Structure of lithium nitride and transition-metal-doped derivatives, Li_3-x-yM_xN (M = Ni, Cu): A powder neutron diffraction study

Structures of lithium nitride (α-Li₃N) and ternary nitridometalates Li_3-x-yM_xN (M = Ni, Cu) have been refined from time-of-flight powder neutron diffraction data using the Rietveld method. The parent binary nitride was synthesized by reaction of lithium metal with nitrogen using liquid sodium as a solvent. The ternary nitrides were prepared by reaction of Li₃N with the respective transition metals under nitrogen. Refined data confirm the hexagonal structure previously reported by single-crystal X-ray diffraction for lithium nitride (P6/mmm, a = 3.6576(1) Å, c = 3.8735(1) Å, Z = 1) and show that the α-Li₃N phase contains close to 3% Li vacancies at room temperature. The substituted nitridometalates, Li_1.36Ni_0.79N and Li_2.21Cu_0.40N, both form structures analogous to α-Li₃N (P6/mmm, a = 3.7697(1) Å, c = 3.5270(1) A and a = 3.6791(1) Å, c = 3.7725(1) Å, respectively) in which the transition metal is disordered with Li between [Li_2-yN] planes and Li vacancies are disordered within these planes. The nickel and copper nitrides contain 43% (y = 0.85) and 16% (y = 0.32) Li vacancies, respectively, within the [Li₂N] planes and consequently contain transition metals predominantly in the +2 oxidation state.