ISSN 0012-5008, Doklady Chemistry, 2009, Vol. 427, Part 2, pp. 194–198. © Pleiades Publishing, Ltd., 2009.Original Russian Text © I.A. Leonidov, I.V. Baklanova, L.A. Perelyaeva, O.N. Leonidova, R.F. Samigullina, V.L. Kozhevnikov, 2009, published in Doklady Akademii Nauk, 2009,Vol. 427, No. 6, pp. 785–789.

194

To increase the lifetime of solid oxide fuel cells, thetemperature at which an acceptable level of oxygenconductivity of a solid electrolyte is achieved should bedecreased. Solid solutions

La

1 –

x

Sr

x

Ga

1 –

y

Mg

y

O

3 –

δ

(LSGM) with a perovskite structure are of particularinterest since they exhibit a considerable ionic conduc-tivity (about 0.1 S/cm) even at

600–800°ë

, which isnoticeably lower than the operation temperature ofwell-studied zirconia-based solid electrolytes [1, 2].The LSGM solid solutions are also more stable at lowpartial oxygen pressures and have lower electrical con-ductivities than solid electrolytes based on

Bi

2

O

3

andGeO

2

[3, 4]. The substitution of divalent cations for lan-thanum and/or gallium in LaGaO

3

is accompanied byformation of anionic vacancies and an increase in theoxygen conductivity. Doping also induces the transitionfrom the orthorhombic to the cubic structure of LSGM.The structural transition is explained in [5] by anincrease in the average lengths of metal–oxygen bonds(La, Sr)–O due to a partial replacement of La

3+

ions bylarger Sr

2+

ions.In this work, the influence of the concentration and

ordering of oxygen vacancies on the crystal structureand phase transitions in

La

1 –

x

å

x

Ga

1 –

x

Mg

x

O

3 –

x

(M =Ca, Sr, Ba) is considered on the basis of calorimetry,electrical conductivity, and thermal expansion data.Successive transformations of the solid solutions struc-

ture, orthorhombic

(

Pbnm

)

→

rhombohedral

(

R c

)

→

cubic (

Pm m

), occur as the number of

GaO

6

octahedrain the

ÄÇé

3

perovskite lattice decreases since thevacancies (

) are localized near gallium to form

GaO

5

pyramids, while magnesium retains its basically octa-hedral oxygen environment. At room temperature, the

3

3

concentration transition

Pbnm

→

R c

occurs when theconcentration of

GaO

6

octahedra in the samplesdecreases by 40%, and the transition to the cubic struc-ture takes place when the concentration of

GaO

6

octa-hedra is about half of the amount of all

Çé

n

polyhedra.An increase in the concentration of

MgO

6

octahedra ondoping accounts for the increase in the temperature of

the

Pbnm

↔

R c

phase transition. The vacancies in theLSGM oxygen sublattice at low temperatures are in thebound state

GaO

5

–

–GaO

5

, which is responsible forthe increase in the activation energy

E

(up to about1 eV) for oxygen conductivity. An increase in the ther-mal expansion coefficients and a decrease in

E

to0.7 eV at the temperatures above

500°ë

are explainedby changes in the defect structure of LSGM caused bydisorder in the oxygen sublattice and formation of defi-cient octahedra

Ga(O

5/6

1/6

)

6

.

The samples

La

1 –

x

Sr

x

Ga

1 –

x

Mg

x

O

3 –

x

(

x

= 0, 0.05,0.1, 0.15, 0.2) and

La

0.95

å

0.05

Ga

0.95

Mg

0.05

O

2.95

(M =Ca, Ba) were synthesized by the glycine–nitratemethod [6]. The phase compositions of the sampleswere determined at room temperature using a DRON-2diffractometer

(Cu

K

α

radiation). The structural param-eters were calculated with the FULLPROF programpackage [7]. Differential scanning calorimetry (DSC)data were obtained with a DSC Q10 calorimeter. Thethermal expansion of the samples was studied using aLinseis L 75 dilatometer. Electrical conductivities weremeasured for cylindrical samples with platinum elec-trodes applied to the end faces on a Solartron 1260impedance analyzer at an ac frequency of 10 kHz.

The X-ray diffraction patterns of the samples

La

1

−

x

Sr

x

Ga

1 –

x

Mg

x

O

3 –

x

(

x

= 0, 0.05, 0.1, 0.15, 0.2) and

La

0.95

å

0.05

Ga

0.95

Mg

0.05

O

2.95

, where M = Ca (LCGM)and Ba (LBGM), obtained at

1400°ë

showed that theywere single-phase samples with a perovskite structure[6]. The X-ray diffraction patterns of the samples with

x

= 0.05 and 0.1 are typical of the orthorhombic struc-

3

3

The Effect of Divalent Cations on the Structural Parameters, Phase Transitions, and Electrical Conductivity

of Oxygen Conductors Based on LaGaO

3

I. A. Leonidov, I. V. Baklanova, L. A. Perelyaeva, O. N. Leonidova, R. F. Samigullina, and

Corresponding Member of the RAS

V. L. Kozhevnikov

Received March 12, 2009

DOI:

10.1134/S0012500809080047

Institute of Solid-State Chemistry, Ural Division, Russian Academy of Sciences, ul. Pervomaiskaya 91, Yekaterinburg, 620041Russia

CHEMISTRY

DOKLADY CHEMISTRY

Vol. 427

Part 2

2009

THE EFFECT OF DIVALENT CATIONS 195

ture of

LaGaO

3

(space group

Pbnm

) [6, 8]. Withincreasing

x

to 0.15, the structure transforms to the

rhombohedral one (space group ). The samplewith

x

= 0.2 has the cubic structure with space group

. The successive transformation of the solidsolution

La

1 –

x

Sr

x

Ga

1 –

x

Mg

x

O

3 –

x

from the orthorhom-bic structure of

LaGaO

3

to the cubic phase

La

0.8

Sr

0.2

Ga

0.8

Mg

0.2

O

2.8

shows that the tolerance factor

t

=

approaches unity with an increase inthe concentration of dopants. According to the aboveexpression, this can imply a relative increase in the

d

AO

bond length or a decrease in the

d

BO

bond length.Figure 1 presents the concentration dependences of thestructural parameters, as well as of the reduced latticeparameter of perovskite

V is the unit cell volume and Z is the number of formulaunits, equal to 4 and 6 for the orthorhombic structureand rhombohedral (hexagonal setting) structure,respectively). The increase in V and a can be due to thedifference between the ionic radii of the dopants, rMg2+

(CN = 6) = 0.72 Å and rSr2+ (CN = 12) = 1.44 Å, andreplaced ions, rGa3+ (CN = 6) = 0.62 Å and rLa3+ (CN =12) = 1.36 Å [9]. Indeed, as have been shown [5], dop-ing leads to an increase in the dAO distances inLa0.8Sr0.2Ga0.8Mg0.2O2.8, and the average B–O bondlength becomes smaller than the dGaO bond length inLaGaO3. The shortening of the average B–O bondlength in La1 – xSrxGa1 – xMgxO3 – x is reflected in an

R3c

Pm3m

dAO/ 2dBO

apVZ---⎝ ⎠

⎛ ⎞1/3

,=

increase in the vibration frequencies of gallium andoxygen atoms with increasing x [6]. This can indicatethat doping in LaGaO3 leads to formation of GaO5pyramids in which the average Ga–O bond length issmaller than in the GaO6 octahedra. Thus, the unitcell volume increases mainly due to an increase inthe volume of ÄO12 polyhedra. This “swelling” ofthe crystal lattice is also due to the fact that there isa stronger Coulomb repulsion between the Ga3+ ionsin the GaO5 pyramids adjacent to oxygen vacancies.This statement is supported by an increase in the vol-ume of LaO12 polyhedra in LaGa0.8Mg0.2O2.9obtained by doping only the B sublattice [5]. In thiscase, the average B–O bond length remains almostthe same as in LaGaO3, whereas the unit cell volumeconsiderably increases.

The endotherms observed in the DSC curves on heat-ing the orthorhombic samples La1 − xSrxGa1 − xMgxO3 – x(x = 0, 0.05, 0.1, 0.15, 0.2) andLa0.95å0.05Ga0.95Mg0.05O2.95 (M = Ca, Ba) are caused bythe phase transition of the low-temperature orthorhom-bic phase to the rhombohedral phase, as in LaGaO3 [8].As is seen in Fig. 2, the minimum temperature (Tpt) andthe maximum enthalpy (∆Hpt) of the phase transitionare observed for LaGaO3. In the LSGM samples, thephase transition temperature increases and the value of(∆Hpt) sharply decreases with increasing x. For theseries of compounds La0.95M0.05Ga0.95Mg0.05O2.95,where M = Ca, Sr, and Ba, the phase transition temper-ature is identical for the samples with calcium andstrontium and is about 100 K lower for LBGM.

A sharp decrease in (∆Hpt) with an increase in theconcentration of anionic vacancies formed upon the

5.49

0.050 0.10 0.15 0.20

5.50

5.51

5.52

5.53

x

3.91

3.90

3.89

Pbnm R3c–

Pm3m–

a, b

, c, Å

a p, Å

apcab

Fig. 1. Concentration dependences of the structural parameters of La1 – xSrxGa1 – xMgxO3 – x.The unit cell parameters of the orthor-

hombic (o), rhombohedral (r), and cubic (c) phases were normalized as follows: a = a0, b = b0, c = co/ ; a = ar, c =cr/ ;

a = ac, and ap =(V/Z)1/3.

2 6

2

196

DOKLADY CHEMISTRY Vol. 427 Part 2 2009

LEONIDOV et al.

introduction of divalent cations of alkaline earth ele-ments of metals and Mg2+ correlates with a decrease inthe number of GaO6 octahedra according to the follow-ing tentative formulas:

La1 – xåx(GaO6/2)1 – 2x(GaO5/2)x(MgO5/2)x, (1)

La1 – xåx(GaO6/2)1 – 3x(GaO5/2)2x(MgO6/2)x, (2)

where the denominator 2 shows that each oxygen atomis shared by two Çén polyhedra. It follows from theseformulas that the number of the GaO6 octahedra rapidlydecreases with increasing x when the anionic vacanciesare localized only near gallium with formation of GaO5pyramids. According to the ∆Hpt(x) curve shown inFig. 2, the enthalpy tends to zero at x about 0.125. Thisconcentration of dopants corresponds to the limitingconcentration for the transition of the orthorhombicLa1 – xSrxGa1 – xMgxO3 – x phases to the rhombohedralphases (Fig. 1). At x = 0.125, the limiting number of theGaO6 octahedra in accordance with formula (2) is closeto 63% of the overall number of Çén polyhedra, that is,this minimum number of the GaO6 octahedra gives riseto rhombohedral distortions of the LSGM structure.

The rise in Tpt with increasing x inLa1 − xSrxGa1 − xMgxO3 – x can be connected with intro-ducing large strontium cations into LaGaO3. Anincrease in the concentration of these cations can leadto steric hindrances for rotations of GaO6 octahedra in

the course of the phase transition Pbnm ↔ R c. How-ever, as is seen in Fig. 2, the introduction of even largerbarium cations (CN = 12, rLJ2+ = 1.61 Å) into lantha-num gallate drastically decreases Tpt. This implies that

3

the rise in the temperature of the Pbnm ↔ R c transi-tion is mainly due to the size of magnesium ions. Inas-much as the ionic radius of Mg2+, even if it is pentaco-ordinated by oxygen ions (r = 0.66 Å for rMg2+ withCN = 5), is larger than that of hexacoordinated Ga3+

ions, the formation of MgO6 and MgO5 polyhedra hin-ders the rotation of GaO6 octahedra, which is reflectedin the increase in Tpt. The considerable rise in Tpt andthe decrease in ∆Hpt are in good agreement with the factthat magnesium ions are predominantly in the octahe-dral oxygen environment in La1 – xSrxGa1 – xMgxO3 – x. Inthis case, the number of the GaO6 octahedra accordingto (2) decreases to smaller values than in the case of for-mation of MgO5 pyramids by formula (1). It can benoted that the temperatures of the phase transition

Pbnm ↔ R c are almost identical and close to 350°ëfor the solid solutions LaGa0.9Mg0.1O2.95 andLaGa0.9SÒ0.1O3 at the same concentrations of thedopants with close ionic radii, rMg2+ (CN = 6) andrSc3+ (CN = 6) (0.72 and 0.745 Å, respectively) [10].The considerable rise in Tpt (by about 200°ë) forLaGa0.9Sc0.1O3 and LaGa0.9Mg0.1O3 is in good agree-ment with the formation of equal numbers of large octa-hedra ScO6 and MgO6 in the structures of the corre-sponding solid solutions La(GaO3)0.9(ScO3)0.1 andLa(GaO3)0.8(GaO2.5)0.1(MgO3)0.1.

As is seen in Fig. 1, the limiting value x for the con-centration transition from the rhombohedral phasesLa1 − xSrxGa1 – xMgxO3 – x to the compositions with thecubic structure is about 0.175. At this value, accordingto (2), the number of the GaO6 octahedra per perovskite

3

3

100

0

200

300

400

500

0.05 0.10 0.15

50

0

100

150

200

250

x

Ba

Ca

Sr

Ca

BaSr

T pt,

°C

∆Hpt

, J/m

ol

Fig. 2. Concentration dependences of (1) the temperature and (2) the enthalpy of the Pbnm ↔ R c transition.3

1

2

DOKLADY CHEMISTRY Vol. 427 Part 2 2009

THE EFFECT OF DIVALENT CATIONS 197

unit cell is about half of all Çén polyhedra. This valueis insufficient for rhombohedral distortions of the per-ovskite structure and formation of …Ga–O–Ga…chains with a bridging angle about 160°, and the struc-ture of LSGM becomes cubic.

The temperature dependences of the relative length-ening (∆L/L0) for the LSGM samples in a range of 30–1000°C is shown in Fig. 3. A kink in the ∆L/L0(T) curvefor LaGaO3 near 150°C corresponds to the Pbnm ↔R c transition and agrees with DSC data. The decreasein the number of the GaO6 octahedra in the LSGM sam-ple with x = 0.1 leads to that the phase transition isobserved as a smooth inflection in the ∆L/L0(T) curve inthe range of 380–420°C.

The observed nonlinear character of the ∆L/L0(T)curves is due to that the thermal expansion coefficients

are a function of temperature. At temperatures aboveTpt, the α values for LaGaO3 increase from 11 × 10–6 to12.8 × 10–6 K–1 at 1000°C. For LSGM with x = 0.1,α increases to an even larger value (from 10 × 10–6 to13 × 10–6 K–1 at 1000°C). The α values for the LSGMcubic phase with x = 0.2 also considerably increase attemperatures above 500°C (from 11 × 10–6 to 14.5 ×10−6 K–1 at 1000°C). These changes in ∆L/L and α withchanging temperature are explained by disorder of the

3

α dLdT L0------------⎝ ⎠

⎛ ⎞=

oxygen sublattice connected with a more random distri-bution of anionic vacancies () and formation of largegallium–oxygen polyhedra.

These polyhedra can be deficient octahedraGa(O5/61/6)6, whose volume is larger than that of theGaO5 pyramids, because the radius of gallium ions inthe octahedra is larger than the radius of Ga3+ ions withthe coordination number five [9]. From this, it followsthat the vacancies in the oxygen sublattice in the solidsolutions based on LaGaO3 are considerably ordered atlow temperatures since they are linked with two neigh-boring pyramids GaO5 to form GaO5––GaO5 com-plexes. The entrapment of vacancies in deficient GaO5––GaO5 clusters results in an increase in the activa-tion energy for oxygen conductivity (to ~1 eV) inLSGM at 400–700°C (Fig. 4). For oxygen migrationto occur in this temperature range, an additionalenergy is required to decompose this complex intotwo deficient Ga(O5/61/6)6 octahedra. At temperaturesclose to 1000°ë, this process is almost complete, and theactivation energy of oxygen conduction decreases to0.7 eV. The nearly parallel curves logσ(1000/T) in Fig. 4is evidence of the same type of disordering of the anionicsublattice and oxygen transfer in La1 – xSrxGa1 – xMgxO3 – x

almost independent of the dopant content, which pro-vides additional confirmation of localization of oxygenvacancies near gallium ions.

0.4

4000

0.8

1.2∆L/L0, %

800 1200T, °C

321

Fig. 3. Temperature dependences of the relative bondlengthening in La1 – xSrxGa1 – xMgxO3 – x with x = (1) 0,(2) 0.1, and (3) 0.2.

–3

0.8

–2

0

–1

–4

0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.61000/T, K–1

log(σ, Ω–1 cm–1)

12

34

Fig. 4. Temperature dependences of the electrical conduc-tivity of La1 – xSrxGa1 – xMgxO3 – x with x equal to (1) 0,(2) 0.05, (3) 0.1, and (4) 0.2.

198

DOKLADY CHEMISTRY Vol. 427 Part 2 2009

LEONIDOV et al.

ACKNOWLEDGMENTS

This work was supported by the Ural Division of theRussian Academy of Sciences (the Program for Sup-port of Interdisciplinary Basic Research, project no. 1).

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