Synthesis, crystal structure and lithium motion of Li8SeN 2 and Li8TeN2

The compounds Li8EN2 with E = Se, Te were obtained in form of orange microcrystalline powders from reactions of Li2E with Li3N. Single crystal growth of Li8SeN2 additionally succeeded from excess lithium. The crystal structures were refined using single-crystal X-ray diffraction as well as X-ray and neutron powder diffraction data (I41md, No. 109, Z = 4, Se: a = 7.048(1) Å, c = 9.995(1) Å, Te: a = 7.217(1) Å, c= 10.284(1) Å). Both compounds crystallize as isotypes with an anionic substructure motif known from cubic Laves phases and lithium distributed over four crystallographic sites in the void space of the anionic framework. Neutron powder diffraction pattern recorded in the temperature range from 3 K to 300 K and X-ray diffraction patterns using synchrotron radiation taken from 300 K to 1000 K reveal the structural stability of both compounds in the studied temperature range until decomposition. Motional processes of lithium atoms in the title compounds were revealed by temperature dependent NMR spectroscopic investigations. Those are indicated by significant changes of the 7Li NMR signals. Lithium motion starts for Li8SeN2 above 150 K whereas it is already present in Li8TeN2 at this temperature. Quantum mechanical calculations of NMR spectroscopic parameters reveal clearly different environments of the lithium atoms determined by the electric field gradient, which are sensitive to the anisotropy of charge distribution at the nuclear sites. With respect to an increasing coordination number according to 2 + 1, 3, 3 + 1, and 4 for Li(3), Li(4), Li(2), and Li(1), respectively, the values of the electric field gradients decrease. Different environments of lithium predicted by quantum mechanical calculations are confirmed by 7Li NMR frequency sweep experiments at low temperatures. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA