Sublattice Short-Range Order and Modified Electronic Structure in Defective Half-Heusler Nb0.8CoSb

NbCoSb, nominally a 19-electron half-Heusler (19-HH) compound, is one of the emerging novel thermoelectric (TE) materials and has attracted much attention in recent years. By introducing a large amount of Nb vacancies, the material will be stabilized at about Nb0.8CoSb, literally becoming an 18-electron-like HH compound. However, the distribution of Nb vacancies is still in debate now. The present work reveals the unique coexistence of the long-range disorder and short-range order of vacancies in the Nb sublattice in the compound. This is obviously beyond the conventional understanding of the crystalline structure with solely long-range ordering and periodicity. The analysis of the short-range order parameters, alpha(hkl), shows that the distribution of vacancies in the sublattice has a characteristic shell distance of about 7.26 angstrom in average, indicating a deviation from nearly uniform distribution of vacancies. The simulated electron diffraction patterns are in good agreement with the experimental observation. The modified band structure of Nb0.8CoSb is also studied based on unfolding the bands of special quasirandom structures. The widening of conduction-band minimum and valence-band maximum, existence of defective states in gap area, and potential observation of gap reduction are predicted. This work may also provide new clues for searching novel TE materials with coexisting order-disorder.