Gearing of nitrate ions in ammonium nitrate.

This article focuses on the magnetic susceptibility measurements of ammonium nitrate (NH4NO3) single crystals to investigate the ordering and dynamics of its polyatomic ions across phase transitions. The study reveals that during the order–disorder transition from phase IV to phase II, two successive sharp steps occur in the magnetic susceptibility along the a- and b-axes, attributed first to a 45° rotation of the NO3− planes and subsequently to the disordering of both NH4+ and NO3− ions. Along the c-axis, only one step is observed, indicating that NO3− ions remain weakly correlated in columns even at high temperatures, while NH4+ ions become magnetically ordered along this axis only in phase V. These findings demonstrate that magnetic susceptibility can sensitively probe molecular gearing and lattice dynamics in solids containing planar ions, offering a method applicable to a broad range of materials with similar ionic structures.