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-Li 3 N) and ternary nitridometalates Li 3 - x - y M x N (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 Li 3 N with the respective transition metals under nitrogen. Refined data confirm the hexagonal structure previously reported by single-crystal X-ray diffraction for lithium nitride ( P 6/ mmm, a = 3.6576(1) Å, c = 3.8735(1) Å, Z = 1) and show that the α-Li 3 N phase contains close to 3% Li vacancies at room temperature. The substituted nitridometalates, Li 1.36 Ni 0.79 N and Li 2.21 Cu 0.40 N, both form structures analogous to α-Li 3 N ( P 6/ mmm, a = 3.7697(1) Å, c = 3.5270(1) Å and a = 3.6791(1) Å, c = 3.7725(1) Å, respectively) in which the transition metal is disordered with Li between [Li 2 - y N] 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 [Li 2 N] planes and consequently contain transition metals predominantly in the +2 oxidation state.