Identification and lattice location of oxygen impurities in $\alpha$-Si$_3$N$_4$*

For over 40 years impurities have been believed to stabilize the ceramic $\alpha$-Si$_3$N$_4$, but there is no direct evidence for their identity or lattice location. In bulk materials electron microscopy can generally image heavy impurities. Here we report direct imaging of N columns in $\alpha$-Si$_3$N$_4$ that suggests the presence of excess light elements in specific N columns. First-principles calculations rule out Si or N interstitials and suggest O impurities, which are then confirmed by atomically-resolved electron energy-loss spectroscopy. The result provides a possible explanation for the stability of $\alpha$-Si$_3$N$_4$ with implications for the design of next-generation structural ceramics.