Structural characterization and Curie temperature determination of a sodium strontium niobate ferroelectric nanostructured powder

The Curie temperature and its correlation with the magnitude of the displacement of the niobium atom from the center of [NbO6] octahedra in NaSr2Nb5O15 nanostructured powder were investigated. A single powder was prepared by high-energy ball milling. A powder with an average crystallite size of 37 nm was prepared by calcining the precursor at 1423 K. The refinement of the structural parameters was carried out by the Rietveld method. NaSr2Nb5O15 exhibits tetragonal symmetry with the tungsten bronze structure (a=b=12.3495 (6) Å, c=3.8911 (2) Å, V=593.432 (5) Å3, and Z=2). The site occupancy of the Na+ and Sr2+ cations and the interatomic distances between the niobium and oxygen atoms were derived. The [NbO6] octahedron undergoes both rotation and tilting depending on the crystallographic site. The Curie temperature of the powder was derived using both the impedance and infrared spectroscopy methods.

Representation of the unit cell of the NaSr2Nb5O15 powder. Nb5+ cations are differentiated as Nb(I) and Nb(II). [Nb(I)O6] octahedral sites are represented by yellow color and [Nb(II)O6] by blue ones.Figure optionsDownload as PowerPoint slideHighlights
► Spontaneous polarization has been based on the off-center displacement of atoms.
► Distortion of the [Nb(II)O6] octahedra occurs due to both to [NbO6] octahedron rotation and tilting.
► Newest another equation to derive the parameter Curie's temperature.
► The ferroelectric transition in the NaSr2NaNb5O15 powder is of displacive type.