The measurements of magnetic susceptibility, EPR and NEXAFS- spectroscopy study of the iron-containing solid solutions of bismuth niobate Bi3NbO7 has indicated that iron atoms are represented by monomeric Fe(III) and Fe(III)-O-Fe(III) exchange bound dimers with the ferromagnetic and antiferromagnetic types of exchange in the solid solutions of cubic modification. The exchange parameter and monomeric and dimeric cluster distribution in Bi3Nb1-xFexO7.s depending on the content of the paramagnetic atoms were calculated according to the model of Heisenberg-Diracvan Vleck. The solid solutions as well as iron oxides FeO, Fe2O3 and Fe3O4 were studied by the NEXAFS spectroscopy in order to determine the degrees of oxidation of iron atoms. The analysis of the NEXAFS Fe2p-spectra of iron-containing solid solutions and iron oxides revealed that the studied Fe atoms were mainly in the +3 oxidation state. The EPR spectrum of the sample with minimum iron content contained a symmetric signal with g = 4.27 with a weak shoulder at g ~ 8. The samples of Bi3Nb1_xFexO7_s solid solutions at 0.02 < х < 0.04 had a low-intensity broad band in the region ofg ~ 2.28 of their spectra. The spectra of EPR of the solutions with x > 0.04 exhibited a broad, slightly asymmetric line centered around g ~ 2.0.
Keywords: iron, clusters, exchange interactions, EPR and NEXAFS-spectroscopy.
На основании данных магнитной восприимчивости и исследований методами ЭПР- и NEXAFSспеκтросκопии железосодержащих твердых растворов ниобата висмута Bi3NbO7 κубичесκой модифиκации установлено, что атомы железа находятся в виде мономеров Fe(III) и обменносвязанных димеров Fe(IH)-O-Fe(In) с ферро- и антиферромагнитным типами обмена. По модели Γейзенберга-Дираκа-ван-Флеκа рассчитаны обменные параметры и распределение κластеров в Bi3Nb1.xFexO7.¡ в зависимости от содержания парамагнитных атомов. С целью определения элеκтронного состояния атомов железа исследованы твердые растворы и оκсиды железа FeO, Fe2O3 и Fe3O4 методом NEXAFS-спеκтросκопии. Анализ Fe2p-спеκтров NEXAFS железосодержащих твердых растворов и оκсидов железа поκазал, что атомы железа имеют степень оκисления +3. В ЭПР-спеκтре образца с минимальным содержанием железа содержится симметричный сигнал с g = 4,27 со слабым плечом при g ~ 8. В спеκтрах образцов твердых растворов Bi3Nb1.xFexO7.¿ (0,02 < х < 0,04) содержится широκая полоса малой интенсивности в области g ~ 2,28. В ЭПР-спеκтрахрастворов (х> 0,04) проявляется широκая, слегκа асимметричная линия, центрированная воκруг g ~ 2,0.
Ключевые слова: железо, κластеры, обменные взаимодействия, ЭПР- и NEXAFSспеκтросκопия.
(ProQuest: ... denotes formulae omitted.)
Introduction
Bismuth niobáte Bi3NbO7 and its solid solutions are perspective basic substances for oxygen sensors and for catalytic reactors as membranes with oxygen-conducting properties [1-3], photocatalysts in UV and visible spectral regions [4, 5]. Bismuth niobate undergoes the reconstructive reversible phase transition from cubic phase to tetragonal one at 830 °C and the cubic phase appears again at 900 °C [6-8]. Cubic bismuth niobate is characterized by defective fluorite-like structure with the following parameters (Fm3m, а = 0.548 nm). Cation positions of bismuth (III) and niobium (V) are distributed in the same crystallographic system [9]. The niobium atoms are arranged in distorted coordination octahedra [6, 10], the niobium-oxygen octahedra are bound by oxygen vertices, forming chains or blocks [3, 6, 10-14]. Previous studies of the magnetic properties of solid solutions of niobate of bismuth of cubic modification containing paramagnetic ions of 3d-elements [15-17] have shown that, due to the charge imbalance and highly distorted coordination polyhedron in the dilute solutions, the paramagnetic atoms were preferably in the oxidized state, e.g. Ni(III), Mn(III), Mn(IV). The concentrated solid solutions Bi3Nbi-xMxO7-5 (M - Ni, Mn) are stabilized by the formation of clusters of paramagnetic atoms with the antiferromagnetic exchange type, which manifestation is possible due to aggregated niobium-oxygen octahedra in the structure.
In this paper, the state and nature of the iron paramagnetic exchange interactions in the Bi3Nb1-xFexO7-8 solid solutions were studied by means of magnetic dilution, EPR and NEXAFS-spectroscopy. The parameters of exchange interactions in clusters of iron atoms were calculated. The solid solution composition as a function of the content of paramagnetic atoms was modelled.
Experimental part
The samples of bismuth orthoniobate solid solutions of cubic modification were synthetized by the solid-phase method from "special pure" grade oxides of bismuth (III), niobium (V) and iron (III). This method included staged calcination at the temperatures of 650 °C and 950 °C. Phase composition of the samples was monitored by means of scanning electron microscopy (electron scanning microscope Tescan VEGA 3LMN, energy dispersion spectrometer INCA Energy 450) and X-ray phase analysis (a DRON-4-13 diffractometer, CuKa emission), parameters of the unit cell of the solid solutions were calculated using the CSD program package [18]. The quantitative measurement of iron content in the samples was performed by atom-emission spectrometry (a spectrometer SPECTRO CIROS of ICP type), the accuracy was ±5% of the parameter x in the formula of the solid solutions. In the study of the solid solution samples, the magnetic susceptibility was measured by the Faraday method at sixteen fixed temperatures in the temperature range of 77-320 K. The relative measurements were characterized by the accuracy of 2%. The spectra of EPR of the polycrystalline bismuth orthoniobate preparations were registered using a RadioPAN SE/X 2547 radiospectrometer of X-diapason (Center for Collective Usage "Geonauka" at the Institute of Geology of Komi Scientific Center of Ural Branch of RAS). Spectra were received at room temperature using a rectangular resonator (RX102, TE 102 mode) in the form of the first derivative at the 100 MHz HF modulation frequency with the 0.25 mT amplitude and the 35 mW SHF field power. A batch of a preparation (-100 mg) was placed into a quartz tube with 4 mm external diameter. The anthracite EPR signal (g0 = 2.0032, Bpp = 0.5 mT) was used for amplification calibration of the instrument. The spectra were recorded in the magnetic field range from 0 to 700 mT and the lines of the reference were separately recorded with the scan step of 5 mT. The total spectra were normalized to the reference line intensity and then to 100 mg of the sample. The near-edge X-ray absorption fine structure (NEXAFS) of the Fe2p-absorption spectra of the iron-containing solid solutions Bi3Nb1-xFexO7-8 and oxides of iron was received using a synchrotron radiation source at the BESSY-II, Russian-German beamline in Berlin [19]. Each spectrum was registered in the mode of total electron yield (TEY) [20].
Results and discussion
The solid solutions Bi3Nb1-xFexO7-8 were obtained in the narrow concentration interval, x < 0.06 [21]. The measurements of magnetic susceptibility allowed us to calculate the paramagnetic components of the magnetic susceptibility for the solid solutions and the values of the effective magnetic moments of iron atoms corresponding to various concentrations of the solid solutions and temperatures. The diamagnetic corrections for calculating the paramagnetic component of the magnetic susceptibility were taken considering the susceptibility of the matrix of bismuth niobate Bi3NbO7 of cubic modification, measured in the analogous temperature range [22]. The temperature dependence of the reciprocal of the magnetic susceptibility paramagnetic component, which was calculated for a mole of atoms of the paramagnetic obeyed the Curie-Weiss law in the investigated temperature range for all iron-containing solid solutions. The isotherms of a paramagnetic component of iron magnetic susceptibility [/para(Fe)] in the solid solutions (Fig. 1) have the form characteristic of antiferromagnets. The effective values of magnetic moment of single iron atoms, that were calculated by extrapolation of the [ /"™(Fe)] values' concentration dependencies to the infinitely diluted solid solutions, decreased with increasing temperature from peff(Fe) = 7.12 MB (90 К) to 6.97 MB (320 К). This indicated the presence of ferromagnetic interactions between paramagnetic iron atoms. The value of the magnetic moment was much greater than the pure spin value of Fe(III) atoms (peff = 5.92 MB, term 6Aig), Fe(II) (peff ~ 4.9-5.7 MB, 5T2g), which indicated the formation of ferromagnetically bound aggregates of Fe(III) atoms in the highly diluted solid solutions.
The ferromagnetic nature of the interaction in iron clusters remained up to x<0.006. The nature of the temperature dependent change in the magnetic moment with increasing paramagnetic atom concentration in the tolid solutions indicated the dominance of the antifertomagnetic type of exchange betwe en iron atoms (Fig. 2) .
The spectra rtf EPR of the Bi 3Nb1-xFexO7-8 solid solutions at 0.006 ft x < 0.06, as -well as the c alculated powder spectra of Fe3+ ions i n c ationic positions of lithium and niobium (V) of lithium ntobate crystal with tho paoameters of the opin Hamiltonian tf the Fot (Fet+(Li)) and Fe 4 (Fet+(Nb)) centers are shown in Fig. 3 [21, 23, 24]. A softwaoe package Easnspin for the MathLab programming eovironment was used for the modeling of the spectra [25]. The narrow line on the experimental spectra with g0 = 2.0032 refers to the standard.
The spectrum of the sample with minimum iron content contained a symmetric signal with g = 4.27 (Л-⅞, 29 mT) with a weak shoulder at g ~ 8. Such a spectrum can be explained by structurally isolated Fe3+ ions located in relatively strong crystal field, characterized by the parameter of axial field D > hvSHF, where vSHF ~ 9.4 MHz, and the maximum degree of orthorhombic distortion of ~ 1/3 [23]. Presumably, the appearance of the octahedral positions of iron with strong rhombic distortion is related with the compensation of excess charge by oxygen vacancies: Fe(III)^Nb(V) +V[O2-]. The V[O2'] vacancies in the nearest surroundings leads to the strong distortion of the octahedral position of iron and to the appearance of the band with 4.27 in the spectrum of EPR. The line with g ~ 8 can be associated with the presence of the paramagnetic atom aggregates [26].
The samples of Bi3Nbi_xFexO7_5 solid solutions at 0.02 < х < 0.04 had a low-intensity broad band in the region of g ~ 2.28 of their spectra, which corresponds to iron (III) atoms occupying the cationic positions of bismuth, because a line with a close effective g-factor value is present in the spectrum of EPR of a resembling complex in monocrystal niobate of lithium (Fig. 3). The low-field part of the broad line of 4.27 probably has a line of axial complexes of Fe (III)(Bi) with g ~ 4.5. The spectra of EPR of the solutions with x > 0.04 exhibited a broad, slightly asymmetric line (dőpp ~ Ш0 mT) centered around g ~ 2.0. The Lo re ntz shape appnsximated well the s egment of the line in the high-field. The lack of noticeable structure of the line with g about 2.0 and the; appearance of tlit signal in the high iron content samples indicated its origination from iron (III) io ns in the octahedral coordination.
The solid s olutiona Bi3 Nb 0.94Fe 0.06O70 at well as iron oxides FeO, Fe2O3 and FerO4 were studied by ohe NEXAFS spectroscopy in order to determine the degrees of oxidation of iron atoms. The analysis of the NEXAFS Fe 2p-spectra of iro n-containing solid solutions and iron oxides (Fig. 4) revealed that the studied Fe atoms were mainly On the +3 oxidation state. The spectra of inon oxides FeO recorded in the present study correspend to the Fe2p-spectra0 which were studied earlier [20].
The solid solution compositions depending on the content of the paramagnetic were modeled by the theoretical estimation of the susceptibility and by the comparison of the calculated values with the experimentaly obtained values.
The computation and plotting of the experimental dependencies of xpara(Fe) on the solid solulron concentrations were carried out within the framework of the dilute soli d solution model, on tire:; basis which, the magnoOic susceptif ility ie define d as the suon oO the conttfbutions fro m patamagnetic atoms whicd are considered to be single (Fe(III), monomers) and their M-O-M aggeegaSes bound by exchange (dimees, FetIn)i°-Fe(nI)). Thf equation for finding the pa ramagneric eompongnt op the iron atom magne Sic s usc uptibility i s t he s um of the c ontaibutions of the magngtsc tusc e ctibilüty of modomese, ms we ll dt 'tins di msrs witho the ¡snít i- end fesio magneüc interaraions:
... (1)
where aŢt"nr) nnd apįno are the monomee fractions s and antifesromaanatėcally bound iron (III) dimern ХОю is tlhe Fe IIn) mhaomet тцй0 ousceptitaUty оЬтХ-мш- and П^Д-р^ш- aire the magnetic susceptibilities of the dimers (Fe (In)-O-Fe(III)) with ferro- and antiferromagnetic types of excaange, determined by the Heisenberg-Dirac-v an Vleck neodīsi [27].
These urn five indepeadent parametars m 1^]b.^ equation (1): die fractions of rndividual Fe(III) atoms nnd Fe(IIt)-O-Ff(III) dimert with an jEtnltL^ and Ferromagnetic exchange types, as well as the parameters ot the anti- aod ferromegneerc exchango btetween -ron (ItIt aeims , 'цппоне whtch aee i^ncd^ı^iî preced- in this equation. ds iit was shown in seveaal of wouks [g5 -19] tűin; number of experimedtat magnetic susceptibility vaiusu as the Snnction of the temperature and the sol-d solution co ncentratlon is sufficiant Oo estimate the exchenge uaoemete r and the fraction of cluhteas o° thd rtoms with prramagnetic propertiec.
Aacording to the Heisedhard-Dirac-van Vleck model [27], Hamiltonian ot the spin-spin intrract-on fod a dmer looks like (2)i
... (2)
Here, J is tfe pcsameter olF isotropic exchange and C is the operator (-IF the toial nngular spin m ome nt.
Then /dim is determined w íÜi the help of the relationships (3) - (6).
... (3)
... (4)
... (5)
where Sa, Sb are the values of spins of the atoms within the dimers, applying to this study, for the Fe(III)-O-Fe(III) dimsr Sa = Sb = 512, g - the Lande factor for atoms of iron ( 111), J - the parameter of exchange, Т - the absolute temperature.
The paramagnetic component oS the magnetic susceptibility of" l7 e(III)-O-Fe(III) dimers was calculated by the equation (6):
... (6)
where x = J /XT, where k - the Boltzmann constant.
The experimental and calculated values achieved the agre ement via the reduction of the value of the function XXX -Xt "XT')2, where X is summation oner rll concentrationis; X is summation over all temperatures; alfC is the calculated vaeue of paramagnetic component of the solid solution magnetic sus ceptibhity, x¡·c is the expe rim-ntal paramagneeic compo-ent value. The feha co freletion lietween the experimentaC valuei and the calculated values was obtained foc Ihe iron-containing sol id yoeutioes with the antiftrromagnetic exchanje pvrameter JyX-o-fX = -1r0 emi 1 and tire; ferromagnetic exchange páramete h J atu -Fh+ = 5 6! cm-1. The compari ton oe ehe txpe rime ntal and theo reticol vnlues of the magnetic susceptibility ot tl- s olid solution if shown in Table 1.
It was establislod as a result nhat the solid solution with infinite dilution contains single atoms of Fe(III) and tlte dimers (Fb(nt)-OeFe(in)) with an antiferro- and ferromagnvtie exchange types. As the solid sohidion inon eencentoatia et ina teased, the fraction of FehIIl) monomers and fe rromagnehcclln bound dimarc decreace along wilii the inaroase in the fracIiono of antiberromagnetically bonnd ones (Table 1). The change in the type of interaction among the atoms with paramagnetic propertier in a clucter wcth inc reasmg oenceotration cen be explained by the dejşree of defectivenesr of the ox^en surroundings and the natuae of the distcfiiution of of iron oe/^r the aationic positionn. It can be assumed that in the heeeeovalentiy-substituted solid solutions, the orcuraence of the oxygen vacanciet leads So nbe suppre ssion ol antifineomagnetic exchaage channels of the type rOf 2 g pt || 17,( and to start the action of the ferromagnetic exchange channenn 75. b pe b drr. With an increase fn the content o8 atoms oS Fe(III) in there -olutions, tig OmcCion of exchnnge aggravates that sTMføe tty stal structure are increa e:i]ri^2; Cue to title localizftion of aggoeartps neat th_ ob^gein vacancies, whict re duces the degret of diftoraion of tha ooagen sucroundingi of the oaramagnatic atomt. Thus io the cause of ihe mcroace in the graction o0 aggaegatot 'wti'^l:i nnsdbrrounipnotic aec_aagn type, realized throuah the ?^r^icíl^^nj^e c-rnnelr df_rr || 45 Il It 2 aod a1z || p° || eg. Thu appeataare af tie dimers instead of more complexly organized aggregates or paramagnetic atom chains is probably can be explained by the fact that these solid solutions have low concentration of the atoms with paramagnetic properties.
Conclusion
The measuring of the magnetic susceptibility, NEXAFS and EPR-spectroscopy of the iron-containing solid solutions of bismuth niobate Bi3NbO7 of cubic modification have indicated that iron atoms are represented by monomers of Fe(III) and exchange bound dimers with the antiferromagnetic type and ferromagnetic type of exchange. The parameters of the exchange in the dimers of atoms of iron (III) with an the anti- and ferromagnetic types of exchange, calculated according to the model of Heisenberg-Dirac-van Vleck, are equal to J = -180 cm'1 and J = 53 cm'1, respectively. The satisfactory convergence was observed between the calculated values of the magnetic susceptibility and the experimental values in the iron-doped Bi3NbO7 ceramics. The modeling of the solid solution compositions and nature of the exchange interactions in clusters allowed us to establish that the increasing solid solution content of the atoms with paramagnetic properties causes the increase in the fraction of dimers of Fe(III) atoms with antiferromagnetic exchange type. Along with it, the fractions of the monomers of Fe(III) and dimers with ferromagnetic type of exchange decrease.
Received 11.03.2018, received in revised form 21.05.2018, accepted 14.08.2018
© Siberian Federal University. All rights reserved
Citation: Zhuk N.A., Belyy V.A., Lutoev V.P., Makeev B.A., Nekipelov S.V., Rychkova L.V. Magnetic properties, EPR and NEXAFS - spectroscopy of iron-doped Bi3NbO7 ceramics, J. Sib. Fed. Univ. Chem., 2018, 11(3), 418-427. DOI: 10.17516/1998-2836-0087.
Corresponding author E-mail address: [email protected]
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Abstract
The measurements of magnetic susceptibility, EPR and NEXAFS- spectroscopy study of the iron-containing solid solutions of bismuth niobate Bi3NbO7 has indicated that iron atoms are represented by monomeric Fe(III) and Fe(III)-O-Fe(III) exchange bound dimers with the ferromagnetic and antiferromagnetic types of exchange in the solid solutions of cubic modification. The exchange parameter and monomeric and dimeric cluster distribution in Bi3Nb1-xFexO7.s depending on the content of the paramagnetic atoms were calculated according to the model of Heisenberg-Diracvan Vleck. The solid solutions as well as iron oxides FeO, Fe2O3 and Fe3O4 were studied by the NEXAFS spectroscopy in order to determine the degrees of oxidation of iron atoms. The analysis of the NEXAFS Fe2p-spectra of iron-containing solid solutions and iron oxides revealed that the studied Fe atoms were mainly in the +3 oxidation state. The EPR spectrum of the sample with minimum iron content contained a symmetric signal with g = 4.27 with a weak shoulder at g ~ 8. The samples of Bi3Nb1_xFexO7_s solid solutions at 0.02 < х < 0.04 had a low-intensity broad band in the region ofg ~ 2.28 of their spectra. The spectra of EPR of the solutions with x > 0.04 exhibited a broad, slightly asymmetric line centered around g ~ 2.0.