X-ray structural determination of the quasi-commensurate phase of barium sodium niobate

Abstract

Inorder to provide anaccurate basis forthe interpretationofthe physical properties of this complex incommensurate system, the structure of the room temperature quasicommensurate phase of barium sodium niobate is determined by taking into account. for the first time, the satellite reflections present in the diffraction spectrum of this material. The quasi-commensurate phase considered is assimilated to a strictly commensurate phase, the satellite reflections being considered as superlattice reflections and referred to a crystalline supercell with a = 35.187 A. b = 17.620 A and c = 7.987 A. This approximation gives an excellent agreement index R = 0.031 for the fit to the intensities of the 2432 basic reflections andthe 1691satellitereflectionscollectedat294 K(AMo Ka = 0.71069A,p, = 111.47cm.'. space group : Bbm2). The structural resultsshow that the commensurate modulation consists of a tilting and a shearing of all the NbO, octahedra in the structure in contrast to previous inferences which had only suggested the occurrence of a shear. The framework of the structure may be described on the basis of the existence of two different Nb50?4 units of five corner-sharing octahedra each. The structure obtained by spatially averaging the modulation is consistent with previous structural studies of the basic reflections of this material. Physical characteristics pertaining to the polar character, or the domain structure, are also deduced.

Relaxor-to-Ferroelectric Crossover and Disruption of Polar Order in “Empty” Tetragonal Tungsten Bronzes

“…Initial inspection of both data sets revealed the deficiency of the previous assignment of tetragonal symmetry based on (lower resolution) ambient temperature laboratory PXRD data; 40 the structure is orthorhombic over the entire measured temperature range between 20 -600 K. Closer inspection of the PND data revealed additional peaks of low intensity at longer d-spacing which do not correspond to likely impurity phases but which indicate the presence of a large orthorhombic superstructure. This superstructure is consistent with the Bbm2 model of Labbé and co-workers 38 which has a ≈ 2√2 a TTB , b ≈ √2 a TTB and c ≈ 2 c TTB expansions, Figure 3(a), and is the same structure reported for Ba 4 La 2 Ti 4 Nb 6 O 30 . 21 Rietveld refinement of the low temperature data PND data indicates that the atom positions in this compound are close to those determined for BNN, see electronic supporting information (ESI) Figures S1-2 and Tables S1-3.…”

X-ray structure refinement of solid solution Ba2.15−xNa0.7+xNb5−xWxO15 and the investigation of the orthorhombic-tetragonal phase transition by Raman spectroscopy

“…The same sequences of transitions were reported for the well known ferroelectric Ba 2 NaNb 5 O 15 TTB-type material [25]. The crystal structure of Ba 2 NaNb 5 O 15 [26] showed the existence of quasi-commensurability at room temperature. In fact, there are several distinct incommensurate phases differing in the number of wave directions of the modulations [25,26].…”

Growth and Characterization of Centimeter‐Sized Ba2LaFeNb4O15 Crystals from High‐Temperature Solution under a Controlled Atmosphere

“…Schneck et al [44] and Labbe et al [45] revealed the incommensurate character of the crystal structure of SBN, which is similar to the previously discovered incommensurability in BNN [46] and appears in combination with ferroelastic properties. [47] Simon et al [48] showed that SBN crystals exhibit anisotropic dielectric behavior.…”