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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Superconductivity in Anti-ThCr2Si2‑type Er2O2Bi Induced by Incorporation of Excess Oxygen with CaO Oxidant Kyohei Terakado,† Ryosuke Sei,†,‡ Hideyuki Kawasoko,† Takashi Koretsune,§ Daichi Oka,† Tetsuya Hasegawa,‡ and Tomoteru Fukumura*,†,∥,⊥ Department of Chemistry and §Department of Physics, Tohoku University, Sendai 980-8578, Japan ‡ Department of Chemistry, The University of Tokyo, Tokyo 113-0033, Japan ∥ WPI-Advanced Institute for Materials Research and ⊥Core Research Cluster, Tohoku University, Sendai 980-8577, Japan Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/29/18. For personal use only.
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S Supporting Information *
ABSTRACT: Recently, superconductivity was induced by expanding interlayer distance between Bi square nets in anti-ThCr2Si2-type Y2O2Bi through incorporation of excess oxygen with increased nominal amount of oxygen. However, such oxygen incorporation was applicable to only Y2O2Bi among R2O2Bi (R = rare earth metal), probably due to a larger amount of oxygen incorporation for Y2O2Bi. In this study, the interlayer distance in Er2O2Bi was increased by cosintering with CaO, which served as an oxidant, indicating that excess oxygen was incorporated in Er2O2Bi. As a result, superconductivity was induced in Er2O2Bi at 2.2 K.
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INTRODUCTION
Er2O2Bi shows superconductivity like Y2O2Bi by increasing caxis length. In this study, we report the emergence of superconductivity in anti-ThCr2Si2-type Er2O2Bi by developing oxygen incorporation method to expand the c-axis length. In contrast with Y2O2Bi,8 increasing oxygen content in starting materials was not effective to incorporate excess oxygen in Er2O2Bi. Instead, cosintering Er2O2Bi with CaO was effective, indicating that CaO serves as not a carrier dopant but an oxidant.
ThCr2Si2-type layered compounds are one of the platforms to explore new superconductors.1−4 For example, on the one hand, ThCr2Si2-type superconducting BaFe2As2 is composed of an alternative stack of electrically conductive Fe2As2 block layer and insulating monatomic Ba square net, indicating the important role of the block layers for superconductivity. On the other hand, anti-ThCr2Si2-type R2O2Bi (R = rare earth metal) is composed of an alternative stack of insulating R2O2 block layer and electrically conductive Bi square net with unusual Bi2− valence. Anti-ThCr2Si2-type compounds exhibited various electronic properties such as metal−insulator transition in R2O2Bi (R = rare earth metal),5 charge density wave state in R2O2Sb (R = rare earth metal),6 and high electron mobility in Bi2O2Se,7 in spite of the absence of superconductivity. Recently, anti-ThCr2Si2-type Y2O2Bi was found to be superconducting by expanding the distance between Bi square nets, which is a half of the c-axis length, through excess oxygen incorporation.8 The expanded c-axis length was suggested to play a key role in emergence of the superconductivity, because the carrier density was essentially unchanged upon the oxygen incorporation. However, this result seems to be inconsistent with the absence of superconductivity in La2O2Bi with larger caxis length reported in previous study.5 Accordingly, further investigation is needed to reveal the relationship between electrical transport properties and lattice constants in R2O2Bi. Particularly, it is interesting whether metallic R2O2Bi such as © XXXX American Chemical Society
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EXPERIMENTAL METHOD
Er2O2Bi specimens were synthesized by solid-state reaction. Er2O3 (99.9%) and CaCO3 (99.9%) powders were heated at 1000 °C in furnace for 10 h to remove moisture and to decompose into CaO, respectively. Er (99.9%), Er2O3 (99.9%), Bi (99.9%), and CaO powders were mixed and pelletized under 20 MPa in nitrogen-filled glovebox to form nominal compositions of Er2OxBi3−x (x = 1.3, 1.4, 1.5, 1.6, 1.7, 1.8) and Er2O1.4Bi1.3 + (CaO)y (y = 0.1, 0.2, 0.3, 0.4, 0.5), respectively (Table S1). Those nominal compositions deviated from the stoichiometric composition (Er2O2Bi) were compensated by the evaporation of excess Bi and the additional oxidation during sintering as in the synthesis of Y2O2Bi.8 The pellets covered with Ta foil were sintered in evacuated quartz tubes at 500 °C for 7.5 h, followed by sintering at 1000 °C for 20 h. The sintered products were ground and pelletized under 30 MPa again in the glovebox, and then the pellets covered with Ta foil were sintered in evacuated quartz tubes at 1000 °C for 10 h. Crystal structures were evaluated by powder X-ray diffraction (XRD) using Cu Kα radiation (D8 DISCOVER, Bruker Received: May 4, 2018
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DOI: 10.1021/acs.inorgchem.8b01199 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry AXS). Rietveld analysis was performed by RIETAN-FP to identify the crystal phases and their lattice constants.9 Surface morphology and chemical composition were investigated by scanning electron microscope equipped with energy dispersive X-ray spectroscopy (SEM-EDX, JEOL JSM-6500F), inductively coupled plasma mass spectrometry (ICP-MS, Agilent 8800), and secondary ion mass spectrometry (SIMS, Cameca IMS-7f). Transport and magnetic properties were evaluated by physical properties measurement system (PPMS, Quantum Design) and magnetic properties measurement system (MPMS, Quantum Design), respectively. Electronic structures were calculated by the Vienna ab initio simulation package (VASP)10,11 within the generalized gradient approximation (GGA). Hereafter, Er2OxBi3−x and Er2O1.4Bi1.3 + (CaO)y are denoted as Er2O2Bi with nominal amounts of oxygen (x) and Er2O2Bi:Ca with nominal amounts of CaO (y), respectively.
Figure 2 shows the results of XRD analyses for a series of Er2O2Bi:Ca with various y. Main phase was Er2O2Bi in all the
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RESULTS AND DISCUSSION Figure 1 shows the results of XRD analyses for a series of Er2O2Bi with various x. XRD pattern of Er2O2Bi (x = 1.3) is
Figure 2. Crystal structure analysis for Er2O2Bi:Ca with different nominal amounts of CaO (y). (a) XRD pattern and its Rietveld analysis for Er2O2Bi:Ca (y = 0.5). (b) Molar fractions of constituent phases as a function of y. (c) a- and c-axes lengths for Er2O2Bi:Ca as a function of y. y = 0 corresponds to Er2O2Bi (x = 1.3).
Er2O2Bi:Ca (Figure 2a). With increasing y, the relative amount of impurity phases (Er2O3 and Bi) increased (Figure 2b). In contrast with Er2O2Bi samples, c-axis length showed a slight increase as a function of y, accompanying a slight decrease in aaxis length (Figure 2c). These changes in lattice constants suggest that excess oxygen was incorporated into Er2O2Bi by cosintering with CaO. Ca was not incorporated in Er2O2Bi phase as described below. The inner wall of quartz tube was considerably covered by yellowish deposit after the synthesis of Er2O2Bi:Ca (inset of Figure 3a). Figure 3a shows EDX spectrum of the inner wall after the synthesis of Er2 O2Bi:Ca (y = 0.5) sample, representing that the specimen was composed of Ca in addition to Si and O from the quartz tube. In scanning electron micrograph of the Er2O2Bi:Ca (y = 0.5), various crystal grains were observed (Figure 3b). In EDX mapping images (Figures 3c−f), Bi, Er, and O signals were almost homogeneously distributed, corresponding to Er2O2Bi phase. In addition, several small areas showed intense Bi and Ca signals on the large grains observed in Figure 3b, indicating the presence of Bi−Ca-rich impurity phase. EDX spectra in selected small areas (I, II, and III in Figure 3b) indicated that the presence of Er−O−Bi, Er−O, and Bi−Ca phases (Figure S1), respectively, corresponding to Er2O2Bi, Er2O3, and Bi (mixed with a little amount of Ca) phases evaluated from the Rietveld analysis (Figure 2c). These elemental mappings were similar to those observed by SIMS (Figure S2). In the EDX measurements, Ca was only detected in such Bi-rich grains. Accordingly, Ca was not incorporated in Er2O2Bi phase but was incorporated in Bi
Figure 1. Crystal structure analysis by Rietveld refinement on X-ray diffraction (XRD) patterns of Er2O2Bi with different nominal amounts of oxygen (x). (a) XRD pattern and its Rietveld analysis for Er2O2Bi (x = 1.3). (b) Molar fractions of constituent phases as a function of x. (c) a- and c-axes lengths for Er2O2Bi as a function of x.
shown in Figure 1a. The detailed results of Rietveld analysis are shown in Table S1. All the XRD peaks were attributed to Er2O2Bi phase, indicating the pure Er2O2Bi phase without any impurities. With increasing x, ErBi and Er2O3 impurity phases appeared as seen in Figure 1b. The relative amount of Er2O3 phase increased with x similar to Y2O3 phase in Y2O2Bi.8 However, c-axis length was not increased with x in contrast with that of Y2O2Bi, as shown in Figure 1c. This result suggests that the excess oxygen was not incorporated in Er2O2Bi by increasing the nominal amounts of oxygen in starting materials. B
DOI: 10.1021/acs.inorgchem.8b01199 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
without superconducting transition (Figure 4a), being consistent with previous study.5 On the one hand, increase in resistivity as a function of x was mainly caused by the overestimation of Er2O2Bi volume owing to the enhanced volume of insulating Er2O3. On the other hand, Er2O2Bi:Ca (y = 0.5) with an increased c-axis length showed zero resistance below 1.9 K (Figure 4b). The superconducting volume fraction evaluated from magnetization measurement was as high as 50% at 1.8 K (Figure S3), indicating bulk superconducting transition of Er2O2Bi phase, taking into account the volume fraction of Er2O2Bi phase in Er2O2Bi:Ca (y = 0.5; Figure 2b). Figure 5a shows the temperature dependence of resistivity at low temperatures for Er2O2Bi:Ca with various y. The
Figure 3. (a) EDX spectrum for inner wall of quartz tube after synthesis. (inset) The photograph of the quartz tube. (b) SEM image of Er2O2Bi:Ca (y = 0.5). The EDX mappings for (c) Bi, (d) Ca, (e) Er, and (f) O.
impurity phase and evaporated on the quartz tube, as also confirmed by ICP-MS (Table S2). Considering the fact that caxis length was increased in Er2O2Bi:Ca, CaO served as an oxidant for Er2O2Bi through self-reduction to Ca metal because of strong ionic interaction between highly electropositive hard acid Er3+ and highly electronegative hard base O2−.12,13 Figure 4 shows temperature dependence of electrical resistivity for Er2O2Bi (x = 1.3−1.8) and Er2O2Bi:Ca (y = 0.5) samples. All Er2O2Bi showed metallic conductivity
Figure 5. (a) Temperature dependence of normalized electrical resistivity for Er2O2Bi:Ca with various y below 2.4 K. (b) Superconducting transition temperature (Tconset) as a function of caxis length. (inset) The y dependence of Tc. Open circles at 1.8 K indicate no superconducting transition down to 1.8 K.
superconductivity was observed at ∼2 K for y ≥ 0.2, and the zero resistance was observed below 1.9 K for y = 0.4 and 0.5. The onset of superconducting transition temperature (Tconset) was a monotonically increasing function of y (inset of Figure 5b). It was concluded from Figure 5b that Er2O2Bi phase is superconducting when the c-axis length was approximately larger than 13.185 Å. Figure 6 shows band structures of stoichiometric Er2O2Bi with different lattice constants corresponding to those of (a) non-superconducting Er2O2Bi (x = 1.3) and (b) superconducting Er2O2Bi:Ca (y = 0.5). Those band dispersion and density of states around Fermi level were almost similar. This result suggests possible factors to induce superconductivity in Er2O2Bi with expanded c-axis length: the modulation of the phonon dispersion and electron−phonon coupling as discussed in non-superconducting R2O2Bi (ref 14) and/or the energy shift of Bi 6px/py bands due to oxygen incorporation as discussed in Y2O2Bi with excess oxygen.15
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CONCLUSION In summary, we expanded c-axis length of Er2O2Bi not by controlling nominal amounts of oxygen but by cosintering with CaO, suggesting that CaO served as an efficient oxidant to incorporate excess oxygen in Er2O2Bi. Er2O2Bi with expanded
Figure 4. (a) Temperature dependence of electrical resistivity for Er2O2Bi with different x. (b) Temperature dependence of electrical resistivity for Er2O2Bi:Ca (y = 0.5) and Er2O2Bi (x = 1.3). C
DOI: 10.1021/acs.inorgchem.8b01199 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Foundation for Science and Technology. The authors acknowledge Dr. T. Miyazaki for SIMS measurements.
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Figure 6. Band structures of stoichiometric Er2O2Bi with different lattice constants corresponding to those of (a) non-superconducting Er2O2Bi (x = 1.3) and (b) superconducting Er2O2Bi:Ca (y = 0.5) along the symmetry lines of the body centered tetragonal Brillouin zone. EF denotes the Fermi level.
c-axis length above 13.185 Å exhibited superconductivity with a transition temperature (Tconset) of 2.2 K at maximum. In R2O2Bi, the expansion of the c-axis length would be a key factor to induce their superconductivity.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01199. Summary of crystal structural data and superconducting transition temperature (Tconset), EDX spectrum, molar ratio measured by ICP-MS, elemental mapping by SIMS, temperature dependence of magnetic susceptibility (PDF)
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REFERENCES
(1) Hosono, H.; Tanabe, K.; Takayama-Muromachi, E.; Kageyama, H.; Yamanaka, S.; Kumakura, H.; Nohara, M.; Hiramatsu, H.; Fujitsu, S. Exploration of New Superconductors and Functional Materials, and Fabrication of Superconducting Tapes and Wires of Iron Pnictides. Sci. Technol. Adv. Mater. 2015, 16, 1−87. (2) Zhang, P.; Zhai, H. Superconductivity in 122-Type Pnictides without Iron. Condens. Matter 2017, 2, 28. (3) Steglich, F.; Aarts, J.; Bredl, C. D.; Lieke, W.; Meschede, D.; Franz, W.; Schäfer, H. Superconductivity in the Presence of Strong Pauli Paramagnetism: CeCu2Si2. Phys. Rev. Lett. 1979, 43, 1892− 1896. (4) Rotter, M.; Tegel, M.; Johrendt, D. Superconductivity at 38 K in the Iron Arsenide (Ba1−xKx)Fe2As2. Phys. Rev. Lett. 2008, 101, 107006. (5) Mizoguchi, H.; Hosono, H. A Metal-Insulator Transition in R2O2Bi with an Unusual Bi2− Square Net (R = Rare Earth or Y). J. Am. Chem. Soc. 2011, 133, 2394−2397. (6) Wang, P. L.; Kolodiazhnyi, T.; Yao, J.; Mozharivskyj, Y. Decoupling the Electrical Conductivity and Seebeck Coefficient in the RE2SbO2 Compounds through Local Structural Perturbations. J. Am. Chem. Soc. 2012, 134, 1426−1429. (7) Wu, J.; Yuan, H.; Meng, M.; Chen, C.; Sun, Y.; Chen, Z.; Dang, W.; Tan, C.; Liu, Y.; Yin, J.; Zhou, Y.; Huang, S.; Xu, H. Q.; Cui, Y.; Hwang, H. Y.; Liu, Z.; Chen, Y.; Yan, B.; Peng, H. High Electron Mobility and Quantum Oscillations in Non-Encapsulated Ultrathin Semiconducting Bi2O2Se. Nat. Nanotechnol. 2017, 12, 530−534. (8) Sei, R.; Kitani, S.; Fukumura, T.; Kawaji, H.; Hasegawa, T. TwoDimensional Superconductivity Emerged at Monatomic Bi2− Square Net in Layered Y2O2Bi via Oxygen Incorporation. J. Am. Chem. Soc. 2016, 138, 11085−11088. (9) Izumi, F.; Momma, K. Three-Dimensional Visualization in Powder Diffraction. Solid State Phenom. 2007, 130, 15−20. (10) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for ab initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (11) Kresse, G.; Furthmüller, J. Efficiency of ab-initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (12) Edelmann, F. T. Lanthanide Amidinates and Guanidinates: from Laboratory Curiosities to Efficient Homogeneous Catalysts and Precursors for Rare-Earth Oxide Thin Films. Chem. Soc. Rev. 2009, 38, 2253−2268. (13) Jeong, N. C.; Lee, J. S.; Tae, E. L.; Lee, Y. J.; Yoon, K. B. Acidity Scale for Metal Oxides and Sanderson’s Electronegativities of Lanthanide Elements. Angew. Chem., Int. Ed. 2008, 47, 10128−10132. (14) Kim, H.; Kang, C.-J.; Kim, K.; Shim, J. H.; Min, B. I. Suppression of the charge density wave instability in R2O2Bi (R = La, Er) due to large spin-orbit coupling. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93, 125116. (15) Cheng, X.; Gordon, E. E.; Whangbo, M.-H.; Deng, S. Superconductivity Induced by Oxygen Doping in Y2O2Bi. Angew. Chem., Int. Ed. 2017, 56, 10123−10126.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hideyuki Kawasoko: 0000-0001-9069-3784 Daichi Oka: 0000-0003-2747-9675 Author Contributions
K.T., R.S., H.K, and D.O. conducted experiments. T.K. calculated band structures. All authors analyzed the results and wrote the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was in part supported by JSPS KAKENHI (26105002, 16H06441), JST-CREST, and Yazaki Memorial D
DOI: 10.1021/acs.inorgchem.8b01199 Inorg. Chem. XXXX, XXX, XXX−XXX
