Local structure of Nb in superconducting Nb-doped Bi2Se3

In the prospect of realizing bulk superconductivity in a topological insulator, metal-doped ${\mathrm{Bi}}₂{\mathrm{Se}}₃$ has been investigated with increased interest, where the Cu-, Sr-, and Nb-doped systems appear particularly promising. It is generally assumed that metal intercalation into the van der Waals (vdW) gap is responsible for the superconductivity. We have investigated the local structure of Nb in samples with nominal composition ${\mathrm{Nb}}_0.25{\mathrm{Bi}}₂{\mathrm{Se}}₃$ and ${\mathrm{Nb}}_0.25{\mathrm{Bi}}_1.75{\mathrm{Se}}₃$ using the x-ray absorption fine structure technique. It is found that that Nb is primarily located in a local environment consistent with that of the misfit layered structure ${(\mathrm{BiSe})}_{1+\ensuremath{\delta}}{\mathrm{NbSe}}₂$, which has a $\ensuremath{\delta}$-dependent superconducting transition in the same temperature range. We explore the possibility of Nb occupancy on various sites in the ${\mathrm{Bi}}₂{\mathrm{Se}}₃$ structure, but neither intercalation nor substitution lead to physically meaningful improvements of the models. Furthermore, we report single crystal x-ray diffraction analysis of Nb-doped ${\mathrm{Bi}}₂{\mathrm{Se}}₃$. Difference density maps are found to show negligible occupancy in the vdW gap. The misfit layer compound has recently been suggested as an alternative origin for superconductivity in the Nb-doped ${\mathrm{Bi}}₂{\mathrm{Se}}₃$ system, in good agreement with the present study. Our findings stress the necessity of thorough structural characterization of these samples. In more general terms, it raises the question of whether metal intercalation is responsible for the superconductivity in the Cu- and Sr-doped ${\mathrm{Bi}}₂{\mathrm{Se}}₃$ systems or phase segregation plays a role as well.