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Synthesis and physical property characterization of LaOBiSe2 and LaO0.5F0.5BiSe2 superconductor
Solid State Communications 205 (2015) 14–18
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Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Synthesis and physical property characterization of LaOBiSe2 and LaO0.5F0.5BiSe2 superconductor S.L. Wu a, Z.A. Sun a,b, F.K. Chiang a, C. Ma a, H.F. Tian a, R.X. Zhang a, B. Zhang b, J.Q. Li a,c,n, H.X. Yang a,n a
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China School of Materials Science and Engineering, Hefei University of Technology, No. 193, Tunxi Road, Anhui Province, Hefei 230009, China c Collaborative Innovation Center of Quantum Matter, Beijing 100084, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 23 September 2014 Received in revised form 19 December 2014 Accepted 20 December 2014 Communicated by E.Y. Andrei Available online 29 December 2014
Bulk polycrystalline samples of LaO1 xFxBiSe2 (x ranges from 0 to 0.5) with nominal composition have been successfully synthesized by solid state reaction. Detailed structural analysis shows that LaOBiSe2 crystallizes in a tetragonal structure (P4/nmm) with lattice parameters of a ¼4.1565(1) Å and c ¼14.1074 (3) Å. Experimental results of electrical transport demonstrate that LaOBiSe2 is a bad metal with an evident anomaly at about 120 K, while the magnetic susceptibility exhibits an evident anomaly around 120 K, suggesting a possible charge density wave (CDW) transition around this temperature. Furthermore, superconductivity is observed in LaO0.5F0.5BiSe2 sample with nominal composition at 3.1 K from magnetic and transport measurements. & 2014 Elsevier Ltd. All rights reserved.
Keywords: A. Superconductor B. Preparation C. Structure D. Charge density wave
1. Introduction Since the discovery of superconductivity with Tc around 8.6 K in layered bismuth oxysulfide Bi4O4S3 [1], superconductivity in the structurally related materials has been extensively investigated. A series of LnO1 xFxBiS2 type (Ln¼ La [2], Nd [3], Ce [4], Pr [5], and Yb [6]) materials have been found to be superconducting at low temperatures with currently the highest Tc at about 10.8 K in LaO0.5F0.5BiS2 sample under high pressure [2]. Instead of substituting F for O, electron doped materials by substitution of tetravalent Th þ 4, Hf þ 4, Zr þ 4, and Ti þ 4 for trivalent La þ 3 have showed superconductivity up to 2.85 K in LaOBiS2 [7]. A new compound SrFBiS2 has also been reported to show superconductivity at 2.8 K by La doping [8]. Furthermore, it was reported that the replacement of sulfur by isovalent selenium leads to the isostructural superconductor LaO0.5F0.5BiSe2 with Tc ¼2.6 K in polycrystalline form [9]. Very recently, single crystals of LaO0.5F0.5BiSe2 have been successfully synthesized by the flux method and the bulk superconductivity were further confirmed to occur with slightly different Tc (Tc ¼3.7 K [10], 3.6 K [11], and 3.5 K [12]). These materials have a layered crystal structure composed of a charge reservoir layer (LnO, where Ln is a lanthanoid) or SrF layer n Corresponding authors at: Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China. E-mail addresses: [email protected] (J.Q. Li), [email protected] (H.X. Yang).
http://dx.doi.org/10.1016/j.ssc.2014.12.016 0038-1098/& 2014 Elsevier Ltd. All rights reserved.
alternating with BiS(Se)2 functional planes that play the same role as CuO2 planes in cuprates or the FeAs and Fe(Se,Te) planes in iron-based superconductors. Despite the similar structural configuration, BiS(Se)2based layered superconductors exhibit some important differences comparing with Fe-pnictide superconductors, and the superconducting mechanism remains unclear. Theoretical calculation suggests that LaO0.5F0.5BiSe2 and LaO0.5F0.5BiS2 belong to conventional electron– phonon coupling induced superconductors [13–16]; g-wave [17] and s-wave [18–20] pairing symmetries have been proposed by different groups. Physical measurement shows that the parent compound LnOBiS2 is a bad metal without detectable antiferromagnetic transition or structure phase transition, implying that magnetism is of less importance to superconductivity in BiS2-based systems [7], and the superconductivity appears in close proximity to an insulating normal state for the optimal superconducting samples, in sharp contrast to the iron-based superconductors where superconductivity grows from a metallic state [21,22]. Ren reported enhanced superconductivity in Fdoped LaO1 xFxBiSSe [23]. Recently, Demura reported the coexistence of bulk superconductivity and ferromagnetism in CeO1 xFxBiS2 [24] and, Cao [25] observed a CDW-like transition occurs at 280 K in EuBiS2F, below which superconductivity emerges at 0.3 K, without any extrinsic doping. So far, there is no detailed work has been carried out on the investigation of the parent compound LnOBiSe2 in literature. In this article, we report the successful synthesis and the detailed characterization of LaO1 xFxBiSe2 (x¼0–0.5) samples. The parent compound LaOBiSe2 exhibits an evident metallic behavior with
S.L. Wu et al. / Solid State Communications 205 (2015) 14–18
a small semiconducting hump at about 117 K. For sample with x¼0.5, the resistivity at normal state still shows the metallic behavior but the anomaly is evidently suppressed, followed by a sharp superconducting transition below 3.1 K. The strong diamagnetic signal provides compelling evidence of bulk superconductivity in LaO0.5F0.5BiSe2 compound with nominal composition.
2. Experimental Polycrystalline samples of LaO1 xFxBiSe2 (x ranges from 0 to 0.5) were obtained via solid state reaction. Mixture of high purity pieces of La and Bi, powders of La2O3, LaF3 and Se with designed composition was pressed into pellets; three pellets with 0.8 g mass were placed in three Al2O3 crucibles, which then further sealed in an evacuated quartz tube of 20 ml. The tubes were placed in a furnace at room temperature. The temperature slowly ramped up to 300 1C and held there for 5 h, then heated to 870 1C and held for 12 h. Additional regrinding and sintering following similar procedure were performed to promote phase homogeneity. The temperature dependence of resistivity was measured by a standard four-probe method. Microstructure analysis was performed on a Hitachi model S-4800 field emission scanning electron microscope (SEM) and a FEI Tecnai-F20 transmission electron microscope (TEM) with double-tilt heating holder. X-ray diffraction
15
(XRD) is performed on a Bruker AXS D8 Advanced diffractometer equipped with Cu Kα radiation at 40 kV and 40 mA. XRD spectrum of powder LaOBiSe2 used for refinement was acquired at room temperature with 2θ ranging from 51 to 1201, a step width of 0.021, and a counting time of 5 s/step. A Rietveld method was applied on XRD data by means of General Structure Analysis System (GSAS) to resolve the crystal structure, in which the Pseudo-Voigt function of Toraya was used as a profile function, and the isotropic atomic displacement parameters Biso, the isotropic Debye–Waller factor, were assigned to all atomic sites. Magnetization measurements
Table 1 Structure parameters of LaOBiSe2 at 293 K.a Site
Wyckoff position
x
y
z
Biso (Å2)
La Bi Se1 Se2 O
2c 2c 2c 2c 2a
1/4 1/4 1/4 1/4 3/4
1/4 1/4 1/4 1/4 1/4
0.0918(1) 0.6233(1) 0.8133(1) 0.3849(1) 0
1.12(3) 1.03(1) 1.25(4) 1.25(4) 2.3(2)
d(La–O)¼ 2.4486(7) Å ( 4), d(Bi–Se)¼ 2.9414(1) Å ( 4), 2.681(2) Å, and 3.363(2) Å. a Space group P4/nmm (No. 129) at the origin choice 2; a¼ 4.1565(1) Å, c¼ 14.1074(3) Å, and V ¼ 243.726(6) Å3, dcalc ¼ 7.16 g/cm3. χ2 ¼ 2.1, Rwp ¼5.65%, and Rp ¼ 3.97%. The occupation (g) of all the sites is unity.
Fig. 1. (Color online) (a) Powder X-ray diffraction pattern at room temperature for LaOBiSe2. The solid line represents the intensities calculated using the Rietveld method. The bottom curves are the differences between the experimental and calculated intensities. The vertical lines indicate the Bragg peak positions of the target compound. The inset shows the crystal structure of the tetragonal compound LaOBiSe2. (b) A SEM image of LaOBiSe2. (c) The EDX spectrum taken on a piece of single crystal, an SAED pattern taken from the [001] zone axis direction is shown as inset.
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S.L. Wu et al. / Solid State Communications 205 (2015) 14–18
between 2 and 300 K were carried out on a commercial superconductor quantum interference device magnetometer. Careful Rietveld analysis at the room-temperature XRD powder pattern indicates that the synthesized LaOBiSe2 material is crystallized into ZrCuSiAs type tetragonal crystal structure with space group P4/nmm (No. 129) as shown in Fig. 1(a). Most of the Bragg peaks can be indexed and some minor peaks (noted by stars) are characterized as impurities, such as La2O3 and Bi2O3, produced during the synthesis process. The resultant lattice parameters are determined as a¼ 4.1565(1) Å and c¼14.1074(3) Å converging the goodness of fit of the refinement (χ2) ultimately to be 2.1 with Rp ¼3.97% and Rwp ¼5.65%. The relatively large χ2 is due to the nontrivial extra peaks originated by impurities. Final lattice parameters, R factors, fractional coordinates, B parameters, and selected bond lengths are listed in Table 1. The structure of LaOBiSe2 is similar to LaOBiS2, which is built up by stacking the rock salt-type BiS2 layer and fluorite-type LaO layer alternatively along the c axis as illustrated as the inset of Fig. 1(a). In order to understand the microstructure features of the LaOBiSe2 materials, we have performed a series of investigations by means of scanning electron microscopy (SEM) and selectedarea electron diffraction (SAED). Fig. 1(b) shows a SEM image illustrating the typical morphological features of LaOBiSe2 crystal with clearly layered structural feature. In Fig. 1(c), we present the energy dispersive X-ray microanalysis (EDX) spectrum taken on a piece of single crystal, which confirmed the presence of the La, O, Bi and Se elements. The composition of the materials is estimated as LaOBiSe2. The crystal structure of the as made samples is further tested by electron-diffraction patterns taken along the various zone axis directions, all results show that all primary diffraction spots with strong intensity in these patterns can be well indexed by a tetragonal unit cell with lattice parameters of a¼ 4.15 Å and c¼ 14.11 Å, and a space group of P4/nmm, which is in good agreement with the XRD data. No additional superstructure spots were detected at room temperature. A typical diffraction pattern taken from the [001] zone axis direction is shown in Fig. 1(c). The temperature dependence of the resistivity ρ (T) for LaOBiSe2 was measured by the standard four-probe method under a zero magnetic field, and the result is displayed in Fig. 2(a). On lowering the temperature, ρ decreases rather linearly and then exhibits an evident upturn between 93 and 128 K. With further cooling, ρ decreases again, indicating conventional metallic behavior below and above the transition temperature. Fig. 2(b) shows experimental results of the zero-field cooled (ZFC) temperature dependent dc magnetic susceptibility of the LaOBiSe2 under applied fields of 5000 Oe. It can be clearly seen that the magnetic susceptibility exhibits an evident anomaly around 120 K, suggesting a possible charge density wave (CDW) transition around this temperature. We also tried to explore the superconductivity in the F doped samples with this new layered system. Fig. 3a shows the X-ray diffraction data for the powdered samples of LaO1 xFxBiSe2 (x ¼0–0.5) with nominal composition. The XRD pattern looks very similar to the standard LaOBiSe2 with a few minor peaks of the impurity phase. The Rietveld fitting result also reveals that the lattice parameters are a ¼ 4.156(2) Å and c ¼ 14.017(9) Å for LaO0.5F0.5BiSe2, which shows a the slightly decrease of the c-axis lattice constant similar as the F doping effect observed in the LnOBiS2 system [6], suggesting that F has been successfully substituted to the O site, as the ionic radius of F is smaller than that of O. In Fig. 3(b), we present the temperature dependence of resistivity for a different doped samples of LaO1 xFxBiSe2 with x¼0.4 and 0.5. We can also realize that the resistivity of these materials increases with the doping of the F concentrations, similar as the doping effect on the LnO1 xFxBiS2 materials. For the F doped samples, we can see that the anomaly at 120 K is gradually suppressed with the increase of doping level and a superconducting transition temperature with
Fig. 2. (a) Temperature dependence of the electrical resistivity for LaOBiSe2. (b) The ZFC magnetization of LaOBiSe2 as a function of temperature at an applied magnetic field of 5000 Oe.
Tc (onset) at 3.1 K and Tc (ρ ¼0) at 2.8 K can be clearly observed in the enlarged low temperature resistivity curves of LaO0.5F0.5BiSe2 sample (Fig. 3c). Superconductivity observed in the LaO0.5F0.5BiSe2 is further confirmed by the magnetization measurement. Fig. 3(d) gives a typical result of the LaO0.5F0.5BiSe2 sample; the field-cooling (FC) temperature dependence of the magnetic susceptibility was measured under an applied field of 1 Oe. The ZFC curve demonstrates an evident diamagnetic transition around 3.1 K, consistent with the Tc (onset) estimated from the ρ (T) curve. According to the density functional theory (DFT) calculations, the parent phase of this kind of material exhibits semiconducting behavior with a band gap of about 0.17 eV (not shown here) which, however, contradicts with the nature of a bad metal deduced from the physical measurements. The metallic behavior of the parent phase may be induced by the strong spin-orbital coupling, which shifts the bottom of the px and py bands below the Fermi energy, as observed in the CeOBiS2 samples [4]. In this metallic state, the Fermi surface topology has a “hidden” quasi-one-dimensional character and a strong Fermi surface nesting which would be found along the (110) direction. In fact, the observed resistivity anomaly at about 110 K for the parent and underdoped LaOBiSe2 samples supports the occurrence of the CDW formation, as shown in Fig. 3. Unlike BiS2-based superconductor in which the superconductivity occurs in the vicinity of the semiconducting state under electron doping, the normal state of the superconducting compound LaO1 xFxBiSe2 at x¼0.5 still exhibits metallic behavior, although the electrical resistivity is relatively large in comparison with the parent compound. In LaOBiSe2, the F doping suppresses the CDW transition, and it is possible that the electron–phonon coupling would be enhanced by the phonon softening in the non-CDW state, since the
S.L. Wu et al. / Solid State Communications 205 (2015) 14–18
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Fig. 3. (Color online) (a) The X-ray diffraction patterns for the powdered samples of LaO1 xFxBiSe2 (x¼ 0–0.5) with nominal composition. Except for several peaks of impurities, all of the peaks can be characterized to the standard LaOBiSe2 with the space group P4/nmm. (b) The temperature dependence of resistivity for LaO1 xFxBiSe2 (x¼ 0.4, 0.5). (c) An enlarged view of the resistivity data in the low-temperature region. (d) Temperature dependence of the FC and ZFC dc magnetization of the superconducting LaO0.5F0.5BiSe2 sample.
strong electron–phonon coupling and phonon softening were predicted. Therefore, the superconductivity would be maximized at specific k points associated with the CDW ordering vectors owing to electron–phonon coupling, as observed in other systems [16,26,27]. In conclusion we have synthesized a nearly single phase LaOBiSe2 compound, which is a bad metal with a possible CDW transition at about 120 K. Superconductivity is observed in LaO0.5F0.5BiSe2 sample with nominal composition at 3.1 K from magnetic and transport measurements. Our data clearly demonstrate a competition between the possible CDW instability and superconductivity in this system.
Acknowledgments This work was supported by the National Basic Research Program of China (973 Program) (Grant nos. 2011CB921703, 2015CB921300, 2011CBA00101, 2010CB923002, and 2012CB821404), the National Natural Science Foundation of China, China (Grant nos. 11474323, 51272277, 91221102,11190022, and 11274368), and the "Strategic Priority Research Program (B)" of the Chinese Academy of Sciences (Grant no. XDB07020000).
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[24] S. Demura, K. Deguchi, Y. Mizuguchi, K. Sato, R. Honjyo, A. Yamashita, T. Yamaki, H. Hara, T. Watanabe, S.J. Denholme, M. Fujioka, H. Okazaki, T. Ozaki, O. Miura, T. Yamaguchi, H. Takeya, Y. Takano, arXiv:1311.4267. [25] H.F. Zhai, Z.T. Tang, H. Jiang, K. Xu, K. Zhang, P. Zhang, et al., Phys. Rev. B 90 (2014) 064518.
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Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Synthesis and physical property characterization of LaOBiSe2 and LaO0.5F0.5BiSe2 superconductor S.L. Wu a, Z.A. Sun a,b, F.K. Chiang a, C. Ma a, H.F. Tian a, R.X. Zhang a, B. Zhang b, J.Q. Li a,c,n, H.X. Yang a,n a
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China School of Materials Science and Engineering, Hefei University of Technology, No. 193, Tunxi Road, Anhui Province, Hefei 230009, China c Collaborative Innovation Center of Quantum Matter, Beijing 100084, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 23 September 2014 Received in revised form 19 December 2014 Accepted 20 December 2014 Communicated by E.Y. Andrei Available online 29 December 2014
Bulk polycrystalline samples of LaO1 xFxBiSe2 (x ranges from 0 to 0.5) with nominal composition have been successfully synthesized by solid state reaction. Detailed structural analysis shows that LaOBiSe2 crystallizes in a tetragonal structure (P4/nmm) with lattice parameters of a ¼4.1565(1) Å and c ¼14.1074 (3) Å. Experimental results of electrical transport demonstrate that LaOBiSe2 is a bad metal with an evident anomaly at about 120 K, while the magnetic susceptibility exhibits an evident anomaly around 120 K, suggesting a possible charge density wave (CDW) transition around this temperature. Furthermore, superconductivity is observed in LaO0.5F0.5BiSe2 sample with nominal composition at 3.1 K from magnetic and transport measurements. & 2014 Elsevier Ltd. All rights reserved.
Keywords: A. Superconductor B. Preparation C. Structure D. Charge density wave
1. Introduction Since the discovery of superconductivity with Tc around 8.6 K in layered bismuth oxysulfide Bi4O4S3 [1], superconductivity in the structurally related materials has been extensively investigated. A series of LnO1 xFxBiS2 type (Ln¼ La [2], Nd [3], Ce [4], Pr [5], and Yb [6]) materials have been found to be superconducting at low temperatures with currently the highest Tc at about 10.8 K in LaO0.5F0.5BiS2 sample under high pressure [2]. Instead of substituting F for O, electron doped materials by substitution of tetravalent Th þ 4, Hf þ 4, Zr þ 4, and Ti þ 4 for trivalent La þ 3 have showed superconductivity up to 2.85 K in LaOBiS2 [7]. A new compound SrFBiS2 has also been reported to show superconductivity at 2.8 K by La doping [8]. Furthermore, it was reported that the replacement of sulfur by isovalent selenium leads to the isostructural superconductor LaO0.5F0.5BiSe2 with Tc ¼2.6 K in polycrystalline form [9]. Very recently, single crystals of LaO0.5F0.5BiSe2 have been successfully synthesized by the flux method and the bulk superconductivity were further confirmed to occur with slightly different Tc (Tc ¼3.7 K [10], 3.6 K [11], and 3.5 K [12]). These materials have a layered crystal structure composed of a charge reservoir layer (LnO, where Ln is a lanthanoid) or SrF layer n Corresponding authors at: Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China. E-mail addresses: [email protected] (J.Q. Li), [email protected] (H.X. Yang).
http://dx.doi.org/10.1016/j.ssc.2014.12.016 0038-1098/& 2014 Elsevier Ltd. All rights reserved.
alternating with BiS(Se)2 functional planes that play the same role as CuO2 planes in cuprates or the FeAs and Fe(Se,Te) planes in iron-based superconductors. Despite the similar structural configuration, BiS(Se)2based layered superconductors exhibit some important differences comparing with Fe-pnictide superconductors, and the superconducting mechanism remains unclear. Theoretical calculation suggests that LaO0.5F0.5BiSe2 and LaO0.5F0.5BiS2 belong to conventional electron– phonon coupling induced superconductors [13–16]; g-wave [17] and s-wave [18–20] pairing symmetries have been proposed by different groups. Physical measurement shows that the parent compound LnOBiS2 is a bad metal without detectable antiferromagnetic transition or structure phase transition, implying that magnetism is of less importance to superconductivity in BiS2-based systems [7], and the superconductivity appears in close proximity to an insulating normal state for the optimal superconducting samples, in sharp contrast to the iron-based superconductors where superconductivity grows from a metallic state [21,22]. Ren reported enhanced superconductivity in Fdoped LaO1 xFxBiSSe [23]. Recently, Demura reported the coexistence of bulk superconductivity and ferromagnetism in CeO1 xFxBiS2 [24] and, Cao [25] observed a CDW-like transition occurs at 280 K in EuBiS2F, below which superconductivity emerges at 0.3 K, without any extrinsic doping. So far, there is no detailed work has been carried out on the investigation of the parent compound LnOBiSe2 in literature. In this article, we report the successful synthesis and the detailed characterization of LaO1 xFxBiSe2 (x¼0–0.5) samples. The parent compound LaOBiSe2 exhibits an evident metallic behavior with
S.L. Wu et al. / Solid State Communications 205 (2015) 14–18
a small semiconducting hump at about 117 K. For sample with x¼0.5, the resistivity at normal state still shows the metallic behavior but the anomaly is evidently suppressed, followed by a sharp superconducting transition below 3.1 K. The strong diamagnetic signal provides compelling evidence of bulk superconductivity in LaO0.5F0.5BiSe2 compound with nominal composition.
2. Experimental Polycrystalline samples of LaO1 xFxBiSe2 (x ranges from 0 to 0.5) were obtained via solid state reaction. Mixture of high purity pieces of La and Bi, powders of La2O3, LaF3 and Se with designed composition was pressed into pellets; three pellets with 0.8 g mass were placed in three Al2O3 crucibles, which then further sealed in an evacuated quartz tube of 20 ml. The tubes were placed in a furnace at room temperature. The temperature slowly ramped up to 300 1C and held there for 5 h, then heated to 870 1C and held for 12 h. Additional regrinding and sintering following similar procedure were performed to promote phase homogeneity. The temperature dependence of resistivity was measured by a standard four-probe method. Microstructure analysis was performed on a Hitachi model S-4800 field emission scanning electron microscope (SEM) and a FEI Tecnai-F20 transmission electron microscope (TEM) with double-tilt heating holder. X-ray diffraction
15
(XRD) is performed on a Bruker AXS D8 Advanced diffractometer equipped with Cu Kα radiation at 40 kV and 40 mA. XRD spectrum of powder LaOBiSe2 used for refinement was acquired at room temperature with 2θ ranging from 51 to 1201, a step width of 0.021, and a counting time of 5 s/step. A Rietveld method was applied on XRD data by means of General Structure Analysis System (GSAS) to resolve the crystal structure, in which the Pseudo-Voigt function of Toraya was used as a profile function, and the isotropic atomic displacement parameters Biso, the isotropic Debye–Waller factor, were assigned to all atomic sites. Magnetization measurements
Table 1 Structure parameters of LaOBiSe2 at 293 K.a Site
Wyckoff position
x
y
z
Biso (Å2)
La Bi Se1 Se2 O
2c 2c 2c 2c 2a
1/4 1/4 1/4 1/4 3/4
1/4 1/4 1/4 1/4 1/4
0.0918(1) 0.6233(1) 0.8133(1) 0.3849(1) 0
1.12(3) 1.03(1) 1.25(4) 1.25(4) 2.3(2)
d(La–O)¼ 2.4486(7) Å ( 4), d(Bi–Se)¼ 2.9414(1) Å ( 4), 2.681(2) Å, and 3.363(2) Å. a Space group P4/nmm (No. 129) at the origin choice 2; a¼ 4.1565(1) Å, c¼ 14.1074(3) Å, and V ¼ 243.726(6) Å3, dcalc ¼ 7.16 g/cm3. χ2 ¼ 2.1, Rwp ¼5.65%, and Rp ¼ 3.97%. The occupation (g) of all the sites is unity.
Fig. 1. (Color online) (a) Powder X-ray diffraction pattern at room temperature for LaOBiSe2. The solid line represents the intensities calculated using the Rietveld method. The bottom curves are the differences between the experimental and calculated intensities. The vertical lines indicate the Bragg peak positions of the target compound. The inset shows the crystal structure of the tetragonal compound LaOBiSe2. (b) A SEM image of LaOBiSe2. (c) The EDX spectrum taken on a piece of single crystal, an SAED pattern taken from the [001] zone axis direction is shown as inset.
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between 2 and 300 K were carried out on a commercial superconductor quantum interference device magnetometer. Careful Rietveld analysis at the room-temperature XRD powder pattern indicates that the synthesized LaOBiSe2 material is crystallized into ZrCuSiAs type tetragonal crystal structure with space group P4/nmm (No. 129) as shown in Fig. 1(a). Most of the Bragg peaks can be indexed and some minor peaks (noted by stars) are characterized as impurities, such as La2O3 and Bi2O3, produced during the synthesis process. The resultant lattice parameters are determined as a¼ 4.1565(1) Å and c¼14.1074(3) Å converging the goodness of fit of the refinement (χ2) ultimately to be 2.1 with Rp ¼3.97% and Rwp ¼5.65%. The relatively large χ2 is due to the nontrivial extra peaks originated by impurities. Final lattice parameters, R factors, fractional coordinates, B parameters, and selected bond lengths are listed in Table 1. The structure of LaOBiSe2 is similar to LaOBiS2, which is built up by stacking the rock salt-type BiS2 layer and fluorite-type LaO layer alternatively along the c axis as illustrated as the inset of Fig. 1(a). In order to understand the microstructure features of the LaOBiSe2 materials, we have performed a series of investigations by means of scanning electron microscopy (SEM) and selectedarea electron diffraction (SAED). Fig. 1(b) shows a SEM image illustrating the typical morphological features of LaOBiSe2 crystal with clearly layered structural feature. In Fig. 1(c), we present the energy dispersive X-ray microanalysis (EDX) spectrum taken on a piece of single crystal, which confirmed the presence of the La, O, Bi and Se elements. The composition of the materials is estimated as LaOBiSe2. The crystal structure of the as made samples is further tested by electron-diffraction patterns taken along the various zone axis directions, all results show that all primary diffraction spots with strong intensity in these patterns can be well indexed by a tetragonal unit cell with lattice parameters of a¼ 4.15 Å and c¼ 14.11 Å, and a space group of P4/nmm, which is in good agreement with the XRD data. No additional superstructure spots were detected at room temperature. A typical diffraction pattern taken from the [001] zone axis direction is shown in Fig. 1(c). The temperature dependence of the resistivity ρ (T) for LaOBiSe2 was measured by the standard four-probe method under a zero magnetic field, and the result is displayed in Fig. 2(a). On lowering the temperature, ρ decreases rather linearly and then exhibits an evident upturn between 93 and 128 K. With further cooling, ρ decreases again, indicating conventional metallic behavior below and above the transition temperature. Fig. 2(b) shows experimental results of the zero-field cooled (ZFC) temperature dependent dc magnetic susceptibility of the LaOBiSe2 under applied fields of 5000 Oe. It can be clearly seen that the magnetic susceptibility exhibits an evident anomaly around 120 K, suggesting a possible charge density wave (CDW) transition around this temperature. We also tried to explore the superconductivity in the F doped samples with this new layered system. Fig. 3a shows the X-ray diffraction data for the powdered samples of LaO1 xFxBiSe2 (x ¼0–0.5) with nominal composition. The XRD pattern looks very similar to the standard LaOBiSe2 with a few minor peaks of the impurity phase. The Rietveld fitting result also reveals that the lattice parameters are a ¼ 4.156(2) Å and c ¼ 14.017(9) Å for LaO0.5F0.5BiSe2, which shows a the slightly decrease of the c-axis lattice constant similar as the F doping effect observed in the LnOBiS2 system [6], suggesting that F has been successfully substituted to the O site, as the ionic radius of F is smaller than that of O. In Fig. 3(b), we present the temperature dependence of resistivity for a different doped samples of LaO1 xFxBiSe2 with x¼0.4 and 0.5. We can also realize that the resistivity of these materials increases with the doping of the F concentrations, similar as the doping effect on the LnO1 xFxBiS2 materials. For the F doped samples, we can see that the anomaly at 120 K is gradually suppressed with the increase of doping level and a superconducting transition temperature with
Fig. 2. (a) Temperature dependence of the electrical resistivity for LaOBiSe2. (b) The ZFC magnetization of LaOBiSe2 as a function of temperature at an applied magnetic field of 5000 Oe.
Tc (onset) at 3.1 K and Tc (ρ ¼0) at 2.8 K can be clearly observed in the enlarged low temperature resistivity curves of LaO0.5F0.5BiSe2 sample (Fig. 3c). Superconductivity observed in the LaO0.5F0.5BiSe2 is further confirmed by the magnetization measurement. Fig. 3(d) gives a typical result of the LaO0.5F0.5BiSe2 sample; the field-cooling (FC) temperature dependence of the magnetic susceptibility was measured under an applied field of 1 Oe. The ZFC curve demonstrates an evident diamagnetic transition around 3.1 K, consistent with the Tc (onset) estimated from the ρ (T) curve. According to the density functional theory (DFT) calculations, the parent phase of this kind of material exhibits semiconducting behavior with a band gap of about 0.17 eV (not shown here) which, however, contradicts with the nature of a bad metal deduced from the physical measurements. The metallic behavior of the parent phase may be induced by the strong spin-orbital coupling, which shifts the bottom of the px and py bands below the Fermi energy, as observed in the CeOBiS2 samples [4]. In this metallic state, the Fermi surface topology has a “hidden” quasi-one-dimensional character and a strong Fermi surface nesting which would be found along the (110) direction. In fact, the observed resistivity anomaly at about 110 K for the parent and underdoped LaOBiSe2 samples supports the occurrence of the CDW formation, as shown in Fig. 3. Unlike BiS2-based superconductor in which the superconductivity occurs in the vicinity of the semiconducting state under electron doping, the normal state of the superconducting compound LaO1 xFxBiSe2 at x¼0.5 still exhibits metallic behavior, although the electrical resistivity is relatively large in comparison with the parent compound. In LaOBiSe2, the F doping suppresses the CDW transition, and it is possible that the electron–phonon coupling would be enhanced by the phonon softening in the non-CDW state, since the
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Fig. 3. (Color online) (a) The X-ray diffraction patterns for the powdered samples of LaO1 xFxBiSe2 (x¼ 0–0.5) with nominal composition. Except for several peaks of impurities, all of the peaks can be characterized to the standard LaOBiSe2 with the space group P4/nmm. (b) The temperature dependence of resistivity for LaO1 xFxBiSe2 (x¼ 0.4, 0.5). (c) An enlarged view of the resistivity data in the low-temperature region. (d) Temperature dependence of the FC and ZFC dc magnetization of the superconducting LaO0.5F0.5BiSe2 sample.
strong electron–phonon coupling and phonon softening were predicted. Therefore, the superconductivity would be maximized at specific k points associated with the CDW ordering vectors owing to electron–phonon coupling, as observed in other systems [16,26,27]. In conclusion we have synthesized a nearly single phase LaOBiSe2 compound, which is a bad metal with a possible CDW transition at about 120 K. Superconductivity is observed in LaO0.5F0.5BiSe2 sample with nominal composition at 3.1 K from magnetic and transport measurements. Our data clearly demonstrate a competition between the possible CDW instability and superconductivity in this system.
Acknowledgments This work was supported by the National Basic Research Program of China (973 Program) (Grant nos. 2011CB921703, 2015CB921300, 2011CBA00101, 2010CB923002, and 2012CB821404), the National Natural Science Foundation of China, China (Grant nos. 11474323, 51272277, 91221102,11190022, and 11274368), and the "Strategic Priority Research Program (B)" of the Chinese Academy of Sciences (Grant no. XDB07020000).
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