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DOI: 10.17516/1997-1397-2022-15-4-450-458 УДК 541.122: 538.214
ESR and 57Fe Mossbauer Spectroscopy Study of Fe-doped SrBi2Nb2O9
Vladimir P. Lyutoev* Andrey Yu. Lysiuk
Institute of Geology of the Komi Science Center UB RAS
Syktyvkar, Russian Federation
Larisa O. Karlova*
JSC "Komi Thermal Company" Syktyvkar, Russian Federation
Dmitriy S. Beznosikov§
Federal State Unitary Enterprise "General Radio Frequency Centre"
Northwestern Federal District Branch Syktyvkar, Russian Federation
Nadezhda A. Zhuk1
Pitirim Sorokin Syktyvkar State University Syktyvkar, Russian Federation
Received 10.09.2021, received in revised form 13.02.2022, accepted 20.04.2022 Abstract. Solid solutions of Bi2SrNb2_2:EFe2;EO9_,5 have been obtained by solid-phase synthesis. The electronic state and nature of the local environment of iron atoms in the SrBi2 Nb2O9 matrix with a layered perovskite-like structure were studied by ESR and Mossbauer spectroscopy. In the ESR spectra of samples of Bi2SrNb2_2^Fe2^O9-g (х < 0.04) solid solutions an intensive asymmetric line in the low-field region with the main feature at g = 4.27, weakly pronounced peak g = 6.15 and shoulder g ~ 9, and also an intensive broad (ABPP ~ 50-150 мТ) band centered around g ~ 2.0 is present. The Mossbauer spectrum of the compound Bi2SrNb2-2^Fe2^O9-5 is represented by an asymmetric doublet with isomer shift (IS) ~ 0.3, and quadrupole splitting (QS) ~ 0.5 mm/s. The shape of the doublet is reproduced by the superposition of two doublets with small and high IS and QS values. About 85% of the spectral area of the paramagnetic part of the spectrum is represented by the doublet Fe3+(1) with IS = 0.31 ± 0.04, QS = 0.45 ± 0.04 mm/s correlated with Fe3+ ions in regular axial positions. The remaining part is represented by the doublet Fe3+(2) with IS = 0.5 ± 0.1, QS = 0.7 ± 0.2 mm/s from the Fe3+ ions in defect environment.
Keywords: Aurivillius phases, iron, ESR, Mossbauer spectroscopy.
Citation: V.P. Lyutoev, A.Yu. Lysiuk, L.O. Karlova, D.S. Beznosikov, N.A.Zhuk, ESR and 57Fe Mossbauer Spectroscopy Study of Fe-doped SrBi2Nb2O9, J. Sib. Fed. Univ. Math. Phys., 2022, 15(4), 450-458. DOI: 10.17516/1997-1397-2022-15-4-450-458.
* vlutoev@geo.komisc.ru https://orcid.org/0000-0003-0231-302X
tandra227@yandex.ru https://orcid.org/0000-0003-3868-4586
tlarisa.karlowa@yandex.ru
§ uvn71p3@gmail.com
^ nzhuck@mail.ru https://orcid.org/0000-0002-9907-0898
© Siberian Federal University. All rights reserved
The unremitting interest of scientists in Aurivillius phases is due to the manifestation of a wide range of practically useful properties [1-3]. They have high Curie temperatures, low degradation rates of residual polarization and piezoelectric properties when exposed to unipolar electric fields, and high dielectric permittivity, which necessitates their use as components of composite film systems for long term information storage (FRAM) processing devices, as well as the manufacture of piezo- and pyroelectric transducers based on them [4,5]. In the crystal structure of Aurivillius phases described by the general formula Am-1Bi2BmO3m+3, fluorite-like [Bi2O2]2+ and perovskite-like[Am-1BmO3m+1]2- layers alternate, where cationic A-positions with cuboctahedral environment are occupied by ions of large radius (Na+, K+, Ca2+, Sr2+, Ba2+, Pb2+, Bi3+, La3+), and in octahedral positions B the highly charged small-radius cations (Cr3+, Ga3+, Fe3+, Co3+, Ti4+, Mn4+, Nb5+, Ta5+, Mo6+, W6+) are located [6-13]. The m parameter corresponds to the number of [Am-1BmO3m+1]2- layers in the perovskite-like block. In the crystal structure of bismuth-strontium niobate Bi2SrNb2O9 (sp. gr. A21am, a = 0.55189(3), b = 0.55154(3) and c = 2.51124(9) nm) bismuth-oxygen layers alternate with perovskite-like blocks of two niobium-oxygen octahedral thickness. The size discrepancy between the cubocta-hedral cavities of the perovskite-like blocks and the strontium atoms leads to geometrical distortions in the structure which manifest in asymmetry and angular inclination of the niobium-oxygen octahedrons [9,14,15], and the partial replacement of Bi3+ ions in fluorite-like layers by Sr2+ ions leads to a blurring of the phase transition and a relaxation character of dielectric polarization (Tc = 420-440 °C) [16]. As shown in [17], the Curie temperature can naturally increase with the Fe3+ ions concentration in SrBi2Nb2-KFeKOg ceramics and for x=0. it increases up to 524 °C. It is shown that the Fe-doped SrBi2Nb2O9 phase-pure ceramics are formed in wide concentration interval x ^ 0.4 and exhibit the properties of magnetoelectric multiferroics. The occurrence of antiferromagnetic properties of the samples is explained by the superexchange interaction (Fe3+-O-Fe3+). Previous studies of magnetic dilution and ESR of iron-containing bismuth niobate solid solutions Bi2BaNb2O9, Bi2SrNb2O9 (m = 2) and Bi5Nb3O15 (m = 1.5) showed [18-23] that iron Fe3+ ions preferentially replace the octahedral positions of Nb5+ ions and tend to aggregate to form high-nucleus antiferromagnetic clusters. This work presents for the first time the results of the study of the electronic state and local environment of iron atoms in solid solutions Bi2SrNb2-2KFe2KO9-a by ESR and Mossbauer spectroscopy.
1. Materials and methods
The samples of the bismuth barium niobate solid solutions were synthesized by the standard ceramic method from the "special purity grade" bismuth (III), niobium (V) and iron (III) oxides by the staged calcination at 650, 850, 950 and 1050 °C. The phase composition of the samples was controlled by X-ray phase analysis (Shimadzu 6000, CuKa-radiation). Microstructure and local chemical composition of the samples were studied by electron scanning microscopy (Tescan MIRA 3LMN, INCA Energy 450). The unit cell parameters of the solid solutions were calculated using the CSD software package [24]. The ESR spectra were recorded with an X-band radiospec-trometer SE/X-2547 (RadioPAN) in the Shared Services Center "Geonauka" at the Institute of Geology FRC Komi SC UB RAS. The spectra recording was done with a rectangular resonator (RX102, mode TE 102) at room temperature as a first-order derivative at high-frequency modulation 100 MHz with amplitude 0.25 mT and microwave field strength 35 mW. A sample powder (approx 100 mg) was put into a quartz tube with external diameter of 4 mm. To calibrate the amplification of the equipment, an ESR signal from a miniature reference sample (anthracite,
singlet line with g = 2.003 and peak to peak width ABpp = 0.5 mT) was used. For each sample the spectrum was recorded within the range of magnetic fields 0-700 mT, and the lines of the reference standard with scanning 5 mT were recorded separately. The samples spectra were adjusted to equal values of the reference standard intensity and normalized on the equal weight of the sample. The 57Fe Mossbauer spectra were obtained on an MS-1104 Em spectrometer at the rates of -11 —h11 mm/s at room temperature. The 6 x 108 Bq 57Co in chrome matrix (Ritverc GmbH, St. Petersburg) was used at RT. To eliminate the texturing effects in the spectra, the samples were prepared in the form of finely ground powder. The duration of spectrum accumulation was about 260 hours. The isomeric shift was determined relative to a-Fe. The spectra were processed using the standard software of a spectrometer "Univem".
2. Results and discussion
The samples of composition Bi2SrNb2_2KFe2KO9_5 (x < 0.08) were obtained by solid-phase synthesis. It is found that grains of all samples are densely packed and form almost porous microstructure (Fig. 1). The samples contain chaotically oriented, mainly lamellar anisotropic grains that is also typical for ceramics of Aurivillius phase [25,26].
Fig. 1. X-ray diffraction (a) and microphotograph (b) of the B^SrNbooFecuoO^^ in the mode of elastically reflected electrons
According to the X-ray phase analysis the iron-containing solid solutions are formed in the limited concentration interval x ^ 0.05. X-ray examination of the obtained solid solutions showed that their crystal structure corresponds to the structure of Bi2SrNb2O9. With increasing iron content in solid solutions, parameters a and b decrease, and c increases: a = 0.55045 nm, b = 0.55066 nm, c = 2.5054 nm (x = 0.003) to a = 0.55017 nm, b = 0.55037 nm, c = 2.5072 nm (x = 0.05). Apparently, the unit cell parameter c slightly increases due to the isomorphic replacement of octahedral Nb5+ cations in perovskite-like layers by Fe3+ ions with a slightly larger radii: R(VINb5+) = 0.064; R(VIFe3+) = 0.0645 nm [27]. This assumption does not contradict our ESR studies.
In the ESR spectra of solid solution samples of Bi2SrNb2_2œFe2œO9_ (x = 0.005, 0.02, 0.04) there is an intensive asymmetric line in the low-field region with the main feature at g = 4.27, weakly pronounced peak g = 6.15 and shoulder g ~ 9, and also an intensive broad (ABOT ~ 50-150 mT) band centered around g ~ 2.0 (Fig. 2). The origin of g = 4.27 component is associated with structurally isolated Fe3+ ions in a strong octahedral crystal field with
D > 10 GHz, and a maximum degree of rhombic distortion E/D ~ 1/3 [21]. At that the effective g-factor of the transition line between the Kramers doublet ± 3/2 levels becomes isotropic and equals 4.27. In amorphous and polycrystalline materials, such as ceramics, the isotropic 4.27 line is not subject to orientational broadening and dominates over the lines of other transitions with anisotropic effective g-factors. The broad prevalence of line 4.27 may be related to distributed values of crystal field parameters in low-ordered matrices. Due to peculiarities of orientation broadening of lines in polycrystalline and amorphous material in the ESR spectrum a small fraction of D and E distribution in the region of D > 10 GHz, E/D ~ 1/3 are mainly observed. Low-intensity lines g = 6.15, 9 probably represent part of the spectrum from isolated Fe3+ ions in the axial crystalline field (E = 0). Relatively low-intensity signal in the region g 2.0 in the spectrum of the sample with x = 0.005 is also a component of the spectrum from the isolated axial Fe3+ ions. At high values of x the band g ~ 2.0 is strongly broadened and belongs mainly to clusters of iron ions.
The axial (E ~ 0) ESR spectrum of Fe3+ in the compound Bi2SrNb2_2KFe2KO9_5 can appear at isovalent substitution of Fe3+ ^ Bi3+, which is unlikely because of the large difference of ionic radii. The axial spectrum can be also connected with regular iron positions by the scheme of heterovalent isomorphism Fe3+ O6 ^ Nb5+O6 + V[O2_]. Compensation of excess negative charge is carried out by axial oxygen vacancy in the second coordination sphere. In defective crystal regions and in near-surface layers the Fe3+O6 complex undergoes strong rhombic distortion (E ~ 1/3), that gives rise to the 4.27 line in the ESR spectrum. The broad band g ~ 2.0 is associated with clusters of these iron ions. Fractions of axial, rhombically distorted and clustered Fe3+ positions cannot be estimated by the intensities of corresponding lines because of their concentration dependence is sharply nonlinear. Even with an insignificant fraction of Fe3+ in the (E ~ 1/3) position, the isotropic narrow line with g = 4.27 will dominate the ESR spectrum.
I-'-1-1-1-1-1-1-1-'-1-'-r
0 100 200 300 400 500 600 B, mT
Fig. 2. ESR spectra of the samples of the Bi2SrNb2_2œFe2œO9_â solid solutions at various values of index x. Reference sample line with g=2.003
The iron atoms clustering was first suggested in [18, 19], a study of magnetic dilution in Fe-doped Bi2SrNb2O9. It was found that in strongly diluted solid solution, iron ions are aggregated
in the form of clusters with antiferroic and ferromagnetic types of exchange. With increasing iron concentration, the proportion of aggregates with the antiferromagnetic type of exchange only increases. Even in strongly dilute solutions of Bi2SrNb2_2KFe2KO9_á, iron atoms tendency to form exchange-bonded aggregates is caused by the considerable covalence of Fe-O bond and the predominance of antiferromagnetic exchange connected with the overlap ferric and oxygen atomic orbitals in clusters. It is shown that geometrical distortions of Bi2SrNb2O9 structure have a significant influence on the nature and intensity of the exchange interaction between paramagnetic atoms in highly dilute iron-containing solid solutions. The presence of clusters with antiferro- and ferromagnetic type of exchange may indicate the presence of Fe3+ ions in crystal fields of different symmetry.
In order to investigate the character of the local environment of iron ions the Mossbauer spectroscopy was used. Mossbauer spectrum of Bi2SrNb2_2KFe2KO9_á compound was obtained at x = 0.08. But even at very long accumulation of spectrum (260 h) the achieved effect error was more than 15%. Fig. 3 shows the paramagnetic (-3 —h3 mm/s) part of the full spectrum (-11 —h11 mm/s). The paramagnetic part of the spectrum is represented by an asymmetric doublet with isomer shift (IS) ~ 0.3 and quadrupole splitting (QS) ~ 0.5 mm/s. Along its edges the features with IS ~ -0.6 and +1.7 mm/s are poorly distinguishable, which we attributed to the internal pair of sextet lines from the magnetically ordered iron oxide. These are the narrowest lines and less susceptible to sextet line broadening. In their spectral position they correspond to Fe2O3 hematite: IS « 0.4 mm/s, QS « -0.3 mm/s and superfine magnetic field at Fe nuclei 400 kOe [28]. It is possible, that these are rudiments of formation of the condensed iron oxide phase on iron (III) clusters observed by ESR at lower iron content in ceramics. This conclusion can contradict results of work [17], at that we deal with poorly resolved spectrum.
Fig. 3. 57Fe Mossbauer spectra of the samples Bi2SrNb2_2KFe2KO9_5 (x = 0.08) in the paramagnetic region. The fitted components: Fe-Ox — internal line of sextet of the magnetically ordered iron oxide; Fe3+(1,2) — paramagnetic doublets of the dissolved in the crystalline structure of ceramics Fe3+ ions
The asymmetry of the paramagnetic doublet IS ~ 0.3, QS ~ 0.5 mm/s is a consequence of its non-elementarity. Its shape is reproduced by is reproduced by the superposition of two doublets with small and high IS and QS values. About 85% of the spectral area of the paramagnetic part of the spectrum falls on the doublet Fe3+(1) with IS = 0.31±0.04, QS = 0.45±0.04 mm/s, the remaining part is represented by the doublet Fe3+(2) with IS = 0.5±0.1, QS = 0.7±0.2 mm/s. The first one, with a lower quadrupole splitting, is reasonable to associate with Fe3+ ions in regular axial positions Fe3+Ü6 ^ Nb5+Ü6 + V[O2~], proposed in the analysis of ESR spectra. This is confirmed by Mossbauer studies of compounds in which Fe3+ ions have octahedral coordination [29-31]. For example, for the compound Bi2FeNbO7 the parameters of the Mössbauer spectrum for VIFe3+: IS = 0.27 mm/s, QS = 0.41 mm/s [29], in the iron-containing Aurivillius phase Bi5FeTi3Oi5 octahedral Fe3+ ions appear in the spectrum with parameters IS = 0.38 mm/s, QS = 0.60 mm/s [30]. Then, the second doublet Fe3+(2) with large quadrupole splitting, refers to the Nb5+ substituted Fe3+ ions in the defect environment. The assumption that Fe3+ ions can substitute the Bi3+ ions positions is not supported by the spectrum parameters. As shown in [30,31], the quadrupole splitting of such Fe3+ ions is higher or is near unity. As shown earlier [32,33], Fe3+ ions are more prone to highly symmetric coordination and, due to the ionic radius and polarization properties, prefer the octahedral coordination of Nb5+ ions from the two alternative cationic positions. It can only be assumed that the Fe3+ ions associated with oxygen vacancies, characterized by low quadrupole splitting and representing about 85% of the total iron content in the niobate lattice, are in an aggregated state, while the other iron ions are predominantly in the monomeric state. The conclusion about the presence of the Fe3+ ions in oxygen environment of different geometries is supported by the results of magnetodilution studies in Fe-doped Bi2SrNb2O9 ceramics [18,19] under the assumption of clusters with antiferro- and ferromagnetic exchange type.
Conclusions
Bi2SrNb2_2œFe2œOg_à solid solutions in the limited concentration range were obtained by ceramic synthesis. The samples are characterized by a porous microstructure formed by chaotically arranged plate-shaped anisotropic grains. Results of ESR and Mössbauer spectroscopy reveal three forms of iron localization in ceramics: isolated Fe3+ ions in regular and defective octahedral positions of Nb5+ substitution and clusters of iron ions up to the formation of iron oxide phase. The major part of Fe3+ ions is located in regular octahedral positions of Nb5+ ions.
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ЭПР и 57Ев-мёссбауэровская спектроскопия SrBi2Nb2O9б допированного железом
Владимир П. Лютоев
Андрей Ю. Лысюк
ИГ ФИЦ Коми НЦ УрО РАН Сыктывкар, Российская Федерация
Лариса О. Карлова
ОАО "Коми тепловая компания" Сыктывкар, Российская Федерация
Дмитрий С.Безносиков
ФГУП "Главный радиочастотный центр", Филиал по Северо-Западному федеральному округу
Сыктывкар, Российская Федерация
Надежда А. ^Кук
Сыктывкарский государственный университет им. Питирима Сорокина
Сыктывкар, Российская Федерация
Аннотация. Твердофазным методом синтеза получены твердые растворы Б12Я^Ь2-2:ЕРе2;Е09-,5. Методами ЭПР и мёссбауровской спектроскопии исследовано электронное состояние и характер локального окружения атомов железа в матрице ЯгВ1^Ь209 со слоистой перовскитоподобной структурой. В спектрах ЭПР образцов твердых растворов Б12Я^Ь2-2:ЕРе2;Е09-,5 (ж < 0.04) присутствует интенсивная асимметричная линия в области низких полей с главной особенностью при g = 4.27, слабо выраженными пиком g = 6.15 и плечом g ~ 9, а также интенсивная широкая (ДВрр ~ 50-150 мТ) полоса с центром около g ~ 2.0. Мёссбауэровский спектр соеди нения Б12Я^Ь2-2:ЕРе2;Е09-,5 представлен асимметричным дублетом с изомерным сдвигом (1Я) ~ 0.3 и квадрупольным расщеплением ^Я) ~ 0.5 мм/с. Форма дублета воспроизводится суперпозицией двух дублетов с малыми и высокими значениями 1Я и QS. Около 85 % спектральной площади парамагнитной части спектра приходится на дублет Ре3+(1)) шНЬ = 0.31 ±0.04, QS = 0.45±0.04 мм/с, соотнесенный с ионами Ре3+ в регулярных аксиальных позициях. Оставшаяся часть представлена дублетом Ре3+(2) с = 0.5 ± 0.1, QS = 0.7 ± 0.2 мм/с от ионов Ре3+ в дефектном окружении.
Ключевые слова: фазы Ауривиллиуса, железо, ЭПР, мёссбауровская спектроскопия.
