B-Site octahedral bridge and A-site polyvalent Cu cation related electron hopping in LiCuNb3O9-based colossal permittivity materials

Abstract

It is widely accepted that localized electron hopping induces colossal permittivity in the bulk of transition-metal-based materials. However, the mechanism and the control of the electron hopping process remain to be explored. In this work, the effect of the B-site environment and multiple Cu valence-states on electron hopping were experimentally revealed in the LiCuNb_2.95M_0.05O₉ (M = Mg, Ti) systems. The results demonstrated that Mg/Ti substitution for Nb⁵⁺ at the B-site effectively causes multivalent states of A-site Cu cations, i.e. Cu⁺/Cu²⁺/Cu³⁺, and distorted NbO₆ octahedra. Upon Mg/Ti substitution, the dielectric permittivity was enhanced by approximately two orders of magnitude (at 1 kHz and 300 K) and an additional dielectric relaxation was induced. More importantly, it was revealed that the B-site substitution effectively adjusted the activation energy and average distances for the electron hopping.