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9Condensed Matter and Interphases (Kondensirovannye sredy i mezhfaznye granitsy)
Original articles
DOI: https://doi.org/10.17308/kcmf.2020.22/2831 ISSN 1606-867X
Received 28 April 2020 elSSN 2687-0711
Accepted 15 May 2020 Published online 25 June 2020
Spectral-Luminescent Properties of Terbium-Containing Zirconomolybdates
©2020 B. G. Bazaroveab, R. Yu. Shendrikcd , Yu. L. Tushinovaab, D. O. Sofichc, J. G. Bazarovaa
aBaikal Institute of Nature Management, Siberian Branch of the Russian Academy of Sciences, 6 ul. Sakhyanovoy, Ulan-Ude 670047, Republic of Buryatia, Russian Federation
bBanzarov Buryat State University,
24a ul. Smolina, Ulan-Ude 670000, Republic of Buryatia, Russian Federation Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, 1a ul. Favorskogo, Irkutsk 664033, Russian Federation
dIrkutsk State University,
20 bulvar Gagarina, Irkutsk 664003, Russian Federation Abstract
To date, double molybdates of mono- and tetravalent elements have been comprehensively studied, and systems with molybdates of mono- and trivalent elements have been studied quite thoroughly. Some materials based on double molybdates, for example, those containing lanthanides, are considered promising for laser technology and electronics. Meanwhile, there is limited information on the properties, especially optical ones, of the molybdates containing rare-earth elements and zirconium. The aim of this work was to study the luminescent properties of self-activated terbium-containing zirconomolybdates with the compositions Tb2Zr3(MoO4)9 (1:3) and Tb2Zr(MoO4)5 (1:1), crystallising in two different structural types.
Powder samples of the studied molybdates were synthesised by ceramic technology. The absorption, excitation, and emission spectra were measured using a Perkin Elmer Lambda 950 spectrophotometer. Luminescence was excited by a 250 W DKSSh-250 xenon lamp through an MDR-2 monochromator and recorded using an SDL-1 double monochromator with a grating of 600 lines/mm. The optical properties of new zirconium molybdates containing Tb3+ ions were studied. They revealed bright luminescence in the green spectral region due to the transitions inside the 4f shell of the rare-earth Tb3+ ion, excited both in the bands associated with the 4f-4f transitions and in the band with a charge transfer. The observed spectral lines as well as luminescence and excitation bands were identified.
It was shown that the position of the wide excitation band associated with the "charge transfer" transitions from O2- in MoO42- groups via Mo-O bonds to luminescent centres (Tb3+) does not depend on the matrix structure. The structure and intensity of the observed spectral lines, indicating a low symmetry of the Tb3+ crystalline environment, correlate with the structural analysis data. The results obtained in this work can be used when creating promising phosphors in the green spectral region under ultraviolet excitation.
Keywords: solid-phase synthesis, luminescence, terbium-containing zirconomolybdate.
Funding: The study was conducted within the framework of the state order by the Baikal Institute of Nature Management, Siberian Branch of the Russian Academy of Sciences, and partially funded by the Russian Foundation for Basic Research (project No. 18-08-799a).
For citation: Bazarov B. G., Shendrik R. Yu., Tushinova Yu. L., Sofich D. O., Bazarova J. G. Spectral-luminescent properties of terbium-containing zirconomolybdates. Kondensirovannye sredy i mezhfaznye granitsy = Condensed Matter and Interphases. 2020;22(2): 197-203. DOI: https://doi.org/10.17308/kcmf.2020.22/2831
Kl Bair G. Bazarov, e-mail: bazbg@rambler.ru
The content is available under Creative Commons Attribution 4.0 License.
1. Introduction
Since lantanoids luminesce in the UV and in visible and near-infrared regions, they can be used in various fields: laser and fibre optics technology, medical diagnostics, and the creation of scintillators and luminophores.
The electrons in lantanoids, located on the 4f shell, are screened by the outer 5s2 and 5p6 shells. As a result, the position of the energy levels is weakly dependent on the environment. In this case, the energetic states of the sublevels are completely determined by the immediate
Fig. 1. Phase diagram of the system Tb2(MoO4)3 Zr(MoO4)2
environment of rare-earth ions due to the Stark splitting effect.
The studies aimed at the search for new materials for matrices activated by rare-earth ions are considered relevant. There are works of Russian and foreign scientists [1-6] dedicated to the study of luminescent properties of double zirconium molybdates and lantanoids, although the luminescent properties of zirconomolybdates with Tb3+ of the Tb2Zr(MO4)5 composition were not studied.
Our study of the Tb2(MoO4)3-Zr(MoO4)2 system allowed establishing for the first time the formation of three new molybdates with the following compositions: Tb2Zr3(MoO4)9 (1:3), Tb2Zr2(MoO4)7 (1:2), and Tb2Zr(MoO4)5 (1:1) (Fig. 1) [7].
The structures of the first two molybdates, 1:3 (space group R3c, Z = 6) and 1:2 (space group C2/c, Z = 4), were determined for monocrystals (Fig. 2a, b) [8-10].
The structure of the 1:1 molybdate was determined through the use of isostructural Er2Zr(MoO4)5, the Rietveld refinement method, and principles of derivative difference minimisation (Fig. 3) [11].
The aim of this work was to study the luminescent properties of self-activated terbium-containing zirconomolybdates with the 1:3 and 1:1 compositions, crystallising in two different structural types.
a b
Fig. 2. Part of the structure Ln2Zr3(MoO4)9 (space group R3c, Z = 6) (Ln = Nd) (a); Part of the structure Ln2Zr2(MoO4)7 (space group C2/c, Z = 4) (b)
Fig. 3. Part of the structure Ln2Zr(MoO4)s (space group Cmc21, Z = 2)
2. Experimental
The absorption, excitation, and emission spectra of Tb3+ were measured in two terbium-containing matrices Tb2Zr(MoO4)5 (space group Cmc21, Z = 4) and Tb2Zr3(MoO4)9 (space group R3c, Z = 6). The molybdates were obtained by ceramic technology [7].
In order to study the optical properties of the investigated samples, the absorption, emission, and excitation spectra were recorded in the integrating sphere at various temperatures.
The absorption spectra were recorded using a Perkin Elmer Lambda 950 spectrophotometer with an integrating sphere. When recording the absorption spectra, the studied sample
was poured into a KU-1 quartz-glass ampoule and placed inside the integrating sphere. The absorption of the test glass was subtracted from the absorption spectra.
Luminescence was excited in the spectral interval of 200-500 nm by a 250 W DKSSh-250 xenon lamp through an MDR-2 monochromator with a ruled grating of 1200 lines/mm. The emissions were recorded using an SDL-1 double monochromator with a grating of 600 lines/mm. The spectral dimension of the monochromator slits varied from 1.2 nm to 0.3 nm. The measurements at temperature 77 К were conducted in the evacuated cryostat. The excitation spectra were corrected by the lumogen excitation spectra.
3. Results and discussion
Two types of bands were observed in the excitation spectra of the studied samples: narrow bands, corresponding to the transitions inside the 4f shell of the rare-earth ion, and wide bands, associated with the bands of charge transfer in the MoO4" complexes to the rare-earth element.
Intensive luminescence was observed in the green spectral region in Tb2Zr(MoO4)5 upon the excitation in the UV region (Fig. 4).
The emission spectrum upon the excitation in the band with the energy 26500 cm-1 (l = 377 nm), measured at a temperature of 77 К, is presented in Fig. 4 (curve 1). The bands observed in the
Fig. 4. Emission (curve 1) and excitation (curve 2) spectra of the Tb2Zr(MoO4)5 sample measured at temperature 77 K
spectrum are related to the electronic transitions inside the 4/ shell from the 5D4 term to the 7Fj (J = 1-6) terms. The greatest intensity in the emission spectrum was found in the band with the maximum in the 18500 cm-1 region (l = 540 nm). The band is related to the magnetic dipole transition of 5D4-7F5. The intensity of this transition changes only slightly depending on the value of the crystalline field. The observed band is split into three lines with the energies of 18280, 18405, and 18460 cm-1.
The luminescence band with the 20500 cm-1 maximum (l = 488 nm) is associated with the electronic dipole transition of 5D4-7F6 in the Tb3+ ion, which is environment-sensitive (but not hypersensitive) and depends on the symmetry of the crystalline field Transitions of 5D4 - 7F1 in the emission of the Tb3+ ion have low intensity. Band intensities related to the /"-/"transitions diminish with a decreasing value of J in the following way: 5D4 ^ 7F6> 7F4> 7F3> 7F2. The presence of the thin structure in the emission spectra of the 5D4-7FJ transitions in terbium ions is associated with their sensitivity to the ligand environment.
The 5D4-7F6 band is more intense as compared to the intensities of other bands (except for 5D4-7F5) and is split into three peaks, which can be indicative of spacial distortion of the nine-peak TbO9 with symmetry decreased to C2v [12], which correlates with the data of the structure. The presence of intensive lines of magnetic dipole and
electronic dipole transitions in the spectrum is also indicative of the presence of several various types of ligands [13].
The emission was excited in the band with energy of 26500 cm-1 (l = 377 nm), corresponding to the 4F0-5D3 transition, and the excitation spectrum was measured for the band with energy of 18500 cm-1 (l = 540 nm), corresponding to the 5D4-4F5 transition. Vertical lines show the energies of the term of the Tb3+free ion. A row of thin bands was observed in the excitation spectrum (Fig. 4, curve 2) that are related to the transitions from the ground state 7F0 to the states split by spinorbital interaction of the 4f term. The band in the 37000 cm-1 region (l = 270 nm) is associated with the charge transfer transition in the (MoO4)2-complexes. The emission spectrum excited in this band is almost the same as the spectrum excited in the region of 4f-4f transitions.
The absorption spectrum of Tb2Zr3(MoO4)9 is presented in Fig. 5 [2]; it consists of a wide absorption band in the ultraviolet region and one narrow low-intensity peak, related to the 4f-4f transition from the ground state of terbium ions 7F6 to the lower excited state 5D4. The Tb3+ transitions are characterised by the low force of the oscillator. As a result, most of the bands of intracentre transitions in the absorption spectrum are not visible as compared to other absorption bands.
Fig. 6 [2] shows the excitation and luminescence spectra Tb2Zr3(MoO4)9. Intensive narrow emission
420 490
Wavelength .A., nm
Fig. 5. Absorption spectrum of Tb2Zr3(MoO4)9
5550 45 40
M M I
"I-1-1-1-1-1-1-1—7/
200 250 300 350 450 500 550 Длина волны, нм
600
650
Fig. 6. Excitation (a) and emission spectra (b) of Tb„Zr,(MoO.)9 at liquid nitrogen temperature
bands were observed in the region of480-680 nm (20800-14700 cm-1), which is typical for the Tb3+ transitions from the D4 level to the lower 7Fj levels (J = 0,1,2,3,4,5). Transitions from the 7F6 ground state were observed in the excitation spectrum. Upon the excitation in the 4f-4f bands, the greatest intensity of luminescence was achieved with the excitation wavelength of 380 nm (7F6-5D3 transition). In the region of 300 nm (33300 cm-1) a wide intensive excitation band was observed. Seven narrow lines in the emission spectrum belong to the transitions Tb3+: 5D4-7F6 (electronic dipole transition, 488 nm (205004 cm6-1)), 5D4-7F5 (magnetic dipole transition, 540 nm (18500 cm-1)), 5D4-7F4 (582 nm (17180 cm-1)), 5D4-7F3 (618 nm (16180 cm-1)), 5D4-7F2 (644 nm (15530 cm-1)), 5D4-7Fj (663 nm (15080 cm-1)), and 5D4-7F0 (673 nm (14860 cm-1)). The most intensive line reaching the peak at 540 nm (18500 cm-1) is responsible for the green colour of Tb2Zr3(MoO4)9.
Decay times of luminescence, corresponding to different transitions inside the f shell with different lengths of the excitation waves, were measured at temperatures 297 K and 77 K (Table).
4. Conclusions
As a result of the conducted studies, we can draw the following conclusions:
Table. Decay times of principal transitions of Tb3+ at 297 and 77 K
(5D4-7FJ) Wave length (nm) Decay time (ps)
J Emission Excitation 297 К 77 К
270 420 400
6 488 352 390 360
370 430 400
380 430 390
270 420 400
5 540 352 460 410
370 450 400
380 450 420
290 420 400
4 582 352 500 390
370 420 370
380 420 410
1. Spectral-luminescent properties of terbium-containing zirconium molybdates of two compositions (1:3 and 1:1) and structures (R3c, Z = 6 and Cmc2p Z = 4) were studied. The observed spectral lines as well as luminescence and excitation bands were identified. Specific features of the matrix structure determine the spectral-luminescent properties of Tb3+ ions.
2. The comparison of the excitation spectra of terbium-containing molybdates with different
structures showed that the position of the wide excitation band associated with the "charge transfer" transitions from O2- in MoO42- groups via Mo-O bonds to luminescent centres (Tb3+) does not depend on the matrix structure and the nature of REE.
3. The structure of the band associated with the electronic dipole transition 5D4-7F6 in the Tb3+ ion is indicative of spacial distortion of TbO9 with decreasing symmetry. The presence of intensive lines of magnetic dipole (5D4-7F5) and electronic dipole (5D4-7F6) transitions is also indicative of the presence of low symmetry. All this data correlates with the data of the structural analysis.
4. The results obtained in this work can be used when creating promising phosphors in the green spectral region under ultraviolet excitation.
Acknowledgements
The studies were carried out using the scientific equipment of the Centre for Collective Use of Equipment "Isotope-geochemical research of the Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences" and of the Centre for Collective Use of Equipment of the Baikal Institute of Nature Management, Siberian Branch of the Russian Academy of Sciences.
Conflict of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Information about the authors
Bair G. Bazarov, DSc in Physics and Mathematics, Leading Researcher, Laboratory of Oxide Systems Baikal Institute of Nature Management, Siberian Branch of the Russian Academy of Sciences (BINM SB RAS), Associate Professor at the Department of Inorganic and Organic chemistry, Banzarov Buryat State University, Ulan-Ude, Russian Federation; email: bazbg@rambler.ru. ORCID iD: https://orcid. org/0000-0003-1712-6964.
Roman Yu. Shendrik, PhD, Senior Researcher of Single Crystal Lab, A. P. Vinogradov Institute of Geochemistry Siberian Branch of the Russian Academy of Sciences, Irkutsk, Russian Federation, e-mail r.shendrik@gmail.com. ORCID iD:https://orcid. org/0000-0001-6810-8649.
Yunna L. Tushinova, PhD in Chemistry, Researcher Fellow, Laboratory of Oxide Systems, Baikal Institute
of Nature Management, Siberian Branch of the Russian Academy of Sciences, Associate Professor at the Department of Inorganic and Organic Chemistry, Banzarov Buryat State University, Ulan-Ude, Russian Federation; e-mail: tushinova@binm.ru. ORCID iD: https://orcid.org/0000-0003-1032-8854 .
Dmitriy O. Sofich, Junior Researcher of Single Crystal lab, A. P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, Irkutsk, Russia Federation, e-mailsofich-dmitriy@live. com. ORCID iD: https://orcid.org/0000-0002-2836-3597.
Jibzema G. Bazarova, DSc in Chemistry, Chief Scientist, Laboratory of Oxide Systems, Baikal Institute of Nature Management, Siberian Branch of the Russian Academy of Sciences, Ulan-Ude, , Russian Federation; e-mail Jbaz@binm.ru.. ORCID iD: https://orcid. org/0000-0002-1231-0116.
All authors have read and approved the final manuscript.
Translated by Marina Strepetova
Edited and proofread by Simon Cox
