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DOI: 10.17516/1997-1397-2020-13-3-342-349 УДК 519.21
Magnetic Susceptibility and EPR Study
of Bi5Nb3_3XCo3xOi5-<5
Nadezhda A. Zhuk*
Pitirim Sorokin Syktyvkar State University Syktyvkar, Russian Federation
Vladimir P. Lutoev^
Institute of Geology, Komi Scientific Center UB RAS Syktyvkar, Russian Federation
Dmitriy S. Beznosikov*
Pitirim Sorokin Syktyvkar State University Syktyvkar, Russian Federation
Andrey N. Nizovtsev§
Institute of Biology, Komi Scientific Center UB RAS Syktyvkar, Russian Federation
Lubov V. Rychkova^
Pitirim Sorokin Syktyvkar State University Syktyvkar, Russian Federation
Received 27.02.2020, received in revised form 06.03.2020, accepted 16.04.2020 Abstract. Magnetic susceptibility, microstructure and EPR of cobalt-containing solid solutions with layered perovskite-like structure BigNb3_3^Co3^Oib-« have been studied. Solid solutions of Bi6Nb3_3^Co3^0i6-i (x < 0.005) can be crystallized in tetragonal syngony (sp. gr. P4/mmm), as cobalt content increases, monoclinic distortion of the unit cell emerges at 0.005 < x < 0.04 (sp. gr. P2/m). The formation of exchange-bound aggregates of Co(III) and Co(II) atoms predominantly with antifer-romagnetic exchange types has been found in the solid solutions. EPR indirectly confirms that cobalt ions are in octahedral positions of substitution of Nb(V) ions.
Keywords: ceramics, magnetic properties, EPR spectroscopy.
Citation: N.A.Zhuk, V.P.Lutoev, D.S.Beznosikov, A.N.Nizovtsev, L.V.Rychkova, Magnetic Susceptibility and EPR Study of BibNbs^^zOw-«, J. Sib. Fed. Univ. Math. Phys., 2020, 13(3), 342-349. DOI: 10.17516/1997-1397-2020-13-3-342-349.
The majority of bismuth-containing compounds with layered perovskite-like structure, analogues of the so called Aurivillius phases, are of practical and theoretical interest owing to their ferroelectric properties [1]. The composition of such compounds is described by the general formula (Bi2O2)(An_iB„O3„+i), where the bismuth-oxygen layers Bi2O2 consist of BiO4 pyramids bound to each other by base edges and An-1BnO3n+1 are perovskite-like fragments consisting of BO6 octahedra bound by vertices. The large cations A are located in the cubic octahedral sites between them [2,3]. The coefficient n in the formula corresponds to the number of BO6 octahedra forming the thickness of the perovskite-like fragment. Alongside with the layered compounds which contain the uniform perovskite-like fragments, there are the so-called mixed
* nzhuck@mail.ru https://orcid.org/0000-0002-9907-0898
tvlutoev@geo.komisc.ru https://orcid.org/0000-0003-0231-302X
t uvn71p3@gmail.com
§ nizovtsev@ya.ru ^ lyu-ba24@mail.ru © Siberian Federal University. All rights reserved
or hybrid layered compounds (Bi2O2)(A„_iB„O3„+i)... (Bi2O2)(Am_iBmO3m+i) [4-6]. Their structure consists of alternating perovskite-like fragments of various widths. Bismuth niobate Bi5Nb3O15 belongs to the group of mixed layered compounds. Its structure is characterized by the ordered alternating of fragments formed by one and two niobium-oxygen octahedra. Therefore, its structure can be described as (Bi2O2)(NbO4)(Bi2O2)(BiNb2O7), with n =1 and m = 3. Oxygen-niobium octahedra are bound by side vertices and are arranged in the a-b plane of the crystal, the Nb-O-Nb bond angle is 180°. The layers of the octahedra are separated by the bismuth-oxygen layers Bi2O2 formed by the BiO4 pyramids and joined by the base edges (Fig. 1). The present work discusses the results of the EPR-spectroscopy and static magnetic susceptibility measurements of electron state and the nature of exchange interactions between cobalt atoms in the solid solutions of bismuth niobate Bi5Nb3O15 with layered perovskite-like structure, which were obtained by heterovalent substitution of niobium (V) with cobalt atoms (II).
1. Experimental
The synthesis of the solid solutions was carried out by the standard ceramic procedure from special-purity grade bismuth(III), niobium(V), and cobalt(II) oxides at 650, 850, 950 and 1050 °C Phase composition of the products was determined by means of electron scanning microscopy (using a Tescan MIRA 3LM electron scanning microscope and a X-ACT energy-dispersive spectrometer) and X-ray diffraction analysis using a DRON-4-13 diffractometer (CuKa radiation). The cell unit parameters of solid solutions were calculated using the CSD software package [7]. The quantitative measurement of the composition of the solid solution samples was performed by atom-emission spectrometry (a SPECTRO CIROS ISP spectrometer). The magnetic susceptibility of the samples of the solid solutions was measured by the Faraday method in the temperature range of 77 — 350 K at 15 fixed temperatures and at the magnetic field strength of 724, 633, 523, and 364 mT. The semicommercial installation created in the laboratory of magne-tochemistry of St. Petersburg State University and consisting of an electromagnet, an electronic balance, and cryostate was used for the magnetic susceptibility measurements. The accuracy of relative measurements was 5%. EPR measurements of finely crushed ceramic samples of the Bi5Nb3_3KCo3KO15_5 solid solutions were carried out in the X-band RadioPAN SE/X 2547 spectrometer (IG Komi SC UB RAS) with 100 kHz field modulation at room temperature. The amplitude of the modulation was 0.25 mT, the microwave field power was 35 mW. A sample (near to 100 mg) was put into a thin-walled quartz test tube (internal diameter of 2.5 mm) together with the reference sample (anthracite, singlet line g0 = 2.003, peak to peak distance ABpp = 0.5 mT) in a ampoule. For each sample, the spectrum in the magnetic field range of 0 — 700 mT and the reference line g0 = 2.003 in the scan range of 5 mT were separately recorded. The intensity of the reference line served as a measure of the gain of the instrument and, when processing spectra, was used to accurately remove background signals from the test tube and ampoule. The spectra were normalized to the reference line intensity and then to 100 mg of the sample.
2. Results and discussion
The cobalt-containing solid solutions Bi5Nb3_3a;Co3a;O15_^ have been studied with 0.005 < x < 0.04. The single-phase nature of the samples was proved by the methods of scanning electron microscopy and X-ray analyses. Solid solutions of Bi5Nb3_3KCo3KO15_a (x < 0.005) can be crystallized in tetragonal syngony (sp. gr. P4/mmm), unit cell parameters with x= 0.005 are: a= 0.5464, c= 2.093 nm (Bi5Nb3O15, sp. gr. P4/mmm, a = 0.547, c= 2.097 nm [6]); as cobalt content increases, monoclinic distortion of the unit cell emerges at 0.005 < x ^ 0.04 (Fig. 1).
Fig. 1. The unit cell of Bi5Nb3O15 and a possible type of distortion of the polyhedron of cobalt (II) atoms
Fig. 1 shows the (Bi5Nb3O15 unit cell, a possible type of distortion of the polyhedron of cobalt (II) atoms is seen to the right of the crystal lattice, given the fact that it represents Jahn-Teller ions. Monoclinic distortion of the tetragonal cell of the solid solutions Bi5Nb3O15 was established in previous works [6,8] and is associated with formation of atomic defects in the structure. The X-ray patterns of the solid solutions were interpreted based on the space group P 2/m [6]. The unit cell unit cell parameters with x= 0.04 are: a= 0.5463 nm, c= 2.084 nm, b= 0.5454 nm, the a angle changes from 90° to 90.7°. Fig. 2 shows the surface of samples of Bi5Nb3_3a;Co3a;O15_,s (x= 0.01, 0.02, 0.04) obtained as secondary or elastically reflected electrons.
Based on the scanning electron microscopy data, the samples are porous compacts with merged melted fine grains 1-3 pm. Using the measured magnetic susceptibility of the solid solutions, we calculated the paramagnetic components of the magnetic susceptibility and effective magnetic moments of cobalt atoms at various temperatures and concentrations of the solid solutions. The isotherms of paramagnetic component of magnetic susceptibility of cobalt atoms in Bi5Nb3_3KCo3KO15_5 are typical for antiferromagnets, their comparison is shown in Fig. 3a.
The effective magnetic moments of single cobalt atoms calculated by extrapolating concentration dependencies of [xpara(Co)] to infinite dilution of the solid solutions exceed pure-spin values and increase as the temperature increases from 6.18 (90 K) to 6.69 (320 K). The magnitude of the magnetic moment exceeds the pure spin values of high-spin cobalt atoms Co(II) (peff = 3.89 ) and Co(III) (peff = 4.92 , therm 5ES), which may indicate the formation of exchange-coupled aggregates with the antiferromagnetic type of exchange out of cobalt atoms in infinitely dilute solid solutions. The formation of aggregates out of paramagnetic atoms in highly dilute solutions did not turn out to be unexpected, it was previously observed in solid solutions of Bi5Nb3O15 containing manganese or iron [8] atoms and is displayed in case of distorted coordination polyhedron caused by heterovalent substitution. Monoclinic distortion of
Fig. 2. Surface photomicrographs of the sample Bi5Nb3_3KCo3KO15_a at x=0.01 (a), 0.02 (b), 0.04 (c) in the mode of secondary and elastic backscattered electrons
the tetragonal structure of the solid solutions of bismuth niobate associated with the incline of the crystallographic axis c to the plane of perovskite layers indirectly indicates such distortions. Apparently, the formation of aggregates of paramagnetic atoms near the oxygen vacancies results in stabilization of the structure of the solid solutions.
The decrease of the paramagnetic component of magnetic susceptibility of the atoms with increasing concentrations of solid solutions may be also associated with low-spin atoms of Co(III) (^eff = 0 hb , 1Ais) and increase of the portion of cobalt clusters with antiferromagnetic type of exchange between atoms [9]. Reduced magnitude of the magnetic moment in more concentrated solid solutions testifies in favour of the suggestion on the described clustering. Growing with higher temperatures dependence of the effective magnetic moment on cobalt atoms in solid solutions of various concentrations indicates the antiferromagnetic type of exchange between atoms (Fig. 3b). The antiferromagnetic type of exchange is supported by cobalt atom electrons in 3d-orbitals and layered perovskite structure of bismuth niobate ensuring indirect exchange between atoms at the angle of 180° and accessibility of exchange channels dxz || pz || dxz, dx2-y2 || py || dx2-y2 and between the layers through the channel - dz2 || pz || dz2. Earlier, the validity of this suggestion was shown at the example of iron-containing solid solutions with layered perovskite structure [8]. The suggestion on diamagnetic cobalt (III) atoms in concentrated solutions can be explained by a number of reasons. The oxidized state of cobalt is likely to
decrease the destabilizing effect of oxygen vacancies on the structure of solid solutions; cobalt (III) atoms in the high-spin state, with their significant magnetic moment, cannot ensure such a sharp reduction of magnetic susceptibility of solid solutions, and, what is more, the accumulated distortions of oxyden polyhedra caused by heterovalent substitution of Nb(V) atoms by cobalt atoms contribute to higher tension of the crystal field of paramagnetic atoms and stabilized low-spin state of cobalt atoms.
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Fig. 3. a — Isotherms of paramagnetic component of magnetic susceptibility of the cobalt-containing solid solutions BisNbs-s*Co3œOi5-«s at 77 K (1), 120 K (2), 180 K (3), 240 K (4) and 293 K (5); b — Temperature dependencies of the effective magnetic moment of cobalt in the Bi5Nb3-3œCo3œO15-5 at x= 0.010 (1), 0.015 (2) and 0.04 (3)
At the top of Fig. 4 shows the EPR spectra of the Bi5Nb3-3œCo3œO15-5 solid solutions for x = 0.005, 0.02, 0.04, and 0.06, reduced to standard registration options. Only one component is reliably established in all spectra. This is an asymmetric narrow line with the value of ABpp = 17—19 mT and a g-factor of about 4.3. The integral intensity of this line monotonously decreases almost twice as x increases from 0.005 to 0.06. For a nominally pure compound, this signal could be attributed to the high-spin state (S = 3/2) of Co2+ ions in a weak, slightly distorted octahedral crystal field at Nb5+ substitution positions. In the limit of a weak crystal field of an ideal octahedral coordination, Co2+ ions in the EPR spectra give an isotropic line with g = 4.33 and the magnitude of hyperfine splitting on 59Co nuclei around 300 MHz, the spin-orbit interaction depending on the strength of the crystal field decreases these values [10,11]. The asymmetry of line 4.3 can be explained by small distortions of the octahedral coordination, and its width corresponds to the unresolved hyperfine structure from 59Co. However, due to the pe-
culiarities of the splitting of the energy levels of the octahedral Co2+ complexes in a weak crystal field, their EPR line 4.33 can be observed in the EPR spectra only at very low temperatures. At room temperature, the spectra of Co2+ ions are observed for low-spin states (S = 1.2) in a strong or strongly distorted crystal field, and the EPR lines are grouped in the g-factor region 2.0, rather than 4.3.
Fig. 4. EPR spectra of solid solutions Bi5Nb3_3a;Co3a;O15_,s for x = 0.005 — 0.06 in comparison with the Fe3+ spectrum of the compounds Bi5Nb3_3a;Fe3a;O15_,s from [8]. The residues of subtraction from the Bi5Nb3_3KCo3KO15_^ spectra of the Bi5Nb3_3KFe3KO15_a spectrum with a weight coefficient k are shown in the bottom. The narrow line with g = 2.003 is the signal of the reference sample
A more adequate explanation of the EPR 4.3 signal in the spectra of cobalt ceramics of the Bi5Nb3O15 follows from our results of the EPR studies of a similar compound doped with Fe3+ ions [8]. In the EPR spectra of Bi5Nb3_3a;Fe3KO15_a at small x, the asymmetric line 4.3 with ABpp = 20 — 22 mT dominates. This is typical EPR signal for a strongly rhombic distorted octahedral oxygen environment Fe3+ ions. We relate it to the NbO6 octahedra of the compound. At high concentrations, the Fe3+ ions in the EPR spectra increase the intensity of a wide band with g = 2.0 from the clusters of these ions. With an increase in the concentration of Fe3+ ions, a wide band g = 2.0 of their clusters develops. A comparison of the EPR spectra of ceramics shows that the spectrum of Bi5Nb3_3KCo3KO15_a is identical to the spectrum of Fe3+ c g = 4.3 of the Bi5Nb3_3KFe3KO15_a ceramics with a low iron content (Fig. 4, bottom). The difference in the shape of the line is reduced only to a slightly larger line width of 4.3 in the ceramics Bi5Nb3_3KFe3KO15_a. Since the line width 4.3 increases with an increase in the Fe3+ content in the compound, we can assume that Bi5Nb3_3KCo3KO15_a ceramics contain uncontrolled traces of Fe3+ ions causing weak 4.3 signals in the EPR spectra. The marked decrease in the integrated intensity of the line g = 4.3 of the minor content Fe3+ in cobalt ceramics with an increase in the cobalt content is explained by the competing occurrence of both iron and cobalt ions in one structural position of the Bi5Nb3O15 compound, namely NbO6. Here is a Fig. 5
Fig. 5. Isotherms of the paramagnetic component of the magnetic susceptibility of the Bi5Nb3-3XFe3KO15— [8] and BisNba-^Co^Ois— solid solutions at 77 K (1), 120 K (2), 180 K (3), 240 K (4) (the solid line is xpara(Co), the dashed line is xpara(Fe))
obtained by applying isotherms of the paramagnetic component of the magnetic susceptibility of the Bi5Nb3-3KFe3KOi5-a [8] and Bis^^^^O^-a solid solutions at 77 K (1), 120 K (2), 180 K (3), 240 K (4) (the solid line is xpara(Co), the dashed line is xpara(Fe)). Comparison of magnetic susceptibilities of both series of solid solutions shows 0.815 to 1 correlation between the susceptibility values of cobalt- and iron-containing solid solutions. Assuming that the magnetic susceptibility of Bi5Nb3-3KCo(Fe)3KO15— is mainly caused by the presence of impurity iron atoms, for any x the proportion of iron atoms should be approximately 4/5x, which can hardly be seen as an impurity. It remains to admit that the absorption band in the EPR spectrum (g = 4.27) can belong to Co(II)s=3/2 ions.
Conclusions
Thus, it was shown that the magnetic behavior of cobalt doping bismuth niobate solid solutions with perovskite-like layered structure is generally similar and is determined mainly by the crystal structure of the solid solutions, the symmetry, and the strength of the crystal field formed by ligands. The cobalt (III), (II) atoms in solid solutions of heterovalent substitution aggregate forming strong clusters of cobalt atoms predominantly with the antiferromagnetic type of exchange, which not disintegrate even at infinite dilution. EPR indirectly confirms that cobalt ions are in octahedral positions of substitution of Nb(V) ions.
References
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Исследсвание магнитнсй вссприимчивссти и ЭПР BieNba-sxСсзх015-в
Надежда А. Жук
Сыктывкарский государственный университет им. Питирима Сорокина
Сыктывкар, Российская Федерация
Владимир П. Лютсев
Институт геологии Коми научного центра УрО РАН Сыктывкар, Российская Федерация
Дмитрий С. Безнссиксв
Сыктывкарский государственный университет им. Питирима Сорокина
Сыктывкар, Российская Федерация
Андрей Н. Низовцев
Институт биологии Коми научного центра УрО РАН Сыктывкар, Российская Федерация
Любсвь В. Рычкова
Сыктывкарский государственный университет им. Питирима Сорокина
Сыктывкар, Российская Федерация
Аннотация. Исследованы магнитная вссприимчивссть, микрсструктура и ЭПР кобальтсодержа-щих твердых растворов Б15УЬ3_3:ЕСо3;Е015_,5 со слоистой перовскитоподобной структурой. Твердые растворы В15УЬ3_3жСо3ж015_а (х < 0.005) кристаллизуются в тетрагональной сингонии (пр. гр. Р4/шшш), с увеличением содержания кобальта возникает моноклинное искажение элементарной ячейки при 0.005 < х < 0.04 (пр. гр. Р2/ш). В твердых растворах обнаружено образование обменно-связанных кластеров из атомов Со(Ш) и Со(11) преимущественно с антиферромагнитным типом обмена. ЭПР косвенно подтверждает, что ионы кобальта замещают октаэдрические позиции ионов УЬ (V).
Ключевые слова: керамика, магнитные свойства, ЭПР.
