Enhanced electron-phonon coupling in NbB₂ by nanoscaling the grain size

Nanocrystalline NbB₂ powder has been synthesized via a single step solid-state reaction of precursor materials Nb₂ O₅ and NaBH₄ . Phase composition, grain morphology, microstructure, phonon vibrations and superconducting properties were investigated by XRD, FESEM, HRTEM, Raman spectroscopy and PPMS. The results show that the grain size of nanocrystalline NbB₂ is very sensitive to the reaction temperature and easy to aggregate to form micron scale of non-morphology powders. A systematic increase of the electron-phonon coupling constant with the nanoscaling grain size is observed, which we explain in terms of crystal defects and phonon vibrations. The zero-temperature critical fields, H_c1 (0) and H_c2 (0), have been estimated at 85 Oe and 7200 Oe, respectively, and the Ginzburg–Landau parameter κ is 5.1, confirming that NbB₂ is a type-II superconductor.