Targeted doping modulation of electrical conductivity and electronic state structure in lithium niobate

This study presents an integrated theoretical approach combining ab initio density functional theory (DFT) simulations with a new analytical model of the ferroelectric phase transition that accounts for interactions between both first and second neighbors in pure and magnesium-doped lithium niobate (LiNbO₃, LN). This framework enables the characterization of the evolution of ionic conductivity as a function of temperature and Mg concentration, as well as the variation of the Curie temperature (TC) with doping level. In parallel, DFT calculations provide insight into the effect of Mg doping on the electronic density of states (DOS) and band structure. By applying vacancy-based models (a mixed model and two models referred to as 1 and 2) to both pure and Mg-doped LiNbO₃, we obtain good agreement with experimental data for ionic conductivity and Curie temperature. At constant temperature, the ionic conductivity of Mg-doped LiNbO₃, increases up to a critical Mg concentration of approximately 3.4%, which is attributed to a reduction in Nb antisite defects (NbLi) and a narrowing of the band gap, leading to an effective n-type behavior. Beyond this threshold, the conductivity gradually decreases due to band gap widening and the formation of additional structural defects. Overall, this integrated approach advances the understanding of transport mechanisms in Mg-doped LiNbO₃, and provides valuable guidance for optimizing acoustic and optoelectronic devices based on this material.