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Phase transitions and ionic conductivity of the Na2SO4MgSO4 system
Solid State Ionics 40/41 North-Holland
( 1990) 31-33
PHASE TRANSITIONS
AND IONIC CONDUCTIVITY
P.W.S.K. BANDARANAYAKE’
OF THE NazS04-MgS04
SYSTEM
and B.-E. MELLANDER
Department ofPhysics. Chalmers University of Technology, S-412 96 GGteborg, Sweden
Differential scanning calorimetry, X-ray diffractrometry and complex formed for the Na2S0.,-MgSO, binary system. The redetermined phase perature phase of Na*SO, is stable up to 35 mol% MgSO, at 680°C. The increasing MgSO, content, the maximum conductivity at 540°C is about For two intermediate compounds, Na2Mg(S04)2 and Na2Mg3(S04)4, 4X 10-6fi-1 cm-’ at 500°C.
impedance spectroscopy measurements have been perdiagram shows that the solid solution of the high-temionic conductivity in this phase increases rapidly with 150 times larger than the conductivity of pure Na2S0,. the ionic conductivity is relatively low, 2x 10m4 and
1. Introduction
2. Experimental
Ionic conductivity studies of the high-temperature phase of sodium sulfate,Naz SO, (I ) , have shown that the conductivity may be considerably improved by adding small amounts of other salts [ 1,2]. For the Na,SO,( I) phase there exists extended solid solubility regions for a number of other cations [2-41 and these systems thus give an opportunity to study the effect of cation substitution in a single phase region over a wide concentration range. The phase diagram of the Na2S04-MgS04 has been investigated in two earlier papers [ 5,6] which both showed a large solid solubility of MgS04 in the NazSO,( I) phase. Very large numbers of cation vacancies may thus be created by adding MgS04 to NazS04( I). The lowtemperature phases have, in general, low solubility for other ions. We have studied the Na2S04-MgSO., system using differential scanning calorimetry (DSC), complex impedance spectroscopy and X-ray powder diffraction. The phase diagram has been redetermined and the ionic conductivity has been studied for both single- and two-phase regions in the system. Electrical conductivity measurements of the solid solution of the Naz SO, (I) phase have earlier been reported in one paper [ 7 ] _
Anhydrous Naz SO, of pro analysi quality (Merck, Germany) and MgS0,*7Hz0 of 99.5% purity (Merck, Germany) were used. Suprapur quality (Merck, Germany) Na2S04 and MgSO, of 99.999% purity (Aldrich, Germany) were used for samples containing less than 5 mol% MgS04. Both compounds were dried at 250°C for 72 h. After drying, the different compositions were carefully weighed and then ground for 15 min for proper mixing. Each sample was heated in a platinum crucible up to 50°C above the melting temperature where it was kept for one hour. All the samples were allowed to cool slowly inside the furnace. Finally they were mechanically ground to a fine powder which was kept in a dry atmosphere when not in use. A Rigaku differential scanning calorimeter was used to detect the phase transitions. Dry A1203 powder, sealed inside a platinum cup, was used as reference. The samples were contained in platinum cups and all measurements were performed in a nitrogen atmosphere. The heating rate was in most cases 10 K min-‘. The X-ray investigations were performed using a Philips powder diffractometer equipped with a high-temperature attachment. Ni filtered Cu Ka radiation was used. Room temperature measurements were made in vacuum while measurements above 100°C were made air. The electrical conductivity was determined using
’
Permanent Peradeniya,
address: Department of Physics, Peradeniya, Sri Lanka.
0167-2738/90/$03.50 (North-Holland )
University
0 Elsevier Science Publishers
B.V.
of
32
P. W.S.K. Bandaranayake, B.-E. Mellander / NalSO,-MgSO,
complex impedance spectroscopy. For measurements in high-conductivity phases cylindrical pellets of 13 mm diameter and about 2 mm thickness were pressed together with thin gold foils of thickness 0.0 1 mm using 200 MPa pressure. Pellets for measurements in low-conducting phases were pressed in the same way but the electrodes were instead painted on after pressing, using graphite dissolved in alcohol (DAG-580, Acheson, Holland). The pellets were heat treated at 500°C for three days. The complex impedance of the samples was measured over a frequency range of 100 Hz to 100 kHz using a computer controlled Hewlett-Packard HP4274A K&meter.
OC 1200
r
I
system
I
1
1
I
/ 1000
/ m+V 800
GO0 n
a:
3. Results and discussion The redetermined phase diagram of the Na2S04-MgS04 system shows, in general, a good agreement with the earlier phase diagrams [ 5,6], see fig. 1. The solid solution of the Naz SO, ( I ) phase has a large stability region, up to 35 mol% MgS04 at 680°C. The difference between the present and the earlier phase diagrams regards the intermediate phases. From our measurements we have identified intermediate phases for 50 and 75 molW MgS04 corresponding to Na,Mg(S04)* and NazMg3(S04)4. We had expected an intermediate phase at 25 mol% MgS04, but no intermediate phase could be detected at this composition. It can be seen from the phase diagram that the transition between the two-phase regions II+111 and I+111 at 205 “C extends up to 50mol% MgS04 according to the DSC measurements, and the X-ray diffraction patterns for samples in this composition range can be indexed as a two-phase mixture. Metastable phases are common in Na,SO,-based systems and have been observed also in the present system. For compositions between 25 and 70 mol% MgS04 transitions related to metastable phases have been observed during cooling runs. Most of the differences compared to the earlier phase diagrams may thus be due to metastable phases. The diffractograms of the intermediate phases have been determined, but a crystal structure determination is beyond the scope of this study. The ionic conductivity in the solid solution of the Nal SO4 (I ) phase is shown in fig. 2 for MgSO, concentrations up to 30 mol%. The ionic conductivity
IV+V 400
III+IV 200
0
NaaSO,
20
GO
40 mol%
30
100 MGO,
Fig. 1. Phase diagram of the Na2S04-MgS04 system determined from DSC and X-ray diffraction measurements.
increases rapidly with increasing concentration of MgS04 and reaches a maximum at about 20 mol% MgS04. The increase in conductivity is in agreement with the results of Guth et al. [ 71 who reported conductivities for MgS04 concentrations up to 10 mol%, our conductivities are, however, somewhat lower. An increase in the conductivity with increasing aliovalent substitution has earlier been reported for di- and trivalent ions in Na,SO, [ 2,3,8]. The conductivity maximum occurs at 5 molO/b for CaS04 substitution [2] and for other NazSO,-based systems the maximum conductivity is obtained for a vacancy concentration of 7% [ 3,8]. The maximum indicates that defect interactions must be present, and the large vacancy concentrations in this phase makes it probable that clusters are formed of dopant ion and vacancies [ 21. The activation energy in the NazS04(I) phase is 0.5 eV at 25 mol% MgS04. The intermediate phases have rather low ionic
P. W.S.K. Bandaranayake, B.-E. Mellander / NaJO,-MgSO,
system
33 2
4
2
cm-’ for NazMg,(S04)4. This can be compared to the conductivity in the solid solution of the Na,SO,(I) phase, e.g., 2~10~~ 0-l cm-’ for 10 mol% MgS04 at the same temperature. The activation energy in the intrinsic temperature range is 1.O eV for NazMg(S04)z and 1.1 eV Na2Mg,(S04),. The ionic conductivity in the two-phase regions in the phase diagram has been found to decrease monotonically with increasing MgS04 content as expected. R-’
6-
4-
2-
Acknowledgement 0
I
0
10
,
I
20
30
Na,SO,
mol% MgSO,
Fig. 2. Ionic conductivity versus composition solid solution of the Na2 SO, (I) phase.
Temperature
at 540°C
for the
PC)
We would like to express our gratitude to Professor A. Lunden for many valuable discussions. One of us (PWSKB) is indebted to the International Science Programs, Uppsala University, for a scholarship which enabled his stay in Gbteborg. This work has been supported financially by the Swedish Natural Sciences Research Council, Adlerbertska Forskningsfonden and Langmanska Kulturfonden.
References
-1t
’ 1.0
I
I
I
1.5
2.0
25
1000/T
(K-l)
Fig. 3. Temperature dependence of the ionic conductivity of the intermediate compounds Na*Mg( SO4)2 (open circles) and NaZMg3(S04)4 (filled circles).
conductivities, especially the 75 mol% MgS04 phase, see fig. 3. Even at 500°C the conductivity is only 2~ lop4 Q-i cm-’ for Na Mg(S0 ) and 4~ lop6
[ 1 ] M.A. Careem and B.-E. Mellander, Solid State Ionics 15 (1985) 327. [2] P.W.S.K. Bandaranayake and B.-E. Mellander, Solid State Ionics 26 ( 1988) 33, and references therein. [ 31 H.H. Hofer, W. Eysel and U. von Alpen, .I. Solid State Chem. 36 (1981) 365. [ 41 W. Eysel, H.H. Hofer, K.L. Keester and T. Hahn, Acta Cryst. B41 (1985) 5. [ 51 R. Nacken, Nachr. Ges. Wiss. Gijttingen ( 1907). [ 61 A.S. Ginberg, Z. Anorg. Chem. 61 ( 1909) 122. [ 71 U. Guth, J. Rosenkranz, P. Schmidt and H.-H. Mobius, Wiss. Z. Ernst-Moritz-Amdt-Univitat Greifswald Math.-Nat. Wiss. Reihe 36 (1987) 42. [ 8) H.H. Hofner, W. Eysel and U. von Alpen, Mat. Res. Bull. 13 (1978) 265.
( 1990) 31-33
PHASE TRANSITIONS
AND IONIC CONDUCTIVITY
P.W.S.K. BANDARANAYAKE’
OF THE NazS04-MgS04
SYSTEM
and B.-E. MELLANDER
Department ofPhysics. Chalmers University of Technology, S-412 96 GGteborg, Sweden
Differential scanning calorimetry, X-ray diffractrometry and complex formed for the Na2S0.,-MgSO, binary system. The redetermined phase perature phase of Na*SO, is stable up to 35 mol% MgSO, at 680°C. The increasing MgSO, content, the maximum conductivity at 540°C is about For two intermediate compounds, Na2Mg(S04)2 and Na2Mg3(S04)4, 4X 10-6fi-1 cm-’ at 500°C.
impedance spectroscopy measurements have been perdiagram shows that the solid solution of the high-temionic conductivity in this phase increases rapidly with 150 times larger than the conductivity of pure Na2S0,. the ionic conductivity is relatively low, 2x 10m4 and
1. Introduction
2. Experimental
Ionic conductivity studies of the high-temperature phase of sodium sulfate,Naz SO, (I ) , have shown that the conductivity may be considerably improved by adding small amounts of other salts [ 1,2]. For the Na,SO,( I) phase there exists extended solid solubility regions for a number of other cations [2-41 and these systems thus give an opportunity to study the effect of cation substitution in a single phase region over a wide concentration range. The phase diagram of the Na2S04-MgS04 has been investigated in two earlier papers [ 5,6] which both showed a large solid solubility of MgS04 in the NazSO,( I) phase. Very large numbers of cation vacancies may thus be created by adding MgS04 to NazS04( I). The lowtemperature phases have, in general, low solubility for other ions. We have studied the Na2S04-MgSO., system using differential scanning calorimetry (DSC), complex impedance spectroscopy and X-ray powder diffraction. The phase diagram has been redetermined and the ionic conductivity has been studied for both single- and two-phase regions in the system. Electrical conductivity measurements of the solid solution of the Naz SO, (I) phase have earlier been reported in one paper [ 7 ] _
Anhydrous Naz SO, of pro analysi quality (Merck, Germany) and MgS0,*7Hz0 of 99.5% purity (Merck, Germany) were used. Suprapur quality (Merck, Germany) Na2S04 and MgSO, of 99.999% purity (Aldrich, Germany) were used for samples containing less than 5 mol% MgS04. Both compounds were dried at 250°C for 72 h. After drying, the different compositions were carefully weighed and then ground for 15 min for proper mixing. Each sample was heated in a platinum crucible up to 50°C above the melting temperature where it was kept for one hour. All the samples were allowed to cool slowly inside the furnace. Finally they were mechanically ground to a fine powder which was kept in a dry atmosphere when not in use. A Rigaku differential scanning calorimeter was used to detect the phase transitions. Dry A1203 powder, sealed inside a platinum cup, was used as reference. The samples were contained in platinum cups and all measurements were performed in a nitrogen atmosphere. The heating rate was in most cases 10 K min-‘. The X-ray investigations were performed using a Philips powder diffractometer equipped with a high-temperature attachment. Ni filtered Cu Ka radiation was used. Room temperature measurements were made in vacuum while measurements above 100°C were made air. The electrical conductivity was determined using
’
Permanent Peradeniya,
address: Department of Physics, Peradeniya, Sri Lanka.
0167-2738/90/$03.50 (North-Holland )
University
0 Elsevier Science Publishers
B.V.
of
32
P. W.S.K. Bandaranayake, B.-E. Mellander / NalSO,-MgSO,
complex impedance spectroscopy. For measurements in high-conductivity phases cylindrical pellets of 13 mm diameter and about 2 mm thickness were pressed together with thin gold foils of thickness 0.0 1 mm using 200 MPa pressure. Pellets for measurements in low-conducting phases were pressed in the same way but the electrodes were instead painted on after pressing, using graphite dissolved in alcohol (DAG-580, Acheson, Holland). The pellets were heat treated at 500°C for three days. The complex impedance of the samples was measured over a frequency range of 100 Hz to 100 kHz using a computer controlled Hewlett-Packard HP4274A K&meter.
OC 1200
r
I
system
I
1
1
I
/ 1000
/ m+V 800
GO0 n
a:
3. Results and discussion The redetermined phase diagram of the Na2S04-MgS04 system shows, in general, a good agreement with the earlier phase diagrams [ 5,6], see fig. 1. The solid solution of the Naz SO, ( I ) phase has a large stability region, up to 35 mol% MgS04 at 680°C. The difference between the present and the earlier phase diagrams regards the intermediate phases. From our measurements we have identified intermediate phases for 50 and 75 molW MgS04 corresponding to Na,Mg(S04)* and NazMg3(S04)4. We had expected an intermediate phase at 25 mol% MgS04, but no intermediate phase could be detected at this composition. It can be seen from the phase diagram that the transition between the two-phase regions II+111 and I+111 at 205 “C extends up to 50mol% MgS04 according to the DSC measurements, and the X-ray diffraction patterns for samples in this composition range can be indexed as a two-phase mixture. Metastable phases are common in Na,SO,-based systems and have been observed also in the present system. For compositions between 25 and 70 mol% MgS04 transitions related to metastable phases have been observed during cooling runs. Most of the differences compared to the earlier phase diagrams may thus be due to metastable phases. The diffractograms of the intermediate phases have been determined, but a crystal structure determination is beyond the scope of this study. The ionic conductivity in the solid solution of the Nal SO4 (I ) phase is shown in fig. 2 for MgSO, concentrations up to 30 mol%. The ionic conductivity
IV+V 400
III+IV 200
0
NaaSO,
20
GO
40 mol%
30
100 MGO,
Fig. 1. Phase diagram of the Na2S04-MgS04 system determined from DSC and X-ray diffraction measurements.
increases rapidly with increasing concentration of MgS04 and reaches a maximum at about 20 mol% MgS04. The increase in conductivity is in agreement with the results of Guth et al. [ 71 who reported conductivities for MgS04 concentrations up to 10 mol%, our conductivities are, however, somewhat lower. An increase in the conductivity with increasing aliovalent substitution has earlier been reported for di- and trivalent ions in Na,SO, [ 2,3,8]. The conductivity maximum occurs at 5 molO/b for CaS04 substitution [2] and for other NazSO,-based systems the maximum conductivity is obtained for a vacancy concentration of 7% [ 3,8]. The maximum indicates that defect interactions must be present, and the large vacancy concentrations in this phase makes it probable that clusters are formed of dopant ion and vacancies [ 21. The activation energy in the NazS04(I) phase is 0.5 eV at 25 mol% MgS04. The intermediate phases have rather low ionic
P. W.S.K. Bandaranayake, B.-E. Mellander / NaJO,-MgSO,
system
33 2
4
2
cm-’ for NazMg,(S04)4. This can be compared to the conductivity in the solid solution of the Na,SO,(I) phase, e.g., 2~10~~ 0-l cm-’ for 10 mol% MgS04 at the same temperature. The activation energy in the intrinsic temperature range is 1.O eV for NazMg(S04)z and 1.1 eV Na2Mg,(S04),. The ionic conductivity in the two-phase regions in the phase diagram has been found to decrease monotonically with increasing MgS04 content as expected. R-’
6-
4-
2-
Acknowledgement 0
I
0
10
,
I
20
30
Na,SO,
mol% MgSO,
Fig. 2. Ionic conductivity versus composition solid solution of the Na2 SO, (I) phase.
Temperature
at 540°C
for the
PC)
We would like to express our gratitude to Professor A. Lunden for many valuable discussions. One of us (PWSKB) is indebted to the International Science Programs, Uppsala University, for a scholarship which enabled his stay in Gbteborg. This work has been supported financially by the Swedish Natural Sciences Research Council, Adlerbertska Forskningsfonden and Langmanska Kulturfonden.
References
-1t
’ 1.0
I
I
I
1.5
2.0
25
1000/T
(K-l)
Fig. 3. Temperature dependence of the ionic conductivity of the intermediate compounds Na*Mg( SO4)2 (open circles) and NaZMg3(S04)4 (filled circles).
conductivities, especially the 75 mol% MgS04 phase, see fig. 3. Even at 500°C the conductivity is only 2~ lop4 Q-i cm-’ for Na Mg(S0 ) and 4~ lop6
[ 1 ] M.A. Careem and B.-E. Mellander, Solid State Ionics 15 (1985) 327. [2] P.W.S.K. Bandaranayake and B.-E. Mellander, Solid State Ionics 26 ( 1988) 33, and references therein. [ 31 H.H. Hofer, W. Eysel and U. von Alpen, .I. Solid State Chem. 36 (1981) 365. [ 41 W. Eysel, H.H. Hofer, K.L. Keester and T. Hahn, Acta Cryst. B41 (1985) 5. [ 51 R. Nacken, Nachr. Ges. Wiss. Gijttingen ( 1907). [ 61 A.S. Ginberg, Z. Anorg. Chem. 61 ( 1909) 122. [ 71 U. Guth, J. Rosenkranz, P. Schmidt and H.-H. Mobius, Wiss. Z. Ernst-Moritz-Amdt-Univitat Greifswald Math.-Nat. Wiss. Reihe 36 (1987) 42. [ 8) H.H. Hofner, W. Eysel and U. von Alpen, Mat. Res. Bull. 13 (1978) 265.