High-pressure study on calcium azide (Ca(N₃)₂): Bending of azide ions stabilizes the structure

The high-pressure structure and elastic properties of calcium azide (Ca(N₃)₂) were investigated using in-situ high-pressure x-ray diffraction and Raman scattering up to 54 GPa and 19 GPa, respectively. The compressibility of Ca(N₃)₂ changed as the pressure increased, and no phase transition occurred within the pressure from ambient pressure up to 54 GPa. The measured zero-pressure bulk modulus of Ca(N₃)₂ is higher than that of other alkali metal azides, due to differences in the ionic character of their metal-azide bonds. Using CASTEP, all vibration modes of Ca(N₃)₂ were accurately identified in the vibrational spectrum at ambient pressure. In the high-pressure vibration study, several external modes (ext.) and internal bending modes (ν₂) of azide anions (${\mathrm{N}}_3^{-}$) softened up to ∼7 GPa and then hardened beyond that pressure. This evidence is consistent with the variation observed in the F_E–f_E data analyzed from the XRD result, where the slope of the curve changes at 7.1 GPa. The main behaviors under pressure are the alternating compression, rotation, and bending of ${\mathrm{N}}_3^{-}$ ions. The bending behavior makes the structure of Ca(N₃)₂ more stable under pressure.