Molecular-Scale Analysis of Barium Azide (Ba(N3)2) Under High Pressure: A Synchrotron X-Ray Diffraction Study on Azide Ion Rotation and Phase Transition

This study presents a comprehensive investigation of the high-pressure behavior of barium azide Ba(N₃)₂ through synchrotron X-ray diffraction, revealing critical insights into its anisotropic compressibility and phase transitions under pressures up to 28 GPa. At ambient conditions, Ba(N₃)₂ crystallizes in a monoclinic structure (space group P2₁/m), exhibiting pronounced anisotropic compression with axial compressibility following the order <i>b</i> > <i>a</i> > <i>c</i>. The distinct compressibility arises from the arrangement of azide ions, where interlayer interactions along the b-axis dominate the response to pressure. A reversible phase transition (Phase I → Phase II) initiates at 2.6 GPa, characterized by a monoclinic-to-monoclinic transformation involving subtle symmetry changes driven by azide ion rotation and lattice plane slippage. Above 11.8 GPa, emergent diffraction peaks suggest a potential secondary transition, though the structure remains stable up to 28 GPa. These findings underscore the unique role of azide ion dynamics in governing structural stability and phase evolution in divalent azides, offering implications for their utility as precursors in polymeric nitrogen synthesis.