Gearing of nitrate ions in ammonium nitrate Available to Purchase

Reorienting polyatomic ions such as NH₄⁺ and NO₃⁻ exhibit weak magnetic fields because the ions at the extremities trace out current loops; if the periodic reorientations become long-range ordered (i.e., gearing of neighboring NO₃⁻), then the magnetic susceptibility should exhibit a unique signature along the different crystallographic axes. For the case of ammonium nitrate (NH₄NO₃), we report the presence of two successive sharp steps in the molar magnetic susceptibility along the a- and b-axes upon crossing its order–disorder phase transition (from phase IV to phase II). We suggest that the first step pertains to the NO₃⁻ planes shifting away from facing only along the b-axis and onto the a-axis by 45°. The second step is attributed to the disordering (ungearing) of the NH₄⁺ and NO₃⁻. In contrast, only one step was observed in the magnetic susceptibility along the c-axis, and its large magnitude suggests that the NO₃⁻ remain weakly correlated even in phase I at 400 K. We also find evidence that the NH₄⁺ become magnetically ordered (geared) along the c-axis only until phase V. The approach employed in this work can be extended to experimentally study the lattice dynamics of other solids possessing planar ions such as amphidynamic crystals.