The search of new non-linear materials and development of technologies for producing optically perfect crystals remain actual up to now. Among solid-state materials an important role play ferroelectric crystals of strontium-barium niobate solid solutions Sr_xBa_1-xNb₂O₆ (SBN:x), which belong to a class of active dielectric, exhibiting qualitatively new properties under influence of external factors. SBN single crystals are characterized by the extremely large electro-optical coefficients and high nonlinear optical properties. Doping of the SBN solid solutions by rare-earth and transition metals allows to control the properties of the crystals and to create new materials for different applications, particularly in the areas of pyroelectricity, piezoelectricity, electro-optics, photorefractive optics and non-linear optics.
The present investigation is directed on investigation of chemical and phase composition of the SBN solid solutions charge and crystals grown from the melt by shaping techniques. We studied influence of non-controlled charge impurities and special dopants introduced into the melt on chemical and phase homogeneities of SBN crystals obtained in profiled configuration. The specific features of the shaping techniques, including form and capillary sizes of dies, were studied, their influence on chemical and optical homogeneities of the crystals were shown.
The SBN charge was prepared by solid phase synthesis at a temperature of 1200°C during 6h according to the reactions:
SBN:61:

0.61SrCO₃+0.39BaCO₃+Nb₂O₅→Sr_0.61Ba_0.39Nb₂O₆+CO₂↑             (1)
SBN:75:

0.75SrCO₃+0.25BaCO₃+Nb₂O₅→Sr_0.75Ba_0.25Nb₂O₆+CO₂↑             (2)
The method of scanning electron microscopy (SEM) combined with microanalysis was used for determination of morphology and the elemental composition of the SBN: 61 and SBN: 75 solid solutions (fig.1).

Fig.1 SEM images of surface micro-relief for SBN:61(a) and SBN:75 (b) solid solutions

It was estimated that the grain sizes in SBN:61 and SBN:75 samples vary from units to tens of microns. Content of Sr, Ba, Nb elements in separate grains does not correspond to the chemical formula. Deviation from the specified element composition was within 10%. The presence of calcium ions as uncontrollable impurities was registered in admixture. The phase composition of the charges was determined by XRD method. It was found that a monoclinic phase and tetragonal phase coexist in the temperature region  of 950-1400°C. The monoclinic phase is stable at temperatures below 850°C. Temperature of the phase transition from monoclinic  into tetragonal form depends on the aggregate state of sample and lies in the region of 1340°C. At higher temperatures the monoclinic phase quickly and completely converted in phase with tetragonal symmetry. The solid phase synthesis of SBN at temperature 1200°C leads to formation of both phases. At the same time the content of monoclinic phase in charge increases with increasing of strontium content in SBN solid solution.
The SBN crystals were grown from the melt by the modified Stepanov technique using special die of capillary type. Influence of constructive peculiarities of dies on chemical homogeneity of profiled SBN crystals was studied. In optimized conditions the crystals (14x24mm) in cross-section and up to 80 mm in length were grown. Effective segregation coefficients for crystals doped with Ce, Cr or Co ions were determined. It was shown that the content of uncontrollable impurities did not exceed 100 ppm, except Na, Al, Si (120-190 ppm). Nevertheless, the presence of non-controlled impurities leads to appearance of absorption band around 400 nm in transmission spectra of the material. The control of optical quality of the crystals was performed by optical and polarization-optical methods, laser radiation scattering control, the method of dynamic holography.

Fig.2 The dependence of full diffuse reflection on wavelength for SBN:61:Ce (a) and SBN:75 (b) crystals

Measurements of the optical homogeneity of crystals were also carried out by photometry method. Diffuse reflection coefficients and diffuse transmittance in the spectral range (250-2500 nm) were determined. The results are given in fig.2. The spectra, obtained in different points of the samples are identical within 1% (experimental error). It should be noted that the introduction of cerium dopant into the congruently melting SBN:61 gives total reflection coefficient less than one for the nominally pure SBN:75 crystal, which is characterized by the most disordered structure among SBN solid solutions.
The regimes of solid phase synthesis and  growth parameters were optimized  to provide the reproducible characteristics of SBN crystals of high homogeneity.

• Legal notice:
  

  Copyright (c) Pielaszek Research, all rights reserved.
  The above materials, including auxiliary resources, are subject to Publisher's copyright and the Author(s) intellectual rights. Without limiting Author(s) rights under respective Copyright Transfer Agreement, no part of the above documents may be reproduced without the express written permission of Pielaszek Research, the Publisher. Express permission from the Author(s) is required to use the above materials for academic purposes, such as lectures or scientific presentations.
  In every case, proper references including Author(s) name(s) and URL of this webpage: https://science24.com/paper/29130 must be provided.

1. Optical images of real structure for oxide crystals grown from the melt
2. Observation of induced birefringence in crystals with scheelite structure
3. Near-electrode surface processes in langatate in connection with growth conditions