Ternary nitrides prepared in the Li 3N–Mg 3N 2 system at 900–1000 K

Abstract

Three phases, (Li x Mg 1− x ) 3N 2− x (0< x<0.20), LiMgN and (Li x Mg 1− x ) 2N (2− x)2/3 (0.5< x<0.6), were prepared in Li 3N – Mg 3N 2 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 Mg 3N 2 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 x Mg 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 Li 3N 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 x Mg 1− x ) 2N (2− x)2/3 .