Effect of Nb cationic substitution on the microstructure and Raman spectra of SrBi2(Ta,Nb)2O9 thin films

Abstract

Micro-Raman spectroscopy and X-ray diffraction have been used to explore the lattice dynamics of Nb-substituted SrBi2(TaxNb1−x)2O9 (SBTN) thin crystalline films annealed at low temperature, 700oC. The relative intensity of the (115), (006) and (200) X-ray diffraction peaks, which correspond to the crystal structure and ferroelectric phase, nonmonotonically increases with the increase of Nb content. It turned out that SBTN films consist of fine-grained spherical structures if the percent of Ta ions substituted by Nb ions is (10–40) wt.%, while the formation of rod-like grains occurs when the percent of Nb ions reaches 50 wt. % due to the stress-induced transformation of the thin film perovskite structure. It was shown that the substitution by Nb of Ta-site in SBT has a pronounced impact on the Raman band around 810 cm− 1, which is assigned to the symmetric stretching modes of the (Ta,Nb)O6 octahedra, leading to the shifting and splitting of this mode. At the same time Nb substitution does not significantly influence on the low frequency Raman modes of SBT. Unexpectedly, we observed the nonmonotonic shift of the Raman band around 810 cm− 1, namely: from 810 cm− 1 (for pure SBT) to ~ 830 cm− 1 (for SBTN with 20–40 wt.% of Nb) and then back to 811 cm− 1 with increasing Nb content up to 50 wt. %. We assume that these changes are conditioned by the ferrodistortion occurring in ferroic perovskites, namely by the tilting distortion of (Ta,Nb)O6 octahedra at (20–50) wt. % of Nb. The octahedral tilting distortion can change the coordination environment of the A-cite cation, as well as it lowers the SBTN symmetry due to the differences in ionic radius and mass between Ta and Nb in the B-sites, that can lead to significant changes of the SBNT crystal structure and symmetry when 50 wt. % of Ta ions is substituted by Nb ions. The same nonmonotonic trends were observed for the ferroelectric perovskite phase fraction and remanent polarization on Nb content in SBTN films. Landau-Devonshire approach is used to explain the experimentally observed nonmonotonic dependence of the ferroelectric perovskite phase fraction and remanent polarization on Nb content in SBTN films, at that the form of the dependence strongly resembles the shift of the Raman band of symmetric stretching modes of the (Ta,Nb)O6 octahedra. Thus, the combination of micro-Raman spectroscopy with X-ray diffraction, ferroelectric measurements and phenomenological description allow to establish the influence of Nb doping on the lattice dynamics, microstructure, phase composition and ferroelectric properties of the SBTN thin films.