In-situ X-ray diffraction study of Nb-doped Bi2Se3 crystal growth revealing unavoidable misfit layer compound

Temperature-resolved in-situ powder X-ray diffraction (PXRD) experiments were performed during synthesis of Nb-doped Bi2Se3 to gain important insights into the phase formation. The crystallization of phases upon heating the pure elements with the nominal composition of Nb0.25Bi2Se3, displays the formation of a misfit layer compound, (BiSe)1.1NbSe2, as a secondary phase. It was also discovered that a misfit-related intermediate phase appears right before Bi2Se3 melts , then misfit (BiSe)1.1NbSe2 crystallizes. This implies that to prevent the formation of the misfit phase, the synthesis temperature should remain well below the melting point of Bi2Se3. Powder samples of Nb-doped Bi2Se3, synthesized at 600 and 650 degrees C, aimed for this, however, they all displayed traces of the misfit phase. Additionally, multiple superconducting transitions were observed as non-quenched samples showed signs of NbSe2, which were removed by quenching the samples. This study has important implications for the synthesis and properties of Nb-doped Bi2Se3, towards the realization of topological superconductors.