The modulated structure of Ba_0.39Sr_0.61Nb₂O₆. I. Harmonic solution

Abstract

The structure of a crystal of Sr0.61Ba0.39Nb2O6 has been solved and refined as an incommensurate structure in five-dimensional superspace. The structure is tetragonal, superspace group P4bm(\,pp1/2,p - p1/2), unit-cell parameters a = 12.4566 (9), c = 7.8698 (6) Å, modulation vectors q 1 = 0.3075 (6) (a* + b*), q 2 = 0.3075 (6) (a* − b*). The data collection was performed on a KUMA-CCD diffractometer and allowed the integration of weak first-order satellite reflections. The structure was refined from 2569 reflections to a final value of R = 0.0479. The modulation affects mainly the positions of the O atoms, which are displaced by as much as 0.5 Å, and the site 4c that is occupied by Sr and Ba atoms. Only a simplified model, in which this atomic position is occupied by an effective atom Sr/Ba, could be refined from the data set. The modulation of displacement parameters has been used to account for the modulated distribution of Sr and Ba. The whole refinement uses only first-order modulation waves, but there are strong indications that for a complete solution the use of higher-order satellites and a more complicated model is necessary.

Crystal growth and characterization of tetragonal tungsten bronze FerroNiobates Ba2LnFeNb4O15

“…As Woike et al observed with a Ba 0.39 Sr 0.61 Nb 2 O 6 single crystal [23], the oxygen ions in the barium and lanthanum layer display strong anisotropic displacements, with a rounded thermal ellipsoid for O1, and lengthened ones for O5. The oxygens ions located in the iron and niobium layer exhibit thermal ellipsoids, which are pointing along the Oz direction (Fig.…”

“…This may be the sign that a modulation takes place in the LaFNO polyhedral framework, as it is the case for Ba 0.39 Sr 0.61 Nb 2 O 6 and similar TTB systems. This may be related to the modulation of the structure observed in some crystals as mentioned above, which required a double unit cell parameter along c direction (a structural model close to the suggested one by Woike et al [23]). The crystal structure of LaFNO we present here will certainly be used as a basis structure for a detailed description of modulated LnFNO crystals with the 3Dþ n approach.…”

“…Although only few accurate crystal structure refinement data are available in the literature, mainly related to the study of barium strontium niobates [23,24], it can be stated that the basic unit cell for the TTB structure is tetragonal (aE12.5Å and cE4.0Å) with a centrosymmetric 4/mmm point group in the paraelectric phase. X-Ray diffraction measurements on a LaFNO single crystal confirmed the P4/mbm space group, with a¼12.513(1)Å and c¼3.938(1)Å at room temperature.…”

Lanthanide doped strontium barium niobate: Optical spectroscopy and local structure at the impurity sites

“…They are tetragonal, with space group P4bm for the ferroelectric phase [18,19]. Since the Curie temperatures are located above 320 K [1], and the present spectral measurements have been carried out at RT or low temperature, all results are relative to the ferroelectric phase.…”

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

“…Recently, the incommensurate structure of SBN was solved and refined as a five dimensional (5D) modulated structure in the superspace formalism. [49] In 2012, Graetsch et al [50][51] [52][53][54][55] demonstrated that both Bi-and La-based compounds exhibit relaxor behavior, whereas Nd-, Sm-, and Gd-based analogs are classical ferroelectrics. In Ba 2 RENb 3 Ti 2 O 15 compounds, deviations from the ideal TTB symmetry caused by an incommensurate modulation was associated with relaxor behavior, whereas a commensurate modulation was observed for phases exhibiting ferroelectric behavior.…”