Structure determination of sodium nitrate near the order-disorder phase transition

Abstract

Two models are proposed to solve the sodium nitrate structure near the order-disorder phase transition (Tc = 276 °C). The first model is built with the static disorder assumption between the two equilibrium positions of NO-3 ions. In this case, disorder is characterized by an order parameter : η. For the second model, a one-dimensional orientational probability function is introduced to take disorder into account. In the latter formalism, a coupling between NO-3 rotations around the three fold axis and translations along the same axis has been introduced. Comparison between the two models shows that the static model can be held to be valid while the square root of the rotational mean square amplitude is lower than 12°. Structure determination by the two models yields equivalent fits for temperatures below ∼ 250 °C, fits with the second model become better for temperatures above 250 °C (Tc - 26 °C). Equiprobability curves display the NO-3 ion jump mechanism between their two equilibrium positions : the rotation around the three-fold axis is made via a translation so that the distance between oxygen atoms and the nearest neighbour Na+ ion stays approximately constant during the jump