This work is devoted to the investigation of GdCr3(BO3)4 crystals by the method of infrared spectroscopy. Incongruently melting borate GdCr3(BO3)4 was obtained as a result of spontaneous crystallization. Crystal structures were identified by the method of infrared spectroscopy. Ab initio calculations in the frame of density functional theory enabled us to separate modes belonging to the R32 and C2/c phases and to estimate the ratio of these phases in GdCr3(BO3)4 crystals. We have found that the content of the rhombohedral R32 (non- centrosymmetric) modification is about 85%. © Published under licence by IOP Publishing Ltd

Boldyrev, K. N.

Borovikova, E. Y.

Chernyshev, V. A.

Dobretsova, E. A.

Leonyuk, N. I.

Maltsev, V. V.

Popova, M. N.

Journal of Physics Conference Series

English

Institutional repository of Ural Federal University named after the first President of Russia B.N.Yeltsin

Journal of Physics: Conference SeriesPAPER • OPEN ACCESSVibrational spectroscopy of GdCr3(BO3)4:quantitative separation of crystalline phasesTo cite this article: E A Dobretsova et al 2016 J. Phys.: Conf. Ser. 737 012035 View the article online for updates and enhancements.Related contentVibrational spectroscopy of Ga+ ionimplanted ta-C filmsM Berova, M Sandulov, T Tsvetkova et al.-Study of sulfur isotopes by vibrationalspectroscopy and quantum chemistryO Kh Poleshchuk, N B Egorov, D VAkimov et al.-Vibrational Spectroscopy of Stretching andBending Modes of Nickel TetraphenylPorphyrin: an Algebraic ApproachSrinivasa Rao Karumuri, JoydeepChoudhury, Nirmal Kumar Sarkar et al.-This content was downloaded from IP address 213.142.35.54 on 20/07/2019 at 09:03      Vibrational spectroscopy of GdCr3(BO3)4: quantitative separation of crystalline phases E A Dobretsova1, K N Boldyrev1, M N Popova1, V A Chernyshev2,  E Yu Borovikova3, V V Maltsev3 and N I Leonyuk3 1Institute for Spectroscopy Russian Academy of Sciences, Moscow, Troitsk, 108480, Russia 2Ural Federal University, Ekaterinburg, 620002, Russia  3Department of Crystallography and Crystal Chemistry, Faculty of Geology, Moscow State University, Moscow, 119991, Russia.  E-mail: elena-dobrecova@yandex.ru Abstract. This work is devoted to the investigation of GdCr3(BO3)4 crystals by the method of infrared spectroscopy. Incongruently melting borate GdCr3(BO3)4 was obtained as a result of spontaneous crystallization. Crystal structures were identified by the method of infrared spectroscopy. Ab initio calculations in the frame of density functional theory enabled us to separate modes belonging to the R32 and C2/c phases and to estimate the ratio of these phases in GdCr3(BO3)4 crystals. We have found that the content of the rhombohedral R32 (non-centrosymmetric) modification is about 85 %. 1.  Introduction Rare-earth (RE) borates described by the general formula RM3(BO3)4 (R = RE or yttrium, M = Al, Ga, Sc ,Fe, or Cr) crystallize in a number of space groups (SG) with structure type of the natural mineral huntite [1]. These compounds are of interest due to their optical, magnetic, and magnetoelectric properties [2, 3]. RE aluminum borates possess excellent luminescent, nonlinear optical, thermal, and mechanical properties, are chemically stable and are widely used in self-frequency doubling and self-frequency summing lasers [4-6]. RE iron [7-11], aluminum [12-15] and gallium [16] borates  demonstrate attractive magnetoelectric properties. Materials with a large magnetoelectric effect have an application potential as magnetoelectric sensors, magneto-electric and electro-magnetic memory elements, etc. It is important to notice that nonlinear optical and magnetoelectric properties are characteristic only for non-centrosymmetric structures.  RE chromium borates have polytype nature and crystalize in two modifications, namely, the rhombohedral one with the non-centrosymmetric SG R32 or the monoclinic one with the centrosymmetric SG C2/c. The crystal type depends on the growth conditions. Moreover, a coexistence of differently ordered structural fragments in one single crystal is a characteristic feature of RE chromium borates. [17, 18].  The objective of our present study are to identify the crystal structure of GdCr3(BO3)4 obtained by the method of spontaneous crystallization and to evaluate its potential for practical applications.  V International Conference of Photonics and Information Optics IOP PublishingJournal of Physics: Conference Series 737 (2016) 012035 doi:10.1088/1742-6596/737/1/012035Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distributionof this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Published under licence by IOP Publishing Ltd 1      2.  Crystal structure of huntite-like borates As was already mentioned in Introduction, RE borates have the structure type of huntite CaMg3(CO3)4. The latter one crystallizes in the SG R32 [19]. Three types of coordination polyhedra are present in this structure, namely, CaO6 trigonal prisms, MgO6 octahedra and two types of CO3 plain triangles [20] The MgO6 octahedra are connected by common edges forming chains along the c axis. The RO6 trigonal prisms are isolated from each other. In the rare-earth double borates, Ca atoms are replaced by RE elements but instead of Mg atoms one finds M cations (which form a quasi-one-dimensional magnetic system in the case of Fe3+ and Cr3+ ions) [21]. The CO32- groups are replaced by the BO33- triangles (figure 1). Besides the rhombohedral R32 structure there is a monoclinic one with SG C2/c. In each of these two structures it is possible to distinguish two different types of layers, L2n and  L2n+1, identical for two modifications (figure 2). The first type layer (L2n) contains pairs of edge-sharing MO6 octahedra which are further connected into a layer via isolated ВО3 triangles. The second one (L2n+1) is built by trigonal RO6 prisms and MO6 octahedra connected in columns by vertices and also connected via the ВО3 triangles. In the rhombohedral (SG R32) polytype, the L2n+1 layers which contain prisms are multiplied around the L2n layers by two-fold axes, whereas in the monoclinic modification (SG C2/c) by centers of symmetry [22, 23].      Figure 1. Crystal structure of GdCr3(BO3)4 with SG R32  Figure 2. Schematic structures of GdCr3(BO3)4 with:  a) SG R32 (D37) and b) SG C2/c (C2h6) 3.  Experimental and computational details  3.1. Sample preparation The incongruently melting borate GdCr3(BO3)4 was obtained from a molten solution on the basis of K2MoO4 - 3MoO3 as a result of spontaneous crystallization. Experiments were conducted with a ratio of the borate to the solvent in the mixture 1:1. The crucible with the charge was maintained in a furnace until a complete homogenization of the melt for 5 hours at 1100 – 1150 °C. Thereafter, the temperature was lowered to 1000 °C at a rate of 2 °C/h, then at a rate of 1°C/h to 840 °C and to 300 °C at a rate of 10 °C/h. Crystallization of the borate occurred at 1050 - 840˚C. For infrared measurements, a single crystal was grinded into a powder in a pobedite mortar.  3.2. Measurements by the methods of infrared spectroscopy Absorption spectra of the powder sample of GdCr3(BO3)4 in the far-infrared (FIR) region 50–550 cm-1 were registered with the help of a Bruker IFS 125HR Fourier spectrometer (the spectral resolution was up to 2 cm-1), using polyethylene disc technique. The infrared absorption spectra in the middle IR region (MIR) 550-1600 cm-1 were registered on the same spectrometer with an attenuated total reflection (ATR) module. The powder sample was pressed to a diamond crystal of the ATR module. The spectral resolution was 2 cm-1. We applied methods of vibrational spectroscopy combined with the group-theoretical analysis [24] in order to understand the peculiarities of the structure of the GdCr3(BO3)4 borate. V International Conference of Photonics and Information Optics IOP PublishingJournal of Physics: Conference Series 737 (2016) 012035 doi:10.1088/1742-6596/737/1/0120352      3.3. Ab initio calculations Ab initio calculations of phonon frequencies and intensities of the infrared-active modes in GdCr3(BO3)4 were performed for both R32 and C2/c structures in a frame of the density functional theory (DFT) with hybrid functional B3LYP. The calculations were carried out within MO LCAO approach implemented in CRYSTAL14 code. Non-relativistic pseudopotential was used to describe the inner shells of the rare-earth ion. Details of the calculations can be found in Ref. [25]. 4.  Results of IR measurements and DFT calculations Figure 3 shows the IR spectrum in the region 50-1600 cm-1. It is typical for RE chromium borates with the SG R32. The majority of the observed modes find their counterparts in the calculated energy spectrum for this phase of GdCr3(BO3)4. It is not possible to extract oscillator strengths from the measured spectra of powder samples. But we have to note that, in general, large calculated oscillator strengths correspond to strong lines in the measured spectrum, whereas for low ones one finds either weak lines or no line at all (see Table 1). Thus, the calculated data for the R32 phase are in good agreement with the experimental ones, which, in turn, confirms the assumption of predominantly rhombohedral structure of gadolinium chromium borate.  However, several features point to the presence of a monoclinic phase fragment. For internal vibrations of BO33- groups, they are (i) a shoulder at 747 cm-1 in the region of the symmetric bending vibrations, (ii) a splitting of the band at 980 cm-1 in the region of the symmetric stretching vibrations, and (iii) shoulders at 1278 and 1372 cm-1 in the region of asymmetric stretching vibrations (Figure 3, Table 1). In the FIR region, translational vibrations of the Gd3+ and Cr3+ ions, as well as translational and librational motions of the BO33- groups are observed. Signatures of monoclinic inclusions in this spectral region are (i) a weak band at 73 cm-1 of the Gd3+ vibrations, (ii) bands at 126, 148, and 275 cm-1, (iii) a weak shoulder at 248 cm-1 due to the BO33- vibrations, and (iv) a splitting of the band at 434 cm-1 in the region of Cr3+ vibrations. (Figures 3 and 4, Table 1).   Figure 3. Infrared spectrum of GdCr3(BO3)4. At the upper part of the figure, results of the ab initio calculations for the R32 (red bars) and C2/c (blue bars) phases are presented.   V International Conference of Photonics and Information Optics IOP PublishingJournal of Physics: Conference Series 737 (2016) 012035 doi:10.1088/1742-6596/737/1/0120353      Table 1. Comparison between calculated and experimentally observed vibrational modes of GdCr3(BO3)4 in the R32 phase. The assignments of particular modes are also given in accordance with the calculated eigenvectors. Experimental extra peaks ascribed to the monoclinic C2/c phase are in bold (their symmetry is indicated in parentheses). The corresponding calculated frequencies are plotted in Figures 3 and 4.  Assignments Ab initio calculations Experiment Wavenumbers, cm-1 Oscillator strength Wavenumbers, cm-1 A1 L BO33- 216   T (BO33-) T (Gr3+) 342   495   ν4 660   ν1 975   1004   ν3 1209   A2 T (Gd3+) 76 2,97 73 sh (Bu) 79 m L (BO33-) 205 0,11 126 w (Bu) 148 w (Bu) 198 w 225 0,001 233 w 261  0,72  265 s 275 w (Au) T (BO33-) T (Cr3+) 289 0,17 290 sh 334 0,78 321 w 391 0,32 400 s 419 0,91 424 sh ν4 657 0,0004 653 sh ν2 690 0,73 707 sh 760 0,12 778 sh ν3 1302 0,02 1278 sh (Bu) 1296 sh E T (R3+) 95 2,08 99 s L (BO33-) 205 0,0005 202 w sh 224 0,0016 223 w 261 0,044 248 sh (Bu) 262 sh  300 0,36 294 s T (BO33-) T (Cr3+) 317 0,05 306 w 351 0,15 347 s 375 0,015 376 m 420  0,33  434 s 440 s (Bu) 454 1,80 453 m 470 0,00007 480 sh ν4 605 0,0011 601 w 650 0,0038 622 w 687 0,053 680 s ν2 752  0,017  739 s 747 sh (Au) ν1 986  0,005  980 w 990 w (Bu) ν3 1187 0,78 1187 s 1210 0,001 - 1318  0,30  1314 s 1372 sh (Au) sh – shoulder, w – weak, s – strong, m – medium   V International Conference of Photonics and Information Optics IOP PublishingJournal of Physics: Conference Series 737 (2016) 012035 doi:10.1088/1742-6596/737/1/0120354       Figure 4. FIR part of the spectrum of GdCr3(BO3)4.  5.  Estimate of the relative quantity of the monoclinic phase in the sample    Figure 5. Long-wavelength part of the absorption spectrum of GdCr3(BO3)4 (dashed grey line). Decomposition into two Gaussian peaks is shown by thin lines.  V International Conference of Photonics and Information Optics IOP PublishingJournal of Physics: Conference Series 737 (2016) 012035 doi:10.1088/1742-6596/737/1/0120355      Figure 5 shows the experimental IR spectrum at long wavelengths where the translational vibrations of Gd3+ are observed. The band at 79 cm-1 has a shoulder 73 cm-1, which is associated with a small admixture of the monoclinic phase. We have fitted the experimental spectrum by two Gaussians and found that the intensity of the strongest band constitutes 82.3%. According to calculations, the oscillator strength of the considered vibration in the monoclinic phase is 1.5 times greater than in the rhombohedral one. Taking into account this value, we could estimate that the content of the rhombohedral phase in the sample was about 87.5%. 6.  Conclusions Good quality single crystals of gadolinium chromium borate with the structure of the mineral huntite were obtained by spontaneous crystallization from the flux. IR spectroscopy delivered experimental data on the IR-active phonons. The data were compared with the results of ab initio DFT calculations. This comparison shows that the crystal has a polytype structure, containing mainly the rhombohedral phase R32 and a small part of the monoclinic phase C2/c. The ab initio calculations allowed us to distinguish between vibrations belonging to the one and the other structural phases. From the relative intensities of the two low-frequency bands corresponding to translational motions of Gd in the R32 and C2/c phases and taking into account the calculated oscillator strengths, we were able to evaluate the content of monoclinic phase in the predominantly rhombohedral single crystal of GdCr3(BO3)4, which was around 15%. Thus, we have shown that the gadolinium chromium borate crystallizes, mainly, in the noncentrosymmetric phase R32. This indicates a potential of owning magnetoelectric properties, by analogy with noncentrosymmetric iron, aluminium and gallium RE borates. Currently, the work is underway to obtain large single-phase crystals of GdCr3(BO3)4 for investigation of the magnetoelectric effect. Acknowledments This work was supported by the Russian Science Foundation under Grant No 14-12-01033 and by the President of Russian Federation (D.E.A. project СП-754.2015.1). References [1] Leonyuk N I and Leonyuk L I 1995 Prog. Cryst. 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St. 58 (8) 1587 V International Conference of Photonics and Information Optics IOP PublishingJournal of Physics: Conference Series 737 (2016) 012035 doi:10.1088/1742-6596/737/1/0120357

Vibrational spectroscopy of GdCr3(BO3)4: Quantitative separation of crystalline phases

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Vibrational spectroscopy of GdCr3(BO3)4: Quantitative separation of crystalline phases

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