High- and low-temperature phases of lithium boron nitride, Li₃BN₂: Preparation, phase relation, crystal structure, and ionic conductivity

Low- (α) and high- (β) temperature phases of Li₃BN₂ were prepared from mixtures of Li₃N BN = 1.1 - 1.0 in molar ratio at 1070 and 1170 K, respectively. Phase relation between these phases was studied by annealing the products at various temperatures and conducting DTA in a stream of nitrogen. The phase transition temperature is at about 1135 K. The melting point of β-Li₃BN₂ is around 1189 K. α-Li₃BN₂ crystallizes directly from the undercooled liquid at 1160 K. The structure of α-Li₃BN₂, which is analyzed in the present study for the first time, has tetragonal symmetry, P4₂2₁2, a = b = 4.6435(2), c = 5.2592(5), Å, Z = 2, D_calc = 1.747 Mg m^-3, μ = 0.082 mm^-1. The structure was determined by 208 unique X-ray reflections with F_o > 3σ(F_o) and refined up to R = 0.042 by a full-matrix least-squares method. The lattice is composed of Li(1), Li(2), and linear (NBN)^3- ions [r(BN) = 1.339(2), Å]. The Li(1) ion is also linearly coordinated by two nitrogen atoms [r(Li(1)N = 1.945(8), Å]. The Li(2) ion is at the center of a tetrahedron of N atoms [r(Li(2)N) = 2.125(18)Å, δ(NLiN) = 103.6(2) and 112.5(9)°]. Lithium ion conductivity of 3 × 10^-5 S m^-1 was measured at 400 K on a polycrystalline α-Li₃BN₂ specimen with an activation energy of 78 kJ/mole.