Understanding how non-conventional systems with low carrier density can result in superconductivity is a new challenge and of fundamental importance. In this part the underlying science-driver is studying the emergence of superconductivity in semiconductors with compositional doping. In this case superconductivity is manifested in systems with intrinsic low carrier density (e.g. 10¹⁹ cm^-3) in contrast to the vast majority of superconductors which have very high carrier densities (10²² cm^-3). Because low carrier density generally leads to a very small density of states at the Fermi energy, historically it has not been thought to be conducive to superconductivity. Therefore such superconducting systems are both interesting and rare.

RbBi_11/3Te₆ is a narrow gap semiconductor featuring Bi vacancies and band gap of 0.25 eV at room temperature and exhibits a sharp superconductivity at 3.2 K. A needle-like crystal of RbBi_11/3Te₆ adopts a layered structure composed of [BiTe₆] octahedral forming defected infinite [Bi₄Te₆] layers which contain one third of Bi³⁺ vacancies balanced by Rb⁺ ions sandwiched between the layers, Fig. 1. The structural flexibility of the [Bi₂Q₃] block to form layers of different orientation and width is a powerful feature in the design of compounds where their composition and structure can be predicted using the concept of homologous relationships. A series of isovalent substitutions of the Te^2- site with Se^2- atoms and the Bi³⁺ site with Sb³⁺ atoms was investigated. T_c changes for every substitution made in the series RbBi_11/3Se_xTe_6-x (0 < x < 2.1), RbBi_11/3-ySb_yTe₆ (y = 0.3, 0.6), and RbBi_11/3-ySb_ySe_xTe_6-x (x,y = 0.3), Fig. 2(A). Interestingly, T_c increases to 3.4 K upon substitution with Se in the RbBi_11/3Se_xTe_6-x series until an optimum concentration is reached (x = 0.55(5)) after which T_c degreases gradually and finally disappears above x ≈ 2.1 where the solubility limit of Se in RbBi_11/3Te₆ is reached. There is a superconducting dome of T_c as a function of composition of the RbBi_11/3Se_xTe_6-x series is shown in Fig. 2(B) which peaks at x = 0.55(5). The discovery of superconductivity in the RbBi_11/3Te₆ system forecasts the potential existence of other interesting superconductors in the class Rb[Bi_2n+11/3Te_3n+6] which is a homology that can be more generally represented as AM_3+mTe_5+m, where A is an alkali metal and M can be a combination of divalent and trivalent metals, e.g., the CsPb_mBi₃Te_5+m series.

“Superconductivity in the narrow gap semiconductor RbBi_11/3Te₆” Malliakas C. D., Chung D. Y., Claus H., Kanatzidis M. G. J. Am. Chem. Soc. 2016,138, 14694-14698. (DOI: 10.1021/jacs.6b08732)