Order-disorder phase transition in NaNO₂ : Local structure and short-range correlation investigation

A prototypical order-disorder system, NaNO₂, undergoes a two successive phase transitions from a room temperature ferroelectric phase to a briefly existing incommensurately modulated antiferroelectric phase at 434 K and finally to a paraelectric phase at T* = 436 K. These phase transitions are influenced by the repositioning of NO₂^- and Na⁺ ions along the crystal b-axis. To investigate the short-range correlation of the ion rearrangement below and above the phase transitions, a single crystal x-ray diffuse scattering experiments were conducted. The reduced reciprocal data were subsequently transformed into real space using a ‘punch and fill’ method generating a three-dimensional pair distribution function (3D-ΔPDF). Our 3D-ΔPDF analysis clearly depicts the fluctuations of NO₂^- and Na⁺ ions above the phase transition, exemplifying a typical order-disorder phase transition.

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.