Crystal Structure and Superconductivity of Tetragonal and Monoclinic

22 hours ago - Synopsis. While monoclinic PrOBiS2 showed no superconductive transition above 0.1 K, tetragonal Ce0.5Pr0.5OBiS2 showed both zero resist...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Crystal Structure and Superconductivity of Tetragonal and Monoclinic Ce1−xPrxOBiS2

Akira Miura,*,† Masanori Nagao,*,‡ Yosuke Goto,§ Yoshikazu Mizuguchi,§ Tatsuma D. Matsuda,§ Yuji Aoki,§ Chikako Moriyoshi,∥ Yoshihiro Kuroiwa,∥ Yoshihiko Takano,⊥ Satoshi Watauchi,‡ Isao Tanaka,‡ Nataly Carolina Rosero-Navarro,† and Kiyoharu Tadanaga† †

Faculty of Engineering, Hokkaido University, Kita 13, Nishi 8, Sapporo 060-8628, Japan Center for Crystal Science and Technology, University of Yamanashi, 7-32 Miyamae, Kofu 400-8511, Japan § Department of Physics, Tokyo Metropolitan University, 1-1 minami-osawa, Hachioji, Tokyo 192-0397, Japan ∥ Department of Physical Science, Hiroshima University, 1-3-1 Kagamiyama, Higashihiroshima, Hiroshima 739-8526 Japan ⊥ MANA, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan ‡

ABSTRACT: Ce1−xPrxOBiS2 powders and Ce0.5Pr0.5OBiS2 single crystals were synthesized and their structure and superconductive properties were examined by X-ray diffraction, X-ray absorption, electronic resistivity, and magnetization. While PrOBiS2 was found to be in a monoclinic phase with one-dimensional Bi−S zigzag chains showing no superconductive transition above 0.1 K, CeOBiS2 was in a tetragonal phase with two-dimensional Bi−S planes showing zero resistivity below 1.3 K. In the range x = 0.3−0.9 in Ce1−xPrxOBiS2, both monoclinic and tetragonal phases were formed together with zero resistivity up to a maximum temperature of 2.2 K. A Ce0.5Pr0.5OBiS2 single crystal, which showed both zero resistivity and a decrease in magnetization at ∼2.4 K, presented a tetragonal structure. Short Bi−S bonding in flat two-dimensional Bi−S planes and mixed Ce3+/Ce4+ were characteristic features of the Ce0.5Pr0.5OBiS2 single crystal, which presumably triggered its superconductivity.

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even in some tetragonal phases via neutron scattering27,28 and extended X-ray absorption fine structure (EXAFS) studies.29,30 Moreover, synchrotron X-ray diffraction studies have shown that nonsuperconductive LaOBiS224 and superconductive LaO0.9F0.1OBiSSe26 present monoclinic phases which are slightly distorted tetragonal phases. It should be mentioned that the abrupt increase in the transition temperature of LaO0.5F0.5BiS2 from 2.5 to 10.7 K at a pressure of 0.7 GPa accompanies the phase transition from tetragonal to monoclinic phase.31 Thus, the relationship betweenthe crystal system and superconductive properties needs further investigation. The relationship between bonding length and superconductive temperature has been systematically investigated in tetragonal Ln(O0.5F0.5)BiS2. A shorter bonding length of Bi− S1 induces more overlap between Bi 6p and S 3p orbitals and increases the superconductive temperature. By consideration of how much the Bi−S/Se orbitals are compressed by the Ln− (O,F) layer or S/Se itself, the effect of bonding length can be understood as an “in-plane chemical pressure effect”: a higher chemical pressure increases the transition temperature of superconductivity.25

INTRODUCTION BiS2-based superconductors have been drawing much attention as a new class of layered superconductors.1−19 The crystal structures of Ln(O,F)BiS2 (Ln = lanthanide) are composed of alternate stacks of Bi−S layers and Ln−O layers, which are similar to those of cuprate-20 and FeAs-based high-transitiontemperature superconductors.21 The conduction bands consist of Bi 6p and S 3p orbitals in two-dimensional Bi−S planes, and electron carriers are doped into these bands. These features allow the control of electronic structures by changing Ln, O, and S. A variety of superconductors have been discovered using this technique. In order to clarify the superconductive mechanism and increase the transition temperature, Tc, the relationship between crystal structure (crystal system, atomic distance/ angle) and superconductivity has been extensively studied using polycrystalline and single-crystalline samples.22−30 There are two crystal systems: tetragonal and monoclinic (Figure 1). The tetragonal phase consists of two-dimensional Bi−S planes, while the monoclinic phase consists of one-dimensional Bi−S1 zigzag chains. Therefore, the crystal systems definitely affect the Fermi surface and anisotropy of conductivity. Most superconductive phases have been reported to be tetragonal. Nonetheless, the disordering of local structure, which can be expressed by one-dimensional Bi−S1 chains, has been proposed © XXXX American Chemical Society

Received: February 7, 2018

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DOI: 10.1021/acs.inorgchem.8b00349 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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Article

EXPERIMENTAL SECTION

(Ce,Pr)OBiS2 powders were synthesized from stoichiometric mixtures of Bi2O3, Bi2S3, Ce2S3, and Pr2S3 through solid−solid reactions in vacuum quartz tubes at 700 °C for 10 h. The calcined mixtures were ground, pressed into a pellet, and then sealed again in a quartz tube. This pellet in the quartz tube was heated at 700 °C for 20 h and then furnace-cooled to room temperature. Single crystals were grown using a CsCl flux.40 Bi2O3, Bi2S3, Ce2S3, and Pr2S3 (0.8 g in total) and CsCl (5.0 g) were mixed using a mortar and pestle and then sealed in a quartz tube under vacuum (∼10 Pa). This mixed powder in the quartz tube was heated at 950 °C for 10 h and cooled slowly to 650 °C at a rate of 1 °C/h. Subsequently, the CsCl flux was washed off with distilled water and plate-shaped crystals were collected. Synchrotron X-ray diffraction (XRD) measurements were performed at room temperature at the BL02B2 beamline at SPring-8 with the approval of 2017B1211 and 2017B1283. The wavelength of the beam was 0.49559 or 0.49692 Å. The diffractions were measured using a high-resolution one-dimensional semiconductor detector.41 Rietveld refinements were performed using RIETAN-FP.42 X-ray absorption of Ce-L3 and Pr-L3 edges of both polycrystalline and single-crystalline samples was performed in the same transmission mode at the Aichi synchrotron BL5S1 (Proposal No. 201704041). The morphology and composition of single crystals were examined by scanning electron microscopy (SEM; Hitachi TM3030Plus) and energy dispersive X-ray spectrometry (EDS), respectively. Crystal structures of single crystals were determined by single-crystal XRD analysis using a Rigaku XTALAB-MINI instrument with Mo Kα radiation. Structural refinement was performed by SHELXL-97. Magnetization and transport properties were measured by a superconducting quantum interference device (SQUID) and a physical property measurement system (PPMS), respectively. Resistivity measurements between 0.1 and 1.8 K were performed using PPMS with an adiabatic demagnetization refrigerator (ADR) option. The schemes of crystal structures were depicted using VESTA.43

Figure 1. Crystal structures of (a, b) tetragonal CeOBiS2 and (c) monoclinic PrOBiS2 at 100 K. The two-dimensional Bi−S1 plane in the tetragonal phase and one-dimensional Bi−S1 chains in monoclinic phase with bond lengths are depicted.

Carrier doping, which is another essential element to trigger superconductivity in BiS2 compounds, has generally been achieved by changing components of Ln−O or Sr−F sites: F doping of Ln−O (e.g., NdO1−xFxBiS2) and La doping of Sr−F (e.g., Sr1−xLaxFBiS2). The carriers are also induced by tetragonal CeOBiS2 (Tc ≈ 1.3 K)32,33 and EuFBiS2 (Tc ≈ 0.3 K),34 where valence fluctuation of Ce3+/Ce4+ or Eu2+/Eu3+ occurs.32−38 Thus, the amounts of Ce/Eu and their valence are important for inducing carriers in CeOBiS2/EuFBiS2. Moreover, long-range ordering of magnetic moments has also been reported in Ce(O,F)BiS2.36−38 These transitions and longrange ordering have shown that Ce orbitals affect transport and magnetic properties. On the other hand, the laboratory X-ray diffraction pattern of PrOBiS2 measured at room temperature has been indexed as having a tetragonal phase, with no superconductivity without F doping;35 nonetheless, in this work, synchrotron X-ray diffraction patterns of PrOBiS2 measured at 100 and 300 K were indexed as having a monoclinic phase as described in Figure 1. In this case, Pr is trivalent.39 The superconductivity of F-doped PrOBiS2 appears up to ∼4 K with the transition from a metallic to a superconductive state.7 Therefore, studying the (Ce,Pr)OBiS2 system can possibly show how Ce affects structure and transport properties. In this work, we investigated the crystal structure and superconductive properties of (Ce,Pr)OBiS2 in order to examine how mixing Ce and Pr ions affects the dimensionality of the Bi−S1 network and carrier doping. In contrast to the tetragonal phase reported in previous studies, PrOBiS2 was found to be in a monoclinic phase with one-dimensional Bi−S1 chains. (Ce,Pr)OBiS2 crystallized in both tetragonal and monoclinic phases in the polycrystalline form and showed zero resistivity below 2.2 K. The superconductivity of Ce0.5Pr0.5OBiS2 was confirmed by a single crystal, presenting a tetragonal crystal structure with a two-dimensional Bi−S1 plane, mixed valence of Ce3+/Ce4+, and a superconductive transition at 2.4 K. The correlation between crystal structure and superconductivity of (Ce,Pr)OBiS2 and other related compounds are presented in the Discussion.

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RESULTS Figure 2 shows synchrotron X-ray diffraction patterns at 100 K of Ce1−xPrxOBiS2. The XRD pattern of CeOBiS2 (x = 0) was indexed as a tetragonal phase. On the other hand, the XRD pattern of PrOBiS2 (x = 1) was indexed as a monoclinic phase; the splitting of the 200 and 020 peaks and moderate fitting of the Rietveld refinement using a tetragonal cell described later indicate a symmetry lowering in the tetragonal cell. Please note that the XRD pattern at 300 K had been incorrectly assigned as a tetragonal phase.35 This is because, for example, the split of 200 and 020 peaks is considerably small. The distortions from a tetragonal phase are rather small, especially near room temperature (a/b = 1.00108 Å, β = 90.200° at 300 K; a/b = 1.00467 Å, β = 90.276° at 100 K), similar to the case of LaO0.9F0.1BiSSe.26 The X-ray diffraction of Ce1−xPrxOBiS2 (x = 0.3−0.9) showed both tetragonal and monoclinic phases. This is clearly shown by the 003 peaks; the peak of the tetragonal phase is located at an angle higher than that of the monoclinic phase. The diffraction peaks of Ce0.9Pr0.1OBiS2 (x = 0.1) were indexed as a tetragonal phase. Nonetheless, there was a very weak peak at 6.22° likely due to a monoclinic phase, while splits of 200 and 020 peaks were not observed. Thus, a tiny amount of the monoclinic phase, whose lattice parameters of a and b axes were close to those of the a axis of the tetragonal phase, likely coexist with a tetragonal phase, although further Rietveld refinement was performed with only the tetragonal phase. Figure 3 shows typical Rietveld refinement profiles of synchrotron XRD patterns measured at 100 K. The refinements were performed with the tetragonal phase for CeOBiS2 (x = 0) and Ce0.5Pr0.5OBiS2 (x = 0.1). The refinement parameters of B

DOI: 10.1021/acs.inorgchem.8b00349 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Synchrotron XRD patterns of Ce1−xPrxOBiS2 powders at 100 K (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1). The bottom figures are enlarged patterns of 003 and 200, 020 diffraction peaks.

Figure 3. Rietveld profiles of synchrotron X-ray diffraction patterns of CeOBiS2, Ce0.5Pr0.5OBiS2, and PrOBiS2 powders at 100 K. Green bars represent the allowed reflections for the tetragonal and/or monoclinic phase. Lines at the bottom give the difference between observed and calculated profiles.

PrOBiS2 (x = 1) using a monoclinic model were lower than those using a tetragonal model: Rwp(monoclinic, 100 K) = 8.90%, Rwp(tetragonal, 100 K) = 15.35%. As described, the patterns of x = 0.3−0.9 contained both tetragonal and monoclinic phases. Since the peaks of these two phases overlapped each other, the refinements of atomic positions and displacement factors were difficult. Thus, these refinements were performed with fixed atomic positions and displacement factors of tetragonal CeOBiS2 and monoclinic PrOBiS2 phases in order to derive the lattice parameters and the fraction of tetragonal and monoclinic phases. Figure 4 shows the fraction of phases and lattice parameters. With increasing Ce/Pr ratio x, the tetragonal phase appears to a lesser extent. The lattice parameters are not constant in both phases. This suggests a change in Ce/Pr ratio of both tetragonal and monoclinic phases, even though the ratios of each phase are unknown. The lattice parameters of both tetragonal and monoclinic phases do not show linear trends. The a and c axes of the tetragonal phase tend to increase with increasing x, but their minima are located at x = 0.5 and x = 0.1, respectively. The β value of the monoclinic phase is approximately 90.2°. The minimal of a, b, c of the monoclinic phase are at x = 0.5. Figure 5 shows the Ce- and Pr-L3 edge absorption spectra of Ce1−xPrxOBiS2 powders. The Ce-L3 edge of Ce1−xPrxOBiS2 shows a peak at around 5736 eV, which can be assigned to Ce4+. This is consistent with the X-ray photoemission spectroscopy (XPS) and X-ray absorption results of an F-free CeOBiS2 single crystal with both Ce3+ and Ce4+ 32,33 but different from the results of Sr1−xCexFBiS2 with Ce3+.44 These spectra are similar, indicating that Ce valence is independent of the Ce/(Ce + Pr) ratio. The Ce4+/(Ce3+ + Ce4+) ratio derived

by a linear combination fitting of Ce2S3 and CeO2 was ∼0.8. On the Pr-L3 spectrum, only the Pr3+ peak around 5966 eV is visible; the peak at around 5978 eV can be assigned to Pr4+. This is consistent with the XPS results showing only Pr3+ in PrOBiS2.39 Therefore, the carrier concentration would be estimated to be one-fifth of Ce/(Ce + Pr). Figure 6 shows the temperature dependence of the relative resistivity of Ce1−xPrxOBiS2 between 0.1 and 15 K. CeOBiS2 (x = 0) shows zero resistivity below ∼1.3 K, as reported.32,33 Ce1−xPrxOBiS2 (x = 0.1) increases the temperature of zero resistivity to ∼1.4 K, and Ce1−xPrxOBiS2 (x = 0.3, 0.5) further increases the temperature to ∼2.2 K. A further increase in x decreases the temperature of zero resistivity, and Ce1−xPrxOBiS2 (x = 0.9) and PrOBiS2 (x = 1) do not show zero resistivity. Since Ce1−xPrxOBiS2 (x = 0.3−0.9) species are made up of both tetragonal and monoclinic phases, the origin of the superconductivity phase cannot be determined. In order to confirm the superconductive phase, we grew single crystals of Ce0.5Pr0.5OBiS2 (x = 0.5) and investigated their structure and properties. Figure 7 shows SEM and EDX mapping of the platelike single crystal, where Bi, Ce, and Pr are distributed homogeneously. The Bi/Ce/Pr ratio was approximately 1/0.5/0.5. Structural refinements are summarized in Table 1. While CeOBiS2 and Ce0.5Pr0.5OBiS2 structures are tetragonal, PrOBiS2 presents a monoclinic structure. The Rietveld refinement of Ce0.5Pr0.5OBiS2 using a monoclinic cell was converged as a tetragonal cell within error. The Bi−S1 lengths in the twodimensional Bi−S1 planes of Ce0.5Pr0.5OBiS2 are shorter than those of tetragonal CeOBiS2 at both 100 and 300 K. The S1− C

DOI: 10.1021/acs.inorgchem.8b00349 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. X-ray absorption spectra at Ce- and Pr-L3 edges of Ce1−xPrxOBiS2 powders. Arrows indicate tetravalent energies.

Figure 4. Fraction of tetragonal phase and lattice parameters of Ce1−xPrxOBiS2 powders at 100 K.

Bi−S1 bond angle in Ce0.5Pr0.5OBiS2 is slightly smaller than that in PrOBiS2. Figure 8 shows X-ray absorption spectra at the Ce- and Pr-L3 edges of a Ce0.5Pr0.5OBiS2 single crystal in comparison with the corresponding polycrystalline sample. The results are very similar; thus, Ce has a mixed valence of 3+ and 4+ but Pr is trivalent. Similar to the case for the corresponding powder, approximately one-fifth of the Ce atoms are tetravalent. Figure 9 shows the temperature dependence of the resistivity and magnetization of a Ce0.5Pr0.5OBiS2 single crystal with a tetragonal structure. The resistivity starts to drop at 3.5 K, and zero resistivity is below ∼2.4 K. The magnetization starts to drop at 2.4 K. There is also a deviation between zero-field cooling and field cooling, suggesting spontaneous magnetization. The large shielding volume fraction indicates bulk superconductivity, although we cannot deny the possibility that

Figure 6. Temperature dependence of resistivity of Ce1−xPrxOBiS2 pellets between 0.1 and 15 K. The inset shows the relationship between composition (x) and temperature of zero resistivity (Tc).

this is enhanced by the demagnetization effect. Thus, tetragonal Ce0.5Pr0.5OBiS2 is a superconductor with a Tc value of ∼2.4 K.

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DISCUSSION The crystal structures and properties of Ce1−xPrxOBiS2 were investigated in powder and single-crystalline samples. While PrOBiS2 crystallized in a monoclinic phase, CeOBiS2 crystalD

DOI: 10.1021/acs.inorgchem.8b00349 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. SEM and EDS images of a Ce0.5Pr0.5OBiS2 (x = 0.5) single crystal.

lized in a tetragonal phase. In Ce1−xPrxOBiS2, for x values in the range of 0.3−0.9, both monoclinic and tetragonal phases were present. With increasing Ce amount, the fraction of the tetragonal phase increased. The lattice parameters of tetragonal and monoclinic phases were modified but did not follow a linear pattern. The minimum of the a axis in both tetragonal and monoclinic phases was found at x = 0.5, and the maximum temperature of zero resistivity was ca. 2.2 K at x = 0.5. In both polycrystalline and single-crystalline samples, the valence of Ce was a mixture of 3+ and 4+ while that of Pr was 3+. The single crystal of Ce0.5Pr0.5OBiS2 (x = 0.5) crystallized in a tetragonal structure, and its superconductivity with a transition temperature of ∼2.4 K was confirmed by both resistivity and magnetization results. Similar transition temperatures and compositions in powder and single-crystal samples suggest that the zero resistivity of Ce0.5Pr0.5OBiS2 of ∼2.2 K found in the polycrystalline sample can most likely be attributed to the tetragonal Ce0.5Pr0.5OBiS2 phase. As described in the Introduction, the conduction bands consist of Bi 6p and S 3p orbitals in Bi−S1 planes, and electron carriers are doped into these bands. In the Ce1−xPrxOBiS2 system, superconductive CeOBiS 2 powder and the Ce0.5Pr0.5OBiS2 single crystal crystallized in a tetragonal system. Thus, the average structure of two-dimensional Bi−S1 planes in

Figure 8. X-ray absorption spectra at Ce- and Pr-L3 edges of a Ce0.5Pr0.5OBiS2 single crystal (solid red line) and polycrystalline samples (dotted black line).

tetragonal structure would be more preferable in this system, although further investigation is necessary to understand their local structure. In the Ce1−xPrxOBiS2 system, the minimum a axis was found in Ce0.5Pr0.5OBiS2 powder. The structural analysis of Ce0.5Pr0.5OBiS2 single crystals showed that the Bi− S1 interatomic distance in two-dimensional Bi−S1 planes of Ce0.5Pr0.5OBiS2 was shorter than that of CeOBiS2. No out-ofplane bending of the linear S1−Bi−S1 bond, i.e. 180° for the S1−Bi−S1 bond angle, indicates a flat Bi−S1 plane. Thus, the Bi−S1 plane in Ce0.5Pr0.5OBiS2 is flatter than that in PrOBiS2 at 100 and 300 K and is slightly zigzag in comparison with that in CeOBiS2 at 300 K. Since approximately one-fifth of tetravalent Ce was detected by X-ray absorption independent of Ce/(Ce + Pr) ratio, a higher carrier concentration was expected in Ce-rich compounds. This is the same relationship between the angle

Table 1. Structural Summary of CeOBiS2, Ce0.5Pr0.5OBiS2, and PrOBiS2a CeOBiS2 cryst syst space group a/Å b/Å c/Å β/deg V/Å3 Bi−S1/Å S1−Bi−S1/deg Tc (zero resistivity)/K a

Ce0.5Pr0.5OBiS2

PrOBiS2

100 K

300 K

100 K

300 K

100 K

300 K

tetragonal P4/nmm 4.00018(5) 4.00018(5) 13.5422(2) 90 216.695(5) 2.8317(5) 174.6(4) 1.3

tetragonal P4/nmm 4.01002(4) 4.01002(4) 13.5602(2) 90 218.052(4) 2.8372(4) 176.1(4) 1.3

tetragonal P4/nmm 3.99299(5) 3.99299(5) 13.5831(2) 90 216.569(5) 2.8272(3) 174.1(2) 2.4

tetragonal P4/nmm 4.00363(4) 4.00363(4) 13.6088(2) 90 218.137(5) 2.8342(3) 174.5(3) 2.4

monoclinic P21/m 4.01231(5) 3.99363(5) 13.8268(2) 90.276(1) 221.554(5) 2.745(5)/ 2.944(5) 168.6(2) -

monoclinic P21/m 4.01269(7) 4.00837(7) 13.8371(1) 90.200(1) 222.560(6) 2.754(6)/2.944(7) 169.1(2) -

Structural analysis of Ce0.5Pr0.5OBiS2 was performed by using powder XRD of ground single crystals. E

DOI: 10.1021/acs.inorgchem.8b00349 Inorg. Chem. XXXX, XXX, XXX−XXX

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can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for A.M: [email protected]. *E-mail for M.N.: [email protected]. ORCID

Akira Miura: 0000-0003-0388-9696 Nataly Carolina Rosero-Navarro: 0000-0001-6838-2875 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by JSPS KAKENHI Grant Numbers 16H04493, 16K05454, 15H05884, and 15H03693.

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(1) Mizuguchi, Y.; Demura, S.; Deguchi, K.; Takano, Y.; Fujihisa, H.; Gotoh, Y.; Izawa, H.; Miura, O. Superconductivity in Novel BiS2-Based Layered Superconductor LaO1‑xFxBiS2. J. Phys. Soc. Jpn. 2012, 81 (11), 114725. (2) Mizuguchi, Y.; Fujihisa, H.; Gotoh, Y.; Suzuki, K.; Usui, H.; Kuroki, K.; Demura, S.; Takano, Y.; Izawa, H.; Miura, O. BiS2-based layered superconductor Bi4O4S3. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86 (22), 220510. (3) Awana, V. P. S.; Kumar, A.; Jha, R.; Kumar Singh, S.; Pal, A.; Shruti; Saha, J.; Patnaik, S. Appearance of superconductivity in layered LaO0.5F0.5BiS2. Solid State Commun. 2013, 157, 21−23. (4) Deguchi, K.; Takano, Y.; Mizuguchi, Y. Physics and chemistry of layered chalcogenide superconductors. Sci. Technol. Adv. Mater. 2012, 13 (5), 054303. (5) Usui, H.; Suzuki, K.; Kuroki, K. Minimal electronic models for superconducting BiS2 layers. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86 (22), 220501. (6) Demura, S.; Mizuguchi, Y.; Deguchi, K.; Okazaki, H.; Hara, H.; Watanabe, T.; Denholme, S. J.; Fujioka, M.; Ozaki, T.; Fujihisa, H.; Gotoh, Y.; Miura, O.; Yamaguchi, T.; Takeya, H.; Takano, Y. New Member of BiS2-Based Superconductor NdO1‑xFxBiS2. J. Phys. Soc. Jpn. 2013, 82 (3), 033708. (7) Jha, R.; Kumar, A.; Kumar Singh, S.; Awana, V. P. S. Synthesis and Superconductivity of New BiS 2 Based Superconductor PrO0.5F0.5BiS2. J. Supercond. Novel Magn. 2013, 26 (3), 499−502. (8) Yazici, D.; Huang, K.; White, B. D.; Chang, A. H.; Friedman, A. J.; Maple, M. B. Superconductivity of F-substitutedLnOBiS2 (Ln = La, Ce, Pr, Nd, Yb) compounds. Philos. Mag. 2013, 93 (6), 673−680. (9) Lin, X.; Ni, X.; Chen, B.; Xu, X.; Yang, X.; Dai, J.; Li, Y.; Yang, X.; Luo, Y.; Tao, Q.; Cao, G.; Xu, Z. Superconductivity induced by La doping in Sr1‑xLaxFBiS2. Phys. Rev. B 2013, 87 (2), 020504. (10) Phelan, W. A.; Wallace, D. C.; Arpino, K. E.; Neilson, J. R.; Livi, K. J.; Seabourne, C. R.; Scott, A. J.; McQueen, T. M. Stacking variants and superconductivity in the Bi-O-S system. J. Am. Chem. Soc. 2013, 135 (14), 5372−4. (11) Zhai, H. F.; Zhang, P.; Wu, S. Q.; He, C. Y.; Tang, Z. T.; Jiang, H.; Sun, Y. L.; Bao, J. K.; Nowik, I.; Felner, I.; Zeng, Y. W.; Li, Y. K.; Xu, X. F.; Tao, Q.; Xu, Z. A.; Cao, G. H. Anomalous Eu Valence State and Superconductivity in Undoped Eu3Bi2S4F4. J. Am. Chem. Soc. 2014, 136, 15386−15393. (12) Chen, H.; Zhang, G.; Hu, T.; Mu, G.; Li, W.; Huang, F.; Xie, X.; Jiang, M. Effect of local structure distortion on superconductivity in Mg- and F-codoped LaOBiS2. Inorg. Chem. 2014, 53 (1), 9−11.

Figure 9. Temperature dependence of (a) resistivity parallel to the ab plane and (b) magnetization under an applied magnetic field of 10 Oe parallel to the ab plane of a Ce0.5Pr0.5OBiS2 single crystal.

and carrier concentration of LaO1−xFxBiS2, observed by singlecrystal X-ray diffraction at room temperature; a higher F concentration shows an S1−Bi−S1 angle close to 180°.22 Thus, the higher transition temperature of Ce0.5Pr0.5OBiS2 (Tc ≈ 2.4 K) in comparison to CeOBiS2 (Tc ≈ 1.3 K) and PrOBiS2 (no transition above 0.1 K) can be explained by more hybridized Bi−S1 orbitals in the flat two-dimensional Bi−S1 plane as well as carrier induced by the mixed valence of Ce3+/Ce4+. Nonetheless, our results do not completely deny superconductivity in their monoclinic phase; thus, the selective syntheses of tetragonal and monoclinic phases are highly needed. Further investigation of local distortion, pressure, and crystal system can give the answer for how the structural dimensions of the phases affect the appearance of superconductivity.

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CONCLUSION We investigated the crystal structures and superconductivity in Ce1−xPrxOBiS2 systems. While monoclinic PrOBiS2 was nonsuperconductive down to 0.1 K, tetragonal Ce0.5Pr0.5OBiS2 was found to be superconductive with a transition temperature of ∼2.4 K. The tetragonal Ce0.5Pr0.5OBiS2 had shorter Bi−S1 bonds in comparison to those found in CeOBiS2; thus, more covalent Bi−S bonds increase the transition temperature. Approximately one-fifth of Ce was tetravalent, which may be responsible for the induction of carriers in flat Bi−S1 planes.

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REFERENCES

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Accession Codes

CCDC 1822467, 1822469−1822472, and 1836380 contain the supplementary crystallographic data for this paper. These data F

DOI: 10.1021/acs.inorgchem.8b00349 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.8b00349 Inorg. Chem. XXXX, XXX, XXX−XXX