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Characteristic Fast Hвˆ' Ion Conduction in Oxygen-Substituted Lanthanum

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Characteristic Fast Hвˆ' Ion Conduction in Oxygen-Substituted Lanthanum ARTICLE https://doi.org/10.1038/s41467-019-10492-7 OPEN Characteristic fast H− ion conduction in oxygen-substituted lanthanum hydride Keiga Fukui1, Soshi Iimura1, Tomofumi Tada 2, Satoru Fujitsu2, Masato Sasase2, Hiromu Tamatsukuri3, Takashi Honda 3,4, Kazutaka Ikeda3,4, Toshiya Otomo3,4 & Hideo Hosono1,2 Fast ionic conductors have considerable potential to enable technological development for energy storage and conversion. Hydride (H−) ions are a unique species because of their 1234567890():,; natural abundance, light mass, and large polarizability. Herein, we investigate characteristic H− conduction, i.e., fast ionic conduction controlled by a pre-exponential factor. Oxygen- −2 −1 doped LaH3 (LaH3−2xOx) has an optimum ionic conductivity of 2.6 × 10 Scm , which to the best of our knowledge is the highest H− conductivity reported to date at intermediate temperatures. With increasing oxygen content, the relatively high activation energy remains unchanged, whereas the pre-exponential factor decreases dramatically. This extraordinarily large pre-exponential factor is explained by introducing temperature-dependent enthalpy, derived from H− trapped by lanthanum ions bonded to oxygen ions. Consequently, light mass and large polarizability of H−, and the framework comprising densely packed H− in LaH3−2xOx are crucial factors that impose significant temperature dependence on the potential energy and implement characteristic fast H− conduction. 1 Laboratory for Materials and Structures, Tokyo Institute of Technology, Yokohama 226-8503, Japan. 2 Materials Research Center for Element Strategy, Tokyo Institute of Technology, Yokohama 226-8503, Japan. 3 Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan. 4 Department of Materials Structure Science, The Graduate University for Advanced Studies, Tsukuba 305-0801, Japan. Correspondence and requests for materials should be addressed to S.I. (email: [email protected]) or to H.H. (email: [email protected]) NATURE COMMUNICATIONS | (2019) 10:2578 | https://doi.org/10.1038/s41467-019-10492-7 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-10492-7 ydride (H−) ions are electrochemically attractive charge determined by a room-temperature neutron powder diffraction Hcarriers not only because they have small ionic radii and a (NPD) measurement. Although the space group is tetragonal P4/ valenceelectronshellsuitabletoionicconductioninsolids nmm, which is a subgroup of fcc, the tetragonality is quite small − √ = but also because of the high standard redox potential of H2/H (c/ 2a 0.9991, see Supplementary Fig. 1 and Supplementary (−2.3 V). These characteristics make H− ions promising for Table 1 for detailed information on the structure)13. We also applications in next-generation electrochemical energy storage applied a Rietveld analysis by using another structure model with systems with high voltage and high energy density. To date, H− a disordered oxygen at the T site of the fcc lattice; however, the conduction has been reported in solid hydrides and oxyhydrides1–3. goodness of fit was worse than that for the tetragonal model. However, high H− conduction has been achieved only at relatively Moreover, electron diffraction done with a transmission electron high temperatures (0.04–0.2 S cm−1 at 450 °C–630 °C for the high- microscope detected no additional spots derived from the fcc 2 temperature phase of BaH2) . Unlocking a wider range of superstructure (see Supplementary Fig. 2), suggesting that oxygen applications in energy storage systems requires materials with a atoms are ordered over a very long range (We conducted the − high H conductivity at lower temperatures. neutron pair distribution function measurement on LaD3−2xOx = Hydrogen is the lightest element. Over the years, diffusion of with xnom 0.25 at 300 K to check the local structure. The pre- hydrogen in metals and that of protons (H+) in hydrogen- liminary result of fitting using the P4/nmm model to the data bonded liquids have attracted significant research attention both sufficiently reproduces the neutron pair distribution function experimentally and theoretically. The diffusion coefficient and pattern.). The LaD2O0.5 structure contains four symmetrically conductivity have many peculiar features in terms of magnitude different deuterium positions: D1 and D2 correspond to the T site and their dependence on isotope mass and temperature. These of LaH3 (the TD1 and TD2 sites, respectively), whereas D3 and D4 features are now understood as a consequence of quantum- correspond to the Oc site (the OcD3 and OcD4 sites, respectively). mechanical processes originating from small mass4. Contrary to the deuterium distributed to both the T and Oc sites, In addition to the smallest mass, a unique and important the oxygen dopant preferentially occupies the TD2 site. As − 5 − feature of H is its large electronic polarizability .H comprises reported for undoped LaH3, the OcD3 and OcD4 sites are not at one proton and two electrons. Contrary to a single proton with the center but are displaced by 0.85 and 0.57 Å from the center of − no electrons, H is regarded as a soft anion due to its large the La6 octahedron toward the TD1 and TD2 sites, respectively. electronic polarizability, which arises from its considerably small The unit cell of the P4/nmm lattice contains two metal sites, four electron affinity. O2− and F− have ionic radii of 1.2–1.3 Å, similar T sites, and two Oc sites for anions. According to the results of 6 − fi to that of H . However, the large polarizability of H is in sharp Rietveld re nement for LaD2.0O0.5 and LaD1.5O0.75, the chemical- contrast with those of anions, which have low polarizability and composition ratios in the unit cell of the P4/nmm are La: D: O = 7 fi = fi are typically considered as hard anions . In terms of ionic con- 2( xed): 3.67(1): 1.32(4) for LaD2.0O0.5 and La: D: O 2( xed): duction, the polarizability of counterpart ions or the framework is 2.76(4): 1.61(1) for LaD1.5O0.75 which are close to the nominal widely recognized as a key factor that reduces the activation ratios each other. energy of ion hopping8. For example, Li+ conduction in sulfides Figure 1c shows X-ray diffraction (XRD) patterns for several 9 2− − is much faster than that in oxides . Similarly, high O and F values of the xnom.. These data were collected after conductivity conduction occurs in solids based on highly polarizable cations, measurements at temperatures up to 340 °C. All XRD patterns δ = e.g., -Bi2O3 and PbF2, respectively, rather than in isostructural except for xnom 1.0 remain unchanged upon heating to high 10 Y2O3-stabilized ZrO2 and CaF2 . However, the polarizability of temperatures and can all be indexed to the P4/nmm structure. mobile ions and the concomitant effects on ionic conduction have At xnom = 1.0, the as-prepared sample synthesized at 750 MPa yet to be elucidated. forms a fcc structure but changes after the conductivity In this paper, we reveal a characteristic fast H− conduction measurement to other tetragonal structure which is reported to arising from its light mass and high polarizability, i.e., fast ionic be an ambient-pressure phase14,15. Rietveld refinements from the conduction controlled by a pre-exponential factor. To demon- XRD profiles indicate that the oxygen preferentially occupies − fi strate this, we choose the material system LaH3, in which H , the T site, which is consistent with the NPD result. The re ned rather than La3+, is the nearest neighbor of each H−. As shown in lattice parameter a multiplied by √2 and c are plotted in the Fig. 1, LaH3 crystalizes in a face-centered-cubic (fcc) structure inset of Fig. 1c. Both values are close each other, indicating the composed of a face-sharing La4 tetrahedron and a La6 octahe- small tetragonality. As xnom. increases, the two values gradually dron. Two of the three H− per La occupy the tetrahedral site increases because of oxygen incorporation at the T site with the 2− − 6,16 (T site) and the other is located at the octahedral site (Oc site). larger ionic radius of O than that of H in LaH3−2xOx . The minimum separation between H− ions at the Tet and at the We use thermal-desorption spectroscopy (TDS) to examine the Oc sites is only 2.4 Å, and the eight H− ions at the T sites and six dopability and occupation of H− in each site. Figure 1d shows fi at the Oc sites form of a rhombic dodecahedron encapsulating TDS pro les of LaH3−2xOx. Stoichiometric LaH3 has two distinct aLa3+ (Fig. 1a). desorption peaks: one near 300 °C and another near 800 °C. The 2− − Partial substitution of O in the H site of LaH3 (LaH3−2xOx) area ratio of the low- to high-temperature peaks is close to 1:2, is required to create a vacancy at either the T or Oc sites and to indicating that these peaks are assigned to the desorption of suppress electronic conduction by deepening the donor level hydrogen occupying the Oc and the T sites, respectively. The total (relative to the vacuum level), which is generated by a tiny defi- amount of hydrogen desorbed from each sample is estimated by ciency in hydrogen content. Because the ionic radius of H− is integrating the area of the two peaks (Fig. 1e), and the result is 2− − similar to that of O , both ions have a wide range of solubility in consistent with the fractional hydrogen content 3 2xnom. Here- various compounds such as LaFeAsO and BaTiO3, where the inafter we denote by x the oxygen content determined by this substituted H− is stabilized at the center of a positively charged analysis.
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