Structural Singularities in Ferroelectric Sr₂NaNb₅O₁₅

The structural features of the ferroelectric compound Sr 2 NaNb 5 O 15 have been carefully studied. A detailed investigation has been performed by means of room temperature (RT) and high temperature (HT) powder X-ray diffraction (XRD), selected area electron diffraction (SAED), high resolution electron microscopy (HREM), and impedance spectroscopy. Comparison is done with the barium homologue compound and the two RT ferroelectric phases can be structurally distinguished. The use of electron diffraction in combination with HREM has revealed the formation of a superlattice structure of tetragonal tungsten bronze (TTB) at RT. While in Ba 2 NaNb 5 O 15 the incommensurate nature of this RT phase seems to be doubtless, the Sr 2 NaNb 5 O 15 ferroelectric phase displays a commensurate superstructure of the TTB structure-type. The RT XRD pattern can be indexed in the Im 2 a space group. The transition to the paraelectric phase occurs at 518 K, and the material can be indexed in the centrosymmetric P 4/ mbm space group. From the Rietveld refinement of this HT phase, disordered distribution of Na and Sr in the A positions of the structure seems to occur, as opposite to the barium compound where barium atoms show clear preference for the largest A2 sites.