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[High-pressure synthesis of bilayer nickelate Sr3Ni2O5Cl2 with tetragonal crystal structure.pdf](https://mdr.nims.go.jp/filesets/2fbfc583-bd31-46e0-ab75-6fb3b8096ca5/download)

## Creator

[Kazuki Yamane](https://orcid.org/0000-0002-0162-5411), [Yoshitaka Matsushita](https://orcid.org/0000-0002-4968-8905), Shintaro Adachi, [Takanobu Hiroto](https://orcid.org/0000-0002-6176-5782), [Ryo Matsumoto](https://orcid.org/0000-0001-6294-5403), [Kensei Terashima](https://orcid.org/0000-0003-0375-3043), [Hiroya Sakurai](https://orcid.org/0000-0003-1964-6023), [Yoshihiko Takano](https://orcid.org/0000-0002-1541-6928)

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## Other metadata

[High-pressure synthesis of bilayer nickelate Sr<sub>3</sub>Ni<sub>2</sub>O<sub>5</sub>Cl<sub>2</sub> with a tetragonal crystal structure](https://mdr.nims.go.jp/datasets/5bfc8f1f-6ca2-4a6c-ae17-7c728d1d0a20)

## Fulltext

High-pressure synthesis of bilayer nickelate Sr3Ni2O5Cl2 with tetragonal crystal structure  Kazuki Yamaneab*, Yoshitaka Matsushitac, Shintaro Adachid, Ryo Matsumotoa,  Kensei Terashimaa, Takanobu Hirotoc, Hiroya Sakuraia and Yoshihiko Takanoab aInternational Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, 1-2-1 sengen, Tsukuba, Ibaraki, 305-0047, Japan bGraduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki, 305-8577, Japan cResearch Network and Facility Services Division (RNFS), National institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan dNagamori Institute of Actuators, Kyoto University of Advanced Science, 18 Gotanda, Yamanouchi, Ukyo, Kyoto, 615-8577, Japan Correspondence email: yamane.kazuki@nims.go.jp  Synopsis A theoretical candidate for a new Ni oxide superconductor, Sr3Ni2O5Cl2, was synthesized for the first time under high-pressure conditions (10 GPa, 1400℃). While it adopts the tetragonal Ruddlesden—Popper phase structure predicted by theoretical calculations, resistance measurements revealed no superconductivity down to 2 K under pressures up to 24 GPa.   Abstract A novel oxychloride, Sr3Ni2O5Cl2, was synthesized for the first time under high pressure of 10 GPa at 1400℃, motivated by a theoretical prediction of its potential superconductivity under ambient pressure. Small single crystals were used to determine the crystal structure and measure the temperature dependence of electrical resistance. The crystal is isostructural with the recently discovered superconductor, La3Ni2O7, in line with the theoretical expectation.   Keywords: Superconductor; Perovskite; Complex Anions; X-ray diffraction 1. Introduction Recently, La3Ni2O7 has been reported to exhibit superconductivity under pressures exceeding 15 GPa (Sun et al., 2023). Remarkably, the superconducting transition temperature reaches as high as 80 K, comparable to that of high-Tc cuprates. The compound adopts a Ruddlesden–Popper phase structure, and the superconductivity occurs in the double layers of NiO2 square lattices like the superconductivity in CuO2 square lattices in the cuprates. These intriguing characteristics have motivated the search for other Ni oxide superconductors (Sakakibara et al., 2024b; Nagata et al., 2024; Ueki et al., 2024), with the hope that one might exhibit superconductivity at ambient pressure. This would achieve a significant breakthrough in the field. The superconductivity of La3Ni2O7 had been theoretically predicted even before its experimental discovery, as the intercoupling between two square lattices in the double layer is thought to be advantageous for superconductivity (Nakata et al., 2017; Sakakibara et al., 2024a; Kaneko et al., 2024). Thus, the key to the occurrence of superconductivity is widely believed to lie in the Ni—Oap—Ni bridging angle (Sun et al., 2023), where Oap represents the oxygen ion between two Ni ions along the c-axis. In fact, the superconductivity appears above the pressure at which the orthorhombic Amam structure transforms into the tetragonal I4/mmm structure (Wang et al., 2024a,b). This transformation causes the bridging angle to change from 168° to 180°, presumably enhancing the intercoupling between the two square lattices. Based on the same theoretical approach used to predict the superconductivity of La3Ni2O7, the previously unreported compound Sr3Ni2O5Cl2 has recently been proposed as a promising candidate for a superconductor under ambient pressure (Ochi et al., 2024). It is expected to exhibit tetragonal I4/mmm symmetry even under ambient pressure, if it exists. Therefore, we decided to synthesize this new Ni oxychloride and successfully obtained a single crystal of it. In this report, we present the method of the synthesis, crystal structure, and electrical resistance measurements.   2.Experimental Single crystals of Sr3Ni2O5Cl2 were synthesized via high-pressure techniques using a stoichiometric mixture of SrO2, SrO, SrCl2, and NiO in a Kawai-type multi-anvil press (Kawai et al., 1970). SrO2 was prepared by precipitation from a reaction between Sr(NO3)2 and H2O2. Specifically, Sr(NO3)2  was dissolved in deionized water that had been pre-degassed by bubbling Ar gas to remove dissolved CO2. A 30% aqueous H2O2 solution was then added, followed by aqueous NH3 to induce precipitation. The precipitate was filtered and dried at 150°C. All steps were carried out in a glove bag filled with Ar gas to minimize CO2 exposure. SrO was obtained by thermal decomposition of SrCO3 at 1200°C under flowing Ar gas, while SrCl2 was prepared by dehydrating SrCl2·6H2O at 300°C under vacuum. The stoichiometric mixture was sealed in a Pt capsule inside an Ar-filled glove box, heated to 1400°C at 10 GPa for two hours in the press, and then the temperature was reduced to room temperature by lowering it in 100 ℃ increments, with a 20-minute hold at each step, before releasing the pressure. The final product included single crystals with typical dimensions of 50 µm × 50 µm × 30 µm. Single crystals were examined using a scanning electron microscope (SEM, JSM-6010LA, JEOL) equipped with energy-dispersive X-ray spectroscopy (EDX) for elemental analysis. Single-crystal X-ray diffraction (SCXRD) data were collected at 300 K using a RIGAKU Synergy Custom DW single crystal diffractometer with VariMax confocal optics for Mo K radiation ( = 0.71073 Å) and a HyPix2000 detector. A crystal with dimensions of approximately 30 × 27 × 19 m3 was isolated under paraffine oil, then immediately mounted in a dry N2 gas stream to prevent degradation, as the compound is highly air-sensitive. Cell refinement and data reduction were carried out by using the program CrysAlis Pro. Preliminary structures were solved using SHELXT (Sheldrick et al., 2015) and refined by full-matrix least squares on F2 using the SHELXL-2018/3 (Sheldrick et al., 2015) in Olex2 program package (Dolomanov et al., 2009). Electrical resistance measurements under pressures were performed using a diamond anvil cell (DAC) equipped with boron-doped diamond electrodes directly fabricated onto the diamond surface (Matsumoto et al., 2016; Sakakibara et al., 2024b). Resistance data were obtained with a Physical Property Measurement System (PPMS, Quantum Design).  3. Results and discussion Figure 1 shows the SEM image of a representative crystal. The plate-like morphology is consistent with the layered structure typical of the Ruddlesden—Popper phase, indicating that the crystal readily cleaves along these planes. Notably, the depicted crystal was cleaved using Scotch tape, highlighting the material’s inherent cleavage planes. EDX analysis determined the composition to be Sr3.1Ni2.1OxCl1.8, in close agreement with the nominal stoichiometry of the starting materials.  Figure 1 SEM image of a single crystal of Sr3Ni2O5Cl2.  The parameters obtained from the SCXRD structure refinements are summarized in Tables 1 and S1. The final cycle of refinement, performed on F2 with 18 variables, 448 averaged reflections [F2 > 2σ (F2)], and 500 reflections [F], converged to residual values of R[F2>2σ(F2)] = 0.0285 and wR(F2) = 0.0757. No significant deviations from full occupancy were observed for any of the atoms. Consequently, the crystal structure was determined to be isostructural with La3Ni2O7 and Sr3M2O5Cl2 (M = Sc, Fe, and Co) (Wang et al., 2024; Su  et al., 2018; Leib et al., 1984; Mcglothlin et al., 2000). Specifically, the compound crystallizes in the Ruddlesden—Popper phase structure, adopting the tetragonal I4/mmm symmetry with lattice parameters a = 3.83990(10) Å, c = 24.2936(12) Å, as shown in Fig. 2 (a). In the structure, there are two oxygen sites, 2a and 8g, referred to as Oap and Oeq, respectively, in this paper. Oap bridges two Ni atoms along the c-axis, while Oeq is located between two Ni atoms in the ab-plane. The bond lengths for Ni—Oap, Ni—Oeq, Ni—Cl are 1.8718(5) Å, 1.9389(4) Å, and 3.1023(14) Å, respectively. Based on these bond lengths, we have estimated the valence state of atoms using the bond-valence-sum (BVS) method (Brown et al., 1985; Brese et al., 1991).  The BVS parameters of Ni2+—Cl－ of 2.02 was used (Brese et al., 1991), following a previous literature procedure for the related compound Sr2NiO3Cl (Tsujimoto et al., 2013).  The Ni valence was estimated to be +3.17.  Similarly, the valences of Sr1 and Sr2 were estimated from the coordination environments, shown in Fig. 2 (d), to be +1.83 and +2.57, respectively. These values are in reasonable agreement with the formal valence expected from the composition, considering that the structure is stabilized only under high-pressure conditions (Ochi et al., 2024). Another high-pressure phase of Ni oxide, Sr2NiO3Cl, (Tsujimoto et al., 2013) also exhibits a larger Sr valence (+2.51) at the Sr2 site, which is surrounded by both O and Cl ions as shown in Fig. 2 (f). The asymmetry in coordination may account for the higher BVS valences. In contrast, the Ni ions are displaced toward the Oap ions from the center of the Oeq coordination plane, reducing the Ni—Oeq—Ni bridging angle from 180° to 164.0° in Sr3Ni2O5Cl2. This value closely matches the theoretical prediction of 162°, further reinforcing the validity of the theoretical calculation.  Interestingly, the Ni—Oap bond length is significantly shorter than the Ni—Oeq bond length. Although this trend is also observed in Sr2NiO3Cl as shown in Fig. 2 (e), it is more pronounced in Sr3Ni2O5Cl2. In fact, the ratio of Ni—Oap to Ni—Oeq is 0.965 in Sr3Ni2O5Cl2 whereas that of Sr2NiO3Cl is 0.975, suggesting stronger covalency between Ni—Oap in Sr3Ni2O5Cl2 than in Sr2NiO3Cl. The larger displacement of the transition metal cations toward Oap in the double-layered structure, compared to the single-layered structure, may be a common characteristic feature of strontium transition-metal oxychlorides (Hector et al., 2001; Leib et al., 1984; Loureiro et al., 2000). Thus, the highly enhanced vertical interlayer hopping in Sr3Ni2O5Cl2 predicted by the theory is presumably attributed to this feature of the strontium transition-metal oxychlorides, being related to the high covalence between Ni and Oap.   Table 1 Wyckoff positions (WP), occupancy, fractional atomic coordinates, isotropic and anisotropic atomic displacement parameters (Å2) of Sr3Ni2O5Cl2. Atom WP Occ. x y z Uiso U11 U22 U33 Sr1 2a 1 0 0 0.5 0.01719(12) 0.01381(13) = U11 0.0239(3) Sr2 4e 1 0 0 0.34453(2) 0.01499(10) 0.01223(10) = U11 0.02051(19) Ni 4e 1 0 0 0.07705(2) 0.01226(11) 0.01015(13) = U11 0.01650(2) Oap 2a 1 0 0 0 0.0193(8) 0.02260(12) = U11 0.01280(17) Oeq 8g 1 0 0.5 0.08818(10) 0.0171(4) 0.0156(8) 0.0101(7) 0.0256(10) Cl 4e 1 0 0 0.24750(5) 0.0208(2) 0.0181(3) = U11 0.0263(5) *) U12 = U13 = U23 = 0   Figure 2 Crystal structures of Sr3Ni2O5Cl2 (a) and Sr2NiO3Cl (b) (Tsujimoto et al., 2013), and local coordination environments around a Ni site (c) and Sr sites (d) in the former compound, and  those in the latter compound (e, f). The boxes in the panels a and b represent unit cells.   The temperature dependence of electrical resistance under various pressures are shown in Fig. 3. Although the resistance increases with decreasing temperature at 0.2 GPa, which is almost the ambient pressure, the compound remains highly conductive, consistent with the metallic nature predicted by the theory. However, no superconductivity is observed down to 2 K. The increase in resistance with decreasing temperature is gradually supressed by applying pressure up to 24 GPa, However, the superconductivity is not emerged. The origin of the absence of the superconductivity is under investigation.   Figure 3 Temperature dependence of electrical resistance of Sr3Ni2O5Cl2 under various pressures. Acknowledgements We thank Profs. Masayuki Ochi (Osaka Univ.), Hirofumi Sakakibara (Tottori Univ.), Hidetomo Usui (Shimane Univ.), and Kazuhiko Kuroki (Osaka Univ.) for fruitful discussions. This work is supported by World Premier International Research Center Initiative (WPI), MEXT, Japan, and JSPS KAKENHI Grant Nos. JP20H05644 and JP24K01333. VESTA was used to generate Figs. 3 (Momma & Izumi, 2011).  Conflicts of interest The authors declare no competing interests. Data availability The authors confirm that the data supporting the findings of this study are available within the article and its supplementary materials. References Brese, N. E., & O’Keeffe, M. (1991). Acta Crystallogr. B47, 192-197. Brown, I. D., & Altermatt, D. (1985). Acta Crystallogr. B41, 244-247. CrysAlis Pro ver. 1.171.43.125a, Rigaku Oxford Diffraction, Tokyo, Japan (2024). Dolomanov, O. V., Bourhis, L. J., Gildea, R. J., Howard, J. A. K., & Puschmann, H. (2009).  J. Appl. Crystallogr. 42, 339-341.  Hector, A. L., Hutchings, J. A., Needs, R. L., Thomas, M. F.,  & Weller, M. T. (2001). J. Mater. Chem. 11, 527-532. Kaneko, T. Sakakibara, H. Ochi, M. & Kuroki, K. (2024). Phys. Rev. B 109, 045154. Kawai, N., & Endo, S. (1970). Rev. Sci. Instrum. 41, 1178-1181. Leib, W., & Müller-Buschbaum, Hk. Z. (1984). Anorg. Allg. Chem. 518, 115-119. Loureiro, S. 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Supporting information  Table S1 Structure refinement of Sr3Ni2O5Cl2  Crystal data  Sr3Ni2O5Cl2 Dx = 4.925 Mr = 531.18 Mo Kα radiation, λ = 0.71073 Tetragonal, I4/mmm (#139) Cell parameters from 4484 reflections a = 3.8399 (1) θ = 3.327 – 44.742° c = 24.2936 (12) μ = 28.061mm−1 V = 358.21 (3) T = 300.8(9) K Z = 2 block, metallic black F (000) = 488 ~30 × 27 × 19 m   Data collection  ROD, Synergy Custom DW system, HyPix 500 Independent reflections Diffractometer 448 reflections with [F2 > 2σ (F2)] Detector resolution: 10.0000 pixels mm−1 Rint = 0.0649 ω scans θmax = 44.860°,θmin = 3.354° Absorption correction: multi-scan h = −7→7 (CrysAlisPro 1.171.43.125a (Rigaku Oxford  k = −7→7 Diffraction, 2024)) l = −48→48 Tmin = 0.34623, Tmax = 0.37985  8872 measured reflections    Refinement  Refinement of F2 w = 1/[σ2(Fo2)+(0.0395P)2+0.6699P] Least-squares matrix: full where P = (Fo2+2Fc2)/3 R[F2>2σ(F2)] = 0.0285 (Δ/σ)max < 0.002 wR(F2) = 0.0757 Δρmax = 3.442 e Å−3 S = 1.123 Δρmin = −1.003 e Å−3 500 reflections  18 parameters  0 restraints  Primary atom site location: dual