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

Abstract

Abstract 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(N3 -) softened up to ~7 GPa and then hardened beyond that pressure. This evidence is consistent with the variation observed in the FE -fE 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 N3 - ions. The bending behavior makes the structure of Ca(N3)2 more stable under pressure.