Magnetism based on nitrate-nitrate interactions: The cases of LiNO${}_3$, K${}_{0.5}$Rb${}_{0.5}$NO${}_3$, Ca(NO${}_3$)${}_2$ and C(NH${}_2$)${}_3$NO${}_3$

Abstract

Long-range magnetic ordering of the orbital motion of oxygen atoms within NO${}_3$${}^{-}$ cations is identified from experimental measurements of the magnetic susceptibility $\chi$($T$) in LiNO${}_3$, Ca(NO${}_3$)${}_2$, K${}_{0.5}$Rb${}_{0.5}$NO${}_3$ and C(NH${}_2$)${}_3$NO${}_3$ at their respective order-disorder, solid-solid phase transitions $T$${}_N$. The observed sharp changes in $\chi$($T$) and accompanying hysteretic behavior indicate the phase transitions to be first order. A model employing the law of conservation of angular momentum is used to explain why the librations between neighboring NO${}_3$${}^{-}$ become geared below $T$${}_N$. Since the periodic motions involve concerted motion of net charges, the associated magnetic moments of the NO${}_3$${}^{-}$ ions indirectly establish an antiferromagnetic structure below $T$${}_N$. Our findings identify a previously unidentified type of molecular interaction which may be exploited to further increase the enthalpy of the widely-popular hydrated salts employed as energy storage devices.

13 pages (single column, 1.5 spaced), 5 figures