Effect of cation arrangement on polaron formation and colossal permittivity in NiNb2O6

Abstract

Localized polaron hopping is one of the common mechanisms of colossal permittivity in oxides; however, its formation still remains to be explored. In this study, the effect of the cation arrangement on polaron formation and dielectric responses was experimentally revealed in two polymorphs of NiNb₂O₆, i.e., the ordered columbite (c-NiNb₂O₆) and the disordered rutile (r-NiNb₂O₆) phases. Nb⁴⁺-type polarons and typical long-range conduction induced dielectric dispersion were observed in c-NiNb₂O₆, while Ni⁺-type polarons and colossal permittivity were found in r-NiNb₂O₆. A broad and temperature-independent (in linewidth) EPR band at g ∼ 1.982 was observed in c-NiNb₂O₆. In contrast, r-NiNb₂O₆ exhibited the coexistence of a broad band (g ∼ 2.816), which showed the dipolar broadening effect, and a sharp band (g ∼ 2.076), which was significantly enhanced at T ∼ 580 K. The correlation between EPR band broadening and an increased conductivity was also evidenced. The results suggest the existence of delocalized Nb⁴⁺ polarons in ordered c-NiNb₂O₆, and that the Ni/Nb disordering facilitated the formation of localized Ni⁺ polaron, which induced colossal permittivity in r-NiNb₂O₆.