The nature of the nitrite disorder in Na3ONO2

Abstract

The nature of the disorder present in Na₃ONO₂ has been studied using variable temperature solid state nuclear magnetic resonance techniques. Using ¹⁵N MAS and static NMR line shape analysis, the disorder could be unambiguously ascribed to a reorientational motion of the nitrite anion. A detailed analysis of the line shapes revealed that at room temperature the dynamic is characterized by a rapid pseudo-isotropic reorientation of the NO₂⁻ group, in which the nitrogen atom occupies the six possible orientations with equal probability. The exchange process is not characterized by a continuous decrease in the exchange frequency with decreasing temperature, but has rather to be described by a dynamic order process. Upon decreasing the temperature, one of the six possible split positions for the nitrogen atom becomes increasingly favorable, leading to a preferential occupation of the two nitrite orientations related to this split position. Thus, the transition from H-Na₃ONO₂ (isotropically reorienting nitrite ions) to the M-phase is continuous and characterized by the continuous order process of the nitrite ion.