$\text{require}mhchem$Misfit phase \ce{(BiSe)_{1.10}NbSe2} as the origin of superconductivity in nobium-doped bismuth selenide

$\text{require}mhchem$Topological superconductivity is of great contemporary interest and has been proposed in doped $\text{ce}Bi2Se3$ in which electron-donating atoms such as Cu, Sr or Nb have been intercalated into the $\text{ce}Bi2Se3$ structure. For $\text{ce}N{b}_{x}Bi2Se3$, with $$\text{T}$_$\text{c}$ \sim 3 \ $\text{K}$$, it is assumed in the literature that Nb is inserted in the van der Waals gap. However, in this work an alternative origin for the superconductivity in Nb-doped $\text{ce}Bi2Se3$ is established. In contrast to previous reports, it is deduced that Nb intercalation in $\text{ce}Bi2Se3$ does not take place. Instead, the superconducting behaviour in samples of nominal composition $\text{ce}N{b}_{x}Bi2Se3$ results from the \ce{(BiSe)_{1.10}NbSe2} misfit phase that is present in the sample as an impurity phase for small $x$ ($0.01\le x\le 0.10$) and as a main phase for large $x$ ($x=0.50$). The structure of this misfit phase is studied in detail using a combination of X-ray diffraction and transmission electron microscopy techniques.