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(N 3 ) 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(N 3 ) 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(N 3 ) 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(N 3 ) 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 ( ν 2 ) of azide anions ( 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 N 3 − ions. The bending behavior makes the structure of Ca(N 3 ) 2 more stable under pressure.