Ternary nitrides prepared in the Li₃N-Mg₃N₂ system at 900-1000 K

Three phases, (Li_xMg_1-x)₃N_2-x (0<x<0.20), LiMgN and (Li_xMg_1-x)₂N_(2-x)2/3(0.5< x<0.6), were prepared in Li₃N - Mg₃N₂ system at 900 and 1000 K, and their crystal structures were investigated by applying the Rietveld method to the X-ray powder diffraction patterns. (Li_x Mg_1-x)₃N_2-x crystallizes in a cubic superlattice (a=9.9767(5) Å at x=0.17). Mg atoms in the Mg₃N₂ structure (space group Ia3) were found to be partially replaced by Li atoms and the resultant structure was distorted with a lower symmetry space group I213. The crystal structure of LiMgN (orthorhombic, a=7.1586(2), b=3.5069(8), c=5.01424(13) Å, space group Pnma) was determined ab initio from the X-ray powder diffraction data. It revealed a cation-ordered antifluorite-type structure. (Li_xMg_1-x)₂N_(2-x)2/3 was prepared by rapid cooling from 1000 K to room temperature. It was also found to be stable at 900 and 1000 K and decomposed into LiMgN and Li₃N on slow cooling in the furnace. Broad tails of the X-ray diffraction peaks suggest a modulated structure. A cation-disordered simple antifluorite type (space group Fm3̄m, a=4.99559(11) Å at x=0.57) was proposed as the fundamental structure of (Li_xMg_1-x)₂N_(2-x)2/3.