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Thermodynamic properties of the SrFeO2.5 and SrMnO2.5 brownmillerite-like compounds by means of EMF-measurements
Solid State Ionics 134 (2000) 265–270 www.elsevier.com / locate / ssi
Thermodynamic properties of the SrFeO 2.5 and SrMnO 2.5 brownmillerite-like compounds by means of EMF-measurements S. Tanasescu*, N.D. Totir, D.I. Marchidan Institute of Physical Chemistry, Splaiul Independentei 202, 77208, Bucharest, Romania Received 26 October 1999; received in revised form 21 February 2000; accepted 22 May 2000
Abstract The solid-oxide electrolyte galvanic cell method has been employed in order to obtain thermodynamic properties of the brownmillerite-like compounds SrFeO 2.5 and SrMnO 2.5 within the temperature range of 1073–1273 K. The standard Gibbs energy, the enthalpy and the entropy of formation, as well as the partial pressures of oxygen have been obtained. The experimental thermodynamic data have been compared to the existing theoretical estimated values. 2000 Elsevier Science B.V. All rights reserved. Keywords: Thermodynamic properties; Oxygen-deficient compounds; EMF-measurements
1. Introduction The compounds in the Sr–Fe–O and Sr–Mn–O systems form an important and interesting object of study owing to their technologically useful electric, magnetic and catalytic properties. This explains the great amount of work with respect to the stoichiometry, crystallographic structure and other physical properties such as electrical conductivity [1–7]. Instead, although a thorough knowledge of the thermodynamics of these materials is very important in technological applications, the literature is lacking as regards the quantitative experimental data. This could be explained by the difficulties met in experimentally approaching these compounds. *Corresponding author. Fax: 140-1-312-1147. E-mail address: [email protected] (S. Tanasescu).
In the absence of a complete set of experimental data, Yokokawa and co-workers have estimated the thermodynamic properties for some alkaline earths or rare earth–transition metal perovskites using an empirical correlation between the stabilization energy and the Goldschmidt tolerance factor. On the basis of these values, the thermodynamic features of the chemical stability of perovskite oxides are discussed in terms of the stabilization energy and the valence stability [8–10]. In order to obtain a rigorous chemical thermodynamic approach, the quantitative experimental thermodynamic data are of fundamental value. In the present study, new measurements have been made with the solid state galvanic cell method previously described [11,12] which ensure a considerable improved precision. The cell includes an yttrium stabi¨ lized zirconia solid electrolyte and an iron–wustite
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00731-1
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S. Tanasescu et al. / Solid State Ionics 134 (2000) 265 – 270
reference electrode. The thermodynamic properties represented by the standard Gibbs energy, the enthalpy and the entropy of formation, as well as the equilibrium oxygen pressures were obtained as a function of temperature in the range of 1073–1273 K.
2. Experimental The design of the apparatus, as well as the theoretical and experimental considerations related to the method applied were already described [11]. Their principal characteristics are as follows. • The geometry of the work cell ensures a compact electrode–electrolyte assembly, so that temperature uniformity is easily reached and the extraneous thermoelectric potentials are minimized. • The measurements are performed in vacuum at a residual gas pressure of 10 25 –10 26 Pa, after a previous rinsing of the installation with purified argon. The insulation of the electrodes in compartments which are separately evacuated and monitored for the gas pressure avoids the possibility of the oxygen transfer between the electrodes through the gas. • Prevention of signal pick-up from external heating sources is achieved by the use of a noninductive furnace and by the adequate shielding of cell and leads.
2.1. Materials The solid state galvanic cell employed in this study can be schematically written as follows: (2) Sample (SrFeO 32x or SrMnO 32x ) / ZrO 2 (Y 2 O 3 ) / ¨ Fe, wustite (1)
(1)
The solid electrolyte is a disc of 12.84 wt.% yttria stabilized zirconia (10 mm diameter, 6 mm thick) supplied by Risø National Laboratory Roskilde, Denmark.
The samples of strontium ferrate and strontium manganite are small pellets of 2 mm thick and 2 mm high and were prepared from the powders supplied ¨ by Forschungszentrum Julich GmbH, Germany. The X-ray diffraction patterns show the starting material of the strontium ferrate to contain a mixture of two nonstoichiometric compounds: SrFeO 2.86 and SrFeO 2.97 . The strontium manganate powder is a stoichiometric SrMnO 3 compound. After each run of measurements under the conditions of our experimental method (high vacuum and temperatures above 1073 K), X-ray diffraction analysis was performed on the samples cooled at room temperature. The diffraction patterns showed the presence of the SrFeO 32x and SrMnO 32x phases with the brownmillerite structure (x 5 0.5). ¨ The iron–wustite reference electrode was prepared from electrolytic iron (Koch-Light iron) and Fe 2 O 3 (Fischer Certified Reagent ferric oxide) powders mixed in a molar ratio of 4:1, pressed at 15 MPa and sintered at 1373 K in vacuum for 12 h. Thus, small cylindrical pieces 2 mm thick and 2 mm high were prepared.
2.2. Measurements After the cell was rinsed with pure argon, the installation was emptied to 10 25 –10 26 Pa and slowly heated. The heating was gradually performed at a constant rate, so that the rest of the pressure in the apparatus did not exceed 10 24 Pa. The temperature gradient across the cell must be less than 1 K. The EMF measurements were undertaken with an accuracy of 1 mV by means of a Keithley 197 Microvoltmeter. The electromotive force was measured at increasing and decreasing temperature, within the 1073–1273 K range. The readings were made at 50-K intervals, every time waiting till the equilibrium values were recorded. The determinations were considered to be satisfactory when values for increasing and decreasing temperatures agreed within 1 or 2 mV. The potential was measured with the furnace on, since tests have shown that the variation of potential on switching off the furnace was only 0.05 mV. Three independent series of measurements were carried out. The measurements lasted 3 days for one sample.
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267
3. Results and discussion
3.1. SrFeO2.5 Previous studies regarding the structural properties showed that strontium ferrate (Fe 31 , Fe 41 ) with the chemical composition SrFeO 32x (x stands for oxygen deficiency) is unique in the sense that it exhibits a wide range of oxygen deficiency and an interesting change in the crystallographic structure, as a function of temperature and oxygen pressure [1,4,5,16–18]. When the material is prepared by the normal solid-state reaction between SrCO 3 and Fe 2 O 3 in air, an anion deficient phase of composition near SrFeO 2.87 is obtained and it crystallises in a cubic (or a slightly distorted tetragonal) perovskite. If this oxygen deficient perovskite is fired in vacuum or in an oxygen free atmosphere, at temperatures above 973 K, its composition changes into that of a most oxygen-deficient perovskite phase SrFeO 2.5 (or Sr 2 Fe 2 O 5 ) with a statistically disordered oxygendeficient perovskite structure [16,17]. By subsequently cooling it down to room temperature, it is no longer a perovskite, but has a structure corresponding to that of brownmillerite [16,18]. Taking into account these considerations and the conditions of our experimental method (high vacuum and temperatures above 1073 K), the SrFeO 2.5 (or Sr 2 Fe 2 O 5 ) composition is anticipated for the sample. Actually, the X-ray diffraction analysis, effected after each run of EMF measurements, confirmed the presence of the brownmillerite structure. The composition of the sample was also controlled both by EMF and coulometric titration measurements. The temperature dependence of the steady state EMF in the range of 1073–1273 K is shown in Fig. 1. For the whole temperature range, reproducible EMF values are obtained on raising and lowering the temperature. The least square line was found to be: E(60.001) (V) 5 2 0.070 1 9 3 10 25 T.
(1)
To demonstrate that the composition of the sample maintains itself under the conditions of our experiments, the coulometric titration method previously described [19] was used and the following experiment at 1173 K was carried out (see Fig. 2). The
Fig. 1. Temperature dependence of EMF of the cells: h (2) ¨ SrFeO 2.5 / ZrO 2 (Y 2 O 3 ) / Fe, wustite (1); s (2) SrMnO 2.5 / ¨ ZrO 2 (Y 2 O 3 ) / Fe, wustite (1).
Fig. 2. Coulometric titration test. Open and closed symbols indicate the data points obtained before and after titration, respectively. h and j, SrFeO 2.5 ; s and d, SrMnO 2.5 .
EMF of the cell before titration is observed to be stable within less than 1 mV over a period of |24 h (only part of which is shown in Fig. 2), indicating a leak-free system and a constant composition of the
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S. Tanasescu et al. / Solid State Ionics 134 (2000) 265 – 270
sample. After reading the steady-state EMF, a titration with a constant current of 50 mA was performed over a period of 10 h. After the coulometric titration is terminated, the same values of the EMF as before the titration are obtained (Fig. 2), which demonstrates that under the respective work conditions we have the same compound, without any change in the oxygen content. Our observations are in accordance with the data of Mizusaki and co-workers [20–22]. The results of their high-temperature gravimetric measurements showed that the tendency of the weight change of SrFeO 32d with log pO 2 at temperatures above 1173 K is similar to that of La 12x Sr x FeO 32d . The authors consider the plateau observed at 1173 K and 1373 K and oxygen pressures comprised between 10 25 – 10 212 Pa as the one corresponding to d 5 x / 2 5 1 / 2, and assigned the point of minimum ≠d / ≠ log pO 2 as exactly the point of d 5 0.5 or the point corresponding to the composition of SrFeO 2.5 [20]. This assignment confirms our results concerning the oxygen content of the sample in the conditions of our experimental method (T .1073 K and oxygen pressures of 10 25 –10 26 Pa). The steady state equilibrium cell voltage E is related to the oxygen equilibrium pressure pO 2 of the sample by Nernst’s equation: pO 2 E 5 2 RT / 4F ln ]] pO 2 (ref)
(2)
where pO 2 and pO 2 (ref) are the oxygen partial pressures of the sample and the reference electrode, respectively, R is the universal gas constant (R5 8.31441 J mol 21 K 21 ) and T is the absolute temperature. Solving this equation for the oxygen partial pressure pO 2 , one obtains the expression: pO 2 5 pO 2 (ref ) .exp (24EF /RT ).
(3)
The partial pressures associated with the iron– ¨ wustite equilibrium (2Fe1O 2 52FeO) were calculated from the equation: log ( pO 2 (ref ) / Pa) 5 2 27636 /T 1 11.7960.04
(4)
evaluated from calorimetrically verified gas equilib-
rium measurements [13–15] and valid between 873 and 1600 K. By using the linear regression analysis, the relationship for the temperature dependence of the log pO 2 was found: log ( pO 2 / Pa) 5 2 27404 /T 1 10.1260.02.
(5)
In the whole temperature range the oxygen partial pressure depends only on temperature. According to the phase rule, the system has only one degree of freedom when three solid phases exist in equilibrium with the gas phase. The value at 1273 K of the log pO 2 fits well with the approximate value corresponding to the stability field of SrFeO 2.5 indicated by Yokokawa in the chemical potential diagram of the Sr–Fe–O system [10]. In the absence of other experimental data and taking into account the diagrams for the stability fields in the Sr–M–O (M5Fe, Mn) systems that Yokokawa constructed by using estimated thermodynamic properties, we consider as a plausible reaction for the dissociation of Sr 2 Fe 2 O 5 : Sr 2 Fe 2 O 5 5 2SrO 1 2FeO 1 1 / 2O 2
(6)
Because of the linear relationship between log pO 2 and 1 /T we suppose that the composition of Sr 2 Fe 2 O 5 in equilibrium with the oxides does not change significantly in the above mentioned temperature range. With the help of the thermodynamic data of SrO and FeO taken from the thermodynamic tables [23] we have calculated the standard Gibbs energies of formation of Sr 2 Fe 2 O 5 . From the temperature dependence, the standard enthalpies and entropies of formation are obtained (Table 1). The overall uncertainty due to the temperature and potential measurement (taking into account the overall uncertainty of a single measurement and also the quoted accuracy of the voltmeter) was 61.5 mV. This was equivalent to 60.579 kJ mol 21 for the free energy change of the cell. Considering the uncertainty 60.523 kJ mol 21 in the thermodynamic data for ¨ the iron–wustite references [10,11], the overall accuracy of the data was thus estimated to be 1.6 kJ mol 21 . For the enthalpies the errors were 60.45 kJ mol 21 and for the entropies 61.1 J mol 21 K 21 .
S. Tanasescu et al. / Solid State Ionics 134 (2000) 265 – 270
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Table 1 Thermodynamic data for Sr 2 Fe 2 O 5 formation (temperature range 1073–1273 K) T/K
2 DG 0f / kJ mol 21
2 DH 0f / kJ mol 21
2 DS 0f / kJ K 21 mol 21
1073 1123 1173 1223 1273
1637.9 1622.1 1606.2 1590.2 1574.1
1838.1
0.533
Errors due to the data taken from the literature are not included in these values because of the unavailability of reliable standard deviations. A third law treatment applied for obtaining DH 0298 of Sr 2 Fe 2 O 5 based on EMF experimental results leads to a value of 2,165,148 J mol 21 which seems to fit well with the theoretical value of 22,118,854 J mol 21 listed in Yokokawa’s data file and the reevaluated value of 22,128,200 J mol 21 indicated by Hilpert [24].
3.2. SrMnO2.5 Also in the system Sr–Mn–O, it is known that the oxygen deficiency of SrMnO 32x is related to the temperature and the oxygen pressure [1,2] affecting at the same time the crystal structure. The temperature of the given polymorphic transformation decreases as the pO 2 decreases. For pressures of about 6 Pa or smaller ones, a compound with x 5 0.5 is found. In our experimental conditions it is expected that SrMnO 2.5 (or Sr 2 Mn 2 O 5 ) should to be present. The composition of the sample was also controlled by coulometric titration measurements (Fig. 2). It is found that EMF maintains the same constant value at a given temperature before and after the coulometric titration. The least square expression for the temperature dependence of the EMF in the range of 1073–1273 K is:
E(60.001) (V) 5 2 0.014 1 2.16 3 10 25 T.
(7)
By using the linear regression analysis, the relationship for the temperature dependence of the log pO 2 was found: log ( pO 2 / Pa) 5 2 27404 /T 1 10.1260.02.
(8)
The values of log pO 2 obtained from our measurements also indicate the formation of the Sr 2 Mn 2 O 5 , according to the chemical potential diagram of the Sr–Mn–O system inferred by Yokokawa [10]. The corresponding values of the standard Gibbs energies of formation of Sr 2 Mn 2 O 5 , as well as the standard enthalpies and entropies of formation are shown in Table 2. In the absence of other experimental values, it would be interesting to compare our data to the theoretical results obtained by using an empirical correlation between the stabilization energy (defined in terms of the enthalpies of formation) and the Goldschmidt tolerance factor, t for the SrMnO 3 [25]. The value used so far in the data file was estimated by the relation DH / kJ mol 21 5 2901750(12t) which is valid for the formation of SrMnO 3 from SrO and MnO 2 within an A(III)M(III)O 3 perovskite matrix. In this way a value of DH5 2122 kJ mol 21 is obtained. Likewise when the formation of SrMnO 3 within an A(II)M(IV)O 3 perovskite matrix is consid-
Table 2 Thermodynamic data for Sr 2 Mn 2 O 5 formation (temperature range 1073–1273 K) T/K
2 DG 0f / kJ mol 21
2 DH 0f / kJ mol 21
2 DS 0f / kJ K 21 mol 21
1073 1123 1173 1223 1273
1873.1 1857.2 1841.5 1825.7 1809.5
2213.7
0.317
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S. Tanasescu et al. / Solid State Ionics 134 (2000) 265 – 270
ered, the relationship DH5 212511000(12t) is used, which leads to the value of 2168 kJ mol 21 . By using our data and additionally considering the second law enthalpy of formation we obtained a value of 2157.038 kJ mol 21 for DH 0 , which is 34 kJ more negative than the estimated value for the A(III)M(III)O 3 matrix.
4. Conclusions In investigating thermodynamic properties of the SrFeO 2.5 and SrMnO 2.5 compounds, the solid electrolyte galvanic cell technique shows large potentialities. The method allowed us to obtain quantitative experimental data, such as the Gibbs energies, enthalpies and entropies of formation, as well as the equilibrium oxygen pressures in the temperature range of 1073–1273 K. The standard heats of formation at 298 K calculated from the data based on EMF experimental results are consistent with the theoretical values estimated by Yokokawa and reevaluated at KFA-Julich.
Acknowledgements The authors are grateful to the Commission of the European Communities (Science, Research and Development) for financial support in the frame of the Contract JOU 2-CT 92-0063. We wish to thank Risø National Laboratory for supplying the solid electrolyte and Forschungszentrum Julich GmbH for preparing the samples used in this study. Many thanks are extended to Professor Hilpert for helpful discussion.
References [1] P.K. Gallagher, J.B. MacChesney, D.N.E. Buchanan, J. Chem. Phys. 41 (1964) 2429.
[2] J.B. MacChesney, R.C. Sherwood, J.F. Potter, J. Chem. Phys. 43 (1907) 1965. [3] T. Negas, R.S. Roth, J. Solid State Chem. 1 (1970) 409. [4] H. Yamamura, R. Kiriyama, Nippon Koyaku Kaishi 2 (1972) 343. [5] S. Shin, M. Yonemura, H. Ikawa, Mater. Res. Bull. 13 (1978) 1017. [6] S. Shin, Y. Hatakeyama, K. Ogawa, K. Shimomura, Mater. Res. Bull. 14 (1979) 133. [7] K. Kuroda, K. Shinozaki, K. Uematsu, N. Mizutani, M. Kato, J. Am. Ceram. Soc. 63 (1980) 109. [8] H. Yokokawa, T. Kawada, M. Dokiya, J. Am. Ceram. Soc. 72 (1989) 152. [9] H. Yokokawa, N. Sakai, T. Kawada, M. Dokiya, J. Electrochem. Soc. 138 (1991) 1018. [10] H. Yokokawa, N. Sakai, T. Kawada, M. Dokiya, Solid State Ionics 52 (1992) 43. [11] S. Tanasescu, N.D. Totir, D.I. Marchidan, Mater. Res. Bull. 32 (1997) 925. [12] S. Tanasescu, N.D. Totir, D.I. Marchidan, Mater. Res. Bull. 32 (1997) 915. [13] L.S. Darken, R.W. Gurry, J. Am. Chem. Soc. 67 (1945) 1398. [14] K.K. Kelley, US Bureau of Mines Bulletin No. 584, 1960. [15] K.K. Kelley, E.G. King, US Bureau of Mines Bulletin No. 592, 1961. [16] S. Shin, H. Arakawa, Y. Hatakeyama, K. Ogawa, K. Shimomura, Mater. Res. Bull. 14 (1979) 633. [17] S. Shin, M. Yonemura, H. Ikawa, Bull. Chem. Soc. Jpn. 52 (1979) 947. [18] T. Sasamoto, J. Mizusaki, M. Yoshimura, W.R. Cannon, M.K. Bowen, Yogyo-Kyokai-Shi 90 (1982) 24. [19] S. Tanasescu, N.D. Totir, D.I. Marchidan, Electrochim. Acta 43 (1998) 1675. [20] J. Mizusaki, M. Okayasu, S. Yamauchi, K. Fueki, J. Solid State Chem. 99 (1992) 166. [21] J. Mizusaki, M. Yoshihiro, S. Yamauchi, K. Fueki, J. Solid State Chem. 58 (1985) 257. [22] J. Mizusaki, M. Yoshihiro, S. Yamauchi, K. Fueki, J. Solid State Chem. 67 (1987) 1. [23] I. Barin, O. Knacke, Thermochemical Properties of Inorganic Substances, Springer-Verlag, Berlin, 1973. [24] Technical Progress Report of KFA for the EC project ‘New SOFC Materials and Technology’, 1994, January. [25] Technical Progress Report of KFA for EC project ‘New SOFC Materials and Technology’, 1994, May.
Thermodynamic properties of the SrFeO 2.5 and SrMnO 2.5 brownmillerite-like compounds by means of EMF-measurements S. Tanasescu*, N.D. Totir, D.I. Marchidan Institute of Physical Chemistry, Splaiul Independentei 202, 77208, Bucharest, Romania Received 26 October 1999; received in revised form 21 February 2000; accepted 22 May 2000
Abstract The solid-oxide electrolyte galvanic cell method has been employed in order to obtain thermodynamic properties of the brownmillerite-like compounds SrFeO 2.5 and SrMnO 2.5 within the temperature range of 1073–1273 K. The standard Gibbs energy, the enthalpy and the entropy of formation, as well as the partial pressures of oxygen have been obtained. The experimental thermodynamic data have been compared to the existing theoretical estimated values. 2000 Elsevier Science B.V. All rights reserved. Keywords: Thermodynamic properties; Oxygen-deficient compounds; EMF-measurements
1. Introduction The compounds in the Sr–Fe–O and Sr–Mn–O systems form an important and interesting object of study owing to their technologically useful electric, magnetic and catalytic properties. This explains the great amount of work with respect to the stoichiometry, crystallographic structure and other physical properties such as electrical conductivity [1–7]. Instead, although a thorough knowledge of the thermodynamics of these materials is very important in technological applications, the literature is lacking as regards the quantitative experimental data. This could be explained by the difficulties met in experimentally approaching these compounds. *Corresponding author. Fax: 140-1-312-1147. E-mail address: [email protected] (S. Tanasescu).
In the absence of a complete set of experimental data, Yokokawa and co-workers have estimated the thermodynamic properties for some alkaline earths or rare earth–transition metal perovskites using an empirical correlation between the stabilization energy and the Goldschmidt tolerance factor. On the basis of these values, the thermodynamic features of the chemical stability of perovskite oxides are discussed in terms of the stabilization energy and the valence stability [8–10]. In order to obtain a rigorous chemical thermodynamic approach, the quantitative experimental thermodynamic data are of fundamental value. In the present study, new measurements have been made with the solid state galvanic cell method previously described [11,12] which ensure a considerable improved precision. The cell includes an yttrium stabi¨ lized zirconia solid electrolyte and an iron–wustite
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00731-1
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S. Tanasescu et al. / Solid State Ionics 134 (2000) 265 – 270
reference electrode. The thermodynamic properties represented by the standard Gibbs energy, the enthalpy and the entropy of formation, as well as the equilibrium oxygen pressures were obtained as a function of temperature in the range of 1073–1273 K.
2. Experimental The design of the apparatus, as well as the theoretical and experimental considerations related to the method applied were already described [11]. Their principal characteristics are as follows. • The geometry of the work cell ensures a compact electrode–electrolyte assembly, so that temperature uniformity is easily reached and the extraneous thermoelectric potentials are minimized. • The measurements are performed in vacuum at a residual gas pressure of 10 25 –10 26 Pa, after a previous rinsing of the installation with purified argon. The insulation of the electrodes in compartments which are separately evacuated and monitored for the gas pressure avoids the possibility of the oxygen transfer between the electrodes through the gas. • Prevention of signal pick-up from external heating sources is achieved by the use of a noninductive furnace and by the adequate shielding of cell and leads.
2.1. Materials The solid state galvanic cell employed in this study can be schematically written as follows: (2) Sample (SrFeO 32x or SrMnO 32x ) / ZrO 2 (Y 2 O 3 ) / ¨ Fe, wustite (1)
(1)
The solid electrolyte is a disc of 12.84 wt.% yttria stabilized zirconia (10 mm diameter, 6 mm thick) supplied by Risø National Laboratory Roskilde, Denmark.
The samples of strontium ferrate and strontium manganite are small pellets of 2 mm thick and 2 mm high and were prepared from the powders supplied ¨ by Forschungszentrum Julich GmbH, Germany. The X-ray diffraction patterns show the starting material of the strontium ferrate to contain a mixture of two nonstoichiometric compounds: SrFeO 2.86 and SrFeO 2.97 . The strontium manganate powder is a stoichiometric SrMnO 3 compound. After each run of measurements under the conditions of our experimental method (high vacuum and temperatures above 1073 K), X-ray diffraction analysis was performed on the samples cooled at room temperature. The diffraction patterns showed the presence of the SrFeO 32x and SrMnO 32x phases with the brownmillerite structure (x 5 0.5). ¨ The iron–wustite reference electrode was prepared from electrolytic iron (Koch-Light iron) and Fe 2 O 3 (Fischer Certified Reagent ferric oxide) powders mixed in a molar ratio of 4:1, pressed at 15 MPa and sintered at 1373 K in vacuum for 12 h. Thus, small cylindrical pieces 2 mm thick and 2 mm high were prepared.
2.2. Measurements After the cell was rinsed with pure argon, the installation was emptied to 10 25 –10 26 Pa and slowly heated. The heating was gradually performed at a constant rate, so that the rest of the pressure in the apparatus did not exceed 10 24 Pa. The temperature gradient across the cell must be less than 1 K. The EMF measurements were undertaken with an accuracy of 1 mV by means of a Keithley 197 Microvoltmeter. The electromotive force was measured at increasing and decreasing temperature, within the 1073–1273 K range. The readings were made at 50-K intervals, every time waiting till the equilibrium values were recorded. The determinations were considered to be satisfactory when values for increasing and decreasing temperatures agreed within 1 or 2 mV. The potential was measured with the furnace on, since tests have shown that the variation of potential on switching off the furnace was only 0.05 mV. Three independent series of measurements were carried out. The measurements lasted 3 days for one sample.
S. Tanasescu et al. / Solid State Ionics 134 (2000) 265 – 270
267
3. Results and discussion
3.1. SrFeO2.5 Previous studies regarding the structural properties showed that strontium ferrate (Fe 31 , Fe 41 ) with the chemical composition SrFeO 32x (x stands for oxygen deficiency) is unique in the sense that it exhibits a wide range of oxygen deficiency and an interesting change in the crystallographic structure, as a function of temperature and oxygen pressure [1,4,5,16–18]. When the material is prepared by the normal solid-state reaction between SrCO 3 and Fe 2 O 3 in air, an anion deficient phase of composition near SrFeO 2.87 is obtained and it crystallises in a cubic (or a slightly distorted tetragonal) perovskite. If this oxygen deficient perovskite is fired in vacuum or in an oxygen free atmosphere, at temperatures above 973 K, its composition changes into that of a most oxygen-deficient perovskite phase SrFeO 2.5 (or Sr 2 Fe 2 O 5 ) with a statistically disordered oxygendeficient perovskite structure [16,17]. By subsequently cooling it down to room temperature, it is no longer a perovskite, but has a structure corresponding to that of brownmillerite [16,18]. Taking into account these considerations and the conditions of our experimental method (high vacuum and temperatures above 1073 K), the SrFeO 2.5 (or Sr 2 Fe 2 O 5 ) composition is anticipated for the sample. Actually, the X-ray diffraction analysis, effected after each run of EMF measurements, confirmed the presence of the brownmillerite structure. The composition of the sample was also controlled both by EMF and coulometric titration measurements. The temperature dependence of the steady state EMF in the range of 1073–1273 K is shown in Fig. 1. For the whole temperature range, reproducible EMF values are obtained on raising and lowering the temperature. The least square line was found to be: E(60.001) (V) 5 2 0.070 1 9 3 10 25 T.
(1)
To demonstrate that the composition of the sample maintains itself under the conditions of our experiments, the coulometric titration method previously described [19] was used and the following experiment at 1173 K was carried out (see Fig. 2). The
Fig. 1. Temperature dependence of EMF of the cells: h (2) ¨ SrFeO 2.5 / ZrO 2 (Y 2 O 3 ) / Fe, wustite (1); s (2) SrMnO 2.5 / ¨ ZrO 2 (Y 2 O 3 ) / Fe, wustite (1).
Fig. 2. Coulometric titration test. Open and closed symbols indicate the data points obtained before and after titration, respectively. h and j, SrFeO 2.5 ; s and d, SrMnO 2.5 .
EMF of the cell before titration is observed to be stable within less than 1 mV over a period of |24 h (only part of which is shown in Fig. 2), indicating a leak-free system and a constant composition of the
268
S. Tanasescu et al. / Solid State Ionics 134 (2000) 265 – 270
sample. After reading the steady-state EMF, a titration with a constant current of 50 mA was performed over a period of 10 h. After the coulometric titration is terminated, the same values of the EMF as before the titration are obtained (Fig. 2), which demonstrates that under the respective work conditions we have the same compound, without any change in the oxygen content. Our observations are in accordance with the data of Mizusaki and co-workers [20–22]. The results of their high-temperature gravimetric measurements showed that the tendency of the weight change of SrFeO 32d with log pO 2 at temperatures above 1173 K is similar to that of La 12x Sr x FeO 32d . The authors consider the plateau observed at 1173 K and 1373 K and oxygen pressures comprised between 10 25 – 10 212 Pa as the one corresponding to d 5 x / 2 5 1 / 2, and assigned the point of minimum ≠d / ≠ log pO 2 as exactly the point of d 5 0.5 or the point corresponding to the composition of SrFeO 2.5 [20]. This assignment confirms our results concerning the oxygen content of the sample in the conditions of our experimental method (T .1073 K and oxygen pressures of 10 25 –10 26 Pa). The steady state equilibrium cell voltage E is related to the oxygen equilibrium pressure pO 2 of the sample by Nernst’s equation: pO 2 E 5 2 RT / 4F ln ]] pO 2 (ref)
(2)
where pO 2 and pO 2 (ref) are the oxygen partial pressures of the sample and the reference electrode, respectively, R is the universal gas constant (R5 8.31441 J mol 21 K 21 ) and T is the absolute temperature. Solving this equation for the oxygen partial pressure pO 2 , one obtains the expression: pO 2 5 pO 2 (ref ) .exp (24EF /RT ).
(3)
The partial pressures associated with the iron– ¨ wustite equilibrium (2Fe1O 2 52FeO) were calculated from the equation: log ( pO 2 (ref ) / Pa) 5 2 27636 /T 1 11.7960.04
(4)
evaluated from calorimetrically verified gas equilib-
rium measurements [13–15] and valid between 873 and 1600 K. By using the linear regression analysis, the relationship for the temperature dependence of the log pO 2 was found: log ( pO 2 / Pa) 5 2 27404 /T 1 10.1260.02.
(5)
In the whole temperature range the oxygen partial pressure depends only on temperature. According to the phase rule, the system has only one degree of freedom when three solid phases exist in equilibrium with the gas phase. The value at 1273 K of the log pO 2 fits well with the approximate value corresponding to the stability field of SrFeO 2.5 indicated by Yokokawa in the chemical potential diagram of the Sr–Fe–O system [10]. In the absence of other experimental data and taking into account the diagrams for the stability fields in the Sr–M–O (M5Fe, Mn) systems that Yokokawa constructed by using estimated thermodynamic properties, we consider as a plausible reaction for the dissociation of Sr 2 Fe 2 O 5 : Sr 2 Fe 2 O 5 5 2SrO 1 2FeO 1 1 / 2O 2
(6)
Because of the linear relationship between log pO 2 and 1 /T we suppose that the composition of Sr 2 Fe 2 O 5 in equilibrium with the oxides does not change significantly in the above mentioned temperature range. With the help of the thermodynamic data of SrO and FeO taken from the thermodynamic tables [23] we have calculated the standard Gibbs energies of formation of Sr 2 Fe 2 O 5 . From the temperature dependence, the standard enthalpies and entropies of formation are obtained (Table 1). The overall uncertainty due to the temperature and potential measurement (taking into account the overall uncertainty of a single measurement and also the quoted accuracy of the voltmeter) was 61.5 mV. This was equivalent to 60.579 kJ mol 21 for the free energy change of the cell. Considering the uncertainty 60.523 kJ mol 21 in the thermodynamic data for ¨ the iron–wustite references [10,11], the overall accuracy of the data was thus estimated to be 1.6 kJ mol 21 . For the enthalpies the errors were 60.45 kJ mol 21 and for the entropies 61.1 J mol 21 K 21 .
S. Tanasescu et al. / Solid State Ionics 134 (2000) 265 – 270
269
Table 1 Thermodynamic data for Sr 2 Fe 2 O 5 formation (temperature range 1073–1273 K) T/K
2 DG 0f / kJ mol 21
2 DH 0f / kJ mol 21
2 DS 0f / kJ K 21 mol 21
1073 1123 1173 1223 1273
1637.9 1622.1 1606.2 1590.2 1574.1
1838.1
0.533
Errors due to the data taken from the literature are not included in these values because of the unavailability of reliable standard deviations. A third law treatment applied for obtaining DH 0298 of Sr 2 Fe 2 O 5 based on EMF experimental results leads to a value of 2,165,148 J mol 21 which seems to fit well with the theoretical value of 22,118,854 J mol 21 listed in Yokokawa’s data file and the reevaluated value of 22,128,200 J mol 21 indicated by Hilpert [24].
3.2. SrMnO2.5 Also in the system Sr–Mn–O, it is known that the oxygen deficiency of SrMnO 32x is related to the temperature and the oxygen pressure [1,2] affecting at the same time the crystal structure. The temperature of the given polymorphic transformation decreases as the pO 2 decreases. For pressures of about 6 Pa or smaller ones, a compound with x 5 0.5 is found. In our experimental conditions it is expected that SrMnO 2.5 (or Sr 2 Mn 2 O 5 ) should to be present. The composition of the sample was also controlled by coulometric titration measurements (Fig. 2). It is found that EMF maintains the same constant value at a given temperature before and after the coulometric titration. The least square expression for the temperature dependence of the EMF in the range of 1073–1273 K is:
E(60.001) (V) 5 2 0.014 1 2.16 3 10 25 T.
(7)
By using the linear regression analysis, the relationship for the temperature dependence of the log pO 2 was found: log ( pO 2 / Pa) 5 2 27404 /T 1 10.1260.02.
(8)
The values of log pO 2 obtained from our measurements also indicate the formation of the Sr 2 Mn 2 O 5 , according to the chemical potential diagram of the Sr–Mn–O system inferred by Yokokawa [10]. The corresponding values of the standard Gibbs energies of formation of Sr 2 Mn 2 O 5 , as well as the standard enthalpies and entropies of formation are shown in Table 2. In the absence of other experimental values, it would be interesting to compare our data to the theoretical results obtained by using an empirical correlation between the stabilization energy (defined in terms of the enthalpies of formation) and the Goldschmidt tolerance factor, t for the SrMnO 3 [25]. The value used so far in the data file was estimated by the relation DH / kJ mol 21 5 2901750(12t) which is valid for the formation of SrMnO 3 from SrO and MnO 2 within an A(III)M(III)O 3 perovskite matrix. In this way a value of DH5 2122 kJ mol 21 is obtained. Likewise when the formation of SrMnO 3 within an A(II)M(IV)O 3 perovskite matrix is consid-
Table 2 Thermodynamic data for Sr 2 Mn 2 O 5 formation (temperature range 1073–1273 K) T/K
2 DG 0f / kJ mol 21
2 DH 0f / kJ mol 21
2 DS 0f / kJ K 21 mol 21
1073 1123 1173 1223 1273
1873.1 1857.2 1841.5 1825.7 1809.5
2213.7
0.317
270
S. Tanasescu et al. / Solid State Ionics 134 (2000) 265 – 270
ered, the relationship DH5 212511000(12t) is used, which leads to the value of 2168 kJ mol 21 . By using our data and additionally considering the second law enthalpy of formation we obtained a value of 2157.038 kJ mol 21 for DH 0 , which is 34 kJ more negative than the estimated value for the A(III)M(III)O 3 matrix.
4. Conclusions In investigating thermodynamic properties of the SrFeO 2.5 and SrMnO 2.5 compounds, the solid electrolyte galvanic cell technique shows large potentialities. The method allowed us to obtain quantitative experimental data, such as the Gibbs energies, enthalpies and entropies of formation, as well as the equilibrium oxygen pressures in the temperature range of 1073–1273 K. The standard heats of formation at 298 K calculated from the data based on EMF experimental results are consistent with the theoretical values estimated by Yokokawa and reevaluated at KFA-Julich.
Acknowledgements The authors are grateful to the Commission of the European Communities (Science, Research and Development) for financial support in the frame of the Contract JOU 2-CT 92-0063. We wish to thank Risø National Laboratory for supplying the solid electrolyte and Forschungszentrum Julich GmbH for preparing the samples used in this study. Many thanks are extended to Professor Hilpert for helpful discussion.
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