Discovery of BiS2-Based Superconductor and Material Design Concept

1. Discovery of BiS₂-Based Superconductor Family

1.1. Layered Superconductors

Since the discovery of the Fe-based superconductor in 2008 [1], many researchers have explored new Fe-based superconductors [2]. During the Fe age, I studied physics, chemistry, and the application of the Fe-chalcogenide superconductors and obtained a Ph.D. in 2010. Then, in 2011, I launched new research group at Tokyo Metropolitan University and started exploring new layered superconductors such as Fe-based or cuprate superconductors [3]. Those layered superconductors have a layered structure composed of alternate stacks of conducting (superconducting) and insulating (blocking) layers. The FeAs layer and CuO₂ plane are the essential structures for the emergence of superconductivity in Fe-based and cuprate superconductors, respectively. Namely, by utilizing the structural flexibility, new superconductors with various stacking structures could be designed by changing the blocking layer and/or the thickness of the superconducting planes. Therefore, I aimed to discover a new layered superconductor with a new type of superconducting layer other than the FeAs layer and CuO₂ plane.

To discover new layered superconductors, experience in the material design of the Fe-based superconductor family is very useful. A typical crystal structure of the Fe-based superconductor is the ZrSiCuAs-type (P4/nmm) structure, for example LaOFeAs [1]. As shown in Figure 1a, the crystal structure of LaOFeAs is composed of the FeAs superconducting layer and the LaO blocking layer. Among layered compounds, the ZrSiCuAs-type structure is one of the typical structures. For example, BiOCuSe, whose structure is composed of a CuSe conducting layer and a BiO blocking layer, is a famous thermoelectric material [4]. When substituting the CuSe layer of BiOCuSe with a CuS layer, we obtain BiOCuS with the ZrSiCuAs-type structure.

1.2. Discovery of the Bi–O–S Superconductor

In the beginning, we explored new superconductors near the BiOCuS system because a superconductivity signal was detected for Cu-deficient BiOCu_1−xS [6]. Although Reference [6] reported superconductivity, the shielding volume fraction of the sample was not high, indicating that the superconductivity was not bulk in nature. In addition, the other research group also examined the BiOCu_1−xS and its Pb-doped samples [7]; superconductivity, however, was not discovered in those samples. Therefore, we modified the composition of Bi-O-Bu-S and finally synthesized Cu-free Bi₂O₂S, which is a layered compound, according to the material database [8]. Surprisingly, the Bi₂O₂S sample prepared at 500 °C showed a superconducting signal at T of ~5 K in the temperature dependence of magnetization. The X-ray diffraction (XRD) pattern of the sample was not what was expected from the Bi₂O₂S structure and indicated that the sample was not a single phase. This suggests the presence of a Bi–O–S superconducting phase stable at around 500 °C. Then, we explored the optimal composition of the unknown Bi–O–S superconducting phase. After synthesizing and evaluating over 100 samples of Bi_xO_yS_z (with various synthesis temperatures), we obtained the optimal composition of Bi₄O₄S₃, in which the superconducting volume fraction is the highest, and the XRD pattern seemed to be a single phase.

Although we obtained the single-phase sample of Bi₄O₄S₃, the crystal structure was unknown, and no similar XRD pattern could be found in the database [8]. The XRD pattern was analyzed by Fujihisa and Gotoh of AIST (National Institute of Advanced Industry Science and Technology), who are specialists in crystal structure determination. From the XRD pattern, the framework of Bi and S was determined. Since the XRD pattern was collected with a powder sample, the determination of the O site was difficult. Therefore, we determined the final structure model with the combination of calculation and Rietveld refinement. Finally, we concluded that Bi₄O₄S₃ had a layered structure as depicted in Figure 1e. In addition, we analyzed the band structure with the support of Kuroki’s group (Osaka University). The crystal structure of Bi₄O₄S₃ is composed of alternate stacking of the BiS₂ bilayer, which is electrically conducting, and the Bi₄O₄(SO₄)_x blocking layer. The parent phase (x = 0) was expected to be a band insulator, but the present phase, Bi₄O₄S₃, corresponded to x = 0.5. Namely, there are 50% SO₄ defects in the blocking layer, and, hence, electrons are doped to the BiS₂ layers, resulting in metallic conductivity and superconductivity [9]. Interestingly, the stable phase (composition) contains defects at the SO₄ site, and the control of the doping level is very difficult in this Bi₄O₄SO₄Bi₂S₄ structure. Motivated by this difficulty, we tried to synthesize other BiS₂-based superconductors with a simpler structure.

1.3. 1112-Type BiS₂-Based Superconductors

After the discovery of the Bi₄O₄S₃ superconductor, we studied LaOBiS₂, the crystal structure of which is composed of a La₂O₂ blocking layer and a BiS₂ bilayer (Figure 1b). Although this material was already known [10], the physical properties and the carrier doping effects had not been examined in detail. Thus, on the basis of the analogies on the crystal structure and electronic structure between Bi₄O₄S₃ and LaOBiS₂, we synthesized electron-doped LaOBiS₂, LaO_1−xF_xBiS₂. The partial substitution of F⁻ for O²⁻ generates electron carriers in the BiS₂ layer [11,12], which will be explained in Section 2.1. The LaO_1−xF_xBiS₂ superconductor shows a T_c of ~3 K with x = 0.5, but the superconductivity is not in bulk, according to the results of the magnetic susceptibility measurements. Bulk superconductivity in LaO_0.5F_0.5BiS₂ can be induced by the pressure effect [13,14,15,16]. Under a pressure above 1 GPa, the T_c reaches 10 K and the superconducting volume fraction is largely enhanced, which indicates the emergence of bulk superconductivity. In addition, high-pressure synthesis also produces the bulk superconductivity phase of LaO_0.5F_0.5BiS₂ [10,17,18]. In order to induce bulk superconductivity in LaO_0.5F_0.5BiS₂, the chemical pressure effect is also effective. By replacing La with smaller rare-earth ions, Ce, Pr, Nd, or Sm, bulk superconductivity can be induced [19,20,21,22,23,24]. Namely, REO_1−xF_xBiS₂ with RE = La, Ce, Pr, Nd, or Sm is basically superconductive, and has a crystal structure as shown in Figure 1b. The detailed explanations on the external and chemical pressure effects will appear in Section 2.2 and Section 2.3.

The R₂O₂ layer can be replaced by the Sr₂F₂ or Eu₂F₂ layer, as depicted in Figure 1c [25,26,27,28]. The SrFBiS₂ compound is a band insulator and superconductivity can be induced by electron doping as well as LaOBiS₂. In this series, the R³⁺ substitution for the Sr (or Eu) site can generate electron carriers in the BiS₂ layer, and superconductivity is induced [26]. EuFBiS₂ shows a superconducting transition at 0.3 K without chemical doping because the valence of Eu is not 2+ but about 2.2+, which results in self-doped electron carriers [27]. Since the lattice constants of SrFBiS₂-type compounds are similar to those of LaOBiS₂-type ones, the physical properties of those two systems are quite similar. Namely, the strategies of carrier doping and the pressure effects can be applied similarly.

1.4. Other Bi-Chalcogenide Layered Compounds

Figure 1d shows the crystal structure of Eu₃F₄Bi₂S₄, which is composed of alternate stacks of the Eu₃F₄ blocking layer and the BiS₂ bilayer [29,30]. This compound also exhibits the mixed-valence state of Eu, and shows superconductivity at low temperatures (T_c ~ 1 K). The T_c can be enhanced by 3.35 K with Se substitution [31].

Recently, a new Bi-S layered compound with a thicker conducting layer was discovered. LaOBiPbS₃ has a La₂O₂ blocking similar to LaOBiS₂, but its conducting layer is Bi₂Pb₂S₆, which is the four-layer-type conducting layer [32]. Although LaOBiPbS₃ is not metallic, carrier doping may induce superconductivity and will open a new playground of Bi-chalcogenide superconductors.

I would like to briefly introduce the existence of the sister phases of Bi₄O₄S₃. In Bi₃O₂S₃, the SO₄ site of Bi₄O₄S₃ is replaced by the S₂ dimer, and superconductivity is observed without substitution [33]. The Bi–O–S series possess quite characteristic structures, and thus, the design of new superconductors in the Bi–O–S series may be one of the strategies for enhancing T_c in the BiS₂-based systems.

2. Emergence of Superconductivity in BiS₂-Based Compounds

2.1. Carrier Doping

As mentioned above, the parent phases of the BiS₂-based compounds, such as LaOBiS₂ or SrFBiS₂, are generally an insulator with a band gap. Figure 2a shows the band structure of LaOBiS₂. The bands just above the Fermi energy are composed of Bi-6p orbitals (p_x and p_y) [12]. Therefore, it is expected that the BiS₂ layer becomes metallic by electron doping. There are several ways to induce metallicity and superconductivity in the BiS₂ layer. In the REOBiS₂-type structure, the most typical way of electron doping is the partial substitution of F⁻ for O²⁻ [11,19,21,22,23]. Figure 2b shows the temperature dependences of electrical resistivity for LaOBiS₂ and LaO_0.5F_0.5BiS₂. By F substitution, the electrical resistivity decreases and the superconducting transition is observed at 2.5 K. Figure 2c shows the temperature dependence of the magnetic susceptibility for LaO_0.5F_0.5BiS₂. Diamagnetic signals corresponding to the emergence of superconductivity are observed. However, the magnitude of the superconductivity signal, 4πχ, is very small as a bulk superconductor. As shown in the inset of Figure 2c, the optimal F concentration, at which the T_c is the highest, is x = 0.5 [11,17,34]. Therefore, the weak superconductivity signal observed in the magnetic susceptibility measurement is caused not by the insufficiency of the carrier concentration but by another factor. To induce bulk superconductivity in LaO_1−xF_xBiS₂, external and chemical pressure effects are essential.

2.2. External Pressure Effect

The superconducting properties of REO_1−xF_xBiS₂ are very sensitive to the external pressure [35]. The first example is the high-pressure synthesis effect. As shown in Figure 2c, the superconductivity in LaO_0.5F_0.5BiS₂ is not perfect (not bulk in nature) on the basis of the shielding fraction observed in the magnetic susceptibility measurement. Surprisingly, the superconducting properties of the LaO_0.5F_0.5BiS₂ sample dramatically change when the sample is prepared under the high pressure of 2 GPa [11,17,18]. Figure 3a shows the temperature dependence of the magnetic susceptibility for two different samples of LaO_0.5F_0.5BiS₂: one was synthesized in an evacuated quartz tube (low pressure) and the other was synthesized at a high pressure of 2 GPa (high pressure). The high-pressure (high-P) phase shows a large shielding fraction, which indicates the emergence of bulk superconductivity, and a higher T_c of ~8 K. Please note that the susceptibility measurements were performed in vacuum (not under high pressure), but the low-P and high-P samples show different properties. Therefore, high-pressure synthesis can modify the crystal structure. Importantly, we can obtain intermediate phases with T_c = 2.5−8 K by modifying the annealing temperature and pressure. Indeed, for LaO_0.5F_0.5BiS₂, there are at least two stable structures which can be transformed by temperature and/or pressure.

The high-P phase can be obtained by applying external pressure during low-temperature measurements. Figure 3b shows the pressure dependences of T_c for REO_1−xF_xBiS₂ with an optimal F doping (LaO_0.5F_0.5BiS₂, CeO_0.3F_0.7BiS₂, PrO_0.5F_0.5BiS₂, and NdO_0.5F_0.5BiS₂) [13,14,15,16,35]. All the REO_1−xF_xBiS₂ superconductors show an increase in T_c by applying external pressure. Interestingly, the pressure dependences of T_c show a transition-like increase at different pressures, which is 0.5–1 GPa for RE = La and about 2 GPa for RE = Nd. The critical pressure seems to be related to the lattice constant of the a-axis of REO_1−xF_xBiS₂. This can be understood with the crystal structure transition from tetragonal (P4/nmm) to monoclinic (P2₁/m) in LaO_1−xF_xBiS₂ [13] under high pressure, as depicted in Figure 3c,d. This structural transition associated with the formation of the Bi-S zigzag chain is shown in Figure 3d. It can be easily assumed that the formation of the zigzag chain takes place when the Bi-S square lattice in the tetragonal structure is not constrained and has some space to distort. In fact, recent crystal structure analysis using synchrotron X-ray showed that the crystal structure of LaOBiS₂ is the monoclinic structure at room temperature [36]. In addition, different three-dimensional stacking sequences of the BiS₂ layers were theoretically proposed [37]. On the basis of these results, the crystal structure of the REO_1−xF_xBiS₂ system can be easily transformed between tetragonal and monoclinic by tuning the lattice volume, and the systematic investigations of the crystal structure are very important for understanding the mechanisms of the emerging superconductivity.

2.3. Chemical Pressure Effect

Another method to induce the bulk superconductivity of LaO_0.5F_0.5BiS₂ is the chemical pressure effect. Generally, the chemical pressure can be applied by substitutions of an isovalent element with a different ionic radius. In the case of LaO_0.5F_0.5BiS₂, there are two methods of applying chemical pressure. The first method is the substitution of La³⁺ by the smaller RE³⁺ ions, such as Ce³⁺, Pr³⁺, Nd³⁺, or Sm³⁺. In REO_0.5F_0.5BiS₂, bulk superconductivity (at above 2 K) is induced at RE = Ce_0.6Nd_0.4. The T_c increases with decreasing the mean RE³⁺ ionic radius. The lattice volume basically decreases with decreasing the mean RE³⁺ ionic radius, as shown in Figure 4c. Therefore, the shrinkage of the BiS₂ layer should be related to the emergence of bulk superconductivity in REO_0.5F_0.5BiS₂. These results seem to be consistent with the external pressure effects, while the crystal structure remains tetragonal up to RE = Sm [23,24]. Then, please note that the filamentary (non-bulk) superconductivity observed in RE = La is suppressed by Ce substitution. Although the reason for this strange behavior is unclear, this should be related to the structural fluctuation between tetragonal and monoclinic on a local scale.

The other method to apply chemical pressure is the substitution of S²⁻ by Se²⁻. This is a direct chemical pressure to the superconducting BiS₂ layer. Since the ionic radius of Se²⁻ is larger than S²⁻, the lattice volume increases with the increasing Se concentration, as shown in Figure 4c. Therefore, the chemical pressure effect by the Se substitution seems to be the opposite to that by the substitution of the smaller RE³⁺. However, the obtained superconductivity phase diagram for LaO_0.5F_0.5BiS_2−xSe_x (Figure 4b) is quite similar to that for REO_0.5F_0.5BiS₂ (Figure 4a). The filamentary superconductivity is suppressed by the small doping of Se, and bulk superconductivity is induced at x ≥ 0.4. T_c increases with the increasing Se concentration [38].

2.4. Importance of In-Plane Chemical Pressure

On the basis of the experimental facts on REO_0.5F_0.5BiS₂ and LaO_0.5F_0.5BiS_2−xSe_x, we explored a factor essential for the evolution of bulk superconductivity in this family. To address it, the crystal structures of Ce_1−xNd_xO_0.5F_0.5BiS₂ and LaO_0.5F_0.5BiS_2−xSe_x, which exhibit a crossover region between non-superconductivity and bulk superconductivity, were examined using synchrotron X-ray [39]. Since the changes in the lattice volume are opposite between these two systems, as shown in Figure 4c, almost all the structural parameters show opposite doping dependences. Therefore, we have introduced the concept of in-plane chemical pressure. The magnitude of in-plane chemical pressure was calculated by the equation of (R_Bi + R_Ch)/(in-plane Bi-Ch distance) where R_Bi and R_Ch are the ionic radii of Bi and the mean ionic radius of the chalcogen (Ch) site [39]. The estimated in-plane chemical pressure is plotted as a function of the Nd or Se concentration (x) in Figure 4e. Surprisingly, the x dependences of the in-plane chemical pressure for the two systems showed the same tendency, and the bulk superconductivity appeared at above the same threshold. The x dependences of the lattice constant of a are also plotted in Figure 4e. It is clear that the lattice constant exhibits opposite x dependences. With the concept of in-plane chemical pressure, we finally obtained the factor correlating to the emergence of superconductivity in the REO_0.5F_0.5BiCh₂ structure.

Figure 4f shows the in-plane chemical pressure dependences of T_c for the REO_0.5F_0.5BiS₂-type and LaO_0.5F_0.5BiS_2-xSe_x superconductors. There is a trend that T_c increases with the increasing in-plane chemical pressure [39]. Indeed, the magnitude of the in-plane chemical pressure becomes the scale for T_c in the tetragonal phase of REO_0.5F_0.5BiCh₂.

3. Material Design Strategies for the BiS₂-Based Superconductor

In the Section 2, the requirements for the emergence of superconductivity in the BiS₂-based compounds were reviewed. Electron carrier doping to the BiS₂ layer is needed to induce superconductivity. However, the carrier doping is not the sole factor related to the emergence of superconductivity in this system. In-plane chemical pressure should be optimized as well. According to these two requirements (the recipe of BiS₂-based superconductivity), new superconductors can be easily designed as demonstrated in Eu_0.5La_0.5FBiS_2−xSe_x [40].

Then, what is the strategy for enhancing T_c in this system? One idea comes when considering the large enhancement of T_c in LaO_0.5F_0.5BiS₂ by the external pressure effect. The crystal structure of the high-P phase is monoclinic, and the Bi-S zigzag chains are formed in the BiS plane (Figure 2d). To link the monoclinic structure to the tetragonal structure, the concept of in-plane chemical pressure can be modified as quasi-one-dimensional (quasi-1D) chemical pressure. Since we have already obtained the trend that T_c increases when higher chemical pressure is created, we consider the shorter Bi-S distance (in-chain Bi-S distance) of the high-P phase of LaO_0.5F_0.5BiS₂ as the origin of the high T_c. As plotted in Figure 5, the calculated quasi-1D chemical pressure for the high-P phase of LaO_0.5F_0.5BiS₂ locates on the extrapolated line of the T_c in-plane chemical pressure relation. This may suggest that a higher T_c can be obtained in a monoclinic structure with an extremely distorted Bi-S chain. In addition, according to the results on the external pressure studies, BiS₂-based compounds with a larger a-axis should be candidates for the extremely distorted phase with a high T_c.

In addition, to design new BiS₂-based superconductors, we have to consider the local-scale crystal structure and physical properties. Tuning the internal strain sometimes results in local phase separation, and the coexistence of local phase separation and superconductivity has been observed in several layered superconductors [41,42,43,44,45]. Thus, understanding the crystal structure and superconducting (electronic) states on the local scale is very important for developing new layered superconductors and increasing T_c. In the BiS₂-based compounds, such a phase separation could occur because the chemical pressure effects, proposed as key strategies for the emergence of superconductivity in this system, yield a poor fit between the conducting layers and blocking layers. In addition, as an experimental fact, local structure instability on the Bi-S plane has been revealed in BiS₂-based compounds under external and chemical pressures [36,46,47,48,49]. Beyond controlling local-scale structures, we will be able to create new BiS₂-based superconductors with a high T_c.

4. Summary

In this review article, we introduced the history of the discovery of new BiS₂-based superconductors, Bi₄O₄S₃ and REO_1−xF_xBiS₂. Although the understanding of the mechanisms of superconductivity has advanced, the precise prediction of the emergence of superconductivity in unknown compounds is very difficult. For this reason, it is very important to stack the experiences of the material design of new superconductors. We are happy if the review on the discovery of the BiS₂-based superconductor is useful for the exploration of new superconductors.

We summarized the effects of electron carrier doping, external pressure, and chemical pressure on the emergence of superconductivity in the BiS₂-based compounds. To induce superconductivity, optimizations of both the electron carrier doping and in-plane chemical pressure are needed. Since we have already obtained some strategies for designing new BiS₂-based superconductors and increasing T_c, we expect that the field of BiS₂-based superconductivity will be a breakthrough for the discovery of new kinds of high-T_c superconductors.