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Electrical conductivity of Na2SO4(I)
Solid State lonics 15 (1985) 327-330 North-Holland, Amsterdam
ELECTRICAL CONDUCTIVITY OF Na2SO4 (I) M.A. CAREEM * and B.-E. MELLANDER Chalmers University of Technology, Department of Physics, S-412 96 GOteborg, Sweden Received 11 March 1985
The electrical conductivity of the solid phase Na2SO4(I) has been measured between the melting point at 8840C and the first order phase transition at about 240°C. The measurements have been performed using complex impedance measurements on pellet samples as well as on U-cells. The electrical conductivity is strongly dependent on sample purity at low temperatures and the activation energy ranges from 0.5 eV to 1.7 eV over the measured temperature range.
1. Introduction
A E = -k[Oln(oT)/a(1/T)]
In sodium sulphate a solid phase, Na2SO4(I), is stable over a temperature range of more than 600°C, from the melting point at 884°C to the solid--solid phase transition at about 2dO°C. Below 240°C a number of stable and metastable phases have been detected [ 1 - 6 ] . The electrical conductivity of Na2SO4(I) is due to the mobile sodium ions and many studies of the ionic conductivity have been reported in the literature [7-18]. The structure of Na2SO4(I) is hexagonal (P63/mmc) [19] and the conductivity is low compared to that of solid electrolytes such as wLi2SO4, but increases considerably with the addition of other cations [9,10,14,15,20]. Na2SO4(I) is thus of great interest for the study of solid electrolytes and due to the scatter in the reported values, a reinvestigation of the electrical conductivity was performed covering a large temperature range including also the region just above the melting point. The ionic conductivity o of a normal ionic compound can be divided into at least two regions: the extrinsic region, where the ionic conductivity is due to defects created by the presence of aliovalent impurities, and the intrinsic region, where thermally generated defects dominate. The activation energy AE calculated from
corresponds in the extrinsic (low temperature) region to a migration enthalpy, and in the intrinsic (high temperature) region to both migration and defect formation enthalpies [21 ].
Present address: Department of Physics, University of Peradeniya, Peradeniya, Sri Lanka. 0 167-2738/85/$ 03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
(I)
2. Experimental Two purity grades of sodium sulphate were used, suprapure and reagent grade purchased form E. Merck, Germany. Before use the salts were dried at 185°C for 48 h. The electrical conductivity was measured using two techniques. Above 500°C the capillary U-ceil method, which is described in detail elsewhere [20], was used. For temperatures below 650°C pressed pellets with evaporated gold electrodes were used. The pellets were pressed in cylindrical dies of 13 or 15 mm diameter using a pressure of 300 MPa. Prior to evaporation of gold the pellets were annealed for more than 24 h at temperatures below 600°C. The lengths of the pellets were 2 - 5 mm and good contact was ensured by spring loading. The pellet assembly was placed in the middle of a 100 cm long 8 cm internal diameter tube furnace. The temperature was measured by a Platinel II thermocouple placed close to the pellet. The electrical conductivity was obtained using impedance spectroscopy [22]. The complex impedance was measured in the frequency range 100 Hz to I00 kHz
M.A. Careem, B.E. MeUander/Electricalconductivity of Na2S04(I)
328
using a Hewlett-Packard 4274A LCR-meter and the signal applied over the sample was 20 mV.
Table 1 Electrical conductivity for Na2SO 4 using U-cell (Ou) a n d pellet m e a s u r e m e n t s . The m e a s u r e m e n t s on pellets were perf o r m e d using suprapure (o s) and reagent grade (Or) Na2SO4.
3. Results and discussion
Temp. (°C)
Fig. 1 shows a typical complex impedance plot for a pellet sample. At low frequencies the points lie on an inclined straight line, and at high frequencies the beginning of the semicircular portion is visible. At temperatures below the phase transition at 240°C only the circular part of the impedance plot was observed, however, the arc was somewhat depressed. The temperature dependence of the electrical conductivity is summarized in table 1 and fig. 2 showing the results of measurements using analytical grade (U-cell and pellet samples) and suprapure Na2SO 4 (pellet samples only). For all types of samples the results shown in fig. 2 represent both increasing and decreasing temperature runs. There is thus no hysteresis nor any change in conductivity between subsequent measurements. Good agreement with earlier measurements [7,8] is obtained in the molten state of Na2SO4. In the solid NaSO 4 (I) phase a more or less continuous curvature
ou (12 cm) -1
250 300 400 500 600 700 800 900
4.0 2.2 1.1 6.9 2.3
x 10-4 X 10 -3 x 10 -2 X 10 -2 (melt)
or (S2 cm) -1
os (S2 cm) -1
1.1 2.4 8.2 3.0 1.7
0.19 0.49 3.2 1.9
X X × × X
10 - s 10 -s 10 -5 10 --4 10 -3
x X X X
10 -s 10 -5 10 -5 10 -4
can be observed in the In oT versus 1/T plot, see fig. 2. In the low temperature range for this phase (the extrinsic region) the conductivity is, as expected, lower for the suprapure samples than for the reagent grade samples. At higher temperatures it is expected that the conductivities for the two purities will be almost the Tern pera±ure (°C) 800 10
600
I
I
400 t
200 I
~-melt-
Q)
T=297.3°C
12-
100 HZ o /
/
10-
8-
°
/
/
o o
5
--o
T
o
o o o o o o
L~ c
rn
-5
q~
~
0
-10 100kHz
2
/ o"
O
-12. 0.8
V 0
12
I
t
I
t
lt.l.
16 Re Z ( k ~ )
18
20
Fig. 1. Complex impedance plot for a pellet sample at 297.3°C.
el
t 1.0
I
I
1.5 1000/T (K -~)
2.0
Fig. 2. ln(oT) versus 1/T plot for Na2SO4. The melting transition and the s o l i d - s o l i d phase transition are included in the diagram. Open circles denote U-cell m e a s u r e m e n t s , triangles represent reagent grade pellet samples and squares suprapure pellet samples.
M.A. Careem, B.E. Mellander/Electricalconductivity of Na2SO4(IJ
329
Table 2 Electrical conductivity of Na2SO4 (I). The literature values were in all cases calculated from conductivity plots. Conductivity (s2 era) -1 300°C
Ref.
500°C -
40 X 10 ..6 1.9 X 10 -6 3.2 x 10-6 88 X 10-6 26 X 10-6 1.4 X 10-6 (5-24) X 10-6
800°C -
4.5 X 1 0 - 4 1.0 X 1 0 - 4 1.8 X 1 0 - 4 2.0 X 10 -4 2.4 x 10 .4 0.073 X 10 -4 (1.9-4.0) X 10 -4
same, since thermally generated defects will dominate over defects created by the presence o f aliovalent impurities. This behaviour is observed in the present measurements and above 500°C the conductivity o f the suprapure samples is only slightly lower than that o f the reagent grade samples. It can also be noted that the U-cell and the pellet measurements are in reasonable agreement. Literature values and the present resuits are compared in table 2. At high temperatures our values are in good agreement with the values o f Josefson and Kvist [7] and Polishchuk et al. [8] while lower values were reported by Saito et al. [15]. At 500°C all the values in table 2 are within the range 1.0 X 10 --4 to 4.5 × 10 -4 except the values o f Singh and Deshpande [17] which are much lower. At 300°C the conductivity is mainly extrinsic resulting in a large scatter in the reported values. E.g. the values o f Imanaka et al. [16] agree with our results for reagent grade samples while the values o f Jakob and Rao [13] are close to our values for suprapure samples. Because of the curvature in the In oT versus l I T plot it is difficult to compare the present activation energies with those reported in the literature. In our measurements we get in the extrinsic region an activation energy o f 0.50 eV for the reagent grade samples and 0.54 eV for the suprapure samples. The activation energy in the high temperature region is 1.68 eV but it has to be pointed out that the conductivity plot does not dearly indicate that only two regions exist. E.g., it can be suggested that three regions are present: two intrinsic regions, one with vacancy and the other
7.1 x
10 -2
--
1.5 x 10 .2 6.9 X 10 -2
[8]
[10] [11,14] [13] [15] [16] [17] Present results (see table 1)
with interstitial mobility, and one extrinsic region. Several other effects may also give rise to a curvature in the I n ( o / ) versus 1/T plot [23 ]. It should also be pointed out that since polycrystaUine samples were used, grain boundary conduction may be of importance in the extrinsic range. In conclusion, it can be noted that differences in purity o f the samples can explain many, though not all, of the discrepancies in the reported electrical conductivity values.
Acknowledgements
We would like to express our gratitude to Professor A. Lund6n for many valuable discussions. One o f us (M.A.C.) is indebted to the International Seminar for Physics for a scholarship which enabled his stay in G6teborg. This work has been supported financially by the Swedish Natural Sciences Research Council, the Erna and Viktor Hasselblad Foundation and Anna Ahrenbergs Fond.
References
[1 ] F.C. Kracek, J. Phys. Chem. 33 (1929) 1281. [2] F.C. Kracek and C.J. Ksanda, J. Phys. Chem. 34 (1930) 1741. [3] G.E. Brodale and W.F. Giauque, J. Phys. Chem. 76 (1972) 737. [4] W. Eysel, Am. Mineral. 58 (1973) 736. [5] C.A. Cody, L. Dicarlo and R.K. Darlington, J. Inorg. Nucl. Chem. 43 (1981) 398.
330
M.A. Careem, B.E. Mellander/Eleetrical conductivity of Na2S04(l)
[6] F. EI-Kabbany, Y. Badr and T. Tosson, Phys. Status Solidi (a) 63 (1981) 699. [7] A.-M. Josefson and A. Kvist, Z. Naturforsch. 24a (1969) 466. [8] A.F. Polishchuk, T.M. Shttrkhal and N.A. Romashchenko, Ukrain. Khim. Zh. 39 (1973) 760. [9] B. Heed, Thesis (Chalmers University of Technology, GOteborg, 1975). [10] Y. Suzuki, T. Takahashi, T. Koizumi, Nippon Kagaku Kaiski 10 (1975) 1701. [11 ] H.H. H6fer, W. Eysel and U. von Alpen, Mat. Res. Bull. 13 (1978) 265. [12] R.M. Murray and E.A. Secco, Can. J. Chem. 56 (1978) 2616. [13] K.T. Jacob and D.B. Rao, J. Electrochem. Soc. 126 (1979) 1842. [14] H.H. H6fer, W. Eysel and U. yon Alpen, J. Solid State Chem. 36 (1981) 365. [15] Y. Saito, K. Kobayashi and T. Maruyama, Solid State Ionics 3/4 (1981) 393.
[16] N. Imanaka, G.-Y. Adachi and J. Shiokawa, Can. J. Chem. 61 (1983) 1557. [ 17 ] K. Singh and V.K. Deshpande, Solid State Ionics 13 (1984) 157. [18] Y. Saito, T. Maruyama and K. Kobayashi, Solid State Ionics 14 (1984) 265. [19] W. Eysel, H.H. H6fer, K.L. Keester and T. Hahn, Acta Cryst. B., to be published. [20] M.A. Cateem, B. Heed, A. Lund6n and B.-E. Mellander, to be published. [21 ] L.W. Barr and A.B. Lidiard, in: Physical chemistry an advanced treatise, eds. H. Eyring, D. Henderson and W. Jost (Academic Pres~, New York, 1970) Vol. X, p. 151. [22] W.I. Archer and R.D. Armstrong, in: Electrochemistry, ed. H.R. Thirsk, Vol. 7 (The Chemical Society, London, 1980) p. 157. [23] B.-E. Mellander and D. Lazarus, Phys. Rev. B. 29 (1984) 2148, and references therein.
ELECTRICAL CONDUCTIVITY OF Na2SO4 (I) M.A. CAREEM * and B.-E. MELLANDER Chalmers University of Technology, Department of Physics, S-412 96 GOteborg, Sweden Received 11 March 1985
The electrical conductivity of the solid phase Na2SO4(I) has been measured between the melting point at 8840C and the first order phase transition at about 240°C. The measurements have been performed using complex impedance measurements on pellet samples as well as on U-cells. The electrical conductivity is strongly dependent on sample purity at low temperatures and the activation energy ranges from 0.5 eV to 1.7 eV over the measured temperature range.
1. Introduction
A E = -k[Oln(oT)/a(1/T)]
In sodium sulphate a solid phase, Na2SO4(I), is stable over a temperature range of more than 600°C, from the melting point at 884°C to the solid--solid phase transition at about 2dO°C. Below 240°C a number of stable and metastable phases have been detected [ 1 - 6 ] . The electrical conductivity of Na2SO4(I) is due to the mobile sodium ions and many studies of the ionic conductivity have been reported in the literature [7-18]. The structure of Na2SO4(I) is hexagonal (P63/mmc) [19] and the conductivity is low compared to that of solid electrolytes such as wLi2SO4, but increases considerably with the addition of other cations [9,10,14,15,20]. Na2SO4(I) is thus of great interest for the study of solid electrolytes and due to the scatter in the reported values, a reinvestigation of the electrical conductivity was performed covering a large temperature range including also the region just above the melting point. The ionic conductivity o of a normal ionic compound can be divided into at least two regions: the extrinsic region, where the ionic conductivity is due to defects created by the presence of aliovalent impurities, and the intrinsic region, where thermally generated defects dominate. The activation energy AE calculated from
corresponds in the extrinsic (low temperature) region to a migration enthalpy, and in the intrinsic (high temperature) region to both migration and defect formation enthalpies [21 ].
Present address: Department of Physics, University of Peradeniya, Peradeniya, Sri Lanka. 0 167-2738/85/$ 03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
(I)
2. Experimental Two purity grades of sodium sulphate were used, suprapure and reagent grade purchased form E. Merck, Germany. Before use the salts were dried at 185°C for 48 h. The electrical conductivity was measured using two techniques. Above 500°C the capillary U-ceil method, which is described in detail elsewhere [20], was used. For temperatures below 650°C pressed pellets with evaporated gold electrodes were used. The pellets were pressed in cylindrical dies of 13 or 15 mm diameter using a pressure of 300 MPa. Prior to evaporation of gold the pellets were annealed for more than 24 h at temperatures below 600°C. The lengths of the pellets were 2 - 5 mm and good contact was ensured by spring loading. The pellet assembly was placed in the middle of a 100 cm long 8 cm internal diameter tube furnace. The temperature was measured by a Platinel II thermocouple placed close to the pellet. The electrical conductivity was obtained using impedance spectroscopy [22]. The complex impedance was measured in the frequency range 100 Hz to I00 kHz
M.A. Careem, B.E. MeUander/Electricalconductivity of Na2S04(I)
328
using a Hewlett-Packard 4274A LCR-meter and the signal applied over the sample was 20 mV.
Table 1 Electrical conductivity for Na2SO 4 using U-cell (Ou) a n d pellet m e a s u r e m e n t s . The m e a s u r e m e n t s on pellets were perf o r m e d using suprapure (o s) and reagent grade (Or) Na2SO4.
3. Results and discussion
Temp. (°C)
Fig. 1 shows a typical complex impedance plot for a pellet sample. At low frequencies the points lie on an inclined straight line, and at high frequencies the beginning of the semicircular portion is visible. At temperatures below the phase transition at 240°C only the circular part of the impedance plot was observed, however, the arc was somewhat depressed. The temperature dependence of the electrical conductivity is summarized in table 1 and fig. 2 showing the results of measurements using analytical grade (U-cell and pellet samples) and suprapure Na2SO 4 (pellet samples only). For all types of samples the results shown in fig. 2 represent both increasing and decreasing temperature runs. There is thus no hysteresis nor any change in conductivity between subsequent measurements. Good agreement with earlier measurements [7,8] is obtained in the molten state of Na2SO4. In the solid NaSO 4 (I) phase a more or less continuous curvature
ou (12 cm) -1
250 300 400 500 600 700 800 900
4.0 2.2 1.1 6.9 2.3
x 10-4 X 10 -3 x 10 -2 X 10 -2 (melt)
or (S2 cm) -1
os (S2 cm) -1
1.1 2.4 8.2 3.0 1.7
0.19 0.49 3.2 1.9
X X × × X
10 - s 10 -s 10 -5 10 --4 10 -3
x X X X
10 -s 10 -5 10 -5 10 -4
can be observed in the In oT versus 1/T plot, see fig. 2. In the low temperature range for this phase (the extrinsic region) the conductivity is, as expected, lower for the suprapure samples than for the reagent grade samples. At higher temperatures it is expected that the conductivities for the two purities will be almost the Tern pera±ure (°C) 800 10
600
I
I
400 t
200 I
~-melt-
Q)
T=297.3°C
12-
100 HZ o /
/
10-
8-
°
/
/
o o
5
--o
T
o
o o o o o o
L~ c
rn
-5
q~
~
0
-10 100kHz
2
/ o"
O
-12. 0.8
V 0
12
I
t
I
t
lt.l.
16 Re Z ( k ~ )
18
20
Fig. 1. Complex impedance plot for a pellet sample at 297.3°C.
el
t 1.0
I
I
1.5 1000/T (K -~)
2.0
Fig. 2. ln(oT) versus 1/T plot for Na2SO4. The melting transition and the s o l i d - s o l i d phase transition are included in the diagram. Open circles denote U-cell m e a s u r e m e n t s , triangles represent reagent grade pellet samples and squares suprapure pellet samples.
M.A. Careem, B.E. Mellander/Electricalconductivity of Na2SO4(IJ
329
Table 2 Electrical conductivity of Na2SO4 (I). The literature values were in all cases calculated from conductivity plots. Conductivity (s2 era) -1 300°C
Ref.
500°C -
40 X 10 ..6 1.9 X 10 -6 3.2 x 10-6 88 X 10-6 26 X 10-6 1.4 X 10-6 (5-24) X 10-6
800°C -
4.5 X 1 0 - 4 1.0 X 1 0 - 4 1.8 X 1 0 - 4 2.0 X 10 -4 2.4 x 10 .4 0.073 X 10 -4 (1.9-4.0) X 10 -4
same, since thermally generated defects will dominate over defects created by the presence o f aliovalent impurities. This behaviour is observed in the present measurements and above 500°C the conductivity o f the suprapure samples is only slightly lower than that o f the reagent grade samples. It can also be noted that the U-cell and the pellet measurements are in reasonable agreement. Literature values and the present resuits are compared in table 2. At high temperatures our values are in good agreement with the values o f Josefson and Kvist [7] and Polishchuk et al. [8] while lower values were reported by Saito et al. [15]. At 500°C all the values in table 2 are within the range 1.0 X 10 --4 to 4.5 × 10 -4 except the values o f Singh and Deshpande [17] which are much lower. At 300°C the conductivity is mainly extrinsic resulting in a large scatter in the reported values. E.g. the values o f Imanaka et al. [16] agree with our results for reagent grade samples while the values o f Jakob and Rao [13] are close to our values for suprapure samples. Because of the curvature in the In oT versus l I T plot it is difficult to compare the present activation energies with those reported in the literature. In our measurements we get in the extrinsic region an activation energy o f 0.50 eV for the reagent grade samples and 0.54 eV for the suprapure samples. The activation energy in the high temperature region is 1.68 eV but it has to be pointed out that the conductivity plot does not dearly indicate that only two regions exist. E.g., it can be suggested that three regions are present: two intrinsic regions, one with vacancy and the other
7.1 x
10 -2
--
1.5 x 10 .2 6.9 X 10 -2
[8]
[10] [11,14] [13] [15] [16] [17] Present results (see table 1)
with interstitial mobility, and one extrinsic region. Several other effects may also give rise to a curvature in the I n ( o / ) versus 1/T plot [23 ]. It should also be pointed out that since polycrystaUine samples were used, grain boundary conduction may be of importance in the extrinsic range. In conclusion, it can be noted that differences in purity o f the samples can explain many, though not all, of the discrepancies in the reported electrical conductivity values.
Acknowledgements
We would like to express our gratitude to Professor A. Lund6n for many valuable discussions. One o f us (M.A.C.) is indebted to the International Seminar for Physics for a scholarship which enabled his stay in G6teborg. This work has been supported financially by the Swedish Natural Sciences Research Council, the Erna and Viktor Hasselblad Foundation and Anna Ahrenbergs Fond.
References
[1 ] F.C. Kracek, J. Phys. Chem. 33 (1929) 1281. [2] F.C. Kracek and C.J. Ksanda, J. Phys. Chem. 34 (1930) 1741. [3] G.E. Brodale and W.F. Giauque, J. Phys. Chem. 76 (1972) 737. [4] W. Eysel, Am. Mineral. 58 (1973) 736. [5] C.A. Cody, L. Dicarlo and R.K. Darlington, J. Inorg. Nucl. Chem. 43 (1981) 398.
330
M.A. Careem, B.E. Mellander/Eleetrical conductivity of Na2S04(l)
[6] F. EI-Kabbany, Y. Badr and T. Tosson, Phys. Status Solidi (a) 63 (1981) 699. [7] A.-M. Josefson and A. Kvist, Z. Naturforsch. 24a (1969) 466. [8] A.F. Polishchuk, T.M. Shttrkhal and N.A. Romashchenko, Ukrain. Khim. Zh. 39 (1973) 760. [9] B. Heed, Thesis (Chalmers University of Technology, GOteborg, 1975). [10] Y. Suzuki, T. Takahashi, T. Koizumi, Nippon Kagaku Kaiski 10 (1975) 1701. [11 ] H.H. H6fer, W. Eysel and U. von Alpen, Mat. Res. Bull. 13 (1978) 265. [12] R.M. Murray and E.A. Secco, Can. J. Chem. 56 (1978) 2616. [13] K.T. Jacob and D.B. Rao, J. Electrochem. Soc. 126 (1979) 1842. [14] H.H. H6fer, W. Eysel and U. yon Alpen, J. Solid State Chem. 36 (1981) 365. [15] Y. Saito, K. Kobayashi and T. Maruyama, Solid State Ionics 3/4 (1981) 393.
[16] N. Imanaka, G.-Y. Adachi and J. Shiokawa, Can. J. Chem. 61 (1983) 1557. [ 17 ] K. Singh and V.K. Deshpande, Solid State Ionics 13 (1984) 157. [18] Y. Saito, T. Maruyama and K. Kobayashi, Solid State Ionics 14 (1984) 265. [19] W. Eysel, H.H. H6fer, K.L. Keester and T. Hahn, Acta Cryst. B., to be published. [20] M.A. Cateem, B. Heed, A. Lund6n and B.-E. Mellander, to be published. [21 ] L.W. Barr and A.B. Lidiard, in: Physical chemistry an advanced treatise, eds. H. Eyring, D. Henderson and W. Jost (Academic Pres~, New York, 1970) Vol. X, p. 151. [22] W.I. Archer and R.D. Armstrong, in: Electrochemistry, ed. H.R. Thirsk, Vol. 7 (The Chemical Society, London, 1980) p. 157. [23] B.-E. Mellander and D. Lazarus, Phys. Rev. B. 29 (1984) 2148, and references therein.