Tetragonal I41/amd crystal structure of Li3BN2 from dehydrogenated Li-B-N-H

We have determined the crystal structure of Li3BN2 formed by dehydrogenation of LiB0.33N0.67H2.67 from powder x-ray diffraction (XRD) data using the Rietveld method. XRD measurements indicate unambiguously that this Li3BN2 polymorph is distinct from any of the previously reported Li3BN2 phases. We find a body-centered tetragonal I4(1)/amd structure (space group No. 141 in the International Tables for X-ray Crystallography) with a=6.60 angstrom and c=10.35 angstrom. We also describe the results of first principles theoretical modeling of the body-centered tetragonal Li3BN2 polymorph as well as the tetragonal P4(2)2(1)2 and monoclinic P2(1)/c Li3BN2 structures previously reported. We obtained excellent agreement between the theoretically calculated I4(1)/amd Li3BN2 lattice constants and atomic positions and those obtained experimentally from XRD. The approximate enthalpy of formation of the I4(1)/amd Li3BN2 phase is Delta H=-495 kJ/mole f.u..