Ammonium ion dynamics in NH₄I at high pressure

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

“…C 4 translations by 90° also occur, but this entails a change of configuration from A to B or vice versa. The C 3 rotations are more energetically favorable, so they become more dominant with cooling. − ,, Akin to the methyl groups in hexamethylbenzene, the NH 4 + cations also possess a net magnetic moment upon experiencing a C 3 rotation if we treat the electrons of the hydrogen atoms to also lie midway the N–H bonds. This is because the area of the orbits of the electrons and protons are different, so their respective magnetic moments do not cancel out.…”

Inducing Ferroelectricity in NH4I and NH4Br via Partial Replacement of Protons by Deuterons

“…Figure a shows χ­( T ) of the partially deuterated crystal (NH 4 ) 0.73 (ND 4 ) 0.27 I measured along the b -axis direction under an applied magnetic field of H = 1 T. The β phase appeared to reversibly phase transition into and out of the γ phase at the transition temperatures of T β‑γ_73:27 = 242 K and T γ‑β_73:27 = 262 K. However, upon further warming past 262 K, χ­( T ) exhibited a minimum in the temperature region between T γ‑β_73:27 and T β‑α_73:27 = 282 K, which we interpret to be a coexistence of the α, β, and γ phases due to the following two reasons. First, the γ phase was observed that it can coexist with the α and β phases up to 293 K according to two independent sets of proton spin–lattice relaxation experiments. , Second, according to our pyroelectric current measurements presented below, the nonzero electric polarization, which is associated with the ordered γ phase, vanishes exactly at T β‑α_73:27 = 282 K.…”

“…Upon warming, the γ phase remained stable up to T γ‑α = 281 K, where the system transformed back to the α phase. The α, β, and γ phases of the ammonium halides have been reported by many researchers to be capable of being supercooled, superheated, and coexisting in very large temperature ranges. ,,− …”

“…Above a certain critical temperature T C , all 14 reorientations are energetically accessible, meaning that the magnetic moments of NH 4 + ions behave “paramagnetically,” but there are only 14 possible directions for μ p to point to. Below T C , the more energetically costly C 4 reorientations seize . The remaining 8 internal degrees of freedom cause the system of NH 4 + to become geometrically frustrated because a fraction of neighboring NH 4 + end up having their μ p pointing along the same direction.…”

“…Below T C , the more energetically costly C 4 reorientations seize. 3 The remaining 8 internal degrees of freedom cause the system of NH 4 + to become geometrically frustrated because a fraction of neighboring NH 4…”