Missing Member in the M II M III Si 4 N 7 Compound Class: Carbothermal Reduction and Nitridation Synthesis Reveal Substitution of Nitrogen by Carbon and Oxygen in CaLu[Si 4 N 7–2 x C x O x ]:Eu 2+ /Ce 3+ ( x ≈0.3)

The nitridosilicate CaLu[Si 4 N 7–2 x C x O x ] ( x ≈0.3) was synthesized by carbothermal reduction and nitridation starting from CaH 2 , Lu 2 O 3 , graphite and amorphous Si 3 N 4 at 1550 °C in a radiofrequency furnace. CaLu[Si 4 N 7–2 x C x O x ] ( x ≈0.3) crystallizes isotypically to many previously known M II M III Si 4 N 7 compounds in the space group P 6 3 mc , as was confirmed by Rietveld refinement based on powder X‐ray diffraction data. Incorporation of carbon into the crystal structure as a result of the carbothermal synthesis route was confirmed by 13 C and 29 Si MAS NMR spectroscopy. For the first time in the M II M III Si 4 N 7 compound class, complementary EDX measurements suggest that simultaneous incorporation of oxygen compensates for the negative charge excess induced by carbon, resulting in an adjusted sum formula, CaLu[Si 4 N 7–2 x C x O x ] ( x ≈0.3). When excited with UV‐to‐blue light, CaLu[Si 4 N 7–2 x C x O x ] ( x ≈0.3) shows an emission maximum in the blue spectral region ( λ em =484 nm; fwhm=4531 cm −1 ) upon doping with Ce 3+ , whereas Eu 2+ ‐doped CaLu[Si 4 N 7–2 x C x O x ] ( x ≈0.3) exhibits a yellow‐green emission ( λ em =546 nm; fwhm=3999 cm −1 ).