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Ionic conductivity of SnCl2
Solid State lonics 3/4 (1981) 385-388 North-Holland Publishing Company
I O N I C C O N D U C T I V I T Y O F SnCl2 N. K I M U R A * Department of Applied Chemistry, Tohoku University, Sendai, Japan and Y. N I I Z E K I Tohoku Institute of Technology, Sendai, Japan The ionic conductivity of nominally pure and doped polycrystalline SnCI2 has been measured, using an ac technique from 50°C to the melting point. Conductivity of the sample showed an activation energy of 1.31 eV for the hightemperature region and 0.41 eV for the low-temperature region. In the latter, the conductivity increased by doping with KCI and decreased by doping with YC13 but the activation energy did not change appreciably. The value of electronic conductivity was observed to be less than 1% of the total conductivity by a dc polarization method at 200°C. The transference number of Sn> ion was found to be negligible from tracer diffusion measurements. Therefore the conductivity of SnC12 crystals is considered to be due to CI- ions, probably via a vacancy mechanism analogous to PbC12 with the same crystal structure.
1. Introduction M a n y h a l i d e c o m p o u n d s of d i v a l e n t c a t i o n s h a v e b e e n r e p o r t e d as a n i o n i c c o n d u c t o r s a n d u n u s u a l l y high c o n d u c t i v i t y of fluoride ion has b e e n i n v e s t i g a t e d r e c e n t l y in t h e / 3 - f o r m of l e a d fluoride [1] a n d s o m e m i x e d fluorides [2]. A s for c h l o r i d e ion c o n d u c t o r s , PbC12 a n d BaC12 h a v e b e e n s t u d i e d in d e t a i l c o n c e r n i n g t h e c o n d u c tivity m e c h a n i s m [3, 4, 5] a n d u s e d for solids t a t e b a t t e r i e s [6]. H o w e v e r , a r e l a t i v e l y high t e m p e r a t u r e of s e v e r a l h u n d r e d d e g r e e s Celsius is n e e d e d b e f o r e t h e s e c h l o r i d e s b e c o m e g o o d ionic c o n d u c t o r s , a n d l o w e r t e m p e r a t u r e s a r e d e s i r e d for p r a c t i c a l uses. S t a n n o u s c h l o r i d e which is i s o m o r p h i c with PbC12 has b e e n r e p o r t e d to exhibit a c o n s i d e r a b l e c o n d u c t i v i t y in t h e e a r l y w o r k of B e n r a t h a n d T e s c h e [7]. H o w e v e r , t h e r e is n o t y e t m u c h i n f o r m a t i o n a b o u t t h e c o n d u c t i v e species. In this p a p e r , dc a n d ac c o n d u c t i v i t i e s a r e o b s e r v e d for the n o m i n a l l y p u r e a n d d o p e d p o l y c r y s t a l * Present address: Department of Chemistry, University of California, Santa Barbara, CA 93106, USA
line s a m p l e s to d e t e r m i n e the ionic p r o p e r t i e s of SnCI2.
2. Experimental and results F o r t h e p r e p a r a t i o n of t h e n o m i n a l l y p u r e s a m p l e , a n a l y t i c a l g r a d e SnCI2.2H20 was d r i e d in a flow of HC1 gas a n d t h e n distilled in v a c u u m . B e c a u s e of its h y g r o s c o p i c c h a r a c t e r , t h e o r d i n a r y p o w d e r press m e t h o d was not u s e d for the e l e c t r o l y t e , but t h e a p p r o p r i a t e a m o u n t of the n o m i n a l l y p u r e s a m p l e a n d d o p a n t w e r e p u t into a p y r e x cell with g r a p h i t e e l e c t r o d e s a n d t h e cell was e v a c u a t e d r a p i d l y . T h e n t h e a t m o s p h e r e was r e p l a c e d by a N2 flow a n d t h e cell was h e a t e d u p to the m e l t i n g p o i n t of t h e s a m p l e . A f t e r t h e d o p a n t was c o n f i r m e d to d i s s o l v e in t h e m e l t , t h e s a m p l e was c o o l e d very slowly to solidification a n d was m a i n t a i n e d at slightly l o w e r t h a n t h e melting t e m p e r a t u r e for a day, b e f o r e the c o n d u c tivity m e a s u r e m e n t . R e p r o d u c i b l e results w e r e o b t a i n e d on b o t h c o o l i n g a n d h e a t i n g cycles
0167-2738/81/0000--0000/$02.50 O N o r t h - H o l l a n d P u b l i s h i n g C o m p a n y
386
N. Kimura, Y. Niizeki I Ionic conductivity of SnCl:
during several times, provided the samples were sufficiently aged in the solidification step. The total resistance of the samples was measured with an ac bridge in the frequency range of 0.2-20 kHz. The observed values were frequency independent at high temperature but were dependent below 150°C. In the latter case the resistance was determined from compleximpedance plots. Using the cell constant obtained from a standard solution of 0.1 M KCI at 25°C, the total conductivities were calculated. Electronic conductivities were measured by means of dc polarization cells of the type: + C[SnCI2[Sn-. The electronic conductivity obtained from the plateau [8] of steady-state current-voltage curves, 3.5 × 10 6 ~'~-1cm (200°C), was less than 1% of the total conductivity. The diffusion coefficient of Sn 2+ ion in the nominally pure sample was 1.0 x 10 -~2 c m : s from radioactive tracer diffusion experiments. Therefore the conductivity of Sn 2+ is negligibly small c o m p a r e d with the total conductivity.
3. Discussion
From the results stated above, it may be concluded that the total conductivity of the sample is mainly due to the chloride ion. The mechanism is further considered. Fig. 1 shows that o'T versus 1/T plots represent two regions for the nominally pure and doped samples. In the lower temperature region the conductivity was observed to increase with the amount of KCI dopant, while it decreased when doped with trivalent YC13. In the present state, the lattice defect of SnCI: crystal is not necessarily clear. Supposing that Schottky defects are predominant, possible lattice defects in the nominally pure sample are vacancies of Sn 2÷ and CI ions. If anionic Frenkel defects are present, chloride ion vacancies and interstitial ions are produced. However, the latter case seems to be less likely considering the large radius of the C1- ion and the analogy with similar crystal structures [9]
l % %
~L L
6 '5 ~4 \3 2
?
I
2.0
3.0
2.5 1finn iT
,,r l
)
Fig. 1. ac conductivity of nominally pure and doped SnCb. (1) Nominally pure; (2) 0.032 m/o KCI doped; (3) 0.074 m/o KCI doped; (4) 0.14 m/o KCI doped; (5) 0.34 m/o KCI doped; (6) 0.63 m/o KC1 doped; (7) (I.031 m/o YCh doped sample.
such as PbC12, which have been shown to exhibit Schottky defects. When impurities are doped into SnCI2, the concentration of chloride ion vacancies will increase by the addition of KCI(la) but it will be consumed when Y C h is added(2a): KC1 = K~. + Vcl + Clc~,
(la)
KCI + Cll = K~. + 2C1o,
(lb)
YCI3 + V~-i = Ys, + 3C1o,
(2a)
YCI3 = Ys, + CI[ + 2 C l o .
(2b)
Even if interstitial chloride ions may exist(lb, 2b), the same situation will be introduced by taking into account the mass-action rule between vacancies and interstitial ions. Regardless of the kind and quantity of dopant, the activation energy of each sample exhibited almost the same value as the nominally pure sample, 0.44 eV. These results show that the
N. Kimura, Y. Niizeki / Ionic conductivity of SnCl2 ionic current was carried by mobile chloride ion vacancies in the low-temperature region. It may not be prudent to try and fit these results to such a polycrystalline material into models designed for a purely bulk effect, but the following discussion has nevertheless been attempted as the observed conductivities showed fairly good reproducibility and the grain boundaries seem to have little effect on the conductivity. Fig. 2 shows trT/e[KC1] versus 1/T relations for KCl-doped samples. The data follow almost a straight line except for the samples with higher concentration, [KC1] = 0.34 and 0.63 m/o. In such an extrinsic conductivity region, the concentration of mobile charge carriers is considered to be dominated by the amount of monovalent dopant and o'T/e[KC1] represents the mobility of the chloride ion vacancy, tzv, under the assumption that the doped amount of KCI is the same as the amount of mobile vacancy, [Vcl ]. Therefore, the activation energy for conductivity in the low-temperature region represents the enthalpy of motion for the chloride ion vacancy and the mobility can be expressed as
v
u
\e 2.0
2.5 IO00/T
3.0 (°K -I )
Fig. 2. log(o'T/e[KC1])versus 1/T relations for SnCI2 doped with KC1, (a)0.032 m/o; (b)0.074 m/o; (c)0.14 m/o; (d) 0.34 m/o; (e) 0.63 m/o.
387
follows: ~v = (1.3 × 104/T) exp(-O.41/kT) cm 2 V -1 s -1 .
(3) With higher amounts of dopant, o'T/e[KC1] has become smaller, but it probably does not represent a decrease in the mobility of the vacancy. Rather it may be considered that part of the chloride ion vacancies associate with the doped K ÷ ions which have been substituted for Sn 2+ ions in the lattice sites and form neutral complex species. In the higher-temperature region the conductivity plots show a steep slope with an activation energy of 1.31 eV for the nominally pure sample. The samples with lower dopant concentration showed almost the same behaviour as the nominally pure one. The high-temperature range may correspond to an intrinsic region where the defect concentration is dominated by thermal generation. In such a situation, the activation energy would be the sum of the enthalpy of m o v e m e n t and of formation for the defect. Introducing the enthalpy for m o v e m e n t of the chloride ion vacancy from eq. (3), the enthalpy for defect formation would be 2.7 eV for the Schottky mechanism and 1.8 eV for the Frenkel. Both of these values are rather high in comparison with values estimated [10, 11] from the melting point, 247°C. On the other hand, a change of conduction mechanism from vacancy to interstitial ion has been reported [12] in the fl-PbF2 crystal which has a Frenkel defect. If such a change would be possible in the present samples, three activation energies should appear in the conductivity behavior, but only one knee was observed up to the melting point. More likely the chloride ion vacancy represents the predominant mobility in Snfl2, presumably via a Schottky defect. The typical C1- ion conductors PbC12 and BaC12 have been used frequently for mediumand high-temperature operation, respectively. But in the t e m p e r a t u r e range lower than 240°C, SnC12 exhibits a conductivity several hundred times higher than PbCl2 and the value is comparable to that of fluoride superionic
388
N. Kimura, Y. Niizeki / Ionic conductivity of Sn(72
conductors. These results show that SnC12 may be a good candidate as an ionic conductor in a practical temperature range; however its tendency to be oxidized and its hygroscopic character may require a special design of solidstate cell.
Acknowledgement We are grateful to Professor J.H. Kennedy, University of California, Santa Barbara, for helpful discussions.
References [1] J.H. Kennedy, R.C. Miles and J. Hunter, J. Electrochem. Soc. 123 (1976) 47.
[2] J.M. Reau and J. Portier, in: Solid electrolytes, eds. P. Hagenmuller and W. van Gool (Academic Press, New York, 1978) p. 313. [3] C. Tubandt, in: H a n d b u c h der Experimentalphysik, Vol. 12.3 (Akademische Verlagsgesellschaft, Leipzig, 1932) p. 383. [4] H. Hoshino, M. Yamazaki, Y. N a k a m u r a and M. Shimoji, J. Phys. Soc. Japan 26 (1969) 1422. [5] Y. Niizeki, Y. Onodera, B. Ise and O. Takagi, Denki Kagaku 47 (1979) 178. [6] A. Sator, Compt Rend. Acad. Sci. (Paris) 234 11952) 2283. [7] A. Benrath and H. Tesche, Z. Physik. Chem. 96 (1920) 474. [8] C. Wagner, in: Proc. CITCE, Vol. 7 (Butterworth, London, 1957) p. 361. [9] J.M. van den Berg, Acta Cryst. 14 (1961) 1002. [10] L.W. Barr and D.K. Dawson, Proc. Brit. Ceram. Soc. 19 (1971) 151. [11] J. Schoonman, G.J. Dirksen and G. Blasse, J. Solid State Chem. 17 (1973) 245. [12] R.W. Bonne and J. Schoonman, J. Electrochem. Soc. 124 (1977) 28.
I O N I C C O N D U C T I V I T Y O F SnCl2 N. K I M U R A * Department of Applied Chemistry, Tohoku University, Sendai, Japan and Y. N I I Z E K I Tohoku Institute of Technology, Sendai, Japan The ionic conductivity of nominally pure and doped polycrystalline SnCI2 has been measured, using an ac technique from 50°C to the melting point. Conductivity of the sample showed an activation energy of 1.31 eV for the hightemperature region and 0.41 eV for the low-temperature region. In the latter, the conductivity increased by doping with KCI and decreased by doping with YC13 but the activation energy did not change appreciably. The value of electronic conductivity was observed to be less than 1% of the total conductivity by a dc polarization method at 200°C. The transference number of Sn> ion was found to be negligible from tracer diffusion measurements. Therefore the conductivity of SnC12 crystals is considered to be due to CI- ions, probably via a vacancy mechanism analogous to PbC12 with the same crystal structure.
1. Introduction M a n y h a l i d e c o m p o u n d s of d i v a l e n t c a t i o n s h a v e b e e n r e p o r t e d as a n i o n i c c o n d u c t o r s a n d u n u s u a l l y high c o n d u c t i v i t y of fluoride ion has b e e n i n v e s t i g a t e d r e c e n t l y in t h e / 3 - f o r m of l e a d fluoride [1] a n d s o m e m i x e d fluorides [2]. A s for c h l o r i d e ion c o n d u c t o r s , PbC12 a n d BaC12 h a v e b e e n s t u d i e d in d e t a i l c o n c e r n i n g t h e c o n d u c tivity m e c h a n i s m [3, 4, 5] a n d u s e d for solids t a t e b a t t e r i e s [6]. H o w e v e r , a r e l a t i v e l y high t e m p e r a t u r e of s e v e r a l h u n d r e d d e g r e e s Celsius is n e e d e d b e f o r e t h e s e c h l o r i d e s b e c o m e g o o d ionic c o n d u c t o r s , a n d l o w e r t e m p e r a t u r e s a r e d e s i r e d for p r a c t i c a l uses. S t a n n o u s c h l o r i d e which is i s o m o r p h i c with PbC12 has b e e n r e p o r t e d to exhibit a c o n s i d e r a b l e c o n d u c t i v i t y in t h e e a r l y w o r k of B e n r a t h a n d T e s c h e [7]. H o w e v e r , t h e r e is n o t y e t m u c h i n f o r m a t i o n a b o u t t h e c o n d u c t i v e species. In this p a p e r , dc a n d ac c o n d u c t i v i t i e s a r e o b s e r v e d for the n o m i n a l l y p u r e a n d d o p e d p o l y c r y s t a l * Present address: Department of Chemistry, University of California, Santa Barbara, CA 93106, USA
line s a m p l e s to d e t e r m i n e the ionic p r o p e r t i e s of SnCI2.
2. Experimental and results F o r t h e p r e p a r a t i o n of t h e n o m i n a l l y p u r e s a m p l e , a n a l y t i c a l g r a d e SnCI2.2H20 was d r i e d in a flow of HC1 gas a n d t h e n distilled in v a c u u m . B e c a u s e of its h y g r o s c o p i c c h a r a c t e r , t h e o r d i n a r y p o w d e r press m e t h o d was not u s e d for the e l e c t r o l y t e , but t h e a p p r o p r i a t e a m o u n t of the n o m i n a l l y p u r e s a m p l e a n d d o p a n t w e r e p u t into a p y r e x cell with g r a p h i t e e l e c t r o d e s a n d t h e cell was e v a c u a t e d r a p i d l y . T h e n t h e a t m o s p h e r e was r e p l a c e d by a N2 flow a n d t h e cell was h e a t e d u p to the m e l t i n g p o i n t of t h e s a m p l e . A f t e r t h e d o p a n t was c o n f i r m e d to d i s s o l v e in t h e m e l t , t h e s a m p l e was c o o l e d very slowly to solidification a n d was m a i n t a i n e d at slightly l o w e r t h a n t h e melting t e m p e r a t u r e for a day, b e f o r e the c o n d u c tivity m e a s u r e m e n t . R e p r o d u c i b l e results w e r e o b t a i n e d on b o t h c o o l i n g a n d h e a t i n g cycles
0167-2738/81/0000--0000/$02.50 O N o r t h - H o l l a n d P u b l i s h i n g C o m p a n y
386
N. Kimura, Y. Niizeki I Ionic conductivity of SnCl:
during several times, provided the samples were sufficiently aged in the solidification step. The total resistance of the samples was measured with an ac bridge in the frequency range of 0.2-20 kHz. The observed values were frequency independent at high temperature but were dependent below 150°C. In the latter case the resistance was determined from compleximpedance plots. Using the cell constant obtained from a standard solution of 0.1 M KCI at 25°C, the total conductivities were calculated. Electronic conductivities were measured by means of dc polarization cells of the type: + C[SnCI2[Sn-. The electronic conductivity obtained from the plateau [8] of steady-state current-voltage curves, 3.5 × 10 6 ~'~-1cm (200°C), was less than 1% of the total conductivity. The diffusion coefficient of Sn 2+ ion in the nominally pure sample was 1.0 x 10 -~2 c m : s from radioactive tracer diffusion experiments. Therefore the conductivity of Sn 2+ is negligibly small c o m p a r e d with the total conductivity.
3. Discussion
From the results stated above, it may be concluded that the total conductivity of the sample is mainly due to the chloride ion. The mechanism is further considered. Fig. 1 shows that o'T versus 1/T plots represent two regions for the nominally pure and doped samples. In the lower temperature region the conductivity was observed to increase with the amount of KCI dopant, while it decreased when doped with trivalent YC13. In the present state, the lattice defect of SnCI: crystal is not necessarily clear. Supposing that Schottky defects are predominant, possible lattice defects in the nominally pure sample are vacancies of Sn 2÷ and CI ions. If anionic Frenkel defects are present, chloride ion vacancies and interstitial ions are produced. However, the latter case seems to be less likely considering the large radius of the C1- ion and the analogy with similar crystal structures [9]
l % %
~L L
6 '5 ~4 \3 2
?
I
2.0
3.0
2.5 1finn iT
,,r l
)
Fig. 1. ac conductivity of nominally pure and doped SnCb. (1) Nominally pure; (2) 0.032 m/o KCI doped; (3) 0.074 m/o KCI doped; (4) 0.14 m/o KCI doped; (5) 0.34 m/o KCI doped; (6) 0.63 m/o KC1 doped; (7) (I.031 m/o YCh doped sample.
such as PbC12, which have been shown to exhibit Schottky defects. When impurities are doped into SnCI2, the concentration of chloride ion vacancies will increase by the addition of KCI(la) but it will be consumed when Y C h is added(2a): KC1 = K~. + Vcl + Clc~,
(la)
KCI + Cll = K~. + 2C1o,
(lb)
YCI3 + V~-i = Ys, + 3C1o,
(2a)
YCI3 = Ys, + CI[ + 2 C l o .
(2b)
Even if interstitial chloride ions may exist(lb, 2b), the same situation will be introduced by taking into account the mass-action rule between vacancies and interstitial ions. Regardless of the kind and quantity of dopant, the activation energy of each sample exhibited almost the same value as the nominally pure sample, 0.44 eV. These results show that the
N. Kimura, Y. Niizeki / Ionic conductivity of SnCl2 ionic current was carried by mobile chloride ion vacancies in the low-temperature region. It may not be prudent to try and fit these results to such a polycrystalline material into models designed for a purely bulk effect, but the following discussion has nevertheless been attempted as the observed conductivities showed fairly good reproducibility and the grain boundaries seem to have little effect on the conductivity. Fig. 2 shows trT/e[KC1] versus 1/T relations for KCl-doped samples. The data follow almost a straight line except for the samples with higher concentration, [KC1] = 0.34 and 0.63 m/o. In such an extrinsic conductivity region, the concentration of mobile charge carriers is considered to be dominated by the amount of monovalent dopant and o'T/e[KC1] represents the mobility of the chloride ion vacancy, tzv, under the assumption that the doped amount of KCI is the same as the amount of mobile vacancy, [Vcl ]. Therefore, the activation energy for conductivity in the low-temperature region represents the enthalpy of motion for the chloride ion vacancy and the mobility can be expressed as
v
u
\e 2.0
2.5 IO00/T
3.0 (°K -I )
Fig. 2. log(o'T/e[KC1])versus 1/T relations for SnCI2 doped with KC1, (a)0.032 m/o; (b)0.074 m/o; (c)0.14 m/o; (d) 0.34 m/o; (e) 0.63 m/o.
387
follows: ~v = (1.3 × 104/T) exp(-O.41/kT) cm 2 V -1 s -1 .
(3) With higher amounts of dopant, o'T/e[KC1] has become smaller, but it probably does not represent a decrease in the mobility of the vacancy. Rather it may be considered that part of the chloride ion vacancies associate with the doped K ÷ ions which have been substituted for Sn 2+ ions in the lattice sites and form neutral complex species. In the higher-temperature region the conductivity plots show a steep slope with an activation energy of 1.31 eV for the nominally pure sample. The samples with lower dopant concentration showed almost the same behaviour as the nominally pure one. The high-temperature range may correspond to an intrinsic region where the defect concentration is dominated by thermal generation. In such a situation, the activation energy would be the sum of the enthalpy of m o v e m e n t and of formation for the defect. Introducing the enthalpy for m o v e m e n t of the chloride ion vacancy from eq. (3), the enthalpy for defect formation would be 2.7 eV for the Schottky mechanism and 1.8 eV for the Frenkel. Both of these values are rather high in comparison with values estimated [10, 11] from the melting point, 247°C. On the other hand, a change of conduction mechanism from vacancy to interstitial ion has been reported [12] in the fl-PbF2 crystal which has a Frenkel defect. If such a change would be possible in the present samples, three activation energies should appear in the conductivity behavior, but only one knee was observed up to the melting point. More likely the chloride ion vacancy represents the predominant mobility in Snfl2, presumably via a Schottky defect. The typical C1- ion conductors PbC12 and BaC12 have been used frequently for mediumand high-temperature operation, respectively. But in the t e m p e r a t u r e range lower than 240°C, SnC12 exhibits a conductivity several hundred times higher than PbCl2 and the value is comparable to that of fluoride superionic
388
N. Kimura, Y. Niizeki / Ionic conductivity of Sn(72
conductors. These results show that SnC12 may be a good candidate as an ionic conductor in a practical temperature range; however its tendency to be oxidized and its hygroscopic character may require a special design of solidstate cell.
Acknowledgement We are grateful to Professor J.H. Kennedy, University of California, Santa Barbara, for helpful discussions.
References [1] J.H. Kennedy, R.C. Miles and J. Hunter, J. Electrochem. Soc. 123 (1976) 47.
[2] J.M. Reau and J. Portier, in: Solid electrolytes, eds. P. Hagenmuller and W. van Gool (Academic Press, New York, 1978) p. 313. [3] C. Tubandt, in: H a n d b u c h der Experimentalphysik, Vol. 12.3 (Akademische Verlagsgesellschaft, Leipzig, 1932) p. 383. [4] H. Hoshino, M. Yamazaki, Y. N a k a m u r a and M. Shimoji, J. Phys. Soc. Japan 26 (1969) 1422. [5] Y. Niizeki, Y. Onodera, B. Ise and O. Takagi, Denki Kagaku 47 (1979) 178. [6] A. Sator, Compt Rend. Acad. Sci. (Paris) 234 11952) 2283. [7] A. Benrath and H. Tesche, Z. Physik. Chem. 96 (1920) 474. [8] C. Wagner, in: Proc. CITCE, Vol. 7 (Butterworth, London, 1957) p. 361. [9] J.M. van den Berg, Acta Cryst. 14 (1961) 1002. [10] L.W. Barr and D.K. Dawson, Proc. Brit. Ceram. Soc. 19 (1971) 151. [11] J. Schoonman, G.J. Dirksen and G. Blasse, J. Solid State Chem. 17 (1973) 245. [12] R.W. Bonne and J. Schoonman, J. Electrochem. Soc. 124 (1977) 28.