Li2CaSi2N4 and Li2SrSi2N4 - a Synthetic Approach to Three-Dimensional Lithium Nitridosilicates

The quaternary lithium alkaline earth nitridosilicates Li2MSi2N4 with M = Ca, Sr were synthesized at 900 degrees C by employment of liquid lithium as fluxing agent in weld shut tantalum ampoules. The title compounds crystallize in space group Pa (3) over bar [Li2CaSi2N4: a = 1056.9(12) pm, Z = 12, R-1 = 0.0439, wR2 = 0.0839, 412 data, 42 parameters; Li2SrSi2N4: a = 1071.37(12) pm, Z = 12, R-1 = 0.0476, wR2 = 0.1227, 528 data, 42 parameters]. The structure is built up from vertex sharing [SiN4 tetrahedra forming a three-dimensional network comprising the alkaline earth and lithium ions in the voids. The network is the first example of a nitridosilicate adopting topology of Net 39 according to O'Keeffe. The two crystallographically independent M2+ sites are highly symmetrically coordinated by six nitrogen atoms. Lattice energy calculations (MAPLE) and EDX measurements confirmed the electrostatic bonding interactions and the chemical compositions. Li-7 and Si-29 solid-state MAS NMR spectra of Li2SrSi2N4 are reported. Electron energy-loss spectroscopy (EELS) elucidates further information upon the lithium incorporation.