Journal of AUoys and Compounds, 182 (1992) L5-L7 JALCOM 131
L5
Letter
Measurement of the thermodynamic stability of lanthanum u s i n g a c a l c i u m f l u o r i d e s o l i d e l e c t r o l y t e g a l v a n i c cell S.
ferrite
Raghavan
Department of Metallurgical Engineering, Indian Institute of Technology, Madras 600 036 (India) (Received September 9, 1991)
1. Introduction
According to the phase diagram [1], the compounds LaFeOa and Fe203 coexist below 1648 K in the system La203-Fe2Oa. Sreedharan and Chandrasekharaiah [2] determined the Gibbs energy of formation of LaFeOa in the temperature range 1 0 9 4 - 1 2 9 9 K from e.m.f, measurements on solid oxide galvanic cells using a ZrO2-CaO solid electrolyte. They employed an F e / " F e O " reference electrode in these cells and obtained the free energies of formation of LaFeOa from knowledge of the experimental cell e.m.f.s and the free energy of formation of "FeO". The oxygen potential in the system LaFeO3/La2Oa/Fe was measured using the gas equilibrium technique by Leontev et al. [3] and Katsura et al. [4], while Nakamura et al. [5] employed thermogravimetry for the same purpose. The Gibbs energies of formation of LaFeOa obtained from the four sets of data are in reasonable agreement with each other. It is possible to measure the Gibbs energy of formation of this c o m p o u n d directly by setting up the following cell incorporating CaF2 as the solid electrolyte: O2(g), Pt/LaeOa, LaFJ/CaF2//LaFeOa, Fe2Oa, LaFa/Pt, O2(g) The two half-cell reactions in this cell are LaeO3 + 6 F - = 2LaFa +~O2(g) + 6 e 2LaFa + Fe2Oa + ~O2(g) + 6 e - = 2LaFe03 + 6 F Therefore the net cell reaction is La203 + Fe203 = 2LaFeOa
(1)
The standard Gibbs energy change for reaction (1) is given by A G ° --
-- 6Fe
where E is the e.m.f, of the cell and F the Faraday.
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2. M a t e r i a l s LaFeO8 was prepared from dried reagent grade La208 and Fe208 by mixing in stoichiometrie proportions and sintering the oxide mixture pellets at 1523 K for 48 h with intermittent grinding. The formation of the ferrite was confirmed by X-ray diffraction. The electrode pellets were prepared b y mixing the appropriate compounds in roughly equimolar proportions. An equal weight of LaFa was then added to the mixture, the mixed p o w d e r was pressed into cylindrical pellets and sintered at 1273 K in dry CO2-free oxygen gas before use. Polyerystalline calcium fluoride solid electrolyte pellets were prepared according to the method of Taylor and Schmalzried [6].
3. E x p e r i m e n t a l d e t a i l s In the cell the calcium fluoride solid electrolyte pellet was sandwiched between the reference electrode and working electrode pellets. The cell was electrically connected to a Keithley 617 electrometer by platinum leads connected to platinum foils pressing against the outer pellets. The whole cell assembly was enclosed inside a sillimanite reaction tube and kept in the even temperature zone of the furnace. The assembly used was similar to that employed by Vecher and Vecher [ 7]. A P t - P t R h (13% Rh) thermocouple welded to the platinum foil in contact with the reference electrode was used
O
HEATING
•
COOLING
A
215 ' " 205 195 I 1130
I 1140
I 1150
I 1160
I 1170
1180
T (K)
Fig. 1. E.m.f. E v s . temperature T for the cell 02(g), Pt/La,203, LaFa//CaF2//LaFeO3, Fe203, LaFa/Pt, O2(g) TABLE 1 Molar Gibbs energy of formation of lanthanum ferrite by the reaction ½La203 + ½Fe203 = LaFeO3 Temperature
AG° (kJ tool -])
(K)
1153
Present work (CaF 2 cell)
Ref. 2 (ZrO2-CaO cell)
Ref. 3 (gas equilibrium measurements)
Ref. 4 (gas equilibrium measurements)
Ref. 5 (thermogravimetry)
--60.9+0.6
--59.4+1.4
-62.9
-66.0+2.5
-63.1+2.5
L7 to m o n i t o r t h e cell t e m p e r a t u r e . T h e t e m p e r a t u r e o f t h e f u r n a c e w a s c o n t r o l l e d b y a p r o p o r t i o n a l p o w e r c o n t r o l l e r to b e t t e r t h a n _+3 K. T h e cell e.m.f, a n d t h e t h e r m a l e.m.f, w e r e m e a s u r e d u s i n g a Keithley 617 digital e l e c t r o m e t e r with a n i n p u t i m p e d a n c e o f 1014 ~ . An a t m o s p h e r e o f dry, CO2-free o x y g e n g a s w a s p r o v i d e d t h r o u g h o u t t h e w o r k i n g p e r i o d o f the cell. T h e r e v e r s i b l e e.m.f.s o f t h e cell w e r e m e a s u r e d in t h e t e m p e r a t u r e r a n g e 1 1 3 3 - 1 1 7 3 K. T h e r e p r o d u c i b i l i t y o f t h e e.m.f, d a t a w a s c h e c k e d b y t h e r m a l cycling a n d c o u l o m e t r i c titration. T h e cell with an La203/LaF3 e l e c t r o d e on e i t h e r side g a v e a n e a r null e.m.f.
4. R e s u l t s a n d d i s c u s s i o n T h e e x p e r i m e n t a l v a l u e s o f t h e e.m.fis o f the cell on h e a t i n g a n d c o o l i n g a r e p l o t t e d as a f u n c t i o n o f t e m p e r a t u r e in Fig. 1. T h e l e a s t - m e a n - s q u a r e analysis o f t h e e.m.f.s in Fig. 1 s u g g e s t s the v a l u e of 2 1 0 . 2 _ + 2 . 0 m V f o r t h e cell at a m e a n t e m p e r a t u r e o f 1 1 5 3 K. This e.m.f, is directly r e l a t e d to t h e s t a n d a r d m o l a r G i b b s e n e r g y c h a n g e o f - 6 0 . 9 _ + 0 . 6 k J tool -1 f o r t h e f o r m a t i o n o f LaFeOa f r o m t h e c o m p o n e n t oxides. This v a l u e is c o m p a r e d with t h e c o r r e s p o n d i n g v a l u e s r e p o r t e d b y o t h e r a u t h o r s in T a b l e 1. C o n s i d e r i n g t h e different e x p e r i m e n t a l t e c h n i q u e s a n d u n c e r t a i n t i e s involved, t h e a g r e e m e n t b e t w e e n t h e different s e t s o f r e s u l t s is r e a s o n a b l y g o o d .
References 1 V. L. Moruzzi and M. W. Shafer, J. A m . Ceram. Soc., 43 (1960) 369. 2 O. M. Sreedharan and M. S. Chandrasekharaiah, Jo Mater. Sci. Lett., 21 (1986) 2581. 3 S. A. Leontev, Yu. P. Vorobev, A. M. Balbechor, A. N. Men, A. Ya. Cherrone, A. Ya. Cherronenkis and G. I. Chufaov, DokL A k a d . N a u k . USSR, 20P (1973) 618. 4 T. Katsura, K. Kitayama, T. Sugihara and N. Kimizuka, Bull. Chem. Soc. Jpn., 48 (1975) 1809. 5 T. Nakamura, G. Petzow and L. J. Gaukler, Mater. Res. Bull., 14 (1979) 649. 6 R. W. Taylor and H. Schmalzried, J. Phys. Chem., 68 (1964) 2444. 7 D. V. Vecher and A. A. Vecher, Zh. Fiz. Khim., 41 (1967) 2916.