  $\require{mhchem}$Topological superconductivity is of great contemporary
interest and has been proposed in doped $\ce{Bi2Se3}$ in which
electron-donating atoms such as Cu, Sr or Nb have been intercalated into the
$\ce{Bi2Se3}$ structure. For $\ce{Nb_{x}Bi2Se3}$, 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 $\ce{Bi2Se3}$ is established. In contrast to
previous reports, it is deduced that Nb intercalation in $\ce{Bi2Se3}$ does not
take place. Instead, the superconducting behaviour in samples of nominal
composition $\ce{Nb_{x}Bi2Se3}$ results from the $\ce{(BiSe)_{$1.10$}NbSe2}$
misfit phase that is present in the sample as an impurity phase for small $x$
($0.01 \leq x \leq 0.10$) 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.

Superconductivity;Materials Science