Variable anisotropy of ionic conduction in lithium nitride: Effect of duplex-charge transfer

The formation and migration of defects relevant to the Li ionic conduction in Li3N have been investigated using first-principles calculations. For undoped Li3N, Frenkel reactions generating the Li vacancy and two types of Li interstitial are found to dominate the defect equilibria. In the layered structure of Li3N, the Li vacancy migrates selectively toward the intralayer direction, whereas the Li interstitial readily moves in both intralayer and interlayer directions. Despite the significant crystallographic anisotropy and the orientation dependence of dominant charge carriers, the resultant activation energy for the Li ionic conduction is nearly isotropic in the undoped system. The presence of H impurities yields Li vacancy-rich defect equilibria, leading to an anisotropic ionic conduction governed by the Li vacancy. These findings elucidate the variable anisotropy in the ionic conductivity of Li3N.