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9Journal of Siberian Federal University. Chemistry 3 (2018 11) 418-427
УДК 541.122:538.214
Magnetic Properties, EPR and NEXAFS -Spectroscopy of Iron-Doped Bi3NbO7 Ceramics
Nadezhda A. Zhuk*a, Vladimir A. Belyyb, Vladimir P. Lutoevc, Boris A. Makeevc, Sergey V. Nekipelova,d and Lubov V. Rychkovaa
aSyktyvkar State University 55 Oktjabrskij, Syktyvkar, Republic of Komi, 167001, Russia bInstitute of Chemistry of the Komi Science Center UB RAS 48 Pervomaiskaya Str., Syktyvkar, Republic of Komi, 167982, Russia cInstitute of Geology of the Komi Science Center UB RAS 54 Pervomaiskaya Str., Syktyvkar, Republic of Komi, 167982, Russia
dInstitute of Physics and Mathematics of the Komi Science Center UB RAS 4 Oplesnina Str., Syktyvkar, Republic of Komi, 167982, Russia
Received 11.03.2018, received in revised form 21.05.2018, accepted 14.08.2018
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-Dirac-van 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 of g ~ 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.
© Siberian Federal University. All rights reserved Corresponding author E-mail address: nzhuck@mail.ru
*
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: 10.17516/19982836-0087.
Магнитные свойства, ЭПР и NEXAFS -спектроскопия керамики Bi3NbO7, допированной атомами железа
Н.А. Жука, В.А. Белый6, В.П. Лютоевв, Б.А. Макеевв, С.В. Некипелова,г, Л.В. Рычковаа
аСыктывкарский государственный университет Россия, 167001, Республика Коми, Сыктывкар,
пр. Октябрьский, 55 бИнститут химии Коми НЦ УрО РАН Россия, 167982, Республика Коми, Сыктывкар, ул. Первомайская, 48
вИнститут геологии Коми НЦ УрО РАН Россия, 167982, Республика Коми, Сыктывкар, ул. Первомайская, 54 гИнститут физики и математики Коми НЦ УрО РАН Россия, 167982, Республика Коми, Сыктывкар, ул. Оплеснина, 4
На основании данных магнитной восприимчивости и исследований методами ЭПР- и NEXAFS-спектроскопии железосодержащих твердых растворов ниобата висмута Bi3NbO7 кубической модификации установлено, что атомы железа находятся в виде мономеров Fe(Ш) и обменносвязанных димеров Fe(Ш)-O-Fe(Ш) с ферро- и антиферромагнитным типами обмена. По модели Гейзенберга-Дирака-ван-Флека рассчитаны обменные параметры и распределение кластеров в Bi3Nb1.xFexO7.s в зависимости от содержания парамагнитных атомов. С целью определения электронного состояния атомов железа исследованы твердые растворы и оксиды железа FeO, Fe2O3 и Fe3O4 методом NEXAFS-спектроскопии. Анализ Fe2p-спектров NEXAFS железосодержащих твердых растворов и оксидов железа показал, что атомы железа имеют степень окисления +3. В ЭПР-спектре образца с минимальным содержанием железа содержится симметричный сигнал с g = 4,27 со слабым плечом при g ~ 8. В спектрах образцов твердых растворов Bi3Nb1.xFexO7.s (0,02 < х < 0,04) содержится широкая полоса малой интенсивности в области g ~ 2,28. В ЭПР-спектрахрастворов (х> 0,04) проявляется широкая, слегка асимметричная линия, центрированная вокруг g ~ 2,0.
Ключевые слова: железо, кластеры, обменные взаимодействия, ЭПР- и NEXAFS-спектроскопия.
Introduction
Bismuth niobate 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, a = 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-xMxO7-8 (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 77320 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 [/para(Fe)] values' concentration dependencies to the infinitely diluted solid solutions, decreased with increasing temperature from MF) = 7.12 MB (90 K) to 6.97 MB (320 K). 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 (^eff = 5.92 MB, term 6A1g), Fe(II) (^eff ~ 4.95.7 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
60000 J0000 40000 30000
d>
fe-20000
-^10000
0.02 x
0,04
so K
140 K
200 K
~260*K
320 K
0,06
Fig. 1. Paramagnetic component isotherms of the magnetic susceptibility of the iron-containing preparations at 90-320 K
imp,
Fo3'(Nb): LiNtOf
9400 M Tu 300 K
100
2Ü0
400
500
Q3C
1 i.iT
Fig. 3. EPR spectra of the samples of iron in the iron-doped Bi3NbO7 at various iron content x. The modelled powder spectrum ofFe(III) in positions ofLi and Nb(V) in the LiNbO3 lattice is below
concentration in tine solid solutions indicated the dominance of this antiferromagnetic typeof exchange betwe en iron atoms (Fig. 2) .
The spectra of EPR of the; Bi3NbaxFexO7_5 solid solutions at 0.006 < x < 0.06, as wetl as the calculated powder spectra of Fe3+ ions in cationic positions of lithium and niobium (V) ot lithium niobate crystal with tho paoameters of the opin Hamiltonian sf the Fol (Fe3+(Li)) and Fe4 (Fe3+(Nb)) centers are shown in Fig; 3 [221, 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 (ABpp « 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 Bi3Nb1-xFexO7-8 solid solutions at 0.02 < x < 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 withx > 0.04 exhibited a broad, slightly asymmetric line (ABW ~ e00 mT) centered around g ~ 2.0. The Lorentz shape appnoximated well the segment of the line in the highsfield. The lack of noticeable structure of the line with. g about 2.0 andtnhe ¡appearance of tlit signal in the high iron content samples indicated its origination from iron (III) ions in the octahedral coordination.
The solid solutionr Bi3Nb0.94Fe0.06O70 at well as iron oxides FeO, Fe2O3 and FerO4 were studied by ohe NEXAFS spectooscopy 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 (Fig. 4) revealed that the studied Fe atoms were mainly in the +3 oxidation state. The spectra of iion oxides FeO necorded 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.
Photon energy, eV
Fig. 4. NEXAFS Fe2p3/2-spectra of the iron-doped Bi3Nb07 ceramics and iron oxides Fe304and Fe203, FeO [20]
The computation and plotting of the experimental dependencies of /pa™(Fe) on the solid solution concentrations were carried out within ths framework of the dilute solid solution model, on the basis which, the magnooir susceptif ility ie dtfintd as the sum oO the contributions from patamagnetic atomi whicd are consideeed to be single (Fe(III), nti^nci^^irss)) and their M-OtM aggsegaies tuound ny exchange (dim^s-is, FeiIII)iO-Fe(nI)). Thn equagion for finding the pa ramagnetic uomponent on ifs iron gtommagne sic susc uptibility is ttlnes sum ol the contributions of iSte magngtsc ousaectibitity of modomnts, ae well at tlie dims with the snt(- antl fesnomagnetic tnteraeaions:
vpara i _ 11 _ mon _ dim(a) ) vdim(f) +
Jtcalc Vt^J — yL uFe(III) UFe(III) )IFe(III)_Fe(III) """
(1)
dim(a) \ F,dim(.a) mon ..mon
+ aFe(in))SeFe(in)sFe(III) + aFe(III)/lFS(ni) '
wters amide red ^1™///) are the monomes fraaliont s and antifeeromagnatically bound iron (III) dlmert, xfo is the FeiIII) monomet mmgne-iic ousseptibilita K^XF-Feinr) and xtiX^- are the magnetic susceptibilities of the dimers (Fe(III)-O-Fe(III)) with ferro- and antiferromagnetic types of exchange, dcterminbdby the Heisenbeng-Dirac-vsn Vleck model [227].
These urn five indepeadent parameters in she equation (1): tiue frecfisns of individual Fe(III) atoms nnd FelIII)-O-Ff(III) dimert with an ^ntti ^ and eeifomagnetic excOange types, as welf ¡is? the parameters ot the anti- and fernomhgneeic exchango ietween iron (itI) i3Lt;or(i_^, J^m^cm), whtch ane i^nd^r^^ctlry preset- en this equation. As it was shown in sevega( of" wouks [ILf!» -li'9], titn number of experimental magnetic sutcepiibility volunu as the tnnction oi ^Ute; temperature and the sohd solution concentration is sufltciant 0o estimate the exchhnge jnii^i^mi.i^ieir a-^d the fraction of clunteas oJT th.e rtoms with paramagnetic propertied.
According to the Heisentard-Dirac-van Vleck model [27], Hamillonian otshe spintspin introaation fod a dmor looks like (22)):
Hexch = _2iJi^3i-"tj. (2)
Heoe, J is tfe pcsameter of isotropic exchange and C is the operatot (of the toial nngular spin mome nt.
Then/dim is determmed weith the help of the relationships (3) - (6).
in torus'a-' H-e)^' -anycftsduo
'i-r = In:_
^ n ^in^ + iy^'^ n '
•S'
EJ, ¿T)= -JS'nO'+h - Sd(Sa+h o O^SS+O)], (4)
SHX+Sb.Oa+Sbol, ... , |Sf_SS, (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) dim)r Sa = Sb = 5f2, g - the Lande factor for atoms of iron (111), J - the parameter of exchange, T - the absolute temperature.
The paramagnetic component of the magnetic susceptibility ol" Fe(III)-O-Fe(III) dimers was calculated by the equation (6):
_ J_ 000a125g + 18Oee25-* +84e-5-5j; -H^Oe^115^ +6e~"-5' (6)
mC"m ~ AT ' 1 hl^ + 9-54 +- le-5 5' + 5_T11L-" + Se^5* +- iT175' '( '
where x = J /kT, where k - tho Boltzmann constant.
Tie experimental nd calculated values achieved the agreement via the reduction af the value off" the funcaion /XX-tsT -XT)2, where 2 is summation oner rll roncentrrtfono; nt is summation over all temperatures; is the calculated vahue of parammgnetic component of the solid solution magnetic susceptiMity, xf,/1 xthe experimental paramagnetic component vahaa. Tire liest correlation between the experimental values and the calculated values was obtained for the iron-containing solid yohutions with the antiftrrommgnetic exchange pvrametei eFhvO-FT+ = -180 em1 and vitrei feraomagnetic sxahangu paramete3arFa+_o-)4+= 50 cmh "^Jtmec compasiton oh the axperimental and H^htie^c^Jtis^iLc;values of trite magoetir susceplibility o( (4e solid solutions ir s^Is^-wiil in Table 1.
It was established as a resula nhat thr snlid solution with infinite dilution contains single atoms of Fe(III) and tlte; dimers (lE^e;^III))-I^eîCI^II^^ weth an antifarro- and foзromagnvtiз exchange types. As the solid soUution boo -encentbOioe iaaoeesed, lhe fcnctiono of Fe(IIl) monomers and aerromagneficagu bound dimnrs decrease algng willi the inaroase in tle aracIlono ot antiOerromaggotécally boucd ones (Table 1). The chnnge in the type of intenaction aimong the atoms winh paramagnetic properties in a clustss wlch incseasmg oancentraCson cen br ^usiij^Pl^ii^iinssKs<il by the degiee of defecrivanesi of tine o^gsn surroundings and the natuoe of the ahstcrtrntion of atomt of iron over 9he aationic positions. It can be assumedthat in the henerovalentiy-substituted solid solutions, the oncuraence of tine oxygen vacanciet lsade So ibe suppce ssioa oT rntlfeeeomagneiic exchaage channels af the ty pe Kmj2 to Il 08an aid to start the action of the feiromagnetic exchange channel; 2 t pe t af,. With am increase tn the confeat oe meoms oSFr(III) in Ooere solutions, tge ftachon 6, exchange bound aggsegutes that staMiize fte crystal structure axe incoeaeing ^ueî to the localization of" aggieggtes aeae ehe oxygen oi-jaLc^jE^r^c^ii;;^^ ws'lnicli reduces tine degree on distortion ot "tJaita; o6ygen sucroundingi of the naramegnetic atomt. Tj'ic.s; in the cause oe the increase in the grsction ou aggaegetot wife nnSieerrimeunoric esre^cssal^iaia^gii^e type, realized tirongh the cxuhanea cnauneli 0,2 _ 2 || 6 || ;2_ 2 anf a9,,.., \\p( ||1xa The appra-aare ctf tbe dimeis
"^^It^i^ i. TTe of calculation of distribution ot atoms of iron in time irsn-dcped B^NbOy1
x ^<Jmn(f) ) fa (Uh )sFe (III ) UFe(m) „ dim(a) i(III )
141OD: 22001 K ^(50 It 35220 ]EC
0.000 0.58 n.40 o.o;?: ni.r(ta.6) 30.88^30.^;) ^4r.;3(2!:5t;) 3^n.8((l ^.CC)
0.006 o.42 ^.nsr-tl- 0.2!(1 a6.2(56.t) 2a. 3-26.4) 3^.:5(:ro.:oi CS.ÇlC^^l)
o.orr eeo 24.a(25V( 39P(t9.8o
o.oaixî O^ncn) ^.rrit 0.^3 5>;;!.O(i:r:2.L0))
o.o;;:o 0.^9) o.-t^ 2^1)). ^^59 .6°) (4(7(14:^.)))
0:0^1 0.36 0.1t 40^(47(26 3^(8;oi8.4i) 141.6(1^.8)
0.341 co;t 0.^411 22.5(23.(( ^■4.41(13.3.
0.059 0.)10 c.58 4^4.(5(44e.^) 17.2f17.5 C i4-.:-i(i^(<i;:
Footnote : 1—et)!) is t letstion ot monoet^ir^ of iron S111 dd, aa,(iii)-Fe(iii) is a fraction af dimeis ot iron (III) with ferromagnetic eochange, aS™—)mmf is a Sraftion off dimerx of iron (illUE) with antiferromagnetic exclenge.
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.
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