Strontium barium niobate (Sr_xBa_1-xNb₂O₆ - SBN) is a ferroelectric material, which belongs to the tetragonal tungsten bronzes (TTB) family. At room temperature SBN crystallizes in a wide solid solution range - 0.25<x<0.75. Below the Curie temperature - T_C, it has the P4bm space group [1]. In ferroelectric phase, SBN has the 4mm point group symmetry, which goes to centrosymmetric 4/mmm point group above T_C. SBN possesses many potential applications owing to its large pyroelectric and electrooptic coefficients and photorefractive properties.

Pure and undoped Sr_0.72Ba_0.25Nb₂O_5.97 single crystals with 72% at. content of strontium grown by Czochralski method were investigated. The real composition of the single crystals was determined using the Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) method [2].   

Basic measurements of the lattice parameter were have been carried out using the X-ray Bond’s method which allows obtaining a accuracy of the order of Δd/d = 10^-5. For this purpose, the metric wavelength of λ CuK_α1 = 1,54059292 ± 4,5x10^-7 Å [3] is required. Measurements of the lattice parameters were performed using 15,4,0 (θ = 73.591°) reflex for an orientation parallel to the crystallographic Z axis and 005 (θ = 77.029°) reflex for an orientation perpendicular to the crystallographic Z axis. Details of measuring principles were presented earlier [4]. Measurements of a and b lattice parameters were repeated 10 times at 298 K. The obtained values of lattice parameters were a = 12.43439 ± 0.00027 Å and b = 12.43451 ± 0.00014 Å. Basing on the analysis of the obtained lattice parameter data it is ascertained that the investigated Sr_0.72Ba_0.25Nb₂O_5.97 single crystals crystallize in the tetragonal system.

Fig. 1. Symmetry along [001]: (a) Laue backscattering pattern taken from (001) plane (b) simulated Laue pattern. Marked squares inside circles are the real equivalent Laue spots.

Large uncertainty of the measured lattice parameters could have been caused by vibrations of atoms when approaching the temperature T_C = 310K of the structural phase transition. Analysis of the diffraction patterns obtained by the Laue method confirmed the occurrence of a four-fold axis of symmetry (Fig. 1).

The additional observations of conoscopic figures using a polarizing microscope technique showed an optical pseudo-biaxiality of the investigated crystals [5]. Such results do not exclude the tetragonal symmetry in the investigated strontium barium niobate single crystals. The results obtained by different X-ray and optical methods are compatible and coincide with the literature data.

References

[1] M. Ulex, R. Pankrath, and K. Betzler, Growth of strontium barium niobate: the liquidus–solidus phase diagram, J. Crystal Growth 271 (2004) 128 - 133.

[2] T. Łukasiewicz, M.A. Świrkowicz, J. Dec, W. Hofman, W. Szyrski, Strontium-barium niobate single crystals, growth and  ferroelectric properties, J. Crystal Growth 310 (2008) 1464 - 1469.

[3] G. Holzer, M. Fritsch, M. Deutsch, J. Hartwig, E. Forster, K_α1,2 and K_β1,3 x-ray emission lines of the 3d transition metals, Phys. Rev. A 56 (1997) 4554-4568.

[4] K. Wokulska, P. Pacek, J. Dec, T. Łukasiewicz, M. Świrkowicz, Characterization of Phase Transition in Strontium Barium Niobate by Bond Method, Solid State Phenomena 163 (2010) 264-267.

[5] J. Dec, T. Łukasiewicz, Crystallooptics of tetragonal tungsten bronze materials, Book of Abstract of the XXII Conference on Applied Crystallography (2012) 73.

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