Microcrystals of the oxonitridosilicate oxide La11Si13N27.636O1.046:Ce3+ were obtained by exploratory high temperature synthesis starting from La, La(NH2)(3), Si(NH)(2), BaH2, and CeF3. Owing to the small size of the crystals, microfocused synchrotron radiation was used for structure investigations (space group Cmc2(1) (No. 36), a = 9.5074(4) angstrom, b = 32.0626(9) angstrom, c = 18.5076(8) angstrom, Z = 8, R1(all) = 0.0267). The crystal structure consists of an unprecedented interrupted three-dimensional network of vertex-sharing SiN4-xOx tetrahedra that form channels of siebener rings along [100]. Moreover, the structure is characterized by layers of condensed sechser rings in a boat conformation and vierer rings, which are alternatingly stacked with layers of vierer and dreier rings. Several split positions indicate two different local structure variants. Infrared spectroscopy confirms the absence of N-H bonds. Powder X-ray diffraction data show that bulk samples contain only a small amount of La11Si13N27.636O1.046:Ce3+. However, once the exact composition was determined from structure analysis, it was possible to optimize the synthesis using fluorides as starting materials. Thereby, bulk samples of the homeotypic compound La11Si13N27.636O0.936F were obtained and investigated.