Li₂CaSi₂N₄ and Li₂SrSi₂N₄ – a Synthetic Approach to Three‐Dimensional Lithium Nitridosilicates

Abstract

Abstract The quaternary lithium alkaline earth nitridosilicatesLi 2 MSi 2 N 4 with M = Ca, Sr were synthesized at 900 °C by employment of liquid lithium as fluxing agent in weld shut tantalum ampoules. The title compounds crystallize in space group Pa $\bar {3}$ [Li 2 CaSi 2 N 4 : a = 1056.9(12) pm, Z = 12, R 1 = 0.0439, wR 2 = 0.0839, 412 data, 42 parameters; Li 2 SrSi 2 N 4 : a = 1071.37(12) pm, Z = 12, R 1 = 0.0476, wR 2 = 0.1227, 528 data, 42 parameters]. The structure is built up from vertex sharing [SiN 4 ] 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 M 2+ 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. 7 Li and 29 Si solid‐state MAS NMR spectra of Li 2 SrSi 2 N 4 are reported. Electron energy‐loss spectroscopy (EELS) elucidates further information upon the lithium incorporation.