Defect structure of the ionic conductor lithium nitride (Li₃N)

The defect structure of Li₃N (space group P6/mmm, a ≃ 3.65, c ≃ 3.88 Å) has been investigated with data sets measured at seven temperatures in the range 153 to 678 K. The data have been used for normal and high-order refinements and for the calculation of the corresponding difference electron densities. The final R_w ranged between 0.7 and 1.3%. The N atom position at z = 0 and the Li(1) positions at z = ½ are completely occupied and remain ordered within the whole temperature range. The Li(2) sites at z = 0 already show a vacancy concentration of 1-2% at low temperatures, which does not change significantly with increasing temperature. The Li(2) ions exhibit strong anharmonic thermal vibrations within the Li₂N layers at z = 0 along the Li(2)-Li(2) connection lines. The high Li ionic conduction is caused by jumps of Li(2) ions from an occupied Li(2) site into an unoccupied one. The anharmonic thermal vibrations are the precursors of these jumps. Interstitial sites are not involved in this mechanism.