On the enthalpic contribution to the redox energetics of SrFeO3�d

Camilla Haavik,a Tooru Atakeb and Svein Stølen*a

a Department of Chemistry, University of Oslo, Postbox 1033 Blindern, N0315, Oslo, Norway.E-mail: svein.stolen@kjemi.uio.no

b Materials and Structures Laboratory, Tokyo Institute of Technology,4259 Nagatsuta-cho, Midori-ku, Yokohama, 226-8503, Japan

Received 23rd October 2001, Accepted 14th December 2001First published as an Advance Article on the web 8th February 2002

The enthalpy of oxidation/reduction of the grossly non-stoichiometric high-temperature phase SrFeO3�d , i.e.the enthalpy of

SrFeO2:50ðperovskiteÞ þ 16 O2ðgÞ Ð SrFeO2:8333ðperovskiteÞ

has been deduced from calorimetric data. The enthalpy of oxidation of vacancy ordered brownmillerite-typeSrFeO2.50 obtained by direct reaction calorimetry combined with extensive heat capacity data (C. Haavik, T.Atake, H. Kawaji and S. Stølen, Phys. Chem. Chem. Phys., 2001, 3, 3863) shows that vacancy ordering in thisparticular case makes a significant contribution to the measured oxidation enthalpy. While it is often arguedthat the vacancy ordering in materials like SrFeO3�d depends to some degree on the thermal history of thesample investigated, such effects give a negligible contribution to the directly determined enthalpy of oxidation.The redox properties of SrFeO3�d are described through the use of a solution model where the presentlydeduced enthalpy of oxidation and the earlier reported entropy of oxidation are the only input parameters. Thepresent study indicates that the redox properties of complex non-stoichiometric perovskite-type oxides can berationalized reasonably well through the use of a simple model description. The main parameters of the modelare the difference in enthalpy and vibrational entropy of formation between the two limiting compositions of thesolid solution (here SrFeO3 and SrFeO2.5) and the configurational entropy.

Introduction

SrFeO3�d is grossly non-stoichiometric at high temperatures,while three reduced vacancy-ordered low-symmetry phasesare formed on cooling.1 These ordered phases will, in the pre-sent paper, be represented by the ideal stoichiometriesSrFeO2.50 (or Sr2Fe2O5), SrFeO2.75 (or Sr4Fe4O11), andSrFeO2.875 (or Sr8Fe8O23). The phase relations at low tempera-tures are complex and not well established. SrFeO2.75,

SrFeO2.875 and SrFeO3 , all appear to be non-stoichiometricat low temperatures. The composition limits of the phasescan, however, only be vaguely indicated based on informationfrom literature. The oxygen rich cubic SrFeO3�d seems to bestable for 2.92� (3�d)� 3.0.2 For the phase with ideal compo-sition SrFeO2.875 , single-phase materials with the same struc-ture are reported between SrFeO2.83

3 and SrFeO2.875 .4 A

single phase with an orthorhombic distortion from the basiccubic unit cell has been reported for the composition rangefrom SrFeO2.68

1 to SrFeO2.75 .3 However, in the course of

the present study, small amounts of SrFeO2.50 are observedin a sample with the nominal composition SrFeO2.725 . Hence,the compositional range of the orthorhombic phase seems tobe narrower than proposed earlier.5 SrFeO2.50 is stoichiometricat low temperatures.1,6 The crystal structures of Sr2Fe2O5 ,Sr4Fe4O11 and Sr8Fe8O23 have recently been reported basedon time-of-flight powder neutron diffraction.4

The non-stoichiometric high-temperature phase has beenmuch studied recently. Composition-partial pressure isothermsof SrFeO3�d have been determined by a number of groups.

7–11

Most studies have focused on the transport properties of thisphase since it is a model system for oxide membranes.Although the defect equilibria in the system have been given

most attention, the models implicitly give enthalpies and entro-pies of the reactions taking place in the system when the oxy-gen fugacity is changed. The enthalpy of oxidationcorresponding to the reaction SrFeO2.50+

14 O2(g) ¼ SrFeO3.00

is assumed to be temperature-independent with DoxH ¼ �160kJ mol O2

�1 for Tmedian ¼ 1248 K.8

In the present paper the enthalpy of oxidation of SrFeO2.50

is determined by high-temperature adiabatic reaction calorime-try. In this newly adapted technique for the study of oxidationenthalpies, the average oxidation enthalpy corresponding to alarge increment in the oxygen content is determined directly.The relative magnitude of the enthalpy of oxidation, enthalpyof vacancy ordering and enthalpy difference between differentordered states is discussed. The average enthalpy of oxidationand the complementary entropy of oxidation of SrFeO3�d ,reported separately,12 is used to calculate the redox propertiesof SrFeO3�d . The agreement with non-stoichiometry iso-therms indicates that the variation of the enthalpy of oxidationwith composition is negligible.

Experimental

Sample preparation and characterisation

Samples of SrFeO3�d were prepared from a-Fe2O3 (Merck,> 99%) and SrCO3 (BDH Laboratory Supplies > 98.5%).Prior to use a-Fe2O3 and SrCO3 were annealed in air at 1073K for 40 h and at 673 K for 24 h respectively, in order toobtain constant mass. Stoichiometric amounts of a-Fe2O3

and SrCO3 were ground together in a ball-mill and calcinedin air at 1250 K for 24 h. After regrinding, the powder waspressed into pellets and annealed in air at about 1500 K for

1082 Phys. Chem. Chem. Phys., 2002, 4, 1082–1087 DOI: 10.1039/b109683d

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24 h and then slowly cooled. The resulting material has com-position near SrFeO2.86 and takes the tetragonally distortedperovskite-type structure reported for SrFeO2.875 .

4 Samplesannealed at high temperatures (1450–1500 K) in N2(g) andcooled with the furnace, have oxygen content near SrFeO2.54 .The samples contain two phases: brownmillerite-typeSrFeO2.50 and a minor amount of SrFeO2.75 . Two phase sam-ples may be used in these studies since average enthalpies ofoxidation are determined and since this study and an earliertreatment of the solution energetics of perovskite-related oxi-des13 indicate that the enthalpy of oxidation varies weakly withthe non-stoichiometry in the oxide systems considered. Theoxygen-content and the unit-cell dimensions of the calori-metric samples are given in Table 1. The unit-cell dimensionsagree favorably with earlier determinations. Unit cell dimen-sions reported by Hodges et al.4 are given in the table for com-parison.The powder X-ray diffraction data were collected with a Sie-

mens D5000, equipped with a primary Ge-monochromatorand a position-sensitive detector, using Cu-Ka1-radiation(l ¼ 154.0598 pm) radiation and Si as internal standard(a ¼ 543.088 pm).14 Unit cell dimensions were determined byleast-squares refinements using the UNITCELL program.15

The oxygen content of the samples was determined by redoxtitration. The SrFeO3�d sample was dissolved in an aqueousHCl solution with a known excess amount of ammonium iro-n(II) sulfate (Mohr salt). Fe(II) from the Fe(II) sulfate reactswith Fe(IV) from SrFeO3�d and forms Fe(III). The reactionchamber is continuously flushed with Ar(g) prior to and duringthe redox reaction, to prevent oxidation of Fe(II) by O2(g) inair. After completion of the redox reaction, the solution wastitrated for excess Fe(II) with standard ceric ammonium sul-fate using ferroin (o-phenanthroline ferrous complex) as indi-cator.

Reaction calorimetry

The step-wise heated adiabatic calorimeter and the measuringtechnique have been described earlier.16,17 The calorimeter is inthe present case primarily used as a reaction calorimeter todetermine the enthalpy of oxidation/reduction of perovskite-type SrFeO3�d according to

SrFeO3�d þx

2O2 ! SrFeO3�dþx

The temperature stability obtainable with accurate shield con-trol makes adiabatic calorimeters ideal for the investigation ofboth the energetic and kinetic aspects of slow phase transfor-mations.18 In a step-wise heated adiabatic calorimeter the heatexchange with the surroundings is small and the temperaturedrift rate of the instrument is, thus, close to zero. Slow exother-mal processes taking place in a sample give rise to small butsignificant positive temperature drift rate contributions in theequilibration periods. The temperature increment associatedwith an intermittent energy input is, hence, due to three effects;the electrical energy supplied, the heat exchange with the sur-roundings and the heat released by the sample. The enthalpyreleased from a sample is, hence, determined through

DsampleH ¼ DsuppliedH þ DexchH � DtotalH

where DtotalH is the enthalpy needed to reach Tfinal if noenthalpy is released by the sample. This contribution isobtained by integration of the observed total heat capacityfrom Tstart to Tfinal

DtotH ¼ðTfinal

Tstart

Cp;totdT

Here CP,tot is the heat capacity of the calorimeter+ sample.DsuppliedH is supplied electrically by a constant current source.The current through the heater is measured every 10 s and thepotential drop across the heater every 1 s in between. Thepotential is measured using a 812 digit Hewlett-Packard digitalvoltmeter. The current is determined by measuring the poten-tial drop over a calibrated resistor. The total time, as well asthe triggering of the potential and current measurements, aremeasured or controlled using a Keithley counter.The heat exchanged with the surroundings is given by

DexchH ¼XTfinal

Tstart

DT

DtCp;totDt

where DT/Dt is the instrumental temperature-drift rate and Dtthe time-interval of each step from Tstart to Tfinal .The samples (masses 22 and 36 g) are contained in open

silica glass containers (40 cm3) that fit tightly into the calori-meter. A central well in the container holds the heater andthe platinum resistance thermometer. The calorimeter is heatedin a step-wise fashion, and electrically heated and electroni-cally controlled adiabatic shields surround the calorimeterproper. A guard shield system is fitted outside the adiabaticshield and the whole assembly is placed in a vertical tube fur-nace. The calorimeter, shield, and guard are all made of silver.The temperature is measured with an ASL F-18 resistance

bridge using locally constructed 25 O platinum resistance ther-mometers. Temperature calibration is performed according tothe recommendation of ITS-90 to 933.473 K. The accuracy intemperature determination is considered to be within 0.02 K.The temperature resolution is 3� 10�5 K.17

Thermogravimetry

Thermogravimetry (TG) was used to determine the change inoxygen stoichiometry during oxidation. Samples from thesame batches as those studied in the calorimetric measure-ments were used. The TG analyses were performed in a Per-kin-Elmer TGA7 using samples with typical mass 100 mgcontained in vitreous silica holders in Pt-cups. The experimentswere carried out in synthetic air with flow rate 20 ml min�1.Temperature was measured with a Pt-Pt/Rh thermocouplesituated close to the sample. The samples were heated at 2 Kmin�1 to 723 K, then held isothermally for 4 h and subse-quently cooled at 2 K min�1 to room temperature. The coolingof the samples took place via 4 h long isothermal steps every 50K to 473 K.

Table 1 Oxygen content and unit-cell dimensions of samples used in the calorimetric experiments. The uncertainties in the compositions are the

standard deviations between the titration parallels

(3�d) in SrFeO3�d a/A b/A c/A Structure

2.540(10)a 5.672(1) 15.570(5) 5.528(1) Brownmillerite type (space group Ibm2)

2.540(1)a 5.676(1) 15.575(2) 5.529(1) Brownmillerite type (space group Ibm2)

Literature values–Hodges et al.4

2.500 5.672(1) 15.59(2) 5.527(1)

a Small amounts of the neighboring phase SrFeO2.75 were present.

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Blank experiments with empty silica glass containers weremade. The blank experiments gave an apparent weight increase(resulting from convection in the oven) with increasing tem-perature. The sample masses were corrected accordingly.

Results and discussion

Thermal behaviour during oxidation

The temperature drift rates observed during two separate seriesof experiments are given as a function of temperature in Fig. 1.In both cases samples with overall composition SrFeO2.54 wereused. The reaction taking place in the calorimeter is

SrFeO2:54 þ0:46� d

2O2 ! SrFeO3�d ð1Þ

The composition of the reaction product was, in both experi-ments, close to SrFeO2.82 at the end of the reaction. Open sym-bols represent the temperature drift rates observed in theequilibrium periods after each individual energy input. Whenthe observed exothermal reaction was believed to be self-dri-ven, no further energy was supplied to the system and the solidpoints represent the temperature drift rate at this stage. Thesolid line gives the instrumental temperature-drift rate. Thiscurve represent the average temperature drift rate observedin several series of separate experiments on non-reacting sam-ples.The thermal behaviour observed in the two series is similar.

The initial increase in the temperature drift rate, reflecting thestart of oxidation, is observed at 550 (series I) and 530 K (ser-ies II). The temperature drift rate returned to the instrumentallevel at 643 K in the first series. In the second series, the calori-metric measurements were terminated at 619 K. The tempera-ture drift rate had at this temperature reached the instrumentallevel and the reaction was believed to be complete. Thisassumption is supported by the fact that no further energyrelease was observed on heating the sample to 670 K in series I.SrFeO2.50 that constitutes the major part of the reactant,

SrFeO2.54 , is a structurally and magnetically ordered com-pound at Tstart

6 whereas the product, SrFeO2.82 , is a structu-rally and magnetically disordered compound at Tfinal . Thestructural disordering that takes place during oxidation is evi-dent from the temperature-time profile after the last energyinput, see Fig. 2. The temperature rises continuously untilabout 630 K, where a plateau in temperature is observed. Aftersome time the temperature starts to increase again and does sountil the oxidation is complete. The sudden decrease in the

temperature drift rate, see insert to Fig. 2, is due to theendothermic transformation of ordered SrFeO2.75 to disor-dered SrFeO2.75 which is followed by a further oxidation ofthe disordered SrFeO2.75 . The order–disorder transition is evi-dent also in series II where the temperature drift rate after thelast energy input has a plateau at 615 K. The temperature ofthe plateau is 15 K lower than observed in series I, which prob-ably implies that the combination of one exothermic and oneendothermic effect makes a correct temperature determinationdifficult.

Calorimetric enthalpy of reaction and enthalpy of oxidation ofperovskite-type SrFeO3�d

The calorimetric results are summarized in Table 2. The heatreleased by the sample, DsampleH, is given by

DsampleH ¼ DsuppliedH þ DexchH � DtotalH ð2Þ

DsupplH is the electrically supplied energy. DexchH, the correc-tion for deviation from true adiabatic conditions, is deter-mined from the observed instrumental temperature drift rate.DtotalH, the enthalpy needed to reach Tfinal if no enthalpywas released by the sample, is obtained by integration of theobserved total heat capacity from Tstart to Tfinal . As an exam-ple, the total heat capacity observed during series I is given inFig. 3. The apparent total heat capacities obtained in the tem-perature range where the oxidation takes place are erroneousdue to the heat released by the sample and the values are esti-mated by interpolation as indicated in Fig. 3.The reaction takes place under non-isothermal conditions

and the reaction enthalpy should, in principle, be correctedfor by the heat capacity of the reaction. A correct evaluationof this contribution demands knowledge of the stoichiometryof the sample versus time during oxidation. This is not easilyderived and this source of systematic error is presently notaccounted for. The uncertainty in DtotH given in Table 2, how-ever, represents the maximum error induced by not taking thechange in composition of SrFeO3�d during oxidation intoaccount. The uncertainty in DexchH in Table 2 is obtained byassuming that the uncertainty of the instrumental temperaturedrift rate is 0.5 mK min�1.The enthalpy of oxidation is determined from DsampleH and

the corresponding oxygen consumption determined by ther-mogravimetric analysis using a temperature program similarto that used in the calorimetric experiments. The mass incre-ment of the calorimetric sample, and the composition of oxi-dised and reduced samples determined by chemical analysisare used to verify the thermogravimetric analyses. The mass

Fig. 1 Temperature drift rates observed in Series I (triangles) and II(circles) as a function of temperature: open symbols represent tempera-ture drift rates after an energy input; closed symbols show temperaturedrift rates at the stage where the reaction is left to itself without exter-nal disturbance. The solid line gives the instrumental temperature driftrate.

Fig. 2 Calorimeter temperature in the final stage of the oxidation ofSrFeO2.54 (Series I) as a function of time t after the last energy input.Inset: Temperature drift rate in the final stage of the oxidation ofSrFeO2.54 (Series I) as a function of temperature T.

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change observed for the sample used in series I is shown as anexample in Fig. 4. The oxidation starts at around 550 K. Amaximum in the oxygen content is observed on heating at near670 K. Further heating results in reduction. Subsequent cool-ing of the sample results in oxidation. Vertical segments in thecurve represent isothermal periods used to ensure that the sam-ple is in equilibrium with the atmosphere. The equilibriummass of the sample at Tfinal was assumed to be equal to thatobtained from the cooling curve. The results of the oxygenconsumption analyses are given in Table 3. The agreementbetween the results obtained by the different techniques isgood. The uncertainty in the oxygen consumption is domi-

nated by the uncertainty in the assignment of Tfinal and theuncertainty of the calorimetrically determined Tfinal is assumedto be 10 K.The reactions taking place in the two series of experiments

and the corresponding enthalpies of reaction are

Series I : SrFeO2:54 þ0:275

2O2 ! SrFeO2:815

� 119 7 kJ ðmol O2Þ�1 ð3Þ

Series II : SrFeO2:54 þ0:299

2O2 ! SrFeO2:839

� 123 10 kJ ðmol O2Þ�1 ð4Þ

The reduced samples contain structurally and magneticallyordered SrFeO2.50 and structurally ordered SrFeO2.75 , whereasthe reaction product is structurally and magnetically disor-dered. The average temperature of the reactions is about 600K.If we assume that the enthalpy of oxidation is independent

of composition in the range between SrFeO2.50 andSrFeO2.833 , the enthalpy of oxidation of high-temperature per-ovskite-type SrFeO3�d , can be derived from earlier reportedthermodynamic data for SrFeO2.50 and SrFeO2.833 . We haveno direct evidence for a variation in the enthalpy of oxidationwith composition. The formation of ordered compounds withintermediate composition suggests a variation, while the lineardependence of the entropy of SrFeO3�d with composition indi-cates that this variation, if present, is small. Under these cir-cumstances, the appropriate thermodynamic cycle is given inFig. 5. The resulting enthalpies of the following reactions

SrFeO2:50ðbrownmilleriteÞþ 1=6O2ðgÞÐ SrFeO2:8333ðperovskiteÞ ð5Þ

SrFeO2:50ðperovskiteÞþ 1=6O2ðgÞÐ SrFeO2:8333ðperovskiteÞ ð6Þ

are given as a function of temperature [per mol O2 (g)] in Fig.6. The enthalpies of reactions (5) and (6) are related throughthe enthalpy of the order–disorder transition in SrFeO2.50

SrFeO2:50ðbrownmilleriteÞ Ð SrFeO2:50ðperovskiteÞ ð7Þ

Two different DTA values have been reported for reaction (7),8.5 1.2 kJ mol�1 (ref. 5) and 6 kJ mol�1 (ref. 19). Theenthalpy of oxidation deduced from equilibration studies byHolt et al.8 is also included in Fig. 6.In this particular case vacancy ordering makes a significant

contribution to the measured oxidation enthalpy that must betaken into account when deriving the enthalpy of reaction (6).The relative importance of the order–disorder enthalpydecreases with increasing enthalpy of oxidation and this termis thus less important e.g. for Cr and Mn-containing systems.13

While it is often argued that the vacancy ordering in materialslike SrFeO3�d depends to some degree on the thermal historyof the sample investigated, the fact that the transition tempera-tures observed for SrFeO2.50 (1120 K5), SrFeO2.725 (660 K12)and SrFeO2.84 (560 K12) are consistent with earlier reportedtemperatures (1103 and 1123 K for SrFeO2.50 ,

1,6 670 and660 K for SrFeO2.75

3 and SrFeO2.7420 and 570 and 523 K for

SrFeO2.853,1), clearly shows that if different ordering schemes

are produced they are of similar energy. This similarity inenergy is also the very reason for the existence of severalordered phases in certain systems. The enthalpy differencebetween different ordered states, if present, thus gives a negli-gible contribution to the directly determined enthalpy of oxi-dation. It should be noted that the unit cell dimensionspresently observed are consistent with those reported in the lit-erature4 and the present samples, hence, seem to take the welldefined structural arrangements discussed by Hodges et al.4

Table 2 Calorimetric data from Series I and II

Series I Series II

Tstart/K 536.60 544.78

Tfinal/K 642.76 618.46

DsuppliedH/J 8992.66 7370.88

DexchH/J 500.(120) 960.(127)

DtotH/J 12639.(100) 10509.(70)

DsampleH/J �3146.(160) �2178.(145)

Fig. 3 Total heat capacity of the calorimeter for Series I as a functionof temperature; (X) determinations during oxidation run; (`) determi-nations in a second experiment after oxidation. The solid line showsheat capacity used in the thermodynamic calculations.

Fig. 4 Reduced mass of SrFeO2.54 during heating and cooling in air.The sample was heated at 2 K min�1 to 723 K, held isothermally for 4h, and then cooled from 723 K to 473 K (2 K min�1) with 4 h isother-mal steps every 50 K. Finally the sample was cooled to room tempera-ture at 2 K min�1.

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Deduced redox properties of SrFeO3�d (0� d� 12)

The redox process involves a simultaneous change in oxygennon-stoichiometry and in the formal valence state of iron. Inthe simplest case SrFeO3�d (0� d� 1

2) is seen as an ideal solidsolution of two limiting compositions with different formalvalence state SrFeO3 and SrFeO2.5 .

13 Fe(III) and Fe(IV) are

assumed to be randomly distributed on the iron sub-latticewhile O2� and VO2� are assumed to be distributed randomlyon the oxygen sub-lattice. The relationship between the partialpressure of oxygen, temperature and d in SrFeO3�d is in thisapproximation

logPO2¼ 4

RT ln 10DfG

� SrFeO3ð Þ � DfG� SrFeO2:5ð Þ½ �

þ 4 logð1� 2dÞ � logð2dÞ½ � � 2 logd

3� d

� �ð8Þ

The first term on the right-hand side is, in this ideal solutionapproach, the Gibbs energy of oxidation

DfG�ðSrFeO3Þ � DfG

�ðSrFeO2:5Þ � DoxG� ¼ DoxH

� � TDoxS�

ð9Þthat corresponds to the reaction

SrFeO2:5ðperovskiteÞ þ 14O2 ! SrFeO3:00ðperovskiteÞ ð10Þ

In the ideal solid solution approach used the enthalpy andentropy of oxidation are independent of composition. Themodel is, thus, compatible with the presently determined aver-age enthalpy of oxidation of perovskite-type SrFeO3�d . A cor-responding, calorimetrically determined entropy of oxidationhas been reported separately.12

Calculated stoichiometry-oxygen partial pressure isothermsat 1073 and 1273 K are compared with experimental non-stoi-chiometry data in Fig. 7. The two curves correspond to twodifferent reported enthalpies of the order–disorder transitionin SrFeO2.50

5,19 needed for the conversion of the presentlydetermined enthalpies of reaction (5) to the enthalpy of reac-tion (6). The agreement between the calculated isothermsand the non-stoichiometry data determined by equilibriummethods is very good when taking into consideration thatthe thermodynamic model is simple, that the equilibrium dataare given for temperatures far above the temperatures reachedin our calorimetric study and when considering the uncertaintyof the enthalpy of oxidation reported here. The uncertainty inthe stoichiometry-log P(O2) data is not discussed by theauthors of those papers but is expected to be significant. Thisis in part illustrated by Fig. 8 which shows a considerable scat-ter in the reported stoichiometry of SrFeO3�d in air at differenttemperatures. The agreement with the non-stoichiometry iso-therms indicates that the variation of the enthalpy of oxidationwith composition is negligible.

Conclusion

The present study and a corresponding investigation of theredox properties of CaMnO3�d and SrMnO3�d

22 indicate thatthe redox properties of complex non-stoichiometric perovskite-type oxides in general can be rationalized reasonably well

Table 3 Oxygen content and change in stoichiometry on oxidation for series I and II of the calorimetric experiments

Chemical analysis Thermogravimetry

Series I

Oxygen content at T ¼ 298, before oxidation SrFeO2.54 —

DO/mol, Tfinal ¼ 642.76 K — 0.0527(10)

DO/mol, T ¼ 298 K, after oxidation 0.0588(6) 0.0588(4)

Oxygen content at T ¼ 298, after oxidation SrFeO2.847 SrFeO2.847

Series II

Oxygen content at T ¼ 298, before oxidation SrFeO2.54 —

DO/mol, Tfinal ¼ 618.46 K — 0.0355(15)

DO/mol, T ¼ 298 K, after oxidation — 0.0378(15)

Oxygen content at T ¼ 298, after oxidation — SrFeO2.86

Fig. 5 Thermodynamic cycle used to derive the enthalpy of reaction(6) from the experimentally determined enthalpies of (5).

Fig. 6 Enthalpy of reaction (5) and reaction (6) as a function of tem-perature. Results from the two series of experiments are compared forreaction (5); (X) series I, (`) series II. For reaction (6) the averagevalue of the enthalpy of reaction (5) is combined with the two differentreported enthalpies of the order–disorder transition in SrFeO2.50 ; (K)Schmidt and Campbell;19 (c) Haavik.5 (S) Enthalpy of reaction (6)deduced from composition-partial pressure of oxygen isotherms.8

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through the use of a simple model description. Hence, the pre-sently used calorimetric approach seems to be an efficientmethod to gain information and to understand trends in theredox energetics of perovskite-related oxides. A general discus-sion of the redox energetics of perovskite-type oxides isreported separately.13

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16 F. Grønvold, Acta Chem. Scand., 1967, 21, 1695.17 S. Stølen, R. Glockner and F. Grønvold, J. Chem. Thermodyn.,

1996, 28, 1263.18 S. Stølen and F. Grønvold, High Temp. High Pressures, 1993, 25,

161.19 M. Schmidt and S. J. Campbell, J. Solid State Chem., 2001, 156,

292.20 O. H. Hansteen, personal communication.21 M. Schmidt, J. Phys. Chem. Solids, 2000, 61, 1363.22 L. Rørmark, A. B. Mørch, K. Wiik, T. Grande and S. Stølen,

Chem. Mater., 2001, 13, 4005.

Fig. 7 Stoichiometry-partial pressure of oxygen isotherms for SrFeO3�d at 1073 and 1273 K. The average value of the enthalpy of (5) is combinedwith the two different enthalpies of the order–disorder transition in SrFeO2.50 ; solid line,19 dotted line5 and the previously reported entropy ofoxidation.12 The symbols represent experimental determinations. (X) Mizusaki et al.;7 (S) Holt et al.;8 (c) Takeda et al.;1 (`)Vashuk et al.9

Fig. 8 Stoichiometry of SrFeO3�d in air as function of T. The solidpoints correspond to the solid line in Fig. 7 using Dord–disordH

�

(SrFeO2.50) reported by Schmidt and Campbell.19 The symbols repre-sent experimental determinations. (X) Schmidt;21 (`) Takeda etal.;1 (c) Mizusaki et al.;7 (¯) Vashuk et al.9

Phys. Chem. Chem. Phys., 2002, 4, 1082–1087 1087

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