Article Cite This: Cryst. Growth Des. 2018, 18, 7435−7440 pubs.acs.org/crystal Development of a Synthesis Technique and Characterization of High-Quality Iron Borate FeBO3 Single Crystals for Applications in Synchrotron Technologies of a New Generation Sergey Yagupov,† Mark Strugatsky,*,† Kira Seleznyova,† Yuliya Mogilenec,† Nikita Snegirev,† Nikita V. Marchenkov,‡,§ Anton G. Kulikov,‡,§ Yan A. Eliovich,‡,§ Kirill V. Frolov,‡ Yulia L. Ogarkova,‡ and Igor S. Lyubutin‡ Downloaded via SHANGHAI UNIV OF ELECTRIC POWER on January 7, 2019 at 13:28:28 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. † Physics and Technology Institute, V.I. Vernadsky Crimean Federal University, 295007 Simferopol, Russia Shubnikov Institute of Crystallography of FSRC “Crystallography and Photonics” RAS, 119333 Moscow, Russia § National Research Center “Kurchatov Institute”, 123098, pl. Akademika Kurchatova, 1, Moscow, Russia ‡ ABSTRACT: A flux growth technique for synthesizing FeBO3 single crystals of high structural perfection was developed. The high structural quality of the synthesized FeBO3 crystals was confirmed by means of double-crystal X-ray diffraction analysis both in Laue and Bragg geometries. The diffraction rocking curves taken locally and integrally over the surface are in excellent agreement with the calculated curves. Some macrodefects were revealed by X-ray topography in the crystal volume. However, the local defects do not prevent the use of defect-free regions of the crystal for synchrotron Mössbauer experiments. 1. INTRODUCTION Traditional Mössbauer spectroscopy, e.g., on 57Fe nuclei, is a widely used and effective experimental method for studying structural, electronic, and magnetic properties of various materials.1 However, the standard procedure is poorly suited for studying small objects (less than 100 μm) and for experiments under extreme conditions, such as strong magnetic fields, high temperatures, and also in high-pressure chambers with diamond anvils.2,3 The traditional method of Mössbauer spectroscopy is based on the use of radioactive sources. However, a major drawback of this method is the ineffective use of the radioactive radiation from the Mössbauer source during the experiments. The Mössbauer radiation is uniformly distributed over the sphere (in 4π), and only a small part of it falls on the sample under study. Thus, durable measurements are required in order to obtain necessary statistics that affects the quality of the results. In this situation, the possibility of using high-intensity focused synchrotron radiation in the traditional Mössbauer scheme instead of the standard nuclear gamma-ray source seems to be the most effective and optimal one. However, first and foremost, it is required to distinguish from the “white” synchrotron radiation the interval of radiation with energy corresponding © 2018 American Chemical Society to Mössbauer resonance. For this purpose, at the initial stage, a system of standard monochromators based on silicon crystals is used.4 However, this system does not provide sufficient tuning accuracy to the resonant frequency. In order to solve this problem, recently it has been proposed to use the effect of nuclear diffraction of Mössbauer radiation in the FeBO3 crystal on the final stages of monochromatization.5 A special tuning leads to the contribution in the diffraction process of only Mössbauer 57Fe nuclei that provide an ideal monochromatization of the diffracted radiation.6−9 This approach has been described in a series of theoretical and experimental studies.10−13 It was proposed to use iron borate crystals 57FeBO3 in order to obtain X-ray radiation with characteristics corresponding to the 57Fe resonance absorption line. FeBO3 has the rhombohedral calcite structure, space group R3̅c.14 For this crystal all reflections (NNN) ((111), (222), (333), etc.) are forbidden for electron diffraction, but are allowed for nuclear diffraction. Such an experimental technique Received: July 26, 2018 Revised: October 13, 2018 Published: October 23, 2018 7435 DOI: 10.1021/acs.cgd.8b01128 Cryst. Growth Des. 2018, 18, 7435−7440 Crystal Growth & Design Article Figure 1. Temperature mode of crystallization used in synthesizing FeBO3 crystals of high structural perfection. is called “synchrotron Mössbauer spectroscopy” (SMS) and has been tested and implemented in European Synchrotron Radiation Facility at beamline ID18 (Grenoble, France)6 and also in SPring-8 (Tsukuba, Japan).15 It was established that X-ray diffraction conditions impose strict requirements on the high quality of such crystals. In addition, in order to provide the symmetry for the necessary reflections, the crystals must have a shape of a basal plate with the plane parallel to the plane of reflection (NNN). Moreover, the required crystals should be enriched up to 95% with the 57Fe isotope. Meanwhile, at present, the deficit of FeBO3 single crystals of high structural perfection that, as mentioned above, are a key element of the SMS, imposes significant limitations on the further development of this technique. Iron borate crystal synthesis can be implemented by (i) vapor, see, e.g.,16,17 and (ii) flux, see, e.g.,18,19 growth. Using technique (i), bulk single crystals of iron borate with large nonbasal faces can be obtained. Meanwhile, technique (ii) allows obtaining single crystals with the shape of basal plates. Such crystals are of high structural perfection19 and possess large basal faces, which is difficult to obtain by technique (i). Thus, for the purposes of the present work the flux growth technique has proved to be most appropriate. However, a standard route of crystal synthesis has to be modified in order to avoid mechanical stresses and destruction of large crystals. The aim of the present work is to develop a technique for synthesis FeBO3 single crystals of high structural perfection suitable for applications in synchrotron technologies and to determine the degree of their structural perfection by X-ray diffraction (XRD) analysis. Iron borate crystals are usually synthesized in the (i) Fe2O3− B2O3−PbO−PbF2, see, e.g.,18−22 and (ii) Fe2O3−B2O3−Bi2O3, see, e.g.,23,24 systems. Meanwhile, the latter system destructively acts on the platinum crucible. Moreover, FeBO3 crystals synthesized using the (ii) system usually possess Bi as an impurity. Thus, the crystallizations were carried out in the (i) system. The crystal-forming reagents are Fe2O3 and B2O3; PbO, PbF2, and B2O3 serve as solvents. Note that B2O3 is both crystal-forming reagent and solvent. A typical composition of the reagents used for successful synthesis of FeBO3 crystals in wt % is as follows: Fe2O3 (5.73), B2O3 (51.23), PbO (29.31), and PbF2 (13.73). After a charge composition was determined, all reagents were dehydrated separately in a drying chamber at 150 °C during 24 h. Next, the reagents were weighed with a high-precision balance and mixed with a laboratory-developed device. For obtaining a homogeneous solution melt, small portions, about 5 g, of the charge were successively adjoined to a platinum crucible (with a volume of 90 cm3) and each time kept for 20 min in a muffle furnace at 870 °C. The weight of the melt solution was 137.1 g. The crucible with the prepared solution melt was covered with platinum foil perforated with small holes and another similar crucible (crucible-lid with a volume of 75 cm3) and installed in the crystallization furnace with uniform temperature distribution. The junction between the crucibles was neatly rolled. Figure 1 shows a typical temperature regime used in synthesizing FeBO3 crystals. It includes the following steps: (i) heating of the furnace from the room temperature; (ii−iii) a few cycles of homogenization of the solution melt with subsequent sharp temperature dropping in order to improve the homogeneity of the solution-melt (in particular, without the application of this technique, solid deposits were found on the bottom of the crucible) and to avoid the emergence of spurious phases, e.g., Fe3BO6; (iv) cooling of the furnace with a speed of 0.5 °C/h (during this step crystal growth occurs); (v) furnace overturn (during this step the solution melt was drained into a crucible-lid); (vi) cooling the furnace. After the last step, a crucible was removed from the furnace and uncovered. The solidified solution melt and several large single crystals of iron borate have turned out to be, respectively, in the crucible-lid and on the foil perforated with small holes. We emphasize the importance of the separation of the liquid solution melt from the synthesized crystals at high temperatures. 2. SYNTHESIS OF FeBO3 SINGLE CRYSTALS OF A HIGH STRUCTURAL PERFECTION BY THE FLUX GROWTH TECHNIQUE The synthesis of FeBO3 single crystals of a high structural perfection by flux growth technique includes the following steps: (i) determining suitable charge compositions and temperature modes; (ii) preparing the charge; (iii) obtaining a homogeneous solution melt in a platinum crucible; (iv) slow cooling of the solution melt according to a predetermined temperature mode; (v) separation of synthesized crystals from the liquid solvent by draining the latter at high temperature. All the parameters of crystallization are interdependent. Therefore, achievement of the best results requires optimization of each technological step. 7436 DOI: 10.1021/acs.cgd.8b01128 Cryst. Growth Des. 2018, 18, 7435−7440 Crystal Growth & Design Article Figure 2. (a−c) Examples of the synthesized FeBO3 single crystals. Figure 3. Schematic diagram of the experimental setup (TXS) for obtaining double-crystal DRCs and topograms. their bigger size and, thus, more convenience of working with them. XRD studies were carried out with a triple-axis X-ray spectrometer (TXS), the principal scheme of which is shown in Figure 3. The sample and the detector were mounted on a tripleaxis goniometer with a set of lateral movements for precise alignment of the sample. An X-ray tube with a Mo anode with a wavelength of characteristic radiation Kα1 λ = 0.7093 Å served as a radiation source. The monochromatization block includes the slits for preliminarily collimation installed before the crystalmonochromator, and the silicon monochromator itself, see Figure 3. In order to detect the scattered X-ray radiation, a scintillation detector was used to measure the double-crystal DRCs. A two-coordinate Bruker detector with the CCD matrix resolution of 1024 × 1024 pixels and a pixel size of 100 μm was used in order to record the topograms. The experiments have been carried out in Laue (for X-ray transmission, reflection (3 3̅ 0) with a diffraction angle of 15.42° and extinction length Lext = 85.8 μm) and Bragg (for X-ray reflection, reflection (0 0 12) with a diffraction angle of 17.09° and Lext = 8.7 μm) geometries. A DRC is a dependence of the intensity of the monochromatic X-ray radiation scattered by the crystal on the diffraction angle, which is varied near the Bragg angle. The shape and half-width of the curve allow judging the presence of a defective structure of the crystal lattice in the volume of interaction with the X-ray beam. In order to measure the double-crystal DRCs we used a symmetric single Si crystal tuned to the (220) reflex. For obtaining integral DRCs the sample was illuminated over the whole surface by a wide monochromatic X-ray beam; for DRCs with a high spatial locality collimating slits were installed in addition after the monochromator to restrict the beam projection on the sample up to several square millimeters. The topogram provides a visual representation of the distribution of the defects basing on the changing of the intensity of If such a separation does not occur, the crystals are exposed to thermal deformations of solidified solution melt that lead to the destruction of large crystals and the occurrence of mechanical stresses in them. Total weight of the synthesized crystals was 5.2 g. The synthesized crystals have the shape of odd crystal pieces and correct (hexagonal) plates with the dimensions up to 15 mm and 5 mm in the basal plane, respectively, and ca. 180 μm in the perpendicular direction. Examples of the synthesized samples are shown in Figure 2. It should be noted that spurious phases, e.g., Fe3BO6, are absent. Our thorough studies have shown that the optimal composition of the reagents is similar to that used previously, see, e.g., refs 18 and 19. However, we significantly improved temperature mode of crystallization, which led to an increase of the degree of structural perfection of the synthesized samples. Moreover, we managed to avoid the appearance of spurious crystalline phases. For this purpose, we have developed a microprocessor system, which allows controlling the temperature mode of crystallization with high accuracy and speed. The mass of the solution-melt and the volume of the crucible that we used exceed that used by the authors in the work,18 which makes it possible to obtain more crystals. It should be noted, that in refs 19 and 20 the procedure of furnace overturn is absent; meanwhile, it is crucial for obtaining large crystals of a high structural perfection. 3. CHARACTERIZATION OF THE SYNTHESIZED SINGLE CRYSTALS In order to determine the degree of crystalline perfection, the synthesized crystals were studied by means of double-crystal XRD analysis. The double-crystal diffraction rocking curves (DRCs) of iron borate with various degrees of localization of the crystal illumination by an X-ray beam, as well as X-ray topograms, were obtained. We have chosen to carry out measurements on odd-shaped crystal pieces, see Figure 2a,b, due to 7437 DOI: 10.1021/acs.cgd.8b01128 Cryst. Growth Des. 2018, 18, 7435−7440 Crystal Growth & Design Article Figure 4. Local experimental (a and b) and calculated (c and d) double-crystal DRCs of FeBO3 crystal obtained with a high spatial locality. diffracted X-ray radiation over the whole area of the crystal.25 Dynamic scanning mode with a simultaneous rotation in angular range of 150 arcseconds and total time exposure of about 300 s was used: the integral intensity of the double-crystal DRC accumulates from each point of the surface.26 This mode shows the differences in the integral intensity associated with the structure heterogeneity. At the same time, it excludes the contribution of a possible influence caused by the crystal holder since the integral intensity is not sensitive to the slight bending of the crystal. The size of the point (without taking divergence into account) is determined by the resolution (pixel size) of the 2D detector. For the X-ray topography method, an asymmetric silicon monochromator of the slice (440) with an asymmetry coefficient of b = 0.025 was used. Asymmetric cut allows creating a spatial broadening of the initial beam in the plane of diffraction that ensures uniform illumination of the whole sample surface with a monochromatic radiation. Figure 4a,b shows the local double-crystal DRCs. As one can see, they have a symmetrical shape and the values of the halfwidth (fwhm) of the DRCs obtained in Laue and Bragg geometries are 12.4 and 16.3 arcsec, respectively. Figure 4c,d shows the results of numerical modeling of the double-crystal DRCs. The calculations were carried out in accordance with the dynamic theory of diffraction taking into account the structural factor of this crystal, the instrumental function of the source ,and the Si monochromator convolution with allowance for the dispersion effect.27,28 The calculated curves are in a good accordance with experimental data, which testifies a high perfection of the obtained FeBO3 crystals. The reason for the broadening of the curve in the Bragg geometry in comparison with Laue geometry lies in different depths of extinction, which in these two cases differs by an order of magnitude, vide supra. According to calculations for selected reflections, in the case of diffraction in Bragg geometry the information is collected from a volume corresponding to a nearsurface layer of about 7 μm in depth due to the small extinction length. As far as crystal surface itself represents a structural defect, the DRCs measured in Bragg geometry have larger values of fwhm It is possible that this broadening is due to the additional defects associated with the reconstruction of the surface of the crystal.29,30 In order to estimate the homogeneity and crystal quality of the whole sample volume, the integral double-crystal DRCs were also measured, see Figure 5. The estimated fwhm values of the DRCs are about 50 arcsec. Thus, local experimental DRCs made it possible to establish that a significant part of the crystal volume has a high structural perfection. Some broadening of integral DRCs may be due to the existence of some localized macroscale defects observed on topograms (see below). The total volume of these defects is relatively small. The following factors can contribute to the broadening of the double-crystal DRCs in both Laue and Bragg measurement schemes: (i) The effect of X-ray beam dispersion due to the difference in the diffraction angles of the monochromator and the sample; (ii) Possible bending of a thin crystal due to the mechanical impact that appears from the crystal holder; 7438 DOI: 10.1021/acs.cgd.8b01128 Cryst. Growth Des. 2018, 18, 7435−7440 Crystal Growth & Design Article Figure 5. (a, b) Integral experimental double-crystal DRCs of FeBO3 crystal. Figure 6. X-ray topograms of the FeBO3 crystals: (a) sample on Figure 2a in the Bragg geometry, (b) sample on Figure 2b in the Bragg geometry, (c) sample on Figure 2b in the Laue geometry. (iii) The presence of different defects in structure and the disrupted near-surface layer. ORCID The cause (i) can be added to the calculations, and the remaining two can only be verified experimentally by the topography. The topograms of two samples, shown in Figure 2a,b, were measured in the Laue and Bragg geometries, see Figure 6. The topography reveals the crystal regions with a different integral intensity. In particular, the clearly visible dark spots indicate the presence of growth defects and microcracks in the sample volume. However, these localized defects do not prevent the use of defect-free regions of the crystal. Notes Mark Strugatsky: 0000-0002-9282-5768 The authors declare no competing financial interest. ACKNOWLEDGMENTS ■ REFERENCES This work was partially supported by the Russian Foundation for Basic Research (RFBR) and the Ministry of Education, Science and Youth of the Republic of Crimea in the framework of scientific project Grant nos. 16-42-910593 and 17-42-92015 “p_a”, by the RFBR in the framework of scientific project Grant no. 18-32-00210 “mol_a” (synthesis of experimental samples) and by the V.I. Vernadsky Crimean Federal University Development Program for 2015−2024. Support by RFBR Grant No. 17-02-00766 in part of the Mössbauer spectroscopy characterization and by the Ministry of Science and Higher Education within the State assignment FSRC “Crystallography and Photonics” RAS in part of the X-ray diffraction analysis are also acknowledged. 4. CONCLUSION We have succeeded in obtaining high-quality FeBO3 single crystals using the flux growth technique. The latter one was substantially modified in order to avoid the destruction of large crystals and the occurrence of mechanical stresses in them, and to eliminate the formation of spurious crystalline phases during the synthesis. By means of XRD analysis, we confirmed a high degree of structural perfection of the synthesized FeBO3 crystals. The double-crystal DRCs taken over the surface are in excellent agreement with the calculated curves for perfect crystals. In the crystals studied, some macrodefects revealed by X-ray topography in the crystal volume may be associated with cracks occurred during the synthesis. However, these defects will not affect the efficiency of Mössbauer diffraction initiated by a synchrotron beam passing in the defect-free regions of the crystal. The synchrotron Mössbauer experiments on the synthesized samples are in progress and will be published elsewhere. ■ ■ (1) Greenwood, N. N.; Gibb, T. C. In Mossbauer Spectroscopy; Chapman and Hall: London, 1978. 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