Gearing of nitrate ions in ammonium nitrate

TL;DR

This study investigates the magnetic susceptibility signatures of ion reorientation and ordering in ammonium nitrate, revealing phase-dependent magnetic behaviors and proposing a method to analyze lattice dynamics in similar solids.

Contribution

It introduces a novel magnetic susceptibility analysis revealing ion gearing and disordering transitions in ammonium nitrate, applicable to other crystalline solids with planar ions.

Findings

Two sharp susceptibility steps along a- and b-axes at phase transition.

No significant change in susceptibility along c-axis, indicating weak correlation of NO3-.

Evidence of magnetic ordering of NH4+ along c-axis in phase V.

Abstract

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 the first step pertains to the NO3- planes shifting away from facing only along the b-axis and onto the a-axis by 45{\deg}. 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 suggest 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.