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 quasi-CuO4 tetrahedra, distorted NbO6 octahedra, and possible polarons. The LiCuNb3O9 ceramic shows room-temperature colossal permittivity (ε′ > 104) but quickly decreases down to around 100 at a temperature range of 80–150 K, a freezing/activation process, which originates from the electrons in Cu+/Cu2+ that exhibit a paramagnetic characteristic but could distort the lattice to form polarons when hopping. This polaron transport deviates from the Arrhenius law, but in the Mott variable-range-hopping mechanism, that can be manipulated by an external magnetic field through magnetoelectric coupling, which could increase the hopping activation energy and hopping distance. Our results therefore provide significant experimental evidence and understanding for the magneto–dielectric correlation in terms of the polaronic hopping phenomenon, which may guide the exploration of a wider range of magnetoelectric oxide materials.