Structure of a new polymorph of lithium boron nitride, Li₃BN₂

A new polymorph of lithium boron nitride, Li₃BN₂, was synthesized by slow-cooling of a mixture of Li₃N and BN from 1200 K. It has monoclinic symmetry, P2₁ c, a = 5.1502(2), b = 7.0824(2), c = 6.7908(2) Å, β = 112.956(2)°, Z = 4, D_m = 1.74, D_calcd = 1.737 g cm^-3, μ = 0.082 mm^-1. The structure was determined from 1352 unique X-ray reflections from a single crystal and refined to R = 0.023 by full-matrix least-squares method. Two kinds of layers alternate parallel to (100) in the structure. One layer includes Li and B atoms, and the other is composed of only N atoms. N(1) and N(2) are coordinated by six Li atoms and one B atom. Each Li atom is in a distorted tetrahedron of N atoms. Boron is linearly coordinated by two N atoms. The N(1)BN(2) bond angle is 179.12(4)°. The bond lengths of N(1)B and N(2)B are 1.3393(5) and 1.3361(5) Å, respectively. Bonding electrons between boron and nitrogen atoms are clearly observed. Lithium ion conductivity of 6 × 10^-5 Sm^-1 was measured at 400 K on a polycrystalline sample and an activation energy was 64 kJ/mole.