-field STEM image of SrCoO 2 . (b) Schematic view of the relationship between tour-legged spin tube structure of SrCoO 2 and spin ladder structure. Room in situ Co L -edge temp. 100 ° C 200 ° C 250 ° C 300 ° C 350 ° C Energy (eV) 770 775 780 785 790 795 800 805 Intensity (arb. units) four-legged spin tube structure Co spin ladder structure 3D a p O4 O1 O5 O3 O2 c p ≈ 0.5 nm 0.5 nm (a) (b) SrCoO 2 b p
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