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5Condensed Matter and Interphases (Kondensirovannye sredy i mezhfaznye granitsy)
Original articles
DOI: https://doi.org/10.17308/kcmf.2020.22/2823 ISSN 1606-867X
Received 28 April 2020 eISSN 2687-0711
Accepted 15 May 2020 Published online 25 June 2020
Synthesis, Structure, and Luminescent Properties of the New Double Borate K3Eu3B4O12
© 2020 E. V. Kovtunets®, A. K. Subanakov, B. G. Bazarov
Baikal Institute of Nature Management, Siberian Branch of the Russian Academy of Sciences, 6, Sakhyanova str., 670047 Ulan-Ude, Republic of Buryatia, Russian Federation
Abstract
The study established the formation of the new double borate K3Eu3B4O12. The Rietveld refinement of the crystal structure revealed that K3Eu3B4O12 crystallises in the monoclinic syngony with unit cell parameters a = 10.6727(7) Á, b = 8.9086(6) Á, c = 13.9684(9) Á, p = 110.388(2)° (space group P2/c). K5Eu5B4O12 has a layered structure with [Eu8(BO3)8]~ sheets which are almost parallel to the ab plane. These sheets are formed by pentagonal EuO7 bipyramids, EuO6 octahedras, and BO3 triangles attached to them through common vertices. Neighbouring layers are interconnected via pentagonal EuO7 bipyramids, BO3 triangles, and potassium cations. The luminescence spectrum demonstrates a noticeable emission band at 611 nm, resulting from the 5D0^7F2 transition of Eu3+ ions.
Keywords: double borates of potassium and rare-earth elements, ceramic technology, Rietveld method, luminescent properties.
Funding: The study received financing within the framework of state order No. 0339-2019-0007 to the Baikal Institute of Nature Management, Siberian Branch of the Russian Academy of Sciences. It was partially funded by the Russian Foundation for Basic Research (project No. 18-08-00985 a).
For citation: Kovtunets E. V., Subanakov A. K., Bazarov B. G. Synthesis, structure and luminescent properties of the new double borate K3Eu3B4O12. Kondensirovannye sredy i mezhfaznye granitsy = Condensed Matter and Interphases. 2020; 22(2): 219-224. DOI: https://doi.org/10.17308/kcmf.2020.22/2823
1. Introduction
Over recent years, a lot of attention has been paid to the synthesis of binary and ternary compounds with boron-oxygen groups. They include a great number of phases with functionally relevant (luminescence, nonlinear-optical, etc.) properties [1, 2].
The study of phase equilibria in systems Rb2O - RE2O3 - B2O3 (RE = Nd, Eu, Ho) revealed two families of isostructural double borates: Rb3REB6O12 [3, 4] and Rb3RE2B3O9 [5]. The present paper continues our systematic research aimed at the identification, preparation, and comprehensive characterisation of double borates
El Evgeniy V. Kovtunets, e-mail: kovtunets@binm.ru
with an alkali metal cation and a rare-earth element in their composition.
The study of the phase formation in K2O-Eu2O3-B2O3 resulted in obtaining the K3Eu3B4O12 compound, isostructural K3Gd3B4O12, for the first time [6].
2. Experimental
Ultra pure K2CO3, Eu2O3 and H3BO3 were used for the synthesis. High temperature annealing was conducted in a Naberthern L3/11/P320 programmable laboratory furnace, cooling was carried out inertially in the furnace.
XRD patterns for the synthesised sample were obtained at room temperature using
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a D8 ADVANCE Bruker AXS powder X-ray diffractometer with a Vantec-1 detector (CuK"a-radiation recording interval 20 = 5-100°, scanning step 0.02076°).
The TOPAS 4.2 software suite was used to analyse the experimental data and to conduct the Rietveld refinement of the obtained compound [7]. All peaks in the K3Eu3B4O12 X-ray powder diffraction patterns were indexed satisfactory by a monoclinic cell (space group P2/c).
The luminescence spectra were recorded with a SDL-1 (LOMO) double monochromator with 600 lines/mm-1 grating and a FEU-106 photoelectron multiplier, excitation was carried out using a high-pressure 150 W DKSH-150 xenon arc lamp through a MDR-2 monochromator with a diffraction grating of 1200 lines/mm-1. A transparent undoped LiF crystal, used as a substrate for the sample, was fixed in a holder.
The K3Eu3B4O12 synthesis was conducted using ceramic technology by stepped annealing of stoichiometric proportions of reagent mixtures.
Calcium carbonate and europium oxide were annealed at 800 °C for 24 hours to remove water. The reaction mixture was carefully ground in an agate mortar for better homogenisation, gradually heated to 200 and 600 °C at 1 °C/min, and incubated for 5 hours at those temperatures. Then, the sample was homogenised and further annealed at 750-850 °C for 48 hours (with intermediate grinding every 8 hours of the heat treatment).
3. Results and discussion
The synthesised sample of double borate is a dry white powder.
The powder X-ray diffraction patterns were used for the Rietveld refinement of the crystal structure of K3Eu3B4O12 The K3Gd3B4O12 structure was used to define the positions of atoms for the initial model. They were refined by isotropic approximation with "soft" limitations of the B-O distance and the B-O-B bond angles. The refinement was carried out by gradually adding refined parameters with the constant graphical simulation of the background. The Pearson VII Function was used to describe the shape of peaks. Isotropic displacement parameters (Biso) for the Eu and K atoms were refined separately, while for the O and B atoms they were taken as equivalent.
The refinement process included corrections for the sample preferred orientation and anisotropy broadening of peaks within the model of spherical harmonics [8]. Occupancy refinement of the positions of the disordered oxygen atoms O1, O9, and O9p was not conducted, as the attempts to refine occupancy of the disordered atoms did not contribute to a better description of the x-ray diffraction patterns.
The refinement results for K3Eu3B4O12 are shown in Table 1, coordinates of atoms and isotropic displacement parameters are given in Table 2, and calculated and experimental x-ray diffraction patterns with difference curves are shown in Fig. 1.
The structure of the synthesised compound is characterised by the [Eu8(BO3)8]œ sheets which are almost parallel to the ab plane. These sheets are formed by pentagonal EuO7 bipyramids, EuO6 octahedras, and BO3 triangles attached to them through common vertices (see Fig. 2). Neighbouring layers are interconnected via pentagonal EuO7 bipyramids, BO3 triangles, and potassium cations.
The luminescence centres in the studied phases are the Eu3+ [9] ions with intensive red luminescence. All the spectra have the five bands characteristic of transitions in the 4/-configuration of the Eu3+ ion from the excited metastable
Table 1. K3Eu3B4O12 crystallographic characteristics and structure refinement parameters
Space group: P2/c
a, Â 10.6727(7)
b, Â 8.9086(6)
c, Â 13.9684(9)
b, ° 110.388(2)
V, Â3 1244.90(14)
Z 4
20-interval, ° 8-100
No. of reflections 1295
No. of refined parameters 120
R , % wp7 1.77
R , % p 1.39
R , % exp7 1.45
c2 1.22
RB, % 0.55
Fig. 1. The measured (circles) and calculated (line), differential, and stroke x-ray diffraction patterns of
K3EU3B4°12
Table 2. Fractional atomic coordinates and isotropic displacement parameters (E2) of K3Eu3B4O12
Atom x y z Occupancy B. , Â2 iso7
Eu1 1/2 0.6925 (8) 1/4 1 0.2 (4
Eu2 0.7884 (6) 1.3014 (5) 0.2502 (4) 1 1.0 (3
Eu3 0.6643 (6) 0.9684 (7) 0.1195 (4) 1 1.0 (3
Eu4 1 0.7648 (8) 1/4 1 1.3 (4
K1 0.418 (2) 0.357 (2) 0.095 (1) 1 1.5 (6
K2 0.887 (2) 1.431 (2) 0.042 (1) 1 1.4 (7
K3 0.798 (2) 0.935 (2) 0.402 (1) 1 1.5 (7
B1 1/2 1.111 (3) 1/4 1 2.0 (15
B2 0.737 (2) 0.594 (2) 0.228 (2) 1 2.0 (15
B3 0.877 (3) 0.758 (3) 0.016 (2) 1 2.0 (15
B4 0.626 (2) 1.221 (2) -0.008 (2) 1 2.0 (15
B5 1 1.101 (3) 1/4 1 2.0 (15
O1 0.517 (3) 0.960 (2) 0.233 (2) 0.5 0.9 (5
O2 0.392 (3) 1.156 (2) 0.274 (3) 1 0.9 (5
O3 0.641 (2) 0.499 (3) 0.236 (2) 1 0.9 (5
O4 0.854 (2) 0.539 (3) 0.220 (3) 1 0.9 (5
O5 0.713 (3) 0.751 (4) 0.230 (3) 1 0.9 (5
O6 0.847 (3) 0.712 (4) -0.086 (2) 1 0.9 (5
O7 0.999 (2) 0.721 (4) 0.088 (2) 1 0.9 (5
O8 0.784 (2) 0.840 (4) 0.042 (2) 1 0.9 (5
O9 0.502 (6) 1.154 (6) -0.027 (3) 0.537 0.9 (5
O9p 0.567 (5) 1.089 (3) -0.058 (1) 0.463 0.9 (5
O10 0.628 (3) 1.348 (2) -0.067 (1) 1 0.9 (5
O11 0.722 (3) 1.214 (2) 0.089 (2) 1 0.9 (5
O12 0.891 (2) 1.025 (3) 0.253 (2) 1 0.9 (5
O13 1 1.254 (2) 1/4 1 0.9 (5
-Q
CO
U)
c Q) "c
3500
3000
£ 2500 -c
3
2000 -I 1500 1000 500
0
Fig. 2. The crystal structure of K3Eu3B4O12
5Do - -7F2
l Xex = 395 nm
5D0 7Fi
\ 5D0 — 7F4
\ I 5Do->7F3 /
5Do-7Fo I Ja I U^x/
1
450 500 550 600 650
Wavelength, (nm)
Fig. 3. Luminescence spectra of K3Eu3B4O12, excitation wavelength of 395 nm
700 750
state 5D0 to the 7Fj multiplet components (J = 0, 1, 2, 3, 4). Fig. 3 shows the emission spectrum K3Eu3B4O12, and Table 3 provides wavelengths of the Eu3+ constituent spectra.
When studying the structure of luminescent materials, the 5D0^7F2 transition of 4/-configura-tion of the Eu3+ ion is of great interest. The fact that there is only one peak within the range of this transition which is characterised by an insignificant broadening indicates close resemblance of the crystallochemical environment of the four sites of the Eu3+ ion in a unit cell. The ratio of maximum intensities of the bands corresponding to the transitions (5D0^7F2)/(5D0^7F1) is 4.482, which
Table 3. Wavelengths (nm) of the Eu3+ constituent spectra of K3Eu3B4O12
5D ^ 7F 0 0 5D ^ 7F 0 1 5D ^ 7F 0 2 5D ^ 7F 0 3 5D ^ 7F 0 4
585.6
586.5
578.1 590.2 610.4 650.7 701.9
592.6 593.8 596.2 619.8 656.5 703.0
indicates a low symmetry of the Eu3+ ion sites in the crystalline structure of K3Eu3B4O12 [10]. This agrees with the results of the structural study.
The excitation spectra (see Fig. 4) have bands of europium 4/6-4/6-transitions from the ground state to the upper levels: 7F0 ^ 5H5 (320.0 nm), 7D0 ^ 5D4 (363.6 nm), 5D0 ^ 5L7 (383.8 nm), 7F0 ^ 5L6 (395.5 nm), 7F0 ^ 5D2 (466.7 nm). The two narrow resonance bands corresponding to the
7Fn ^ 5L, m 7Fn ^ 5D„ transitions are the most
0 6 0 2
intensive.
4. Conclusions
Thus, a ceramic technology and stepped annealing were used to synthesise a new double borate of potassium-europium K3Eu3B4O12. According to the results of the Rietveld refinement of the crystal structure, the synthesised phase is isostructural to the K3Gd3B4O12 compound and crystallises in the monoclinic syngony with unit cell parameters a = 10.6727(7) A, b = 8.9086(6) A, c = 13.9684(9) A, p = 110.388(2)° (space group P2/c).
Luminescent properties of K3Eu3B4O12 were studied. Luminescence is due to optical
Fig. 4. The excitation spectra of K3Eu3B.O, emission wavelength of 610 nm
transitions in the 4f-configuration of the Eu3+ ion. The brightest luminescence of a bright red monochromatic colour is seen in the 5D0^7F2 electric dipole transition band, which is convenient for the production of screen luminophores with colours of high contrast.
Acknowledgements
XRD data were obtained using the equipment of the Centre for Collective Use of Buryat Scientific Centre of Siberian Branch of Russian Academy of Sciences. Luminescence spectra were obtained using the equipment of the Centre for Collective Use of Vinogradov Institute of Geochemistry of Siberian Branch of Russian Academy of Sciences.
Conflict of interests
The authors declare that they have no known competing financial interests or personal relationships that could have influenced the work reported in this paper.
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Information about the authors
Evgeniy V. Kovtunets, Postgraduate Student, Laboratory of Oxide Systems, Baikal Institute of Nature Management, Siberian Branch of the Russian Academy of Sciences, Ulan-Ude, Russian Federation; e-mail: kovtunets@binm.ru.ORCID iD: https://orcid.org/0000-0003-1301-1983.
Alexey K. Subanakov, PhD in Chemistry, Senior Researcher, Laboratory of Oxide Systems, Baikal Institute of Nature Management, Siberian Branch of the Russian Academy of Sciences, Ulan-Ude, Russian Federation; e-mail: subanakov@binm.bscnet.ru. ORCID iD: https://orcid.org/0000-0002-1674-283X.
Bair G. Bazarov, DSc in Chemistry, Leading Researcher, Laboratory of Oxide Systems, Baikal Institute of Nature Management, Siberian Branch of the Russian Academy of Sciences, Ulan-Ude, Russian Federation; e-mail: bazbg@rambler.ru. ORCID iD: https://orcid.org/0000-0003-1712-6964.
All authors have read and approved the final manuscript.
Translated by Irina Charychanskaya
Edited and proofread by Simon Cox
