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48ISSN 2072-5981 doi: 10.26907/mrsej
aänetic Resonance in Solids
Electronic Journal
Volume 21 Special Issue 4 Paper No 19415 1-6 pages
2019
doi: 10.26907/mrsej-19415
http: //mrsej. kpfu. ru http: //mrsej. ksu. ru
Established and published by Kazan University Endorsed by International Society of Magnetic Resonance (ISMAR) Registered by Russian Federation Committee on Press (#015140),
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"Magnetic Resonance in Solids. Electronic Journal" (MRSey) is a
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Editors
Vadim Atsarkin (Institute of Radio Engineering and Electronics, Moscow) Yurij Bunkov (CNRS, Grenoble) Mikhail Eremin (KFU, Kazan) David Fushman (University of Maryland, College Park) Hugo Keller (University of Zürich,
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Yoshio Kitaoka (Osaka University,
Osaka)
Boris Malkin (KFU, Kazan) Alexander Shengelaya (Tbilisi State University, Tbilisi) Jörg Sichelschmidt (Max Planck Institute for Chemical Physics of Solids, Dresden) Haruhiko Suzuki (Kanazawa University, Kanazava) Murat Tagirov (KFU, Kazan) Dmitrii Tayurskii (KFU, Kazan) Valentine Zhikharev (KNRTU,
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* In Kazan University the Electron Paramagnetic Resonance (EPR) was discovered by Zavoisky E.K. in 1944.
Short cite this: Magn. Reson. Solids 21, 19415 (2019)
doi: 10.26907/mrsej-19415
Paramagnetic centres in crystals YAlO3: Eu, Si
V.A. Vazhenin1, A.V. Fokin1'*, A.P. Potapov1, G.S. Shakurov2, A.G. Petrosyan3, M.Yu. Artyomov1
1Ural Federal University, Institute of Natural Sciences and Mathematics, Mira 19, Ekaterinburg 620002, Russia 2Zavoisky Physical-Technical Institute, FRC Kazan Scientific Center of RAS,
Sibirsky trakt 10/7, Kazan 420029, Russia 3Institute for Physical Research, National Academy of Sciences of Armenia,
Ashtarak-2 0203, Armenia * E-mail: Andrej.Fokin@urfu.ru (Received May 26, 2019; accepted May 28, 2019; published June 6, 2019)
In the spectra of the electron paramagnetic resonance of YAlO3:151Eu single crystals, the signals of the Eu2+, Gd3+, Cr3+, and Mo3+ paramagnetic centers were observed. Using high-frequency EPR, it was shown that the signals of Mo3+ ions observed in the X-range belong to two intra-doublet transitions of the same center with S = 3/2.
PACS: 76.30.-v, 76.30.He.
Keywords: yttrium aluminium perovskite, insulators, point defects, EPR.
Dedicated to Boris Malkin on the occasion of his 80th birthday
1. Introduction
Interest in the study of YAlO3 crystals doped with ions of the iron group and rare earths is associated with their applications in laser physics and optoelectronics. To optimize the characteristics of these materials, information about the nature, structure, and number of defects in real crystals is needed. The presence of uncontrolled impurities during the growth can lead to changes in the properties of single crystals, and therefore their concentration is sought to be minimized. For yttrium orthoaluminate crystals grown in molybdenum containers, molybdenum ions can be such an uncontrolled impurity. One of the direct and most informative methods for obtaining information about paramagnetic impurities is the electron paramagnetic resonance (EPR) method.
The EPR spectra of Mo3+ ions (electron configuration is 4d3, the electron and nuclear spins are S = 3/2, I = 5/2) in YAlO3 doped with Er, Nd, Ce were first studied in [1]. The authors of [2] suggested that the Mo3+ spectra observed by them in YAlO3 belong to two paramagnetic molybdenum centers with large zero-field splittings, one of which is caused by the Mo3+ ion in the aluminum position, and the second by the Mo3+ ion in the same position, but with a nearby defect. The orientational behavior of the signals of these Mo3+ centers was described in [2] in three crystallographic planes by rhombic spin Hamiltonian with an effective spin S = 1/2.
In [3,4] monoclinic centers Eu2+ and Gd3+ (S = 7/2) were observed in YAlO3, and the parameters of the fine structure were determined. However, the crystal was doped with europium with a natural abundance of isotopes, and its EPR spectrum demonstrated a complex weakly resolved hyperfine structure.
In the present work, the YAlO3 sample doped with europium enriched with the 151Eu isotope is studied at different frequencies by the EPR method.
2. Materials and methods
The crystals were grown by the method of vertical directional crystallization, using crystalline sapphire (99.95%), high-purity yttrium oxide (ITO-B brand) and oxide of the europium isotope 151Eu2Os (97.5% 151Eu and 2.5% 153Eu). The SiO2 oxide was additionally introduced into the melt to stabilize the Eu2+ centers in the lattice. The composition of the initial melts corresponded to Y1-xEuxAl1-ySiyO3 (x = 0.02; y = 0.04). The quality control was carried out on a polarization microscope MPS-2. The presence of europium ions in crystals (Eu2+ and Eu3+) was observed in the absorption spectra recorded on a SPECORD200 PLUS spectrophotometer for the f-f transitions of Eu3+ ions and the charge-transfer band Eu3+-O2-.
The EPR spectra were measured at room temperature using an EMX Plus Bruker X-band spectrometer. The orientation of the sample in the magnetic field was carried out using a standard automatic goniometer and a device for rotating the sample in the plane perpendicular to the axis of rotation of the goniometer. For measurements in the high-frequency range at 4.2 K, a home-built wide-band EPR spectrometer based on microwave oscillators (backward-wave tubes) was used.
3. Results and discussion
Yttrium orthoaluminate crystals have a distorted perovskite structure with the space group Pbnm (D2h ) [5]. The unit cell parameters are a = 5.176 A, b = 5.332 A, c = 7.356 A [6]. In the EPR spectra of the samples under study at room temperature in the orientations: B||a, B||b, B||c (Fig. 1, B - magnetic field induction, a, b, c - crystallographic axes), intense groups of signals are observed, which can be attributed to the centers Cr3+, Eu2+, Gd3+ and Mo3+. Impurity ions of molybdenum and chromium replace aluminum Al3+ in the crystal lattice and have magnetic multiplicity of 4 (local symmetry group 1(Cj)). These positions are related to each other by the reflection operation in the planes normal to the axes a, b, c as a result of
B||c '
0 100 200 300 400 500
B, mT
Figure 1. The EPR spectrum of YAlO3 doped with 151Eu2+ at B||c, T = 300K; the frequency is 9833 MHz, the lower arrows indicate the transitions of the Cr3+ centers, the upper ones are Mo3+; the bottom part of the figure shows the result of calculating the positions and integral intensities of the transitions: the blue bars are for Gd3+, the red ones are for the centers of the hyperfine structure of the Eu2+ transitions.
which two pairs of Cr3+ and Mo3+ centers become equivalent in the indicated planes. This leads to the fact that in the orientations of the magnetic field parallel to the crystallographic axes B || a, b, c the signals of the four centers merge.
The rare-earth impurity ions Eu2+ and Gd3+ (electron spin S = 7/2) replace the Y3+ ions (the point symmetry group is m (Cs)) and have magnetic multiplicity of 2. If the charge compensation for Eu2+ by silicon ions is nonlocal, then there will be two magnetically nonequivalent centers that become equivalent in the ca and cb planes. The four positions of yttrium ions in the crystal structure are pairwise connected by inversion and reflection operations in these planes.
The signals of the listed paramagnetic centers were identified taking into account their hyper-fine structure. For the orientation of the sample in a magnetic field, the angular dependences of the transition positions of the Cr3+, Mo3+ and Gd3+ paramagnetic centers were used.
The Eu2+ transitions overlap with each other and with the signals of Gd3+ or Cr3+, making it difficult to analyse the complex hyperfine structure of 151Eu2+ center (I = 5/2). The reason for the intense chromium signals in the sample can most likely be considered to be low quality molybdenum tubes used in growing, containing chromium contaminants.
The positions of aluminum ions with point symmetry [7], which are replaced by molybdenum ions Mo3+, are surrounded by six oxygen ions [8]. Since the charge states of the impurity and matrix ions are the same, the association of the molybdenum ion with any other defect in the aluminum position seems unlikely. At the same time, there are no valid arguments in favor of the existence of alternative localization of Mo3+ in the YAlO3 crystal lattice. Thereby, in contrast to [2], it was suggested that two transitions of molybdenum centers with magnetic multiplicity 4, observed in the X-range, belong to two intra-doublet transitions of one Mo3+ center with spin S = 3/2. To describe such a center, we used the spin Hamiltonian in the following definition [9]
Hsp = P (BgS) + 1/3 (620020 + 621021 + 622 022 + C21^21 + C22^22) , (1)
where g is the g-factor, P is the Bohr magneton, Onm are the Stevens spin operators [9], and bnm are the fine structure parameters. The parameters of the spin Hamiltonian in the x||a, y|| b, z|| c coordinate system, describing the experimental angular dependences, were obtained by the least-squares method on the set of experimental transition positions and are listed in the Table 1. To describe the positions and intensities of the Gd3+ and Eu2+ signals (Fig. 1), we used the monoclinic spin Hamiltonian in the definiton [9], which includes terms of the fourth rank with the parameters from [3]. With the parameters given in the Table 1, the angular dependence of the transition positions of the Mo3+ centers in the cb plane for the frequency of 9.8 GHz was built (Fig. 2). The splitting of the experimental angular dependences in Fig. 2 can be explained by a small deviation (« 3 deg) of the magnetic field from the cb plane.
The high-field transition (between levels 3-4 in the indicated orientation) in magnetic fields of about 1500 mT (see Fig. 2) was predicted by the obtained parameters. Due to the large zero-field splitting for Mo3+ centers, it is impossible to observe inter-doublet resonant transitions
Table 1. The spin Hamiltonian parameters of Mo3+ triclinic centers in the x||a, y||b, z||c coordinate system, T = 300K; 6nm, cnm, and F(N) are given in MHz, F(N) - the standard deviation of the calculated frequencies from experimental, N - the number of experimental signal positions, the sign of 62o was not determined [4].
gx gy gz 620 621 622 C21 C22 F (N)
1.970 1.975 1.970 -10350 -23030 -9940 113560 -20560 32(163)
0, deg
Figure 2. The orientational dependence of the positions of the resonant transitions of Mo3+ centers at a frequency of 9833 MHz when the magnetic field rotates near the zy (cb) plane, the points show the experimental data, the curves are the calculation (the red curves indicate the 3^4 transition, the blue ones are the 1^2 transition). The slight splitting of the observed signals is due to the deviation of the magnetic field from the bc plane.
B, mT
Figure 3. The frequency-field dependence of the inter-doublet resonant transitions of Mo3+ ions in a YAlO3 crystal at 4.2 K; the signals' splitting with an increase in the magnetic field is due to the existence of four magnetically nonequivalent centers.
in the X-band, which can adversely affect the accuracy of the determined experimental spin Hamiltonian parameters. Therefore, in order to obtain additional experimental information and to observe inter-doublet transitions, measurements of high-frequency EPR were performed.
In Fig. 3, the experimental frequency-field dependence of the inter-doublet resonant transitions of Mo3+ ions, at a temperature of 4.2 K, is shown. From this, we can determine the magnitude of the zero-field splitting, which turns out to be about 75.8 GHz. In turn, the calculation of the zero-field splitting using the parameters of the spin Hamiltonian determined from measurements in the three-centimeter range (see Table 1) for paramagnetic molybdenum centers
2500 2000 15 0 0
H
E
CQ
10 0 0
500
0 30 60 90
<9, deg
Figure 4. The angular dependence of the positions of the Mo3+ center transitions when the magnetic field rotates in the zy (cb) plane at 4.2 K at a frequency of 86250 MHz, the points are experimental values, the curves are calculation with parameters from the Table 1, the numbers indicate the energy level numbers.
showed the value of 75 GHz, which is close to the zero-field splitting obtained from the frequency-field dependence. Thus, despite the fact that high-frequency measurements and measurements in the X-range were performed at different temperatures (4.2K and 300K, respectively), it can be concluded that the parameters of the fine structure of the centers of Mo3+ ion predict well the splitting value in zero-field.
The angular dependences of the positions of the resonant transitions of molybdenum centers measured at a frequency of 86250 MHz at 4.2 K are shown in Fig. 4. The calculated curves demonstrate the quality of the description by the obtained parameters of the orientational behavior of the inter-doublet transitions of all four molybdenum centers. This, as well as experimental data from the X-range, confirm the adequacy of the chosen approach and allow us to conclude that only one Mo3+ triclinic center with spin S = 3/2, which is localized in the Al3+ position, exists in yttrium orthoaluminate.
4. Summary
In single crystals of yttrium aluminate YAlO3: 151Eu, the EPR spectra of centers with a complex hyperfine structure Cr3+, Eu2+, Gd3+, and Mo3+ were observed. Magnetic resonance measurements were performed at high frequencies and in the X-band. From the frequency-field dependence of the inter-doublet resonant transitions of the Mo3+ centers, the zero-field splitting for paramagnetic molybdenum centers was determined, the value of which is close to that calculated using the fine structure parameters. The angular dependence of the positions of the resonant inter-doublet transitions of Mo3+ centers at a frequency of 86.25 GHz, which are well described by the calculated curves, was measured.
All this, together with the results of the EPR of the three-centimeter range, confirms the conclusion that the observed signals of Mo3+ centers are due to two intra-doublet transitions of one center with electron spin S = 3/2.
Acknowledgments
The work was performed with a partial financial support from the Ministry of Science and Education of the Russian Federation (No. 3.6115.2017/8.9). ShakurovG.S. thanks the Program of Presidium of RAS No. 5. The measurements were carried out on the spectrometer of the Center for Collective Use of Modern Nanotechnologies of the Ural Federal University.
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