La₆Ba₃[Si₁₇N₂₉O₂]ClAn Oxonitridosilicate Chloride with Exceptional Structural Motifs

The oxonitridosilicate chloride La₆Ba₃[Si₁₇N₂₉O₂]Cl was synthesized by a high-temperature reaction in a radiofrequency furnace starting from LaCl₃, BaH₂, and the ammonolysis product of Si₂Cl₆. Diffraction data of a micrometer-sized single crystal were obtained using microfocused synchrotron radiation at beamline ID11 of the ESRF. EDX measurements on the same crystal confirm the chemical composition. The crystal structure [space group <i>P</i>6₃/<i>m</i> (no. 176), <i>a</i> = 9.8117(14), <i>c</i> = 19.286(6) Å, <i>Z</i> = 2] contains an unprecedented interrupted three-dimensional network of vertex-sharing SiN₄ and SiN₃O tetrahedra. The SiN₄ tetrahedra form <i>dreier</i> rings. Twenty of the latter condense in a way that the Si atoms form icosahedra. Each icosahedron is connected to others via six SiN₄ tetrahedra that are part of <i>dreier</i> rings and via six Q³-type SiN₃O tetrahedra. Rietveld refinements confirm that the final product contains only a small amount of impurities. Lattice energy (MAPLE) and bond-valence sum (BVS) calculations show that the structure is electrostatically well balanced. Infrared spectroscopy confirms the absence of N–H bonds