ISSN 0012-5008, Doklady Chemistry, 2007, Vol. 417, Part 2, pp. 285–288. © Pleiades Publishing, Ltd., 2007.Original Russian Text © O.V. Mel’nikov, O.Yu. Gorbenko, A.R. Kaul’, 2007, published in Doklady Akademii Nauk, 2007, Vol. 417, No. 5, pp. 642–645.

285

Perovskite manganites of the general formula

La

1

−

x

A

x

MnO

3 +

δ

(A is a dopant singly charged cation)are of interest as materials with a colossal magnetore-sistance (CMR) at room temperature [1, 2]. Amongthem,

La

1 –

x

Ag

x

MnO

3 +

δ

stands out inasmuch as, due tothe closeness of its Curie temperature to room temper-ature, it is promising for treating oncological diseasesby the cell hyperthermia method [3] and as a workingsubstance in high-efficiency Freon-free refrigerators[4]. Synthesis of new materials based on lanthanummanganite doped with

Ag

+

requires a comprehensiveknowledge of the thermodynamic properties of suchsolid solutions since incorrectly chosen

P

(O

2

)–

T

syn-thesis conditions (oxygen pressure–temperature) oftenlead to formation of a two-phase mixture containingmetallic silver [5, 6]. The question arises as to whethersynthesis conditions are optimal for the preparation ofsingle-phase

La

1 –

x

Ag

x

MnO

3

with a given microstruc-ture [7, 8]. Previously, we showed the existence of

La

1

−

x

Ag

y

MnO

3

solid solutions with a wide homogene-ity range for a silver content

y

that is no more than thelanthanum deficiency

x

(

y

≤

x

)

[9]. The compositionswith

y

<

x

are thermodynamically more stable withregard to formation of metallic silver compared withthe compositions with

x

=

y

. The most reliable andinformative method for investigation of the thermody-namics of solid-phase systems is the electromotive force(EMF) method [10]. This work presents a study of thethermodynamic properties of lanthanum manganite dopedwith silver by the EMF method with a solid electrolyte.

An optimal solid electrolyte for studying oxidephases doped with silver is

Ag–

β

-alumina, which is a

silver polyaluminate of variable composition

Ag

2

O

·

n

Al

2

O

3

(

n

= 5.5–9.5)

with a high ionic conductivity(Ag+) and a low electronic conductivity in the range

300–900°ë

[11, 12].

Ceramic samples of lanthanum silver manganite

La

0.8

Ag

0.1

MnO

3 +

δ

were obtained by a chemical homog-enization method as described in [9] and were charac-terized by scanning electron microscopy (SEM) withX-ray microanalysis and X-ray powder diffraction.According to the SEM data (a LEO Supra 50VP elec-tron microscope), the samples obtained had a homoge-neous porous ceramic microstructure with an averagegrain size of 0.1–1

µ

m. The homogeneity of the cat-ionic composition of the ceramics was demonstrated byX-ray microanalysis. The X-ray powder diffraction pat-terns were recorded on a Rigaku D/MAX 2500 diffracto-meter (generator operated at 12 kW, rotating anode,

Cu

K

α

radiation) using a high-temperature stage with a platinumcell. The heating rate to the temperature required was10 K/min; the time of exposure to this temperature beforerecording patterns was 5 min, and the scanning rate was5 deg/min with a step of 0.02

°

. The samples obtained weresingle-phase with a rhombohedral perovskite structure

(space group

R c

). The hexagonal unit cell parameters for

La

0.8

Ag

0.1

MnO

3 +

δ

at

20°ë

were

a

= 5.500

±

0.002

Å

,

c

= 13.354

±

0.009

Å, and

V

= 349.8

Å

3

.

The study of the thermodynamics of the reaction ofsilver insertion was performed in electrochemical cell(1) with auxiliary electrochemical cell (2) in a widetemperature range of 850–1123 K:

3

(Pt, O

2

), La

0.8

Ag

0.1

MnO

3 +

δ

|

Ag–

β

-Al

2

O

3

|

Au

0.988

Ag

0.012

, (Pt, O

2

). (1)

Pt, Ag

|

Ag–

β

-Al

2

O

3

|

Au

0.988

Ag

0.012

, Pt. (2)

A Study of the Thermodynamic Properties of the La

0.8

Ag

0.1

MnO

3 +

d

Solid Solution by the EMF Method with a Solid Electrolyte

O. V. Mel’nikov, O. Yu. Gorbenko, and A. R. Kaul’

Presented by Academician Yu.D. Tret’yakov July 13, 2007

Received July 11, 2007

DOI:

10.1134/S0012500807120038

Moscow State University, Vorob’evy gory, Moscow, 119992 Russia

CHEMISTRY

286

DOKLADY CHEMISTRY

Vol. 417

Part 2

2007

MEL’NIKOV et al.

Overall reaction (3) formally corresponds tocell (4):

(3)

(Pt, O

2

), La

0.8

Ag

0.1

MnO

3 +

δ

|

Ag–

β

-Al

2

O

3

|

Ag,(Pt, O

2

). (4)

Inasmuch as metallic silver is highly plastic and dif-fusively mobile at a temperature close to the meltingpoint, we used an Ag–Au alloy as a reference electrode,which allowed us to extend the temperature range stud-ied. The cell scheme is shown in Fig. 1. A commercialAg–

β

-alumina ceramic (Ionotec) was used as a solidelectrolyte. The values of EMF and temperature, mea-sured with a chromel–alumel thermocouple, wererecorded at a frequency of 1 Hz and oxygen pressures

P

(O

2

) of 0.21 and 1 atm in a flow system using anE14-140 multichannel analog-to-digital converter(L-Card) with an input resistance of each line above1 M

Ω

using the PowerGraph software.Initially, the reproducibility of measurements was

insufficient and the EMF values were unstable at tem-peratures above 1000 K. In addition, the EMF values ofcell (1) drifted in different directions when Ag–

β

-alu-mina samples of different composition were used. Weassumed that the ion exchange between

La

1

−

x

Ag

y

MnO

3 +

δ

and the

Ag2O · nAl2O3 electrolytechanges the silver equilibrium activity in the samplebecause both the solid electrolyte and theLa0.8Ag0.1MnO3 + δ electrode studied are phases of vari-able composition with respect to silver; the silver activ-ity is constant at constant phase composition.

La0.8Ag0.1 – zMnO3 + δ – z/2 + zAg + z/4O2

= La0.8Ag0.1MnO3 + δ,

The problem of instability of EMF values wassolved by using the solid electrolyte in the form of apiece of ceramic with a mass much smaller than themass of the sample studied (the approximate molarratio of the sample to the electrolyte was 100 : 1). Thisallowed us to fix the silver activity in the electrode dueto the establishment of diffusion equilibrium betweenthe electrode and solid electrolyte at a negligiblechange in the silver content of the electrode. A changein the silver content in the solid electrolyte itself had noeffect on the EMF of the cell.

The EMF values were determined during heatingand cooling with a step of 20–30 K. The system waskept at each temperature for a long time to reach a con-stant EMF value (the minimum exposure time was 3 hat constant T and P(O2)). The EMF values reproducedwithin ±1 mV when the oxygen pressure was repeat-edly switched between 0.21 and 1 atm were taken to beequilibrium. The obtained dependence was reproducedwell for different samples of the same compositionLa0.8Ag0.1MnO3 + δ. The temperature dependences ofEMF for La0.8Ag0.1MnO3 + δ are shown in Fig. 2.

Inasmuch as oxides tend to dissociate at elevatedtemperatures, the temperature dependence of the EMFof the cell under study is a descending function. Thezero EMF for reaction (3) corresponds to equal silveractivities on each side of the solid electrolyte, i.e., to thetemperature of decomposition of the lanthanum silvermanganite solid solution with precipitation of themetallic silver phase. Based on the data on the temper-ature dependence of the EMF, we determined the ther-modynamic parameters of reaction (3) shown in thetable.

The reaction of silver dissolution in the perovskitelattice is described by Eq. (3). Inasmuch as the activityof the manganite phase is equal to 1 in this experiment,

Vol

tmet

erThermo-

Loadé2

Pt

Au0.988Ag0.012

Ag–β-Al2O3

La0.8Ag0.1MnO3

Pt

couple

50

900

E, mV

í, ä1000 1050 1200 1300

100

150

200

250

300

0

1 atm

0.21 atm

Fig. 1. The scheme of electrochemical cell (1).

Fig. 2. Temperature dependence of EMF (E) for reaction (3).

DOKLADY CHEMISTRY Vol. 417 Part 2 2007

A STUDY OF THE THERMODYNAMIC PROPERTIES 287

the equilibrium constant of reaction (3) can be written(assuming the constancy of δ in this reaction) as

(5)

It follows that an increase in the oxygen pressuredecreases the activity of silver in the manganite, i.e.,increases the EMF of reaction (3), which was actuallyobserved in the experiment. Even a small increase inP(é2) considerably increases the stability of theLa0.8Ag0.1MnO3 + δ solid solution. This increase in sta-bility is due to a considerable decrease in the entropycontribution. The change in the entropy of reaction (3)as the oxygen pressure changes from 1 to 0.21 atm was

calculated to be ln = 3.2 J/(mol K). At the same

time, the observed difference is noticeably larger (18 ±2 J/(mol K)), which is caused by an additional changein the oxygen nonstoichiometry of the manganite, i.e.,in the index δ in reaction (3). Using the Nernst equationfor cell (4) and taking into account the equilibrium con-stant K(n), we obtain for a fixed nonstoichiometry

E(1 atm) – E(0.21 atm) = ln ≈ 0.033T mV.

K1

aAgP O2( )14---

-------------------------.=

R4--- 1

0.21----------

RT4F------- 1

0.21----------

At 900 K, the calculated difference between theEMF values was 30 mV, and the corresponding experi-mental value was 32 mV. However, the closer the tem-perature to the decomposition point, the greater the dif-ference in the oxygen nonstoichiometry; the calculationfor 1120 K at constant oxygen nonstoichiometry givesthe EMF difference 38 mV, and the experimental valuewas 76 mV.

This discrepancy is not due to any structuralchanges in the manganite phase, which follows fromcomparison of these data with the data of high-temper-ature X-ray diffraction (Fig. 3), which show only aslight decrease in the rhombohedral distortion ofLa0.8Ag0.1MnO3 + δ with increasing temperature. It isworth noting that the thermal expansion coefficientderived from the data of high-temperature X-ray dif-fraction was 10.3 × 10–6 K–1, which is close to the liter-ature data for structural analogues in the La1 – xSrxMnO3system [13] and completely coincident with the thermalexpansion coefficient of ZrO2(8%Y2O5). This can be ofinterest for design of electrodes for high-temperatureelectrochemical devices on the basis of this solid elec-trolyte.

Thus, our findings allow us to evaluate the P(é2)–Tconditions for the synthesis of new materials based onLa1 – xAgyMnO3 + δ (y ≤ x). We also improved the proce-dure for studying the thermodynamic properties ofsolid solutions by the EMF method with an Ag-ion-conducting solid electrolyte.

ACKNOWLEDGMENTS

This work was partially supported by the HumanFrontier Science Program (grant no. RGP47/2007)and the Russian Foundation for Basic Research(project no. 07–03–01019a).

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Fig. 3. Temperature dependence of the rhombohedral unitcell parameters of La0.8Ag0.1MnO3 + δ.

Thermodynamic parameters of reaction (3)

P(O2), atmE = A – BT, mV ∆H0,

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J/(mol K) T, K*A B

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í, ä400 600 800 1000

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a, Å

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DOKLADY CHEMISTRY Vol. 417 Part 2 2007

MEL’NIKOV et al.

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