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Solid State Sciences 10 (2008) 438e443www.elsevier.com/locate/ssscie

Oxygen nonstoichiometry and defect structure of perovskite-typeoxides in the LaeSreCoe(Fe, Ni)eO systems

Vladimir Cherepanov*, Tatyana Aksenova, Eugenie Kiselev, Ludmila Gavrilova

Department of Chemistry, Ural State University, Lenin Avenue, 51, Ekaterinburg 620083, Russia

Received 22 August 2007; received in revised form 6 December 2007; accepted 10 January 2008

Available online 25 January 2008

Abstract

Oxygen nonstoichiometry of La1�xSrxCo1�yMeyO3�d (Me¼Ni, Fe) has been studied as a function of temperature and oxygen partial pres-sure (1 to 10�3 atm) by TGA. The defect structure of complex oxides was described within the point defects approach by fitting the parametersof theoretical equations logðPO2

Þ ¼ f ðdÞ to the set of experimental data. Partial molar enthalpy and partial molar entropy of oxygen inLa1�xSrxCo1�yMeyO3�d (Me¼Ni, Fe) were calculated.� 2008 Elsevier Masson SAS. All rights reserved.

Keywords: Doped lanthanum cobaltate; Oxygen nonstoichiometry; Defect structure; Partial molar enthalpy and entropy

1. Introduction

The oxides with the perovskite structure exhibiting highconductivity and catalytic activity have attracted considerableattention as potential materials for electrodes in fuel cells, ox-ygen membranes, etc. [1e5]. Since the transport propertiesstrongly depend on the defect structure of these oxides it isdesirable to have quantitative knowledge of the oxygen non-stoichiometry dependences upon temperature and oxygenpressure. Oxygen nonstoichiometry and defect structure ofLaCoO3�d [6,7], strontium substituted lanthanum cobaltatesLa1�xSrxCoO3�d have been extensively studied previously[8e10]. However, the oxygen nonstoichiometry in perovskiteswith substitution on the cobalt site or simultaneous substitu-tion on both lanthanum and cobalt sites have been studiedonly for a limited number of compositions [11e13].

In the present work the oxygen nonstoichiometry ofLa1�xSrxCo1�yMeyO3�d (Me¼Ni, Fe) has been studied asa function of temperature and oxygen partial pressure (1 to10�3 atm) by thermogravimetric analysis (TGA). The defectstructure of complex oxides was described within the point

* Corresponding author. Tel./fax: þ7 343 261 7411.

E-mail address: vladimir.cherepanov@usu.ru (V. Cherepanov).

1293-2558/$ - see front matter � 2008 Elsevier Masson SAS. All rights reserved.

doi:10.1016/j.solidstatesciences.2008.01.009

defects approach by fitting the parameters of theoretical equa-tions logðPO2

Þ ¼ f ðdÞ to the set of experimental data.

2. Experimental

The series of samples La1�xSrxCo1�yMeyO3�d (Me¼Ni,Fe): LaCo0.9Fe0.1O3�d (LCF91), LaCo0.9Ni0.1O3�d (LCN91),LaCo0.7Ni0.3O3�d (LCN73), La0.9Sr0.1Co0.9Fe0.1O3�d (LSCF9191), La0.7Sr0.3Co0.9Fe0.1O3�d (LSCF7391), La0.9Sr0.Co0.9

Ni0.1O3�d (LSCN9191), La0.7Sr0.3Co0.9Ni0.1O3�d (LSCN7391)were prepared by either standard ceramic technique or the citricroute. All samples were identified by X-ray powder diffractionusing Cu Ka radiation and found to be single phases with theperovskite-type structure.

The changes of the oxygen nonstoichiometry were deter-mined using thermogravimetric analysis (TGA). TGA mea-surements were made depending on the composition in thetemperature range 960e1473 K and at oxygen partial pres-sures ðPO2

Þ 10�3 to 1 atm. Different oxygen partial pressureswere obtained by mixing of argon and air in appropriateratios and controlled using the oxygen sensor (ZrO2 dopedby Y2O3), which was located just under the sample in theTGA cell. The accuracy of the control of the thermodynamic

Table 2

The coefficients of equations for the oxygen nonstoichiometry isotherms

versus oxygen pressure for La1�xSrxCo1�yMeyO3�d (Me¼ Fe, Ni)

d ¼ y0 þ A�eðlog PO2Þ=t

T, K y0 A t Correlation

factor

LaCo0.9Ni0.1O3�d

1223 0.00688 0.00069 1.21383 0.983

1273 0.00719 0.00091 1.1182 0.982

1323 0.00851 0.00132 1.03465 0.994

1373 0.00379 0.00523 1.38292 0.997

1423 0.00178 0.01016 1.53239 0.998

LaCo0.7Ni0.3O3�d

1273 �0.01406 0.02122 3.17399 0.971

1323 �0.01171 0.02018 2.4511 0.966

1373 0.01672 0.00203 0.71132 0.988

1423 0.00302 0.01653 1.41469 0.999

1473 �0.16662 0.18489 5.73408 0.994

LaCo0.9Fe0.1O3�d

1273 �0.00416 0.00414 3.03959 0.999

1323 �0.00107 0.0017 1.41448 0.998

1373 �0.00197 0.00273 1.37763 0.999

1423 �0.00144 0.00388 1.28586 0.999

La0.9Sr0.1Co0.9Fe0.1O3�d

1423 �0.00292 0.03623 2.35439 0.99961

1373 �0.01656 0.04482 3.63129 0.99967

1323 0.00852 0.01764 2.50919 0.99779

1273 0.01104 0.01336 2.68846 0.99571

1223 0.02167 0.00276 1.39445 0.98703

1176 0.02248 0.00129 1.07809 0.99845

La0.7Sr0.3Co0.9Fe0.1O3�d

1423 �0.37674 0.49164 12.52707 0.99894

1373 �1.27599 1.37502 36.67297 0.9986

1323 �0.3491 0.43453 13.61123 0.99947

1273 �0.09577 0.16936 6.55524 0.99945

1223 �0.06141 0.12465 5.67375 0.9997

La0.9Sr0.1Co0.9Ni0.1O3�d

1373 0.04 0.02038 1.83064 0.9978

1323 0.03323 0.02092 2.22471 0.9981

1273 0.02728 0.02159 2.72594 0.9958

1223 0.03616 0.01021 2.07383 0.99852

1173 0.03742 0.00761 2.26909 0.99247

1123 0.04126 0.00312 1.79046 0.99561

La0.7Sr0.3Co0.9Ni0.1O3�d

1360 0.03763 0.08498 2.87643 0.9994

1310 0.04207 0.06686 2.67412 0.99472

1260 0.0475 0.04654 2.25663 0.99585

1210 0.0335 0.044838 2.30958 0.99598

1160 �0.00554 0.06852 3.25796 0.99527

1110 �0.09689 0.1461 6.45228 0.9961

439V. Cherepanov et al. / Solid State Sciences 10 (2008) 438e443

parameters was D logðPO2=atmÞ ¼ �0:05, DT¼�0.5 K; the

maximum error during the measurements of weight changeswas �2� 10�3%.

The absolute values of oxygen nonstoichiometry d inLa1�xSrxCo1�yMeyO3�d (Me¼Ni, Fe) were determined bythe complete reduction in hydrogen flow in thethermobalances.

3. Results and discussion

According to the results of XRD all samples were rhombo-hedrally distorted perovskites. The unit cell parameters arelisted in Table 1.

Using the absolute values of oxygen nonstoichiometryobtained for certain T and PO2

¼ 0:21 atm, the dependenciesof d versus oxygen partial pressure were calculated accordingto the following equation:

d¼ y0 þA�expð� log PO2=tÞ ð1Þ

The coefficients for each isotherm and the correlation coef-ficients, which show the degree of convergence between theexperimental data points and the fitting equation, are listedin Table 2. In order to compare the oxygen nonstoichiometryfor different samples, the experimental data at 1373 K togetherwith the results for undoped LaCoO3�d [6] are shown in Fig. 1.Strontium substitution for lanthanum and nickel for cobaltsignificantly increase oxygen deficiency while iron substitu-tion for cobalt slightly decreases it. Sr- and Ni-substitutionpromote oxygen desorption since they act as acceptors of elec-trons Sr0La and Ni0Co (using Kroger and Vink notation). On thecontrary iron acts as donor of electrons ðFe�CoÞ and preventoxygen vacancy formation.

Within the methodology of randomly distributed pointdefects, the defect structure of studied complex oxides canbe described by two alternative approaches or their modifica-tions, as has been previously suggested [14,15]. The main dif-ference of these approaches consists in the treatment of theelectronic defects (electrons and/or holes) as non-localized(Eq. (2)) or localized (Eq. (3)) on the cobalt sites. These alter-native processes and their equilibrium constants can be repre-sented as follows:

nil¼ 1

2O2 þV��O þ 2e0 K1 ¼

�V��O�n2P0:5

O2ð2Þ

Table 1

The unit cell parameters of complex oxides

Formula a, b (A) c (A) RBr Rf Rp

LaCo0.9Ni0.1O3�d 5.447(2) 13.095(3) 3.85 3.47 9.68

LaCo0.7Ni0.3O3�d 5.457(2) 13.105(3) 3.77 2.98 9.91

LaCo0.9Fe0.1O3�d 5.443(5) 13.103(4) 4.60 3.64 10.3

La0.9Sr0.1Co0.9Fe0.1O3�d 5.444(1) 13.142(2) 3.48 3.03 9.61

La0.7Sr0.3Co0.9Fe0.1O3�d 5.442(1) 13.197(1) 3.38 2.63 9.52

La0.9Sr0.1Co0.9Ni0.1O3�d 5.444(1) 13.135(1) 3.40 3.20 10.2

La0.7Sr0.3Co0.9Ni0.1O3�d 5.434(1) 13.186(2) 2.66 2.10 10.2

1060 �0.11212 0.15059 7.68403 0.99733

1010 �0.11578 0.14871 9.22263 0.98589

960 �0.0084 0.03808 3.89127 0.99396

or

O�O þ 2Me�Co ¼1

2O2þV��O þ 2Me0Co K2 ¼

�V��O��

Me0Co

�2P0:5

O2�O�O��

Me�Co

�2

ð3Þ

In the previous works [16e18] it has also been shown that inaddition to the atomic disordering process, an intrinsic

-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.00.00

0.04

0.08

0.12

0.16

0.20

0.24

log (Po2/atm)

LCF91LCLCN91LCN73LSCF9191LSCN9191LSCF7391LSCN7391

Fig. 1. Oxygen nonstoichiometry of complex oxides at 1373 K versus oxygen

partial pressure.

0.030 0.045 0.060 0.075 0.090 0.105-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

LSCF9191

log(

Po 2

/atm

)

0.05 0.06 0.07 0.08 0.09 0.10

-0.8

-0.4

LSCN9191

-3.2

-2.8

-2.4

-2.0

-1.6

-1.2

Log

(Po 2

/atm

)

T, K

1423 1373 1323 1273

137313231273122311731123

T, K

a

c

Fig. 2. Oxygen nonstoichiometry of La1�xSrxCo1�yMeyO3�d (Me¼Ni, Fe) as a fun

on Eqs. (2) and (4).

440 V. Cherepanov et al. / Solid State Sciences 10 (2008) 438e443

electronic disordering should be taken into account for thesetypes of complex oxides. Within the suggested approaches(of non-localized and localized electron defects) these canbe presented as follows:

nil¼ e0 þ h� Kdis1 ¼ np ð4Þ

or

2Me� ¼Me0 þMe� Kdis2 ¼�Me0Co

��Me�Co

��Me�Co

�2ð5Þ

Furthermore taking into account the difference in the values ofelectronegativity for iron, cobalt and nickel ions (cFe¼ 1.64,cCo¼ 1.7, cNi¼ 1.75 according to the Allred and Rochowscale [19]) the process of charge disproportionation could bedescribed as follows:

Ni�CoþCo�Co5Ni0Co þCo�Co Kdis2a ¼�Ni0Co

� �Co�Co

��Ni�Co

� �Co�Co

� ð6Þ

0.06 0.09 0.12 0.15 0.18 0.21 0.24

-3.6

-3.0

-2.4

-1.8

-1.2

-0.6

0.0

LSCF7391

Log

(Po 2

/atm

)

0.06 0.09 0.12 0.15 0.18 0.21-2.7

-2.4

-2.1

-1.8

-1.5

-1.2

-0.9

-0.6

LSCN7391

Log

(Po 2

/atm

)

T, K

14231373132312731223

T, K

136013101260121011601110

b

d

ction of PO2. The points obtained experimentally, solid lines were fitted based

0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11-3.0

-2.5

-2.0

-1.5

-1.0

-0.5 1423137313231273

log(

Po 2

/atm

)

T, K

0.06 0.09 0.12 0.15 0.18 0.21 0.24

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5T, K

14231373132312731223

Log

(Po 2

/atm

)

0.05 0.06 0.07 0.08 0.09 0.10

-3.2

-2.8

-2.4

-2.0

-1.6

-1.2

-0.8

-0.4T, K

137313231273122311731123

Log

(Po 2

/atm

)

0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22

-2.6

-2.4

-2.2

-2.0

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

Log

(Po 2

/atm

)

T, K

136013101260121011601110

a b

c d

Fig. 3. Oxygen nonstoichiometry of La1�xSrxCo1�yMeyO3�d (Me¼Ni, Fe) as a function of PO2. The points obtained experimentally, solid lines were fitted based

on Eqs. (3) and (5).

441V. Cherepanov et al. / Solid State Sciences 10 (2008) 438e443

and

Fe�Co þCo�Co5Fe�CoþCo0Co Kdis2a0 ¼�Co0Co

� �Fe�Co

��Fe�Co

� �Co�Co

� ð7Þ

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

log

(pO

2/atm

)

- LCF91- LCN91- LCN73

Fig. 4. Oxygen nonstoichiometry of LaCo1�yMeyO3�d (Me¼Ni, Fe) as a func-

tion of PO2at 1373 K. The points obtained experimentally, dash lines were fitted

based on Eqs. (3) and (5), solid lines were fitted based on Eqs. (3), (5) and (8).

In addition to the mentioned above processes one should notexclude the possibility of the Shottky-type disordering.

nil5V000

LaþV000

Coþ 3V��O KS ¼�V000

La

�$�V000

Co

�$�V��O�3 ð8Þ

This is likely to be significant especially at relatively hightemperatures.

Taking into account all possible types of point defects, theelectroneutrality condition can be represented as follows:

3�V000

La

�þ 3�V000

Co

�þ�Me0Co

�þ n¼ pþ

�Me�Co

�þ 2�V��O�

ð9Þ

Based on the suggested possible disordering processes a numberof defect models could be proposed. Models 1 and 1a included ei-ther combination of Eqs. (2) and (4) or combination of Eqs. (3)and (5). In models 2 and 2a the atomic disordering on the oxygensublattice (Eq. (2) or (3)) was combined with the disproportion-ation processes as described by the Eq. (6) or (7), respectively.Models 3 and 3a can be regarded as improvements to the Model1 and 1a by considering the reactions (6) or (7) to be completelyshifted to the right side. In model 4 and 4a the Shottky-type dis-ordering (Eq. (8)) is incorporated to the Models 1 and 1a.

For each model a set of equations, which included the equi-librium constants of corresponding reactions, the electroneutrality

442 V. Cherepanov et al. / Solid State Sciences 10 (2008) 438e443

condition and the mass balance equation, was solved to obtaina general equation for oxygen nonstoichimetry in the follow-ing form:

log ðPO2Þ ¼ f ðdÞ ð10Þ

Eq. (10) specific for each Model was fitted to the experimentaldata and corresponding equilibrium constants were refined bynon-linear least square fitting using software package Origin 7.0.

First of all, it should be noted that the results of the oxygennonstoichiometry measurements did not allow us to distin-guish between the localized electrons or non-localized elec-trons. The differences in the correlation coefficients obtainedfor these models were insignificant.

Secondly, a comparison of the calculated correlation coef-ficients together with the refined parameters shows thatalthough acceptor doping (Sr0La and Ni0Co) considerably in-creases oxygen nonstoichiometry donor doping ðFe�CoÞ slightlydecreases it, the predominant disproportionation according toEqs. (6) and (7) is not significant. In some cases taking into

-2.7 -2.4 -2.1 -1.8 -1.5 -1.2 -0.9 -0.60.00

0.05

0.10

0.85

0.90

0.95

LSCF-9191

LSCN-9191

log(Po2/atm)

poin

t de

fect

con

cent

rati

on, [

i]

VoMe3+

Me2+

Me4+

VoMe3+

Me2+

Me4+

T= 1373 K

-2.8 -2.4 -2.0 -1.6 -1.2 -0.8 -0.4 0.00.00

0.05

0.10

0.15

0.20

0.70

0.75

0.80

0.85

LSCF-7391

VoMe3+

Me2+

Me4+

T= 1373 K VoMe3+

Me2+

Me4+

LSCN-7391

log(Po2/atm)

poin

t de

fect

con

cent

rati

on, [

i]

a

b

Fig. 5. Isothermal (1373 K) dependencies of the point defect concentration

calculated from Eqs. (3) and (5) versus oxygen partial pressure; (a)

La0.9Sr0.1Co0.9Me0.1O3�d and (b) La0.7Sr0.3Co0.9Me0.1O3�d.

account Eqs. (6) and (7) slightly improves the quality of fitat relatively low temperatures. Although the electronic dispro-portionation of 3d-transition metal appears to take place(ignoring of process (5) leads to a very poor fitting), the effectof the nature of 3d metal on the disproportianation is negligi-ble under the studied conditions. Furthermore the concentra-tion of acceptor (nickel) and donor (iron) dopants do notsignificantly affect the fitting especially for y¼ 0.1. Figs. 2and 3 illustrate good agreement between the experimentaldata and the results calculated according to the models 1 or1a, which takes into account Eqs. (2) and (4) or (3) and (5).

Finally, it should be noted that the addition of the Shottky-type disordering improves the quality of the fitting only in theregion where oxygen stoichiometry is close to 0 (Fig. 4).

The refined values of parameters for disordering processesallowed us to calculate the equilibrium concentrations of alldefects as a function of oxygen partial pressure. As an exam-ple, Fig. 5 shows the dependencies of defect concentrationsversus oxygen partial pressure. These can be very useful forthe analysis of the electrical or other transport properties instudied complex oxides.

Thermodynamics of oxygen nonstoichiometry can becharacterized by the values of oxygen partial molar enthalpyDH0 and partial molar entropy DS0. The process of oxygenexchange between solid and gaseous phases can be representedas follows (the process of oxygen dilution in crystal lattice):

nLa1�xSrxCo1�yMeyO3�d þ ½O2

¼ nLa1�xSrxCo1�yMeyO3�dþ1=n; ðn / NÞ ð11Þ

As a result, the change of the chemical potential of oxygen atstandard conditions can be written as follows:

Dm0O ¼

RT

2ln PO2

ð12Þ

Taking into account well known GibbseHelmholtz equation

Dm0 ¼ DH0� TDS0 ð13Þ

0.00 0.04 0.08 0.12 0.16

-320

-280

-240

-200

-160

-120

Δ H

O(k

J/m

ole

O)

LSCN7391LSCN9191LSCF7391LSCF9191LCF91LCN91LCN73

Fig. 6. Partial molar enthalpy of oxygen versus oxygen nonstoichiometry for

LaCo1�yMeyO3�d (Me¼Ni, Fe).

0.00 0.04 0.08 0.12 0.16-220

-200

-180

-160

-140

-120

-100

-80

ΔSo

(J/m

ole

O)

LCF91LCN91LCN73LSCN9191LSCN7391LSCF9191LSCF7391

Fig. 7. Partial molar entropy of oxygen versus oxygen nonstoichiometry for

LaCo1�yMeyO3�d (Me¼Ni, Fe).

443V. Cherepanov et al. / Solid State Sciences 10 (2008) 438e443

one can easily obtain the expressions for partial molarenthalpy DH0 and partial molar entropy DS0.

DH0O ¼

R

2

�v ln pO2

vð1.

TÞ

�d

and DS0O ¼�

R

2

�vðT ln pO2

ÞvT

�d

ð14Þ

The values of partial molar enthalpy (Fig. 6) demonstrate thatthe process of oxygen dissolution in the lattice is energeticallymore favorable for lanthanum cobaltates doped with iron (do-nor impurity) rather than for nickel doped samples (acceptorimpurity). An increase in strontium content also hampers ox-ygen dissolution in the lattice. The effect of the nature ofthe substitution on partial molar entropy is more complicated(Fig. 7), although the absolute values are generally higher forthe donor-doped oxides.

Acknowledgements

This work was financially supported in parts by RussianFoundation for Basic Researches (projects nos. 05-03-32477and 06-08-08120_ofi) and RFBR-Urals (project no. 07-03-96079).

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