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Thermodynamic stabilities of LaVO3 and LaVO4 by e.m.f. methods
July 1995
ELSEVIER
Materials Letters 24 ( 1995) 247-25
Thermodynamic
1
stabilities of LaVO, and LaVO, by e.m.f. methods
R. Pankajavalli,
O.M. Sreedharan *
Metallu yqy Division, Indira Gandhi Centrefor Atomic Research, Kalpakkam,
Received
10 January
Tamil Nadu 603 102. India
1995; revised 25 April 1995; accepted 27 April 1995
Abstract The e.m.f. of the galvanic cell: Pt, La203, LaF3, 0, ( 1 atm) 1CaF, 1O2 ( 1 atm), LaF,, LaVO,, V,OS (s or I), Pt. was measured as a function of temperature. The melting temperature and the standard enthalpy of fusion of V,05 was derived to be 957 K and 68.5 kJ mol- ’ in agreement with the literature. The standard Gibbs energies of formation, A G&x, of LaVOj and LaVO, from the constituent binary oxides were calculated to be: AG&,(LaV03) + 3.3 (kJ mol- ’ ) = - 17.54 -0.02652 T (K) and A G$,( LaVO,) + 0.78 (kJ mol ’ ) = - 125.93 + 0.02210 T (K). These values are valid over the range 960 to 1073 K. Using the literature data on the binary oxides, the Aq of LaVO, was calculated to be - 1269 kJ mol- ’ at 1273 K (by extrapolation) and found to exhibit a systematic trend with other LaMO, compounds (where M = Cr, Mn, Fe, Co, Ni and Cu) when plotted against the atomic number of the transition metal. A phase diagram of La-V-O system at 1073 K could be suggested based on the obtained results.
1. Introduction Perovskites in the ternary system of lanthanum-transition metal-oxygen I:LaMO,) had often found applications as oxidation catalysts in pollution control [ 11, as electrode materials for MHD generators [2] and recently as matrix materials related to ceramic superconductors [ 3,4]. The standard Gibbs energies of formation of LaMO,, wlhere M is Mn, Fe and Co, were reported in the literature [ 5-81 by employing the solid oxide electrolyte e.m.f. method. Using these Gibbs energy data and that reported by Nakamura et al. on LaNiO, [ 91, the standard Gibbs energy of formation, AGP of LaCu03 coul’d be predicted by Sreedharan et al. [4]. In order to determine the thermodynamic stability of LaCrO,, Azad et al. [lo] had employed the CaF, e.m.f technique under an atmosphere of unit * Corresponding
author.
0167-577x/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDlO167-577x(95)0~0096-8
fugacity of oxygen. Earlier, the oxygen potential in the system LaV03/LaV04/La20, was determined by Pankajavalli et al. [ 111. For the purpose of determining the AGF of either of the ternary phases LaVO, or LaVO,, that of one of the phases was needed. It was found that the CaF, e.m.f. method adopted for the evaluation of stability of LaCrO, could well be employed for the measurement of AGF of LaVO, after suitable phase equilibrium measurements. The results of the CaF, e.m.f. studies on LaVO,/V,O, (s or 1) /LaF3 in pure oxygen at 1 atm pressure are reported here.
2. Experimental
procedure
The compounds La,O, (Indian purity better than 99.99%), V20, UK, purity better than 99.99%) UK, electronic grade) were used
Rare Earths, India, (Johnson Matthey, and LaF, (Reacton, as the starting mate-
248
R. Pankajavalli, 0. Sreedharan /Materials Letters 24 (1995) 247-251
rials. The ternary compound LaVO, as well as an equimolar mixture of LaVO,/V,Os were prepared from intimate mixtures of La*O, and V,O, in the mole ratios 1:l and 1:2 respectively. These mixtures were compacted into cylindrical pellets of 10 mm diameter and 3-5 mm thickness in a hydraulic press under a pressure of 100 MPa. The pellets were sintered in static oxygen atmosphere at 1073 K for 24 h. This procedure was repeated three times to ensure completion of the reaction. It should be noted that the two batches of pellets were to be heated separately throughout in order to avoid thermal transpiration of V,05 between the biphasic mixture and the pure compound LaVO,. Phase analyses were carried out by powder X-ray diffraction (XRD) and the products were found to be LaV04 and LaVO,/V,OS respectively for the two batches within the 5 mass% limit of detection of impurities by XRD. The electrode pellets were made by compacting equal weight ratios of the component materials for the reference electrode La,O,/LaF, and for the test electrode LaVO,/V,OJLaF, at a pressure of 100 MPa followed by sintering as described above. Cylindrical discs of single crystal CaF,, 10 mm in diameter and 3 mm thickness (Harshaw/Filtrol, USA) had served as the electrolyte for the galvanic cell. For the purpose of establishing equilibrium nature of the e.m.f. values, two sets of test electrodes were made differing by about 5 mass% on either side of equal mass ratios and were designated as A and B in the e.m.f. runs. An open cell stacked pellet assembly was used for the e.m.f. measurements. High purity oxygen gas (IOL, India, better than 99.99% ~01%) was used as the cover gas for the galvanic cell in order to fix the oxygen fugacity at unity. The absence of thermal and other asymmetric potentials was verified by the observation of null e.m.f. ( k 1 mV) over the range 800-l 100 K in a symmetric galvanic cell with CaF, as electrolyte and with identical electrodes comprising of Pt/La,O,/ LaF,/O, ( 1 atm) . A standard thermocouple of type S (Pt-10% Rh/Pt) calibrated at the freezing points of Sn, Zn, Sb and Ag was used for temperature measurements. The e.m.f. measurements on the following galvanic cell were carried out over the range 887 to 1059 K: Pt, La,O,, LaF,, O2 ( 1 atm) 1CaF, IO, ( 1 atm), LaF,, LaVO,, V205 (s or l), Pt.
The equilibrium nature of the e.m.f. was verified not only by thermal cycling and micropolarization but also by marginal variation in the ratio of the co-existing phases as stated earlier. Other experimental details are as given in earlier publications [ 12,131.
3. Results and discussion The e.m.f. results on cell I listed in Table 1 are plotted as a function of temperature in Fig. 1. The leastsquares analyses of the e.m.f. values over the ranges 887-960 K and 982-1059 K corresponding to solid and liquid ranges of V20, could be represented by the following equations: EC,, f 1.1 (mV) =316.7+0.04731
T(K)
(887-960
K),
(1)
1059 K).
(2)
Eo,, + 2.7 (mV) =435.1-0.07634
T (K)
(982-
3.1. Gibbs energy data on LaV04 and AHzs of V,O, The two half cell reactions of cell I are given by: LapOj + 6F- + 2LaF, + $0, + 6e-, 2LaF3+V,0, + 2LaV0,
(3a)
(s or 1) +$0,+6e+ 6F-.
(3b)
For the passage of 6 F of electricity, reaction could be represented by:
the overall cell
La,O, + V,05 (s or 1) + 2LaV0,.
(3)
Table 1 Measured e.m.f. results for cell (I) Run
T(K)
E(mV)
T(K)
E(mV)
A
903.2 960.5 1026.9
358 363 359
936.2 982.5 1044.3
360 351 356
B
887.2 950.5 1058.2
360 362 351
919.3 1013.4
360 361
249
R. Pankajavalli, 0. Sreedharan /Materials Letters 24 (1995) 247-2.51
could expect for an ideal solid-solid reaction. This observation together with that of fair agreement of the enthalpy of fusion data with the literature values upholds the reliability of the values of the slope of e.m.f. results. The AC; of LaVO, could be derived using the following expression: A Gy( LaV04) = A G&,( LaVO,)
T/K
Fig. 1. The e.m.f. of the cell: Pt. La203. LaF,, O2 ( 1 atm) O2 ( 1 atm, LaF,, LaVO,, V,!O, ( s or 1), Pt.
ICaF, 1
+ iAGT(La,O,)
+ fAG;(V,OS).
(7)
mThus, the standard Gibbs energy change for reaction had directly yielded the standard Gibbs (3), AG& energy of formation A,G&, of 2 mol of LaVO, from the constituent binary oxides La,O, and V,O, (s or 1). Hence, the AG&, of :LaVO,, was thus derived from Eqs. ( 1) and (2) to be as follows:
Expressions (5) and (7) were combined with the literature data for A GF of La,,O, and V,O, (1) (taking into account the existence of the equilibrium phases VO, V,03 and VO,) compiled by Kubaschewski [ 141 in order to yield the following expressions for the standard Gibbs energy of formation of LaVO, over the range 960 to 1060 K:
AG&,,(LaV04)
AGT(LaV0,)
kO.3 (kJ mol-‘)
= -91.67-0.01369 AG&,(LaV04)
T (K),
+0.78
(4)
(W mol-‘)
= - 125.93+0.02210
T (K).
(5)
Eqs. (4) and (5) are valid for the equilibrium solid and liquid ranges of e:xistence of V205 respectively. Combining the above two equations, the standard Gibbs energy change, AG& for the melting transformation of V,Os was derived to be: AG&(V,O,)
=68.53-0.07158
1’ (K).
(6)
The melting point and the standard enthalpy of fusion of V,O, in 1 atm of oxygen were thus found to be 957 K and 68.5 kJ mol-’ which are in fair agreement with 943 K and 65.3 kJ mol-’ reported for the same in the ambient air [ 141. The somewhat larger values of T,, and AH& of V205 might be either due to the inherent limitations in the method of slopes of e.m.f. data for such determinations olr due to the possible effect of stoichiometry on T,, and AH&, when the sample environment happened to be pure oxygen as in the present studies instead of the a:mbient air. The entropy change AS&, for the formation of 1 mol of LaVO, from the binary oxide was found to be a small positive value of
(W mol-‘)
= - 1862.72+0.35305
T (K).
(8)
The precision quoted in Eq. (8) did not include the uncertainities in the Gibbs energy data on the binary oxides. 3.2. Gibbs energy data on L.aVO, The standard Gibbs energy change for the following reaction: LaVO,(s)
+ 1.1 (kJ mol-‘)
+0.8
+&O,(g)
*LaVO,(s),
(9)
had been reported earlier by Pankajavalli et al. [ 111 using oxygen potential measurements on the co-existing three-phase mixture La,O,/LaVO,/LaVO, over the range 1133 to 1373 K. Combining this with Eq. (8) and AC: of V20, and V20, from Kubaschewski, the following expressions were derived: AG&,(LaVO,)
k3.3
= - 17.54-0.02652 AGp(LaV03)
f3.3
(kJ mol-‘) T (K),
(10)
(W mol-‘)
= - 1558.62+0.22724T
(K).
(11)
Though the value of 27 J K- ’ mol- ’ for AS;,, of LaVO, is somewhat larger than the theoretical value of about 10 J K-’ mol-’ for ideal solutions, still it may be considered as acceptable in view of the derived
R. Pankajavalli, 0. Sreedharan /Materials Letters 24 (1995) 247-251
250
nature of the property instead of direct determination. In addition, it should be borne in mind that the Gibbs energy corresponding to reaction (9) had to be extrapolated to lower temperature in order to facilitate the derivation of Eqs. ( 10) and ( 1I ) . Therefore, the temperature range of validity could be taken to be corresponding to the liquidus range of V,05, namely 960 to 1060 K.
3.3. Comparison of A Gf”of LaM03
Sreedharan et al. [ 111 had earlier reported a systematic trend in the values of A G; of LaM03 (when plotted against the atomic number of M) at 1273 K, where M was Mn, Fe, Co and Ni. This trend in A G; values was used to estimate the AGF of LaCu03 at 1273 K. The same systematic trend could be seen in the value of AGF of LaV03 from Eq. ( 11) and in that of LaCrO, [ lo] seen tions ionic
(when extrapolated to the same temperature) as from Fig. 2. This information may find applicain the crystallo-chemical calculations based on radii and crystal structure.
3.4. Phase diagram of the La-V-0
system
There are only two ternary compounds reported in the system La-V-O. The co-existence of La,O,/ LaVOJLaVO, had been earlier established by Pankajavalli et al. [ 111. In the present investigation, the co-existence of LaV04/V,05( 1) had been established at 1073 K though the e.m.f. studies had been limited to about 1059 K owing to the high volatility of Vz05 (P( V,05) = 1.6 mm Hg at 1073 K) [ 141. The tie lines corresponding to these co-existences are shown in the isothermal section of the La-V-O equilibrium diagram (Fig. 3). The remaining tie lines are based on the analysis of the Gibbs energy data on the binary and ternary oxides in this system. For instance, the following reactions: LaVO,(s)
+V,O,(l)
+LaVO,(s)
+2VO,(s), (12)
La(s) +LaVO,(s)
*La,O,(s)
+V(s)
(13)
and 3LaVO,( s) + $V( s) + $La,O,( s) + qVO( s), ( 14) had been considered at 1073 K. In each of the above reactions four phases were involved at the four comers of the quadrilateral; which of the diagonals would form the tie line is decided by the lower value of the sum of AGY as per stoichiometry. These considerations had led to the phase diagram represented in Fig. 3. The Ellingham diagram of the relevant co-existing phases is plotted in Fig. 4. It could be seen that LaVO,
-8OO-
-900 -
2
z
2
z
-lOoo-
0
0 -4 _ 8
-llOO-
c
-1200-
-l%O
ATOMIC
26
NUMBER
27
26
29
OF M
Fig. 2. Systematic trend in A GF of LaM03 against atomic number of M (where M is V, Cr. Mn, Fe, Co, Ni and Cu).
Fig. 3. Isothermal section of La-V-O
phase diagram at 1073 K.
R. Pankajavalli, 0. Sreedharan /Materials Letters 24 (1995) 247-251 T/K
251
Metallurgy and Materials Group and Dr. P. Rodriguez, Director, Indira Gandhi Centre for Atomic Research for their keen interest and constant encouragement throughout the course of this work.
1280 1288
References
9
9
11
10 184/T
12
[KY
Fig. 4. Ellingham diagram :Yorthe relevant co-existing V-O system.
phases in La-
would co-exist with V (instead of VO) and La,O, as in the case of LaFeO,, and LaCrO, which were found to co-exist with Fe/La203 and Cr/LaCrO, in contrast to the LaMnO and LaCoO, analogues. Acknowledgements The authors are indebted to Shri J.B. Gnanamoorthy, Head Metallurgy Division, Dr. Baldev Raj, Director,
[l] W.F. Libby, Science 171 ( 1970) 199. [2] D.B. Meadowcroft, P.G. Meier and A.C. Warren, Energy Conversion 12 (1972) 145. [ 31 J.G. Bednorz and K.A. Mttller, Z. Phys. B 64 ( 1986) 189. [4] O.M. Sreedharan, C. Mallika and K. Swaminathan, J. Mater. Sci. 23 (1988) 2735. [5] O.M. Sreedharan, R. Pankajavalli and J.B. Gnanamoorthy, High Temp. Sci. 16 (1983) 251. [6] O.M. Sreedharan and MS. Chandrasekharaiah, J. Mater. Sci. 21 (1986) 2581. [ 71 O.M. Sreedharan and M.S. Chandrasekharaiah, Mater. Res. Bull, 7 ( 1972) 1135. [ 81 O.M. Sreedharan and R. Pankajavalli, J. Mater. Sci. Letters 3 (1984) 388. [9] T. Nakamura, G. Petzow and L.J. Gauckler, Mater. Res. Bull. 14 (1979) 649. [lo] A.M. Azad, R. Sudha and O.M. Sreedharan, J. Less-Common Metals 166 (1990) 57. [ 1 l] R. Pankajavalli, O.M. Sreedharan and J.B. Gnanamoorthy, Trans. Ind. Inst. Metals 36 (1983) 250. [ 121 A.M. Azad and O.M. Sreedharan, J. Appl. Electrochem. 17 (1987) 949. [ 131 A.M. Azad, R. Sudha and O.M. Sreedharan, Thermochim. Acta 194 (1992) 129. [ 141 0. Kubaschewski and C.B. Alcock, Metallurgical thermochemistry, 5th Ed. (Pergamon Press, Oxford, 1983).
ELSEVIER
Materials Letters 24 ( 1995) 247-25
Thermodynamic
1
stabilities of LaVO, and LaVO, by e.m.f. methods
R. Pankajavalli,
O.M. Sreedharan *
Metallu yqy Division, Indira Gandhi Centrefor Atomic Research, Kalpakkam,
Received
10 January
Tamil Nadu 603 102. India
1995; revised 25 April 1995; accepted 27 April 1995
Abstract The e.m.f. of the galvanic cell: Pt, La203, LaF3, 0, ( 1 atm) 1CaF, 1O2 ( 1 atm), LaF,, LaVO,, V,OS (s or I), Pt. was measured as a function of temperature. The melting temperature and the standard enthalpy of fusion of V,05 was derived to be 957 K and 68.5 kJ mol- ’ in agreement with the literature. The standard Gibbs energies of formation, A G&x, of LaVOj and LaVO, from the constituent binary oxides were calculated to be: AG&,(LaV03) + 3.3 (kJ mol- ’ ) = - 17.54 -0.02652 T (K) and A G$,( LaVO,) + 0.78 (kJ mol ’ ) = - 125.93 + 0.02210 T (K). These values are valid over the range 960 to 1073 K. Using the literature data on the binary oxides, the Aq of LaVO, was calculated to be - 1269 kJ mol- ’ at 1273 K (by extrapolation) and found to exhibit a systematic trend with other LaMO, compounds (where M = Cr, Mn, Fe, Co, Ni and Cu) when plotted against the atomic number of the transition metal. A phase diagram of La-V-O system at 1073 K could be suggested based on the obtained results.
1. Introduction Perovskites in the ternary system of lanthanum-transition metal-oxygen I:LaMO,) had often found applications as oxidation catalysts in pollution control [ 11, as electrode materials for MHD generators [2] and recently as matrix materials related to ceramic superconductors [ 3,4]. The standard Gibbs energies of formation of LaMO,, wlhere M is Mn, Fe and Co, were reported in the literature [ 5-81 by employing the solid oxide electrolyte e.m.f. method. Using these Gibbs energy data and that reported by Nakamura et al. on LaNiO, [ 91, the standard Gibbs energy of formation, AGP of LaCu03 coul’d be predicted by Sreedharan et al. [4]. In order to determine the thermodynamic stability of LaCrO,, Azad et al. [lo] had employed the CaF, e.m.f technique under an atmosphere of unit * Corresponding
author.
0167-577x/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDlO167-577x(95)0~0096-8
fugacity of oxygen. Earlier, the oxygen potential in the system LaV03/LaV04/La20, was determined by Pankajavalli et al. [ 111. For the purpose of determining the AGF of either of the ternary phases LaVO, or LaVO,, that of one of the phases was needed. It was found that the CaF, e.m.f. method adopted for the evaluation of stability of LaCrO, could well be employed for the measurement of AGF of LaVO, after suitable phase equilibrium measurements. The results of the CaF, e.m.f. studies on LaVO,/V,O, (s or 1) /LaF3 in pure oxygen at 1 atm pressure are reported here.
2. Experimental
procedure
The compounds La,O, (Indian purity better than 99.99%), V20, UK, purity better than 99.99%) UK, electronic grade) were used
Rare Earths, India, (Johnson Matthey, and LaF, (Reacton, as the starting mate-
248
R. Pankajavalli, 0. Sreedharan /Materials Letters 24 (1995) 247-251
rials. The ternary compound LaVO, as well as an equimolar mixture of LaVO,/V,Os were prepared from intimate mixtures of La*O, and V,O, in the mole ratios 1:l and 1:2 respectively. These mixtures were compacted into cylindrical pellets of 10 mm diameter and 3-5 mm thickness in a hydraulic press under a pressure of 100 MPa. The pellets were sintered in static oxygen atmosphere at 1073 K for 24 h. This procedure was repeated three times to ensure completion of the reaction. It should be noted that the two batches of pellets were to be heated separately throughout in order to avoid thermal transpiration of V,05 between the biphasic mixture and the pure compound LaVO,. Phase analyses were carried out by powder X-ray diffraction (XRD) and the products were found to be LaV04 and LaVO,/V,OS respectively for the two batches within the 5 mass% limit of detection of impurities by XRD. The electrode pellets were made by compacting equal weight ratios of the component materials for the reference electrode La,O,/LaF, and for the test electrode LaVO,/V,OJLaF, at a pressure of 100 MPa followed by sintering as described above. Cylindrical discs of single crystal CaF,, 10 mm in diameter and 3 mm thickness (Harshaw/Filtrol, USA) had served as the electrolyte for the galvanic cell. For the purpose of establishing equilibrium nature of the e.m.f. values, two sets of test electrodes were made differing by about 5 mass% on either side of equal mass ratios and were designated as A and B in the e.m.f. runs. An open cell stacked pellet assembly was used for the e.m.f. measurements. High purity oxygen gas (IOL, India, better than 99.99% ~01%) was used as the cover gas for the galvanic cell in order to fix the oxygen fugacity at unity. The absence of thermal and other asymmetric potentials was verified by the observation of null e.m.f. ( k 1 mV) over the range 800-l 100 K in a symmetric galvanic cell with CaF, as electrolyte and with identical electrodes comprising of Pt/La,O,/ LaF,/O, ( 1 atm) . A standard thermocouple of type S (Pt-10% Rh/Pt) calibrated at the freezing points of Sn, Zn, Sb and Ag was used for temperature measurements. The e.m.f. measurements on the following galvanic cell were carried out over the range 887 to 1059 K: Pt, La,O,, LaF,, O2 ( 1 atm) 1CaF, IO, ( 1 atm), LaF,, LaVO,, V205 (s or l), Pt.
The equilibrium nature of the e.m.f. was verified not only by thermal cycling and micropolarization but also by marginal variation in the ratio of the co-existing phases as stated earlier. Other experimental details are as given in earlier publications [ 12,131.
3. Results and discussion The e.m.f. results on cell I listed in Table 1 are plotted as a function of temperature in Fig. 1. The leastsquares analyses of the e.m.f. values over the ranges 887-960 K and 982-1059 K corresponding to solid and liquid ranges of V20, could be represented by the following equations: EC,, f 1.1 (mV) =316.7+0.04731
T(K)
(887-960
K),
(1)
1059 K).
(2)
Eo,, + 2.7 (mV) =435.1-0.07634
T (K)
(982-
3.1. Gibbs energy data on LaV04 and AHzs of V,O, The two half cell reactions of cell I are given by: LapOj + 6F- + 2LaF, + $0, + 6e-, 2LaF3+V,0, + 2LaV0,
(3a)
(s or 1) +$0,+6e+ 6F-.
(3b)
For the passage of 6 F of electricity, reaction could be represented by:
the overall cell
La,O, + V,05 (s or 1) + 2LaV0,.
(3)
Table 1 Measured e.m.f. results for cell (I) Run
T(K)
E(mV)
T(K)
E(mV)
A
903.2 960.5 1026.9
358 363 359
936.2 982.5 1044.3
360 351 356
B
887.2 950.5 1058.2
360 362 351
919.3 1013.4
360 361
249
R. Pankajavalli, 0. Sreedharan /Materials Letters 24 (1995) 247-2.51
could expect for an ideal solid-solid reaction. This observation together with that of fair agreement of the enthalpy of fusion data with the literature values upholds the reliability of the values of the slope of e.m.f. results. The AC; of LaVO, could be derived using the following expression: A Gy( LaV04) = A G&,( LaVO,)
T/K
Fig. 1. The e.m.f. of the cell: Pt. La203. LaF,, O2 ( 1 atm) O2 ( 1 atm, LaF,, LaVO,, V,!O, ( s or 1), Pt.
ICaF, 1
+ iAGT(La,O,)
+ fAG;(V,OS).
(7)
mThus, the standard Gibbs energy change for reaction had directly yielded the standard Gibbs (3), AG& energy of formation A,G&, of 2 mol of LaVO, from the constituent binary oxides La,O, and V,O, (s or 1). Hence, the AG&, of :LaVO,, was thus derived from Eqs. ( 1) and (2) to be as follows:
Expressions (5) and (7) were combined with the literature data for A GF of La,,O, and V,O, (1) (taking into account the existence of the equilibrium phases VO, V,03 and VO,) compiled by Kubaschewski [ 141 in order to yield the following expressions for the standard Gibbs energy of formation of LaVO, over the range 960 to 1060 K:
AG&,,(LaV04)
AGT(LaV0,)
kO.3 (kJ mol-‘)
= -91.67-0.01369 AG&,(LaV04)
T (K),
+0.78
(4)
(W mol-‘)
= - 125.93+0.02210
T (K).
(5)
Eqs. (4) and (5) are valid for the equilibrium solid and liquid ranges of e:xistence of V205 respectively. Combining the above two equations, the standard Gibbs energy change, AG& for the melting transformation of V,Os was derived to be: AG&(V,O,)
=68.53-0.07158
1’ (K).
(6)
The melting point and the standard enthalpy of fusion of V,O, in 1 atm of oxygen were thus found to be 957 K and 68.5 kJ mol-’ which are in fair agreement with 943 K and 65.3 kJ mol-’ reported for the same in the ambient air [ 141. The somewhat larger values of T,, and AH& of V205 might be either due to the inherent limitations in the method of slopes of e.m.f. data for such determinations olr due to the possible effect of stoichiometry on T,, and AH&, when the sample environment happened to be pure oxygen as in the present studies instead of the a:mbient air. The entropy change AS&, for the formation of 1 mol of LaVO, from the binary oxide was found to be a small positive value of
(W mol-‘)
= - 1862.72+0.35305
T (K).
(8)
The precision quoted in Eq. (8) did not include the uncertainities in the Gibbs energy data on the binary oxides. 3.2. Gibbs energy data on L.aVO, The standard Gibbs energy change for the following reaction: LaVO,(s)
+ 1.1 (kJ mol-‘)
+0.8
+&O,(g)
*LaVO,(s),
(9)
had been reported earlier by Pankajavalli et al. [ 111 using oxygen potential measurements on the co-existing three-phase mixture La,O,/LaVO,/LaVO, over the range 1133 to 1373 K. Combining this with Eq. (8) and AC: of V20, and V20, from Kubaschewski, the following expressions were derived: AG&,(LaVO,)
k3.3
= - 17.54-0.02652 AGp(LaV03)
f3.3
(kJ mol-‘) T (K),
(10)
(W mol-‘)
= - 1558.62+0.22724T
(K).
(11)
Though the value of 27 J K- ’ mol- ’ for AS;,, of LaVO, is somewhat larger than the theoretical value of about 10 J K-’ mol-’ for ideal solutions, still it may be considered as acceptable in view of the derived
R. Pankajavalli, 0. Sreedharan /Materials Letters 24 (1995) 247-251
250
nature of the property instead of direct determination. In addition, it should be borne in mind that the Gibbs energy corresponding to reaction (9) had to be extrapolated to lower temperature in order to facilitate the derivation of Eqs. ( 10) and ( 1I ) . Therefore, the temperature range of validity could be taken to be corresponding to the liquidus range of V,05, namely 960 to 1060 K.
3.3. Comparison of A Gf”of LaM03
Sreedharan et al. [ 111 had earlier reported a systematic trend in the values of A G; of LaM03 (when plotted against the atomic number of M) at 1273 K, where M was Mn, Fe, Co and Ni. This trend in A G; values was used to estimate the AGF of LaCu03 at 1273 K. The same systematic trend could be seen in the value of AGF of LaV03 from Eq. ( 11) and in that of LaCrO, [ lo] seen tions ionic
(when extrapolated to the same temperature) as from Fig. 2. This information may find applicain the crystallo-chemical calculations based on radii and crystal structure.
3.4. Phase diagram of the La-V-0
system
There are only two ternary compounds reported in the system La-V-O. The co-existence of La,O,/ LaVOJLaVO, had been earlier established by Pankajavalli et al. [ 111. In the present investigation, the co-existence of LaV04/V,05( 1) had been established at 1073 K though the e.m.f. studies had been limited to about 1059 K owing to the high volatility of Vz05 (P( V,05) = 1.6 mm Hg at 1073 K) [ 141. The tie lines corresponding to these co-existences are shown in the isothermal section of the La-V-O equilibrium diagram (Fig. 3). The remaining tie lines are based on the analysis of the Gibbs energy data on the binary and ternary oxides in this system. For instance, the following reactions: LaVO,(s)
+V,O,(l)
+LaVO,(s)
+2VO,(s), (12)
La(s) +LaVO,(s)
*La,O,(s)
+V(s)
(13)
and 3LaVO,( s) + $V( s) + $La,O,( s) + qVO( s), ( 14) had been considered at 1073 K. In each of the above reactions four phases were involved at the four comers of the quadrilateral; which of the diagonals would form the tie line is decided by the lower value of the sum of AGY as per stoichiometry. These considerations had led to the phase diagram represented in Fig. 3. The Ellingham diagram of the relevant co-existing phases is plotted in Fig. 4. It could be seen that LaVO,
-8OO-
-900 -
2
z
2
z
-lOoo-
0
0 -4 _ 8
-llOO-
c
-1200-
-l%O
ATOMIC
26
NUMBER
27
26
29
OF M
Fig. 2. Systematic trend in A GF of LaM03 against atomic number of M (where M is V, Cr. Mn, Fe, Co, Ni and Cu).
Fig. 3. Isothermal section of La-V-O
phase diagram at 1073 K.
R. Pankajavalli, 0. Sreedharan /Materials Letters 24 (1995) 247-251 T/K
251
Metallurgy and Materials Group and Dr. P. Rodriguez, Director, Indira Gandhi Centre for Atomic Research for their keen interest and constant encouragement throughout the course of this work.
1280 1288
References
9
9
11
10 184/T
12
[KY
Fig. 4. Ellingham diagram :Yorthe relevant co-existing V-O system.
phases in La-
would co-exist with V (instead of VO) and La,O, as in the case of LaFeO,, and LaCrO, which were found to co-exist with Fe/La203 and Cr/LaCrO, in contrast to the LaMnO and LaCoO, analogues. Acknowledgements The authors are indebted to Shri J.B. Gnanamoorthy, Head Metallurgy Division, Dr. Baldev Raj, Director,
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