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Ionic conductivity of polycrystalline PbCl2·RbCl double salt
SOLID STATE IOWICS
Solid State Ionics 66 (1993) 49-52 North-Holland
Ionic conductivity of polycrystalline PbC12-RbCl double salt Y. Niizeki Department of Science, Tohoku Institute of Technology, 3.5-l Kasumi-cho Yagiyama. Taihaku-ku. Sendai 982, Japan
Received 29 November 1992; accepted for publication 8 June 1993
PbCl,.RbCl showed large ionic conductivities of the order of 10e3 S,cm-’ at temperatures higher than its transition point (310°C). The ionic transference numbers of PbC12.RbC1 were approximately equal to unity at both temperatures lower and higher than the transition temperature. This high temperature modification seems to take the perovskite structure.
1. Introduction The existence of three types of double salts, 2PbC12.RbCl (mp 423”C), PbCl,.RbCl (mp 440°C) and PbC12.RbCl (mp 448”C), in the lead (II) chloride-rubidium chloride system was reported by Treis [ 11. However, no information concerning the ionic conductivities of these double salts in the high temperature region has been reported. The ac conductivity measurements of these sintered polycrystalline double salts have revealed that the conductivities of PbClz.RbCl steeply increased up to the order of lo-) S.cm-’ as large as ten times that of pure PbCl, [2] in the vicinity of the transition point (310°C) [ 11. Only two compounds of PbClz and PbCLCsCl [ 3 ] have been reported as the relatively high chloride ion conductor, but these conductivities are of the order of 10m4 at about 3 10°C. Therefore, PbCL.RbCl with the high conductivity is particularly interesting as an applicable chloride ion conductor. The high ionic conduction properties of the sintered polycrystalline PbCl*.RbCl are discussed in this paper.
salts, appropriate amounts of these two materials were intimately mixed, pestled in a silicon carbide mortar, transferred into a quartz tube, heated at about 300°C for 3-5 h, and then melted at temperatures higher than their melting points by about 20°C followed by cooling to room temperature in the furnace. All of the melts were pale greenish yellow and the congelations, translucent milk white, wax-like. The sample powders were prepared by grinding the congelations in the mortar. About 1 g of each powder was weighed and pressed into a pellet with about 0.3 cm thickness under a pressure of 4.5 x 10’ kgcm-* in a 1.0 cm diameter steel die. Graphite coatings thoroughly evaporated on both sides of the pellet were adopted as the electrodes. A cell for measurement of the total conductivity was constructed by sandwiching the pellet between two graphite blocks equipped with a platinum leading wire. Before the conductivity measurement, the pellets were sintered at about 425°C for several minutes. After cooling, the 10 kHz ac conductivities were measured by means of Solartron 1250 frequency response analyzer in the temperature range from 150°C to 430°C. An ionic transference number, tion, was calculated according to the following formula:
2. Experimental Analytical grade PbC12 and RbCl were used as starting materials. PbCll was recrystallized from its aqueous solution and further purified by the zone melting method. RbCl was used without further purification. In order to prepare three kinds of double Elsevier Science Publishers B.V.
where Eexp is the electromotive force (EMF) of the chlorine-concentration cell and Ecalo the EMF calculated on the basis of Nernst equation from partial pressures of chlorine in both the anodic and cathodic chambers of the cell.
50
Y. Niizeki / Polycrystaliine PbCI, RbCl double salt
3. Results In fig. 1, the temperature dependence of the conductivities of the three kinds of double salts composed of PbClz and RbCl is compared with that of their components [ 41. The conductivities of PbC12.RbCl increase steeply by a factor of about 10 in the vicinity of 3 10” C and attain the order of 1Oe3 S.cm-’ at the temperatures above it. The conductivities of PbC12.2RbCl show a small steep enhancement at about 350°C to attain 10m3 S.cm-‘. However, these conductivities at low temperatures were 300 600
1.OT!
400
.
200
go
‘C
O-
-1.o -
comparatively small and only as large as the order of 10-4-10-6 S.cm-i. On the other hand, 2PbC12.RbCl shows very low conductivities of the order of lop4 Scm-’ even at the high temperatures. The conductivities of PbC12.RbCl are smaller than those of PbQ, whereas much larger than those of RbCl at the low temperature range. At high temperatures the conductivities of PbCl*.RbCl are about six times higher than those of PbC12, and about lo6 times higher than those of RbCl. Table 1 shows the ionic transference numbers of PbCl,.RbCl by means of a chlorine-concentration cell at different temperatures. The values of 0.997 and 1.006 are obtained at 273°C lower than the transition temperature (3 10” C), and at 345 “C, higher than it, respectively. The ionic transference numbers of PbCl,.RbCl are approximately equal to 1.O within the experimental errors at the temperatures covered.
4. Discussion
x ;E " -2 .o-
ui
ti; m-3 ;.o. 0
-4
.o.
-5
.o.
-6 ‘8 -
1.5
1.0
2.0 lOfT-l/K-'
2.5
3.0
Fig. I. Dependence of ionic conductivity on temperature three kinds of double salts in PbC12-RbCI system. Table 1 Ionic transference than its transition
for the
The steep enhancement of the conductivity of PbCl,.RbCl is considered to be due to the change of its crystal structure by a transition, because the temperature at the steep enhancement was in fair agreement with its transition temperature, 311 “C, reported by Treis [ 11, at which the structure of PbC12.RbCl is transformed from the rhombic (not confirmed) to the cubic system. In the present investigation a clear strong endothermic peak responsible for the transition obtained at 3 14.5 ‘C on the DTA curve without changes in the TG curve. The peak area, i.e., the enthalpy change of the transition, on the DTA curve went up about 60% of that of the melting. This means that the transition is accompanied by a relatively large change of the structure. The diffraction patterns of the sample powder syn-
numbers of PbC12.RbCl measured by means of a chlorine-concentration point, 272°C and 345°C respectively.
tIO” 0.997 1.006
Temp. (“C)
Pi (Cl21 (Pa)
272 345
5.07 x lo4 5.07 x lo4
1.013x lo4 0.841 x lo4
cell at both temperatures
E”calf. (mV)
E’ew (mV)
37.8 41.8
37.7 48.1
lower and higher
Y. Niizeki / Polycrystalline PbCI, .RbCI double salt
thesized in the present study and the ones of pure PbCl*, and RbCl at room temperature differed from one another greatly. This fact shows that the synthesized powder is a double salt of PbC&.RbCl and not merely a mixture of PbC& and RbCl. Fig. 2 shows the high temperature X-ray diffraction patterns of the PbC12.RbCl, of which the conductivity was measured in the present study. The pattern at the temperature lower (264°C) than the transition point resembled that at room temperature very closely, whereas at the temperature higher (345°C) than the transition point, a simple pattern of the cubic system such as that of the high temperature modification in Treis report [ 1 ] was obtained. The lattice constant of the sample was calculated to be 5.54 8, as the cubic system from the X-ray diffraction data at 345°C. The indexes of the reflexion
51
surface corresponding to each peak are shown in fig. 2. As a general rule, the compound, BX,.AX, with the tolerance factor ( =t) of 0.8-1.0 defined by the following equation gives r*+rx=fit(rr3+rx)
)
where r,, r, and r, are the ionic radii of A, B and X, respectively. The perovskite structure is very stable and, consequently, shows a high ionic conductivity owing to maintaining the original structure even if a very large number of lattice defects are introduced into the structure. For example, PbC12.CsCl is the only one double salt composed of PbC12 and alkali metal chlorides which has the perovskite structure with a tolerance factor of 0.822 and shows the high chloride ion conductivity at high temperatures [ 41. On the other hand, PbC12.RbCl cannot have the perovskite structure at room temperature since its tolerance factor, 0.773, is slightly smaller than the lower limit of 0.8 (the ionic radii, r,-,=O. 169 nm, r,,=O.148 nm, rpb=0.120 nm, r,,=0.181 nm). However, if a thermal expansion of the lattice constant at high temperature results in a crystallographical increase in the apparent radius ratio, r-Jr,, PbCl,.RbCl may have the tolerance factor larger than 0.8 and, consequently, its structure may be converted to the perovskite structure. This is consistent with the experimental result that PbC12.RbCl has a high ionic conductivity at high temperature. In this case, Rb+ in the A site and Pb2+ in the B site may undergo 6- and 12-coordination of Cl-, respectively. The apparent activation energies from Arrhenius plot of conductivities in the low and high temperature regions were about 64 kJ.mol-’ and 55 kJ.mol-‘, respectively, and the former energy was smaller by 9 kJ.mol-’ than the latter. This means that ions can migrate more easily in the high temperature modification (cubic system) than in the low temperature one (rhombic system, probably).
5. Conclusion 20
30
40 281”
50
Fig. 2 . X-ray diffraction patterns of PbC12.RbCl lower and higher than its transition point.
60 at temperatures
It was found that PbC12.RbCl shows the considerably large ionic conductivity of the order of 1O-3 Scm-’ at temperatures higher than its transition point (3 10” C). Other double salts, PbC1,.2RbCl and
52
Y. Niizeki / Polycrystalline PbCl,.RbCI doublesalt
2PbC12.RbCl, did not show the higher conductivity than that of PbC12*RbCl at the temperatures covered. This high temperature modification seems to have the perovskite structure. The temperature region of high conductivity of PbC12.RbCl extends to a relatively low temperature region from 3 10°C to 420 oC. Therefore, this solid electrolyte is applicable to the reactors for the chlorination of the organic compounds which are easily subject to thermal decompositions. Further, it may be used for chlorine gas sensors, Cl*-Hz rechargeable batteries and so on.
Acknowledgement The author is grateful to 0. Takagi, Emeritus
Pro-
fessor of Tohoku Institute of Technology, for reading the manuscript and providing the helpful discussions. Also, the author wishes to express appreciation to H. Iwahara, Professor of Nagoya University, and T. Takahashi, Professor Emeritus of Nagoya University, for their helpful suggestions.
References [ 1] K. Treis, Neues Jahrb. Mineral. Geol. 37 (1914) 784. [2] G.-M. Schwab and G. Eulitz, Z. Physik. Chem. (NF) 55 (1967) 179. [ 31 J. Mizusaki, K. Arai and K. Fueki, Solid State Ionics 11 (1983) 203. [4] W. Lchfelt, Z. Phys. 85 (1933) 720.
Solid State Ionics 66 (1993) 49-52 North-Holland
Ionic conductivity of polycrystalline PbC12-RbCl double salt Y. Niizeki Department of Science, Tohoku Institute of Technology, 3.5-l Kasumi-cho Yagiyama. Taihaku-ku. Sendai 982, Japan
Received 29 November 1992; accepted for publication 8 June 1993
PbCl,.RbCl showed large ionic conductivities of the order of 10e3 S,cm-’ at temperatures higher than its transition point (310°C). The ionic transference numbers of PbC12.RbC1 were approximately equal to unity at both temperatures lower and higher than the transition temperature. This high temperature modification seems to take the perovskite structure.
1. Introduction The existence of three types of double salts, 2PbC12.RbCl (mp 423”C), PbCl,.RbCl (mp 440°C) and PbC12.RbCl (mp 448”C), in the lead (II) chloride-rubidium chloride system was reported by Treis [ 11. However, no information concerning the ionic conductivities of these double salts in the high temperature region has been reported. The ac conductivity measurements of these sintered polycrystalline double salts have revealed that the conductivities of PbClz.RbCl steeply increased up to the order of lo-) S.cm-’ as large as ten times that of pure PbCl, [2] in the vicinity of the transition point (310°C) [ 11. Only two compounds of PbClz and PbCLCsCl [ 3 ] have been reported as the relatively high chloride ion conductor, but these conductivities are of the order of 10m4 at about 3 10°C. Therefore, PbCL.RbCl with the high conductivity is particularly interesting as an applicable chloride ion conductor. The high ionic conduction properties of the sintered polycrystalline PbCl*.RbCl are discussed in this paper.
salts, appropriate amounts of these two materials were intimately mixed, pestled in a silicon carbide mortar, transferred into a quartz tube, heated at about 300°C for 3-5 h, and then melted at temperatures higher than their melting points by about 20°C followed by cooling to room temperature in the furnace. All of the melts were pale greenish yellow and the congelations, translucent milk white, wax-like. The sample powders were prepared by grinding the congelations in the mortar. About 1 g of each powder was weighed and pressed into a pellet with about 0.3 cm thickness under a pressure of 4.5 x 10’ kgcm-* in a 1.0 cm diameter steel die. Graphite coatings thoroughly evaporated on both sides of the pellet were adopted as the electrodes. A cell for measurement of the total conductivity was constructed by sandwiching the pellet between two graphite blocks equipped with a platinum leading wire. Before the conductivity measurement, the pellets were sintered at about 425°C for several minutes. After cooling, the 10 kHz ac conductivities were measured by means of Solartron 1250 frequency response analyzer in the temperature range from 150°C to 430°C. An ionic transference number, tion, was calculated according to the following formula:
2. Experimental Analytical grade PbC12 and RbCl were used as starting materials. PbCll was recrystallized from its aqueous solution and further purified by the zone melting method. RbCl was used without further purification. In order to prepare three kinds of double Elsevier Science Publishers B.V.
where Eexp is the electromotive force (EMF) of the chlorine-concentration cell and Ecalo the EMF calculated on the basis of Nernst equation from partial pressures of chlorine in both the anodic and cathodic chambers of the cell.
50
Y. Niizeki / Polycrystaliine PbCI, RbCl double salt
3. Results In fig. 1, the temperature dependence of the conductivities of the three kinds of double salts composed of PbClz and RbCl is compared with that of their components [ 41. The conductivities of PbC12.RbCl increase steeply by a factor of about 10 in the vicinity of 3 10” C and attain the order of 1Oe3 S.cm-’ at the temperatures above it. The conductivities of PbC12.2RbCl show a small steep enhancement at about 350°C to attain 10m3 S.cm-‘. However, these conductivities at low temperatures were 300 600
1.OT!
400
.
200
go
‘C
O-
-1.o -
comparatively small and only as large as the order of 10-4-10-6 S.cm-i. On the other hand, 2PbC12.RbCl shows very low conductivities of the order of lop4 Scm-’ even at the high temperatures. The conductivities of PbC12.RbCl are smaller than those of PbQ, whereas much larger than those of RbCl at the low temperature range. At high temperatures the conductivities of PbCl*.RbCl are about six times higher than those of PbC12, and about lo6 times higher than those of RbCl. Table 1 shows the ionic transference numbers of PbCl,.RbCl by means of a chlorine-concentration cell at different temperatures. The values of 0.997 and 1.006 are obtained at 273°C lower than the transition temperature (3 10” C), and at 345 “C, higher than it, respectively. The ionic transference numbers of PbCl,.RbCl are approximately equal to 1.O within the experimental errors at the temperatures covered.
4. Discussion
x ;E " -2 .o-
ui
ti; m-3 ;.o. 0
-4
.o.
-5
.o.
-6 ‘8 -
1.5
1.0
2.0 lOfT-l/K-'
2.5
3.0
Fig. I. Dependence of ionic conductivity on temperature three kinds of double salts in PbC12-RbCI system. Table 1 Ionic transference than its transition
for the
The steep enhancement of the conductivity of PbCl,.RbCl is considered to be due to the change of its crystal structure by a transition, because the temperature at the steep enhancement was in fair agreement with its transition temperature, 311 “C, reported by Treis [ 11, at which the structure of PbC12.RbCl is transformed from the rhombic (not confirmed) to the cubic system. In the present investigation a clear strong endothermic peak responsible for the transition obtained at 3 14.5 ‘C on the DTA curve without changes in the TG curve. The peak area, i.e., the enthalpy change of the transition, on the DTA curve went up about 60% of that of the melting. This means that the transition is accompanied by a relatively large change of the structure. The diffraction patterns of the sample powder syn-
numbers of PbC12.RbCl measured by means of a chlorine-concentration point, 272°C and 345°C respectively.
tIO” 0.997 1.006
Temp. (“C)
Pi (Cl21 (Pa)
272 345
5.07 x lo4 5.07 x lo4
1.013x lo4 0.841 x lo4
cell at both temperatures
E”calf. (mV)
E’ew (mV)
37.8 41.8
37.7 48.1
lower and higher
Y. Niizeki / Polycrystalline PbCI, .RbCI double salt
thesized in the present study and the ones of pure PbCl*, and RbCl at room temperature differed from one another greatly. This fact shows that the synthesized powder is a double salt of PbC&.RbCl and not merely a mixture of PbC& and RbCl. Fig. 2 shows the high temperature X-ray diffraction patterns of the PbC12.RbCl, of which the conductivity was measured in the present study. The pattern at the temperature lower (264°C) than the transition point resembled that at room temperature very closely, whereas at the temperature higher (345°C) than the transition point, a simple pattern of the cubic system such as that of the high temperature modification in Treis report [ 1 ] was obtained. The lattice constant of the sample was calculated to be 5.54 8, as the cubic system from the X-ray diffraction data at 345°C. The indexes of the reflexion
51
surface corresponding to each peak are shown in fig. 2. As a general rule, the compound, BX,.AX, with the tolerance factor ( =t) of 0.8-1.0 defined by the following equation gives r*+rx=fit(rr3+rx)
)
where r,, r, and r, are the ionic radii of A, B and X, respectively. The perovskite structure is very stable and, consequently, shows a high ionic conductivity owing to maintaining the original structure even if a very large number of lattice defects are introduced into the structure. For example, PbC12.CsCl is the only one double salt composed of PbC12 and alkali metal chlorides which has the perovskite structure with a tolerance factor of 0.822 and shows the high chloride ion conductivity at high temperatures [ 41. On the other hand, PbC12.RbCl cannot have the perovskite structure at room temperature since its tolerance factor, 0.773, is slightly smaller than the lower limit of 0.8 (the ionic radii, r,-,=O. 169 nm, r,,=O.148 nm, rpb=0.120 nm, r,,=0.181 nm). However, if a thermal expansion of the lattice constant at high temperature results in a crystallographical increase in the apparent radius ratio, r-Jr,, PbCl,.RbCl may have the tolerance factor larger than 0.8 and, consequently, its structure may be converted to the perovskite structure. This is consistent with the experimental result that PbC12.RbCl has a high ionic conductivity at high temperature. In this case, Rb+ in the A site and Pb2+ in the B site may undergo 6- and 12-coordination of Cl-, respectively. The apparent activation energies from Arrhenius plot of conductivities in the low and high temperature regions were about 64 kJ.mol-’ and 55 kJ.mol-‘, respectively, and the former energy was smaller by 9 kJ.mol-’ than the latter. This means that ions can migrate more easily in the high temperature modification (cubic system) than in the low temperature one (rhombic system, probably).
5. Conclusion 20
30
40 281”
50
Fig. 2 . X-ray diffraction patterns of PbC12.RbCl lower and higher than its transition point.
60 at temperatures
It was found that PbC12.RbCl shows the considerably large ionic conductivity of the order of 1O-3 Scm-’ at temperatures higher than its transition point (3 10” C). Other double salts, PbC1,.2RbCl and
52
Y. Niizeki / Polycrystalline PbCl,.RbCI doublesalt
2PbC12.RbCl, did not show the higher conductivity than that of PbC12*RbCl at the temperatures covered. This high temperature modification seems to have the perovskite structure. The temperature region of high conductivity of PbC12.RbCl extends to a relatively low temperature region from 3 10°C to 420 oC. Therefore, this solid electrolyte is applicable to the reactors for the chlorination of the organic compounds which are easily subject to thermal decompositions. Further, it may be used for chlorine gas sensors, Cl*-Hz rechargeable batteries and so on.
Acknowledgement The author is grateful to 0. Takagi, Emeritus
Pro-
fessor of Tohoku Institute of Technology, for reading the manuscript and providing the helpful discussions. Also, the author wishes to express appreciation to H. Iwahara, Professor of Nagoya University, and T. Takahashi, Professor Emeritus of Nagoya University, for their helpful suggestions.
References [ 1] K. Treis, Neues Jahrb. Mineral. Geol. 37 (1914) 784. [2] G.-M. Schwab and G. Eulitz, Z. Physik. Chem. (NF) 55 (1967) 179. [ 31 J. Mizusaki, K. Arai and K. Fueki, Solid State Ionics 11 (1983) 203. [4] W. Lchfelt, Z. Phys. 85 (1933) 720.