Electronic structure of diluted SrFexTi1-xO3-δ solid solutions

Journal of Solid State Chemistry 274 (2019) 259–264

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Electronic structure of diluted SrFexTi1-xO3-δ solid solutions N. Chezhina a, *, D. Korolev a, R. Bubnova a, b, Y. Biryukov b, O. Glumov a, V. Semenov a a b

St. Petersburg State University, Universitetskaya Emb. 7/9, St. Petersburg, 199034, Russia Institute of Silicate Chemistry of the Russian Academy of Sciences (ISC RAS), Makarova Emb. 2, St. Petersburg, 199034, Russia

A R T I C L E I N F O

A B S T R A C T

Keywords: Doped strontium titanate High-temperature X-ray powder diffraction (HTXRD) Magnetic susceptibility M€ ossbauer spectra Electron-hole-ionic conductors

The study of diluted solid solutions SrFexTi1-xO3-δ (0.01 < x < 0.10). High-temperature X-ray powder diffraction (HTXRD), M€ ossbauer spectra and magnetic susceptibility measurements proved that up to x ¼ 0.015 in the solid solutions there are clusters of Fe(III) atoms, which take part in ferromagnetic exchange through oxygen vacancy starting from 200 K via F-centers and a small fraction of single Fe(IV) atoms. An increase in iron content results in the larger clusters with ferromagnetic exchange both between Fe(III)-[O]-Fe(III) and Fe(III)-O-Fe(IV). The solid solutions are mixed electron-hole-ionic conductors and may be used as sensors of the humidity of gases.

1. Introduction Nowadays the problems of global energy deficit as well as of pollution of the Earth resulted in an intensive search for alternative energy sources, solid oxide fuel cells, particularly [1–4]. Therefore the necessity aroused to obtain materials for SOFC meeting several requirements: The electrolytes must demonstrate good ionic (oxygen) conductivity, whereas cathodes and anodes must have both ionic and electronic conductivity. At the same time these materials operating at rather high temperature must have close coefficients of thermal expansion. Recently the interest of researchers has been centered on oxides with perovskite or perovskite like structure due to the fact that such complex oxides are tolerant to various kinds of substitution with dia- and paramagnetic elements, which makes possible the variations of electrophysical characteristics over a very large scale – from electron-ionic conductors to relaxors [5]. Perhaps the first electrolyte alternative to conventional yttrium doped zirconium dioxide and having greater oxygen conductivity was obtained on the basis of doped lanthanum gallate – La0.9Sr0.1GaO3-δ [6] and La0.9Sr0.1Mg0.2Ga0.8O3-δ [7]. This resulted in an intensive search for cathodes and anodes based on doped lanthanum gallate [8–10]. Here we must emphasize two obstacles – first, lanthanum gallate is rather expensive, which limits its application, and the search for the compositions with optimal electrophysical characteristics is going on mostly empirically without regard to the electronic structure of such materials and its correlation with physical performance. Recently the materials based on doped strontium titanate attracted attention of scientists. Strontium titanate doped with d-elements may

serve as an electron ionic conductor. Its advantages are that whereas to create the vacancies in the oxygen crystallographic orbit in lanthanum gallate it is necessary to dope it with a bivalent element, no additional doping with bivalent element is necessary upon doping strontium titanate with iron(III) and the very material is much cheaper than materials based on lanthanum gallate. In the most part of works the researchers study the conductivity of strontium titanate doped with iron [11–16]. Much less attention is focused on electronic structure of these systems [14,17], where Fe2þ was found on doping SrTiO3. All the above sited works are united by one common drawback – the approach is not systematical. There is no works where a series of solid solutions was studied with a small step by concentrations over a wide concentration range was examined. Whereas the study of electronic structure of iron in such systems and its correlation with electro physical characteristics would make possible an efficient search for the compositions with predetermined electro physical properties. It is very difficult to reveal the electron state of a transition element in concentrated solid solutions, since it is masked by interatomic interactions, which, as was shown in Refs. [18,19], influence the conductivity. Therefore the method of magnetic dilution was used, which allows us to study the states of paramagnetic atoms and the exchange interactions between them. The aim of this work was a thorough study of crystal and electronic structure of diluted SrFexTi1-xO3-δ solid solutions (0.01 < x < 0.1) and to correlate the obtained data with electro physical characteristics.

* Corresponding author. E-mail addresses: [email protected], [email protected] (N. Chezhina). https://doi.org/10.1016/j.jssc.2019.03.029 Received 18 January 2019; Received in revised form 15 March 2019; Accepted 16 March 2019 Available online 25 March 2019 0022-4596/© 2019 Elsevier Inc. All rights reserved.

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Journal of Solid State Chemistry 274 (2019) 259–264

2. Experimental

Table 1 Data of atomic emission analysis.

Solid solutions SrFexTi1-xO3-δ (0.01 < x < 0.1) were synthesized by ceramic procedure. The starting substances were analytical grade SrCO3 (99.5%), special pure grade titanium and iron oxides (99.9%). Strontium carbonate and titanium dioxide were checked for the absence of ferromagnetic impurities by magnetic susceptibility method. Thoroughly ground stoichiometric mixtures of oxides and strontium carbonate were pelleted and sintered in corundum crucibles in air at 1200  C for 30 h and at 1300  C for 30 h with three intermediate regrindings. After the end of sintering the samples were cooled in air in the cooling oven to the room temperature. To ensure the homogeneity of the obtained samples their magnetic susceptibility was measured after the synthesis (single phase according to X-ray) and after additional sintering for 10 h under the same conditions. The susceptibility being constant shows that the distribution of paramagnetic atoms is close to the equilibrium. Powder diffraction data of the solid solutions were collected using a Bruker « D2 Phaser » diffractometer at room temperature (CuKα, Nifilter, 2θ ¼ 10–120 , step 0.02 ). The phase composition was determined using PDXL integrated X-ray powder diffraction software and PDF2 2016 (ICDD) database. The Rietveld refinement of the X-ray diffraction patterns was performed using TOPAS software. The HTXRD experiment was conducted using a Rigaku Ultima IV diffractometer with a thermal attachment (CoKα, 40 kV and 35 mA, reflection geometry, D/teX Ultra high-speed detector, low vacuum, 2θ ¼ 10–110 , temperature range 93–443 K, step 10 K in the range 93–293 K, further – 5 K, heating rate 2 K/min). Experimental data processing by the Rietveld refinement was performed using DIFFRACplus Topas, approximation of temperature dependencies of lattice parameters, thermal expansion coefficients (CTE) calculation and drawing its figures were performed using ThetaToTensor [20]. The content of iron in the systems under study was determined by the method of atomic emission analysis with inductive coupled plasma on a Shimadzu ICPE-9000 optical emission spectrometer. The error in determining iron content did not exceed 5% from x in the solid solution formula. We did not determine the value of δ, since it must be only 0.05 for the most concentrated solution (x ¼ 0.1), and taking into account the fact that, as will be shown below. There is a fraction of Fe(IV), it substantially decreases. The magnetic susceptibility was measured by Faraday method in the temperature range 77–400 K. The diamagnetic corrections were inserted with respect to the susceptibility of SrTiO3 diamagnetic matrix measured over the same temperature range. The accuracy of specific susceptibility measurements was 1%. M€ ossbauer spectra were recorded on a WISSEL spectrometer in the transmission geometry at room temperature. The source of Mossbauer emission was 57Со in a rhodium matrix (57Co/Rh) with the activity 30 mCu. The isomer shifts are given relative to the isomer shift of standard absorber α-Fe. Electrophysical studies were carried out by the method of impedance spectroscopy on an Autolab PGSTAT302N device. The frequency range in the measurement was 1 MHz–100 Hz. Silver deposited on the samples by the vacuum spraying method were used as the electrodes. The measurements were carried out in a quartz electrochemical cell, which made it possible to carry out measurements at different compositions of the gas phase (dry nitrogen, wet nitrogen, air).

x(Fe) before synthesis x(Fe) after synthesis

0.01

0.015

0.02

0.03

0.04

0.06

0.08

0.009

0.012

0.015

0.023

0.037

0.050

0.071

cell parameter practically does not depend on the concentration of iron (Table 1S). This is associated with the fact that, in spite of a difference in sizes of Fe3þ, Fe4þ and Ti4þ (r(Fe3þ HS)VI ¼ 0.645 Å, r(Fe4þ)VI ¼ 0.585 Å, r(Ti4þ)VI ¼ 0.605 Å [21]), an increase in concentration of iron results in new vacancies emerging in the oxygen crystallographic orbit, which adds up to an almost constant parameter. In the plots of inverse paramagnetic component of magnetic susceptibility calculated per 1 mol of iron atoms vs temperature (1/χ Fe – T) (Fig. 2) a break is observed at about 210–220 K. These dependences for all other solid solutions are given in Fig. 2S. The first assumption was that we have a phase transition at this temperature resulting in some changes in the crystal structure. So we undertook the investigation of X-ray patterns at low temperatures. Taking into account that the bending of the curves of the magnetic susceptibility with an increase in temperature (Fig. 2) was obtained for all the SrFexTi1-xO3-δ solid solutions and in order to obtain the changes of the crystal structure caused by the temperature growth (such as the possible change of symmetry from cubic to tetragonal) the sample with the highest content of Fe (x ¼ 0.071) in the composition was chosen for the HTXRD experiment. Generally, such changes are more expectable when the highest concentration of the substituent atom is in chemical composition of sample. The examination the X-ray powder diffraction patterns in the temperature range 93–443 K shows that the sample does not undergo any phase transitions. However, the analysis of the temperature dependencies of the unit cell parameter a and unit cell volume V showed that the inflection point exists at about 210 K, just in the region of the change in the magnetic susceptibility plot. It allows one to approximate the temperature dependencies of the cell parameter and volume by splitting them into two intervals (93–210 and 210–443 K). The unit cell parameter and volume were approximated using quadratic polynomials in both temperature ranges: 93  210K   a ¼ 3:909 þ 0:044  103 T þ 0:088  106 T 2 A ;   V ¼ 59:67 þ 2:03  103 T þ 4:08  106 T 2 A 3 ; 210  443K   a ¼ 3:909 þ 0:035  103 T þ 0:021  106 T 2 A ;   V ¼ 59:74 þ 1:63  103 T þ 1:01  106 T 2 A 3 : The coefficients of thermal expansion at some temperatures calculated by the polynomials given above are given in Fig. 3 and Table 2. Temperature dependence of unit cell parameter given in Fig. 3S. Since in a cubic ABO3 perovskite all the (Fe,Ti)–O–(Fe,Ti) angles between octahedra are constant and equal to 180 , the mechanism of thermal expansion must be an increase in the B–O and A–O bond lengths as the temperature increases, both A and B atoms nature affects the thermal expansion coefficient aa, as is discussed in Ref. [22]. Therefore the only possible reason for the break in the 1/χFe - T plots must the change in the exchange interactions between paramagnetic iron(III) atoms.

3. Results and discussion According to atomic emission analysis, the concentration of iron in solid solutions before and after the synthesis is somewhat different (Table 1). We operate with a real concentration of iron given by atomic emission analysis. According to the X-ray data the obtained solid solutions have the structure of cubic perovskite (Pm-3m) (Fig. 1). X-ray patterns for all samples are given in Fig. 1S According to Rietveld refinement the unit 260

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Journal of Solid State Chemistry 274 (2019) 259–264

Fig. 1. Rietveld refinement for solid solution SrFexTi1-xO3-δ (x ¼ 0.071) at room temperature in coordinates (Normalized intensity)1/2 – 2 theta (red line - calculated data, blue line – experimental data, grey line – difference). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 2. Plots of inverse paramagnetic component of magnetic susceptibility vs temperature for the SrFexTi1-xO3-δ solid solutions for х ¼ 0.037, 0.050, 0.071.

Fig. 3. Plot of the thermal expansion coefficient αa vs temperature for the SrFexTi1-xO3-δ (x ¼ 0.071) solid solution.

Now we turn our attention to the exchange interactions in our solid solutions. The dependences of χ Fe on iron atom concentration in the solid solutions are characterized by two special features (Fig. 4). First, at low concentrations (x ¼ 0.009, 0.012, 0.015) the paramagnetic component of magnetic susceptibility does not depend on iron content. We emphasize that these data were obtained for two series of solid solutions, so there is no doubt of their reliability. And second, a further increase in iron atom concentration results in an abrupt increase in the paramagnetic component of magnetic susceptibility, which points to strong ferromagnetic

interactions. In perovskite structure there can be only superexchange interactions via oxygen atoms at the angle 180 . For iron atoms the exchange Fe3þ–O–Fe3þ must be antiferromagnetic according to the exchange channel model [23] owing to the presence of unpaired electrons in eg orbitals and their strong overlapping with p-orbitals of oxygen. However, upon heterovalent doping of stannic dioxide with iron Coey [24] using M€ ossbauer spectra found that iron atoms are high spin Fe3þ, in spite of vacancies in the oxygen crystallographic orbit. He suggested that 261

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Journal of Solid State Chemistry 274 (2019) 259–264

Table 2 Thermal expansion coefficients of SrFexTi1-xO3-δ (x ¼ 0.071) at some temperatures. TEC∙106 (K1)

T, K 93

200

210

220

253

293

443

αa αV

3.6(3) 10.9(3)

7.9(3) 24.1(1)

8.1(4) 24.4(4)

8.4(2) 25.2(2)

8.9(1) 26.6(2)

9.3(7) 27.9(2)

10.9(4) 32.8(4)

temperature range 77–220 K, but being lower than the spin only value for Fe3þ (5.92 μB, 6A1g) (Fig. 5). As the temperature increases the effective magnetic moment begins to increase abruptly. Such a dependence of μeff on temperature could be explained in two ways. First it can be due to antiferromagnetic exchange between two iron(III) atoms. But taking into account the fact that there must be a vacancy between these two atoms the distance between them is 3.90 Å too large for direct exchange. Second, there could be an admixture of other oxidation states of iron atoms – Fe2þ as suggested in Ref. [17] or Fe4þ, which would compensate for the charge misbalance and does not require any vacancies. This put forward the problem of determining the oxidation states of iron atoms. The best method for this appeared to be M€ ossbauer spectroscopy. M€ ossbauer spectra were recorded for the solid solutions containing x ¼ 0.015 and 0.071 of iron. The spectra are given in Figs. 6 and 7. It can be seen from the spectra that iron atoms are in three local surroundings. The data of M€ ossbauer parameters – isomer chemical shift and the quadruple splitting are given in Table 3, where we could conclude that a fraction of iron atoms are in the oxidation state IV. The main conclusion of M€ ossbauer study is that there is a fraction of Fe(IV) of about 15–17%, but no Fe(II) is observed. There are Fe(IV) in an undistorted surrounding and two different surroundings for Fe(III). We believe that the first doublet is for Fe(III) in an almost undistorted octahedral surrounding and the second doublet belongs to Fe(III) having a neighboring Fe(III) atom via a vacancy in the oxygen crystallographic orbit therefore being in a distorted octahedral surrounding. But Fe(IV) high spin (d4, term of ground state 5Eg) must have also a magnetic moment independent on temperature. It is reasonable to assume that Fe(IV) must be in the low spin state for the following reasons: the crystal field strength increases as the oxidation state of a transition element increases, for cobalt and nickel low spin states are often observed even in trivalent state, and the size of low spin Fe(IV) must better suit the size of Ti(IV). Then we tried to simulate the effective magnetic moments of diluted solid solutions at low temperatures as μ2exp ¼ aFe(IV)μ2Fe(IV) þ (1- aFe(IV))μ2Fe(III), where μFe(III) ¼ 5.92 μB, μFe(IV) can be calculated with respect to the fact that for 3T1g ground term it is the function of kT/λ by the formula given in Ref. [25]. Now we have two

Fig. 4. Plots of χFe vs iron content in the solid solutions for 100, 140, 180, 260, and 320 K.

ferromagnetic exchange is associated with an electron in a vacancy between two iron atoms passes to higher energy levels allowing for ferromagnetic exchange between two iron atoms and an F-center. Therefore we can suggest that at low temperatures, up to about 210 K this electron does not take part in the exchange being on a low energy levels and only at higher temperatures begins to participate in the ferromagnetic exchange between iron atoms. The effective magnetic moments (μeff) of the infinitely diluted solid solution were obtained upon extrapolating χFe to x ¼ 0. In essence, they are equal to μeff of three most diluted solid solutions, since their magnetic susceptibilities coincide with each other with the accuracy not more than 5%. The effective magnetic moment at x ¼ 0 slightly increases in the

Fig. 5. Plot of the effective magnetic moment vs temperature for the infinite dilution. Dashed line – spin only effective magnetic moment for Fe(III).

Fig. 6. M€ ossbauer spectrum for SrFexTi1-xO3-δ (x ¼ 0.071) solid solution. 262

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Journal of Solid State Chemistry 274 (2019) 259–264

Fig. 7. M€ ossbauer spectrum for SrFexTi1-xO3-δ (x ¼ 0.015) solid solution.

Fig. 8. Plot of conductivity of the solid solutions SrFexTi1-xO3-δ (x ¼ 0.037; 0.071) in air vs inverse temperature.

Table 3 M€ ossbauer spectra parameters for the SrFexTi1-xO3-δ (x ¼ 0.015; 0.071) solid solutions. x(Fe) 0.071

0.015

Singlet Doublet Doublet Singlet Doublet Doublet

1 2 1 2

Isomer chemical shift, mm/s

Quadruple shift, mm/s

Relative fraction of iron atoms, %

0.17  0.01 0.18  0.01 0.32  0.03 0.16  0.01 0.19  0.076 0.41  0.073

– 0.37  0.03 0.75  0.05 – 0.450  0.157 0.898  0.254

16.69 64.48 18.82 15.60 70.23 14.17

dependence a transfer is observed from ionic conductivity to the hole conductivity with activation energies 1.13 and 0.65 eV. A comparison of activation energies of conductivity with the results of [13] shows a good agreement for our samples with x ¼ 0.037. In Ref. [13] the single crystals with concentrations of Fe2O3 0.017, 0.033 and 0.17 mol% were studied. Moreover, the conductivity of our samples was two-four orders of magnitude higher, which is in good agreement with the scheme of oxygen vacancy emerging in the synthesized ceramics (see eq. (1)). For the SrFexTi1-xO3-δ sample with x ¼ 0.071 the measurements were carried out at a higher temperature region in air (Fig. 8). In such a manner we were able to find three linear parts in the temperature dependence of conductivity related to oxygen-ionic, hole, and mixed electron-hole conductivity. The activation energy of the third electronhole part of conductivity was 0.37 eV. These results are in good agreement with the ideas about the changes in the transfer numbers of the charge carriers as the temperature increases [13]. The results of impedance measurements of SrFexTi1-xO3-δ samples with x ¼ 0.037 carried out in dry and moist nitrogen at 706 K are given in Fig. 9.

variables for calculating μ2exp - μFe(IV) (low spin, t42g, 3T1g) and aFe(IV). Having 9 experimental points in the dependence of μeff – T we can estimate both variables with certainty. The best agreement between experimental and calculated data we obtained for aFe(IV) ¼ 0.2  0.03 and λ ¼ 120  5 cm1. Both values are reasonable since the fraction of iron(IV) according to M€ ossbauer spectra is about 0.17 and λ for coordination compounds according to Ref. [25] is 178 cm1. But in solids the spin-orbit coupling is always somewhat frozen [23] and its absolute value is lower, as was shown in Refs. [26,27]. The study of electrical conductivity of the solid solutions SrFexTi1xO3-δ (x ¼ 0.037; 0.071) showed the following. Strontium titanate doped with acceptor admixture of Fe2O3 is a mixed electron ionic conductor [13,28]. In the region of room temperatures ionic conductivity dominates at the expense of oxygen vacancies present in the perovskite structure. The substitution of titanium for iron upon doping with Fe2O3 results in an emergence of an equivalent quantity of oxygen vacancies:   2½VO  ¼ Fe'Ti

(1)

The ionization of the acceptor admixture also results in the emerging of equivalent quantity of hole charge carriers: FeTix ¼ Fe'Ti þ h

(2)

The concentration of holes h is proportional to the ½FeTix =½Fe'Ti  ratio, and the temperature of the transition from low temperature ionic conductivity to the high temperature hole conductivity depends on the ionization energy of the acceptor. The results of measuring the conductivity of the synthesized SrFexTi1-xO3-δ samples with x ¼ 0.037 and 0.071 are given in Fig. 8. Low temperature parts of the conductivity belong to the regions, where the charge transfer is performed at the expense of mobile oxygen vacancies. The activation energy of ionic conductivity is 0.69 and 0.50 eV respectively. For the second part of temperature

Fig. 9. Impedance spectra of SrFexTi1-xO3-δ (x ¼ 0.037) in dry and wet nitrogen at 706 K. 263

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Journal of Solid State Chemistry 274 (2019) 259–264

References

At such a temperature strontium titanate is a hole conductor. The sample under study was first in the atmosphere of moist nitrogen, and then it was ventilated with dry nitrogen and after this moist nitrogen was again fed into the measuring cell. As was shown by the measurements the resistance of SrFexTi1-xO3-δ (x ¼ 0.037) changed from 6 kOhm to 12 kOhm and reversibly returned to the starting value after feeding the cell with moist nitrogen. Therefore SrTiO3 doped with iron may act as a resistive sensor material for determination of the humidity of gases.

[1] [2] [3] [4] [5] [6]

4. Conclusions

[7]

The data of structural, magnetic and M€ ossbauer study of the SrFexTi1(0.01  x  0.1) allowed the following concept of the electronic structure of strontium titanate doped with iron to be advanced. At the infinite dilution of the solid solutions there remain a small (about 20%) fraction of single iron(IV) atoms and Fe(III)-[O]-Fe(III) clusters surrounded by titanium atoms. Iron atoms do not take part in the exchange at low temperatures (up to 200 K) and at higher temperatures the exchange becomes ferromagnetic via F-centers. An increase in concentration of iron results in the increase in ferromagnetic exchange owing to the clusters enlargement. They can include Fe(IV) close to Fe(III) with the vacancy, and this also results in ferromagnetic exchange between iron atoms with difference oxidation states. The electrical conductivity of strontium titanate doped with iron is close to the conductivity of lanthanum gallate doped with iron strontium and magnesium [29]. Moreover, SrTiO3 doped with iron may be used as a sensor for determination of the humidity of gases.

[8]

xO3-δ

[9]

[10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

[20] [21] [22] [23]

Acknowledgements We are grateful to student A. Paderina for the synthesis of samples and their sample preparation for chemical analysis. Scientific researches were performed at the Research park of St. Petersburg State University «Center for X-ray Diffraction Studies» and «Center for Chemical Analysis and Materials Research».

[24] [25] [26] [27] [28]

Appendix A. Supplementary data

[29]

Supplementary data to this article can be found online at https://do i.org/10.1016/j.jssc.2019.03.029.

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