3D-ΔPDF analysis on the structural phase transitions in NaNO₂

NaNO₂ is a ferroelectric with a simple crystal structure that undergoes a phase transition from a ferroelectric body centered orthorhombic (Im2m) phase at room temperature to a paraelectric centrosymmetric orthorhombic (Immm) phase at T* = 436 K. There is an intermediate incommensurately modulated antiferroelectric phase in between ferroelectric and paraelectric phase for a very narrow temperature range of about 2 K at 434 K. The ferroelectric to paraelectric phase transition is modulated by the rearrangement of triangular NO₂^- and Na⁺ ions along the crystal b-axis. To probe the short-range order and local correlations behind the process of the proto-typical order-disorder phase transition of this system, x-ray diffuse scattering measurements performed on a single crystal of were transformed into real space producing a three-dimensional pair distribution function using the ‘punch and fill’ method. This 3D-ΔPDF analysis shows unambiguous evidence of temperature-independent local displacements of NO₂^- ions resulting in its polarization reversal below T*.

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