A hidden role of the nitrate ions in nitrate-based energy storage materials

Abstract

Nitrate-based salts exhibit anomalously large changes of entropy near some of their phase transitions. When accounting for the possible sources of entropy, usually only the positional and orientational aspects of the ions are considered while temporal ordering has been largely ignored. In-plane librations of polyatomic ions such as NO₃⁻ possess a periodically oscillating orbital magnetic moment so if there is an onset of gearing (temporal ordering) between neighboring NO₃⁻, then the equivalent of an antiferromagnetic structure is established and the magnetic susceptibility χ(T) should exhibit a pronounced change. In the cases of LiNO₃, Ca(NO₃)₂, K_0.5Rb_0.5NO₃ and C(NH₂)₃NO₃, we experimentally identify sharp discontinuities in χ(T) at their respective order-disorder phase transitions T_N indicating gearing of NO₃⁻. A model employing the law of conservation of angular momentum is used to explain the possible origin of gearing below T_N. Our findings identify a previously unidentified type of molecular interaction and source of entropy which may be exploited to further increase the enthalpy of the widely-popular hydrated salts employed as energy storage devices.