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 Li3NBN = 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, Dcalc = 1.747 Mg m−3, μ = 0.082 mm−1. The structure was determined by 208 unique X-ray reflections with Fo > 3σ(Fo) 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(BN) = 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(NLiN) = 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.