Colossal-Permittivity Behaviors in A-Site Distorted Double-Perovskite LiCuNb3O9 with Correlated Magnetoelectric Effect and Variable-Range Hopping

There is a general perception that the colossal permittivity in nonferroelectric oxides is usually ascribed to the barrier layer capacitor (BLC) effect, but the states of the involved charges as well as corresponding physical picture for the charge transport in the materials are not yet clear, which hinders the explorations and applications for new colossal-permittivity materials and devices. Here, we present the polaronic conduction mechanism in terms of Mott variable-range hopping coupled with the magnetoelectric effect for apparent colossal permittivity in double-perovskite LiCuNb3O9. Valence-state and defect analysis reveals that A-site Cu ions have mixed valences with a determined ratio of Cu+/Cu2+ equal to 1:9 and an intrinsic A-site deficiency that is balanced through oxygen vacancies. Structural refinement combined bond valence sum (BVS) calculation indicates that Cu+ (BVS = 1.3476) is overbonded, while both Cu2+ (BVS = 1.9140) and Nb5+ (BVS = 4.7998) are underbonded, permitting the formation of q...