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Erika Fukushi, Fumiya Mori, Kota Munefusa, [Takayuki Harada](https://orcid.org/0000-0002-8657-2258), Hiroyuki Oguchi
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This document is the unedited Author’s version of a Submitted Work that was subsequently accepted for publication in ACS Applied Energy Materials, copyright © 2024 American Chemical Society after peer review. To access the final edited and published work see https://doi.org/10.1021/acsaem.3c03188[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)
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[Epitaxial Thin Film Growth of Perovskite Hydrides MLiH3 (M : Sr, Ba) for the Study of Intrinsic Hydride-Ion Conduction](https://mdr.nims.go.jp/datasets/4ed5dfcf-ff07-44c8-b340-18b12c612151)
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Template for Electronic Submission to ACS JournalsEpitaxial Thin Film Growth of Perovskite Hydrides MLiH3 (M : Sr, Ba) for The Study of Intrinsic Hydride-Ion Conduction Erika Fukushi1, Fumiya Mori1, Kota Munefusa1, Takayuki Harada2 and Hiroyuki Oguchi1* 1Department of Applied Chemistry, Graduate School of Engineering and Science, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo, Japan 2Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, 1-1 Namiki, Tsu-kuba-shi, Ibaraki, Japan Perovskite hydride, Hydride-ion conduction, Epitaxial thin film, Radical hydrogen ABSTRACT: Perovskite hydrides, with their excellent elemental substitution ability, are ideal materials for stimulating re-search on the physical properties of hydrides through material design. For the material design, it is necessary to correctly understand the physical property of perovskite hydrides and to elucidate its origin. In this regard, studies using single-crystal epitaxial thin films that enable observation of the intrinsic properties of the materials are effective. In this study, MLiH3 (M : Sr, Ba) epitaxial thin films were synthesized by radical hydrogen reactive infrared laser deposition, which is a highly efficient method for growing hydride phases using the high reactivity of radical hydrogen. Furthermore, the intrinsic hydride-ion con-ductivity, which is not affected by grain boundaries, was successfully determined for SrLiH3 using the synthesized epitaxial thin film. These results established the basic technology for the epitaxial growth of perovskite hydrides and paved the way for the research on the control of physical properties by elemental substitution in perovskite hydrides. 1. Introduction Hydrides are a series of compounds in which a negatively charged hydride-ion (H-) is bonded to other elements. Hy-drides are attractive targets for the development of func-tional materials because they can often exhibit special prop-erties derived from hydrogen. Examples of hydrogen-de-rived properties include hydrogen storage properties,1, 2 high-temperature superconductivity,3, 4, 5, 6and hydride-ion conduction.7, 8 These properties have the potential to be used to develop energy-saving materials such as supercon-ducting power cables. The development of energy devices such as new types of batteries and fuel cells using hydride-ion conduction is also expected. Therefore, hydrides are ma-terials that could revolutionize the energy field. Perovskite hydrides9, 10, 11, 12, 13 are ideal materials for stim-ulating research on the physical properties of hydrides. This is because perovskite hydrides, which have a perovskite structure with high elemental substitution ability, are ex-pected to create and improve various physical properties. In fact, in the perovskite hydride AB(BH4)3, the electronic structures and luminescence properties were changed by lattice deformation through the combination of various ele-ments.14 Furthermore, in the perovskite hydride MLiH3 (M : Ba, Sr), the hydride-ion conductivity were increased by sub-stituting divalent elements M with monovalent elements.15 To accelerate research on perovskite hydrides, accurate characterization of physical properties is necessary since it can provide us clues to clarify the origin of physical proper-ties. However, accurate characterization of physical proper-ties has often been difficult in perovskite hydride research to date. For example, in the case of MLiH3 introduced earlier, the experiments were conducted on a powder sample with a high percentage of grain boundaries, so the impedance measurement data was strongly affected by the grain boundaries, which obscured the original hydride-ion con-ductivity of the MLiH3 crystal. One effective means of accurately characterizing the in-trinsic properties of a material is to use epitaxial thin films,16 which are single-crystalline thin films. Fortunately, research on epitaxial thin films of hydrides, which had pre-viously been reported in only three cases,17, 18, 19 has re-cently been revitalized, and the number of reported synthe-ses is increasing. So far, as epitaxial thin films, LiH has been synthesized by vapor-liquid-solid growth20 and pulsed laser deposition,21 TiH2 by pulsed laser deposition,22 and TiH2 MgH2, YH2, NbH3 and EuH2 by sputtering,23, 24, 25, 26, 27 respec-tively. In addition to these, the complex hydrides LiBH4 and NaBH4 with molecular [BH4]2- complex anions in their struc-tures have been synthesized by infrared laser evapora-tion.28, 29 However, these are all binary or quasi-binary hy-drides, and there have been no reports of epitaxial thin film synthesis of perovskite hydrides, which are ternary hy-drides that are expected to be more difficult to synthesize than binary hydrides. In this study, we firstly determine the synthesis tempera-ture suitable for MLiH3 epitaxial thin film synthesis. Next, the effect of radical hydrogen on metal residues in MLiH3 thin films will be examined. Epitaxial thin film synthesis is then performed under conditions determined to be appro-priate based on the results of these investigations. Finally, the epitaxial films will be used to evaluate the intrinsic hy-dride-ion conductivity of SrLiH3. 2. EXPERIMENTAL SECTION MLiH3 epitaxial thin films were synthesized by a radical hydrogen reactive infrared laser deposition method that we developed for this study (Figure 1). This method is an im-provement version of the infrared laser evaporation, which is a powerful method for organic thin film synthesis28, 29, 30. The most outstanding feature of the new technique is that deposition can be carried out in a radical hydrogen atmos-phere, which strongly promotes the reaction between the metal and hydrogen. Therefore, it is expected to synthesize single-phase hydride thin films by completely hydrogenat-ing the metal that tends to remain in the film. The films were deposited in an ultra-high vacuum chamber with a back pressure of less than 1.0 × 10-8 Torr. Radical hydrogen was generated by flowing H2 gas at 1.0 × 10-2 Torr through a high-temperature tungsten filament attached inside the chamber. The substrates used in this study were MgAl2O4(100) (lattice mismatch is -0.5 % for BaLiH3 and -5.2 % for SrLiH3) and LaAlO3(100) (lattice mismatch is +0.3 % for SrLiH3). The substrate temperature during dep-osition was set in the range of 100 - 170 °C to promote film crystallization and prevent hydrogen desorption from the films. The target was a disk-shaped pellet with a diameter of 20 mm and a thickness of about 1.5 mm, obtained by uni-axially pressurizing a powder mixture of MH2 and LiH at a molar ratio of 1:1. A non-focused continuous wave infrared laser (wavelength 808 nm) (LIMO32-F-400-DL808-EX2024) was used to heat the target. The target was rotated at a speed of approximately 60 - 120 rpm to ensure homo-geneous heating and to stabilize the deposition rate. The target-substrate distance was about 60 mm and the deposi-tion rate was 0.5 - 1.0 Å / sec. The crystallinity and orientation of the thin films were evaluated by X-ray diffraction (XRD) measurements (SmartLab, Rigaku and Empyrean 3, Malvern). To prevent sample degradation during the measurements, Cu with a thickness of 0.5 μm to 1 μm was deposited on the films as a protective layer. Surface conditions were evaluated with an optical microscope (ME-LUX2, KYOWA OPTICAL CO.), scan-ning electron microscope (SEM) (JSM-7800F, JEOL), and atomic force microscope (AFM) (MultiMode 8, Bruker AXS). Film thickness was determined by cross-sectional observa-tion by SEM. In-plane hydride-ion conductivity was evalu-ated using an electrochemical impedance measurement system (SP-150, Biologic) in a vacuum chamber (back pres-sure: 1 × 10-7 Torr). The measurement temperature range was from room temperature to 200 °C, and the applied fre-quency range was 1 Hz ~ 1 MHz. Comb-shaped electrodes made by Mo were deposited at room temperature in a vac-uum chamber using a patterned shadow mask (Figure S1). To prevent degradation of the thin films, all of the above ex-perimental manipulations and characterizations were per-formed in a non-air-exposure environment. 3. RESULTS AND DISCUSSION Initially, we searched for a suitable synthesis temperature for the growth of BaLiH3 thin films by radical hydrogen re-active infrared laser evaporation. The H2 gas pressure in the Figure 1. Schematic of MLiH3 epitaxial thin film synthe-sis by radical hydrogen reactive infrared laser evapora-tion. On the substrate, metal M and metal Li react with hydrogen to grow MLiH3 phase. Hydrogen supplied by the MH2-LiH target are used in film growth, too. Figure 2. The 2θ/θ XRD patterns of BaLiH3 films deposited at 100, 130, 150, and 170 °C. The vertical axis is offset to make it easier to see the patterns for different tempera-tures. radical hydrogen generator was set to 1.0 × 10-2 Torr and the W filament temperature Tf to 800 °C. Figure 2 shows 2θ/θ XRD diffraction patterns of BaLiH3 thin films grown on MgAl2O4(100) substrates at temperatures between 100 and 170 °C. BaLiH3-derived diffraction peaks were observed at all temperatures. The most clearly observed peaks were h00 (h = 1, 2, 3) diffraction ones. In addition to those peaks, the films synthesized at 100 ºC and 130 ºC also exhibited 110 (2θ = 31.5°), 111 (2θ = 38.8°), 210 (2θ = 50.5°), 211 (2θ = 56.0°), and 220 (2θ = 65.7°) diffraction peaks. At these temperatures, orientation was incomplete, probably be-cause the crystal growth rate was not fast enough. At a dep-osition temperature of 150 °C, all but the diffraction peaks from the (h00) plane was very small, and a film with almost (100) orientation was obtained. However, when the tem-perature was further increased to 170 °C, diffraction peaks other than those from the (h00) plane became prominent again, suggesting a decrease in orientation. It may due to hy-drogen desorption from the film, causing compositional de-viations that distorted the crystals, resulting in a decrease in orientation. Based on these results, we concluded that 150 °C was appropriate for BaLiH3 film deposition. Next, to verify the effect of radical hydrogen on BaLiH3 deposition, films were deposited when H2 and radical hy-drogen H gases were used as the reaction gas. In the latter case, amount of radical hydrogen was changed by changing the W filament temperature Tf in the radical hydrogen gen-erator to 800 °C and 2000 °C. The experiment at Tf = 2000 °C was expected to generate a larger amount of radical hydro-gen than at Tf = 800 °C.31 32 For all three cases, H2 gas pres-sure flown into the chamber was 1.0 × 10-2 Torr. MgAl2O4(100) was used as the substrate, and the substrate temperature was set at 150 °C during deposition. Figure 3 (a)-(c) shows optical microscope images of the films depos-ited under each condition. According to theoretical calcula-tions,9, 10, 11, 33 a band gap of the BaLiH3 is 1 – 4 eV and thus it should be transparent. However, the films synthesized with H2 and radical hydrogen (Tf = 800 °C) were opaque and shiny. When these films were observed under a microscope, light-reflecting areas that appeared to be Ba metal or Li metal were dispersed all over the observed image. The pres-ence of metal was also confirmed by the fact that the re-sistance of these films shown in the upper right corner of each figure was less than 40 Ω. XRD 2θ/θ patterns of these films (Figure S2) showed diffraction peaks attributed to Ba-LiH3, suggesting that the films were synthesized as a mix-ture of BaLiH3 and metal phases. In contrast, the film syn-thesized under radical hydrogen (Tf = 2000 °C) was almost transparent, and the light-reflecting areas disappeared from the microscopic images, suggesting that the entire film was BaLiH3. For this sample, the resistance was above the multimeter's measurement range. These results indicate that a nearly single-phase BaLiH3 film with no metal con-tamination could be obtained by supplying a large amount of radical hydrogen to the film. Figure 3. Optical microscope images of BaLiH3 thin films synthesized under (a) H2 gas, (b) radical hydrogen gas with low concentration (The W filament temperature Tf = 800 °C), and (c) radical hydrogen gas with high concentration Tf = 2000 °C). For (a) – (c), hydrogen gas pressure PH2 was set to 1.0 × 10-2 Torr. The upper right of these figures shows a photograph of the sample and the film resistance measured with a multimeter. (d) AFM image of the film in (b). (e) AFM image of the film in (c). AFM observations (Fig. 3(d) and (e)) of these films showed that high concentrations of radical hydrogen also contrib-uted to the improvement of the flatness of the BaLiH3 film. At Tf = 800 °C, the RMS average roughness of the entire im-age was as high as about 160 nm due to the scattered islands with diameters of 1 - 2 μm and heights of up to about 1 μm. On the other hand, at Tf = 2000 °C, the islands disappeared from the surface, resulting in a decrease in RMS average roughness to 13 nm. The BaLiH3 epitaxial thin film was successfully grown on a MgAl2O4(100) substrate under the conditions (substrate temperature 150 °C, PH2 = 1.0 × 10-2 Torr, Tf = 2000 °C) that yielded a flat, nearly single-phase BaLiH3 film. Figure 4(a) shows the XRD 2θ/θ diffraction pattern. The h00 diffraction peaks of BaLiH3 are the only peaks observed except for the substrate and the Cu protective layer, indicating that the Ba-LiH3 epitaxial film is almost a singleFigure 4. (a) 2θ/θ XRD diffraction pattern and (b) XRD rocking curve of BaLiH3 thin film synthesized under optimal conditions. In the inset of (a), the area around the LiH 200 diffraction peak is enlarged. (c) φ scan pattern of a 111diffraction plane for BaLiH3 thin film and MgAl2O4 substrate. (d) Atomic arrangement of the epitaxial thin film on the substrate expected from the results of (a) and (b). phase film with a (100) orientation. A shoulder peak on the side of the 200 diffraction peak of BaLiH3 suggesting the presence of small amount of LiH. The rocking curve FWHM of the BaLiH3 100 diffraction was 1.61°(Figure 4(b)). This FWHM value is relatively large when it is compared with other epitaxial films of hydrides.20, 21, 24, 27, 28, 29 Possibly the oxide substrate surface was reduced and roughened by highly reductive radical hydrogen and crystallinity of the BaLiH3 was lowered. The XRD φ scan shows 111 diffraction peaks at the same rotation angle for the film and substrate, indicating that the in-plane epitaxial relationship is [001]Ba-LiH3 ‖ [001]sub (Figure 4(c)(d )). Cross-sectional SEM observations and energy dispersive spectroscopy (EDS) measurements of the BaLiH3 epitaxial film indicated that it was a dense film of almost constant thickness and that there was almost no elemental diffusion between the film and the substrate. These characteristics make these films suitable for accurate evaluation of hy-dride-ion conduction. Figure 5 shows a cross-sectional SEM image of a BaLiH3 epitaxial film prepared by cracking a MgAl2O4(100) substrate after growth of the film. The film thickness was about 1 μm at all positions on the substrate. In the EDS elemental mapping, Ba was detected only at the location of the film in the SEM image, and other elements (O, Mg, Al) were detected only at the location of the substrate. The synthesis of SrLiH3 was performed under the same conditions that produced high-quality epitaxial BaLiH3 films, and high-quality SrLiH3 epitaxial films were also success-fully obtained. Figure 6(a) and (b) show XRD 2θ/θ diffrac-tion patterns of samples grown on two different substrates (MgAl2O4 and LaAlO3). SrLiH3-derived diffraction peaks were observed on both substrates. Comparing two samples, Fig. 5 A cross sectional SEM image (a) and EDS elemental mappings for (b) Ba, (c) O, (d) Mg, and (e) Al of a BaLiH3 epitaxial thin film grown on a MgAl2O4(100) substrate. Inset in (a) shows a surface SEM image of this film. Figure 6. A set of XRD measurements for the SrLiH3 films grown on a MgAl2O4 substrate ((a), (c), (e)) and a LaAlO3 substrate ((b), (d), (f)). (a), (b) The XRD 2θ/θ patterns. Inset shows magnified view around LiH 200 diffraction peak. (c), (d) XRD rocking curves. (e), (f) XRD φ scan patterns. the peak intensity was higher on the lattice-matched LaAlO3 substrate than MgAl2O4. The half-width of the rocking curve was also greatly affected by the lattice matching between the films and substrates, with the half-widths of 0.74° and 2.38° for the LAO and MgAl2O4 substrates, respectively (Fig-ure 6(c) and (d)). The h00 diffraction peak of the films on the LaAlO3 substrate was more dominant than that on the MgAl2O4 substrate, indicating that the (100) orientation of the films was also higher. In the XRD φ scan profiles, the 111 diffraction peaks appeared at the same rotation angle for the SrLiH3 films and the substrates, indicating that the in-plane epitaxial relationship is [001]SrLiH3 ‖ [001]sub (Figure 6(e) and (f)). SEM observations and EDS measurements of the film cross section confirmed that the SrLiH3 film, simi-larly for the BaLiH3 film, was a uniform and dense film suit-able for the evaluation of hydride-ion conduction (Figure S3). The LiH 200 diffraction peak observed in the BaLiH3 film was again observed in the SrLiH3 films. Finally, the hydride-ion conductivity of SrLiH3 epitaxial films was evaluated by electrochemical impedance method to demonstrate that the intrinsic hydride-ion conduction of the material can be observed with the epitaxial films. The reason why we chose SrLiH3 epitaxial films here is that it is known to exhibit higher hydride-ion conductivity than Ba-LiH3.15 Figure 7(a) shows the Nyquist plot obtained by measurement at 200 °C. Except for a slight deviation on the low frequency side, the plot was ideal semi-circular. This re-sult contrasted with the earlier report for the SrLiH3 pow-der.15 The capacitance value calculated from the data analy-sis (circle fitting) is 1.1 × 10-10 F, which confirms that we are indeed observing hydride-ion conduction15 within the crys-tal grains. Fitting to the low-frequency side of the Nyquist plot yielded a capacitance value of 3.2 × 10-8 F, suggesting hydride-ion conduction at the grain boundary. The Nyquist plot obtained by measurement at 300 °C (Figure 7(b)) showed a straight line in the low-frequency region suggest-ing hydride-ion diffusion at the electrode interface. The in-trinsic hydride-ion conductivity of the SrLiH3 crystal was determined by fitting Nyquist plots obtained in the temper-ature range from 70 °C to 300 °C (Figure 7(c)). The hydride-ion conductivity σ was 2.6 × 10-8 S cm-1 at 70 °C and 4.4 × 10-5 S cm-1 at 300 °C, about one-fifth lower than in the powder study. The activation energy was about 57 kJ mol-1, which was similar to the powder study.15 Comparing films on dif-ferent substrates (MgAl2O4 and LaAlO3), the conductivity was almost the same, indicating that the epitaxial films ena-bles us to accurately evaluate the hydride-ion conductivity of perovskite hydrides even if their crystallinity differs slightly. 4. CONCLUSION In this study, epitaxial thin films of BaLiH3 and SrLiH3 were successfully synthesized by using radical hydrogen reactive infrared laser evaporation. The success verified that radical hydrogen promoted the reaction between the metal and hy-drogen during film growth and played an essential role for obtaining high-quality perovskite hydride epitaxial thin films. For the SrLiH3 epitaxial thin film, electrochemical im-pedance measurements were performed. The shape of the obtained Nyquist plots was almost semicircular, corre-sponding to hydride-ion conduction in the SrLiH3 crystal. This result demonstrated that intrinsic hydride-ion conduc-tion can be evaluated using the epitaxial thin film. Conduc-tivity of the SrLiH3 epitaxial thin film was about one-fifth of the value reported in the powder study. In the future, this study may lead to the synthesis of epitaxial thin films of var-ious hydrides. In addition, the intrinsic physical properties of perovskite hydrides will be elucidated using epitaxial Figure 7. (a) A Nyquist plot obtained at 200 °C for the SrLiH3 epitaxial thin film deposited on a MgAl2O4 (100) substrate. The blue and pink semicircles were obtained by circle fitting of the Nyquist plot. A capacitance value calculated from the blue sem-icircle was 1.1 × 10-10, indicating that the semicircle was for the hydride-ion conduction of SrLiH3 crystal grains. In the same way, the pink semicircle with a capacitance value of 3.2 × 10-8 F indicated that it was for the grain boundary. (b) A Nyquist plot obtained at 300 °C. (c) Hydride-ion conductivity of SrLiH3 crystal grains determined by the Nyquist plots obtained at different temperatures. The conductivity of the film grown on the MgAl2O4 and LaAlO3 substrates are colored in red and blue, respec-tively. The black dashed line corresponds to conductivity of SrLiH3 powder reported in the previous study.15 thin films. Understanding of intrinsic physical properties will shed light on its origin and facilitate material design re-search of the perovskite hydrides, which should construct basis for the development of innovative functional materi-als that exhibit hydrogen-derived unique properties. ASSOCIATED CONTENT Electrochemical impedance measurement technique, addi-tional XRD data, crystal structure, SEM images. These materials are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Department of Applied Chemistry, Graduate School of Engi-neering and Science, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo, Japan E-mail: h-oguchi@shibaura-it.ac.jp Author Contributions E.F. and H.O. planned the experiments; E.F. performed synthe-sis, evaluation, and data analysis; F.M. and K.M. assisted with synthesis; and T.H. assisted with XRD measurements. All au-thors discussed the results and commented on the manu-script. Funding Sources Grant-in-Aid for Challenging Exploratory Research (7K19 K22225), the 56th Kowa Lithium Award from the Liberace Re-search Fund, the S-SPIRE project at Shibaura Institute of Tech-nology, and the NIMS Collaboration Center Promotion Pro-gram (2022-87) (2023-024). ACKNOWLEDGMENT This work was supported by Grant-in-Aid for Challenging Ex-ploratory Research (7K19 K22225), the 56th Kowa Lithium Award from the Liberace Research Fund, the S-SPIRE project at Shibaura Institute of Technology, and the NIMS Collaboration Center Promotion Program (2022-87) (2023-024). We thank Associate Professor Masaki Nakano of the Quantum Phase Elec-tronics Research Center, Department of Physics and Engineer-ing, Graduate School of Engineering, the University of Tokyo for his assistance with XRD measurements. Dr. Kyosuke Matsu-shita and Dr. Makiko Oshida of the Battery Research Platform, National Institute for Materials Science (NIMS) for their assis-tance with AFM and SEM measurements, and Dr. Masatomo Sumiya of the National Institute for Materials Science (NIMS) for development of a radical hydrogen generator. REFERENCES (1) Orimo, S.; Nakamori, Y.; Eliseo, J. R.; Züttel, A.; Jensen, C. R. Complex Hydrides for Hydrogen Storage. Chem. Rev. 2007, 107, 4111. 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