Magnetic Ordering of Ammonium Cations in NH₄I, NH₄Br, and NH₄Cl

Different types of magnetism arise mainly from how electrons move and interact with each other. In this work, we show how protons (H⁺) also exhibit magnetic behavior. We measured the magnetic susceptibility of the ammonium halides and identified pronounced increases at 232, 233, and 243 K for NH₄I, NH₄Br, and NH₄Cl, respectively, all of which coincide with the geometric ordering of their ammonium cations. With extensive literature establishing the fact that the ammonium cations exhibit rotational motion even toward the lowest temperatures, we take into account that the orbital motion of the protons carries a magnetic moment and find it to be larger than that of the paired electrons. Consequently, the structural phase transitions are magnetically driven as the system attempts to lift 8-fold energy degeneracies of the proton orbitals via Jahn-Teller distortions. Our findings identify that NH₄⁺ cations are capable of comprising magnetism which appears to be ubiquitous in ammonia-based molecular solids.