Tunable Defect Structure in the Li-Mg-N Ternary Phase System: A Powder Neutron Diffraction Study

Defect structures in the Li-Mg-N system can be tuned by control of composition and temperature. A series of compounds in the Li3N-Mg3N2 system have been synthesized via solid state reaction of the binary nitrides. The resulting ternary nitrides have been structurally characterized by variable temperature powder neutron diffraction. LiMgN is orthorhombic (space group Puma), at room temperature. It undergoes a structural transition above 673 K to a simple cubic anti-fluorite structure (space group Fm (3) over barm). Nitrogen-deficient, magnesium-rich Li0.24Mg2.76N1.92 is isostructural with Mg3N2 and retains a cubic anti-bixbyite structure, I2(1)3, from room temperature to 873 K. Li1.11Mg0.89N0.96 retains a simple cubic anti-fluorite structure, (space group Fm (3) over barm), over the entire temperature range investigated. However, Li1.09Mg0.91N0.97 crystallizes with the orthorhombic Puma LiMgN-type structure, providing evidence for the accommodation or variable metal and nitrogen stoichiometry in that structure and expansion of the structure stability field beyond a LiMgN "line phase".