Local structure of Nb in superconducting Nb-doped Bi2Se3

In the prospect of realizing bulk superconductivity in a topological insulator, metal-doped Bi2Se3 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 Nb0.25Bi2Se3 and Nb0.25Bi1.75Se3 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 (BiSe)1+δNbSe2, which has a δ-dependent superconducting transition in the same temperature range. We explore the possibility of Nb occupancy on various sites in the Bi2Se3 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 Bi2Se3. 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 Bi2Se3 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- A nd Sr-doped Bi2Se3 systems or phase segregation plays a role as well.