**1. Introduction**

Superconductivity was first discovered in the resistivity measurement of mercury by Kamerlingh Onnes in 1911. Its resistance abruptly vanishes at 4.1 K. Zero resistance means no energy loss in electric transport, which could greatly solve the energy crisis in the future. Since then, superconductivity has been a long-lasting hot topic in condensed matter physics. Exploring room temperature superconductors is one of the ultimate dreams.

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

However, so far, only two kinds of unconventional superconducting systems have exceeded the Macmillan limit at ambient pressure, i.e., the cuprate and iron-based superconductors. In general, the correlation of structure and typical properties is always a useful guideline for effectively searching for special functional materials. In fact, the structure of both cuprate and iron-based superconductors can be characterized as a sandwiched "hamburger" model. It consists of superconducting layers (CuO2 plane, Fe2 M2 (M = As, P, S, Se, and Te) layer) and spacer layers, which stack alternatively along the c-axis [1, 2]. Superconductivity occurs when the charged carriers are generated by the defects or substitution in superconducting layers or more commonly provided by the space layers; namely, a new superconducting layer probably means a new superconducting system. The spacer layer can be easily tuned by doping, substitution, intercalation, and pressure, which could affect superconductivity [3]. Therefore, materials with layered structure have been regarded as the most promising playground for exploring new high-Tc superconductors.

In 2010, superconductivity arising from the topological insulator Bi2 Se3 by Cu intercalation was first reported [4]. It has drawn much attention since Cu<sup>x</sup> Bi2 Se3 is proposed as a topological superconductor candidate, as evidenced by the zero-bias conductance peak and quantum oscillation experiment [5, 6]. Very recently, superconductivity with topological states was also reported in its isostructural compounds, Srx Bi2 Se3 and Nbx Bi2 Se3 [7, 8]. In 2012, an exotic superconductivity was discovered in a new layered structure Bi4 O4 S3 with zero-resistance superconducting temperature at about 4.5 K [9]. Soon, another new BiS2 -based superconductor LaO0.5F0.5BiS2 was reported, whose structure is more definite and the zero-resistance superconducting temperature is about 8 K for the samples annealed under high pressure [10]. As its structure is very similar to the iron-based superconductor LaOFeAs, this system has been intensively researched, and lots of isostructural superconductors have been synthesized, including ReO1−xFx BiS2 (Re: Ce, Pr, Nd, Yb), Sr1−xRex FBiS2 (Re: La, Ce), EuBiS2 F, and Eu3 Bi3 S4 F4 [11–15]. These researches are focused on tuning the spacer layers. The attempts to explore new superconducting layers only succeed in LaOx Fx BiSe2 and Sr0.5La0.5FBiSe2 [16–18]. So far, the superconducting layer of this system has been extended to BiCh2 (Ch: S, Se). In this chapter, the crystal structure and superconducting properties of Bi─O─S superconductors, LaO1−xFx BiSe2 single crystals, and Srx Bi2 Se3 single crystals are briefly reviewed.

However, the chemical composition studies show that it probably contains two new Bi─O─S

O4 S3 [21].

, and (c) Bi4

sample is improved, the superconducting volume fraction will be enhanced with its zero-

O4 S3

O2 S3

, BiO1−xFx

) [9, 19–21]. Besides, these samples can only be synthesized in a narrow

BiS2

. Then we can see it is isostructural with LaOBiS2

O4 S3

conductivity is likely to be induced by introducing carriers into spacer layer. In fact, F-doped

temperature region. Another difficulty in detecting their actual composition and structure is that several strong diffraction peaks in the powder XRD patterns are very close to each other.

. Their schematic structures can be seen in **Figure 1(a)** and **(b)**.

Emerging Superconductivity and Topological States in Bismuth Chalcogenides

http://dx.doi.org/10.5772/intechopen.73057

113

O4 S3 O2 S3

. The chemical composition of Bi2


is the main phase and

O2 S3

2− lay-

OS2 can

with P4/nmm

samples. We

, Bi, and

O3

O2 S3 , and

. And the superconductivity can

with the same I4/mmm space group,

has the simplest structure and com-

, and Bi2

O4 S3 and Bi3


OS2

phases, i.e., Bi2

Bi2 OS2

Bi2 OS2

Bi2 S3

400°C for Bi2

OS2

**Figure 1.** Crystal structures of (a) Bi2

be suppressed by the amount of Bi2

The crystal structure of Bi3

also be expressed as BiOBiS2

ers replacing the vacancy of SO4

(O,F)S2

and Bi3

O2 S3

OS2 , (b) Bi3 O2 S3

likely accounts for the 4.5 K superconductivity in Bi4

O2 S3

space group, a = b = 3.9744 Å and c = 13.7497 Å. BiOBiS2

**Figure 2** shows the powder XRD patterns of Bi<sup>3</sup>

position, then it is probably the parent compound of this BiS2

is an insulating phase and its content is less than 10%. Bi3

resistance superconducting temperature increased up to 4.9 K [20].

OS2

is similar to Bi4

2− layers in Bi4

a = 3.9674 Å and b = 41.2825 Å. The electron carriers are believed to be generated from S2

has been reported to exhibit bulk superconductivity below 5 K [21, 22].

can see that samples of Bi─O─S compounds tend to contain impurities such as Bi2

, because their synthesis temperature is relatively low (520°C for Bi4
