Figure 2.

a) The enthalpy per atom for Ca₂N in R $\overline{3}$ m, I $\overline{4}$2d, and Cc symmetry as a function of pressure with respect to Fd $\overline{3}$ m‐type Ca₂N. Inset: the enthalpy per atom for Ca₂N in Cc symmetry as a function of pressure with respect to I $\overline{4}$2d type Ca₂N. b,c) Rietveld refinements on XRD patterns of Ca₂N under different pressure and the pressure dependence of formula unit volume. The marked peaks around 9° and 10.6° are caused by CaO with Fm $\overline{3}$ m structure (see Figure S6). d–g) Crystal structures of R $\overline{3}$ m (d), Fd $\overline{3}$ m (e), I $\overline{4}$2d (f), and Cc (g) Ca₂N. The blue and green spheres represent Ca and N atoms, respectively. The red guide lines in Fd $\overline{3}$ m structure are showing tetrahedral configurations, not bonding.