Li4Ca3Si2N6 and Li4Sr3Si2N6 - Quaternary Lithium Nitridosilicates with Isolated [Si2N6]10- Ions

The isotypic nitridosilicates Li4Ca3Si2N6 and Li4Sr3Si2N6 were synthesized by reaction of strontium or calcium with Si(NH)(2) and additional excess of Li3N in weld shut tantalum ampoules. The crystal structure, which has been solved by single-crystal X-ray diffraction (Li4Sr3Si2N6: C2/m, Z = 2, a = 6.1268(12), b = 9.6866(19), c = 6.2200(12) angstrom, beta = 90.24(3)degrees, wR(2) = 0.0903) is made up from isolated [Si2N6](10-) ions and is isotypic to Li4Sr3Ge2N6. The bonding angels and distances within the edge-sharing [Si2N6](10-) double-tetrahedra are strongly dependent on the lewis acidity of the counterions. This finding is discussed in relation to the compounds Ca5Si2N6 and Ba5Si2N6, which also exhibit isolated [Si2N6](10-) ions.