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The ionic conductivity of (Ag, Na)-nitrite sodalites
Solid State Ionics 46 ( 1991 ) 341-345 North-Holland
The ionic conductivity of (Ag, Na)-nitrite sodalites M.R.M. Jiang and M.T. Weller Department of Chemistry, University of Southampton, Southampton, S09 5NH, UK Received 7 January 1991; accepted for publication 25 March 1991
Ionic conductivity data for mixed Ag/Na-nitrite sodalites, AgxNas_x(AISiO4)6 (NO2)2, have been determined over the temperature range 250 to 550°C in ambient atmosphere using an ac impedance method. The activation energy for conduction is reported for five samples with silver contents ranging from x = 0 to 7.5 and was found to be a monotonic function of silver content, with a m i n i m u m at the highest silver content. Conductivity isotherms for the mixed A g / N a nitrite sodalites were constructed, showing a direct relationship of total ionic conductivity as a function of Silver content, with fastest ionic motion at the highest silver content. A transition of conduction m e c h a n i s m from Na + ion predominance to Ag + ion predominance with increasing x was confirmed from variations in the interfacial impedance characteristics. Ionic conductivity of nitrite sodalites is strongly dependent upon the ionic radius of the mobile cations incorporated.
i. INTRODUCTION Zeolites consist of a three dimensional network of SiO 4 and AIO 4 tetrahedra connected by shared oxygen atoms and charge balanced in the so formed cages by positively charged organic or inorganic ions. Sodalite is a special member of the zeolite family comprising exclusively of close-packed B-cages (or sodalite cages). Applications of this material have included its use as a cathodochromic/photochromic material as coloration takes place upon exposure to daylight or electron beams [ i ] . More recently, other applications, such as use as a fast ionic conductive electrolyte, as a high resolution imaging/printing material and as a high density optical data storage material, have been suggested by Ozin [2]. We have synthesised, in our own course of research, a range of sodalites with different guest inorganic ions occupying the B-cages [36]. One type of material of particular interests to us are the mixed (Ag,Na)- nitrite sodalites of general formula, AgxNaa_x(AiSiO4)8(NO2)2. In an earlier article, we reported the synthesis and characterisation of the sodalite series using Xray and NMR techniques [6]. The purpose of this paper is to study the general ionic conductivity behaviour of the sodalite family and to investigate the effect of silver substitution on the overall ionic conductivity of the nitrite sodalite. Results obtained in this study are compared, wherever possible, to similar work on
Author to w h o m all correspondence should be addressed.
Elsevier Science Publishers B.V. ( North-Holland )
mixed metal ionic conductivity reported for Band B"-alumina, and for other members of the zeolite family.
2. EXPERIMENTAL Full details of the synthetic process for the mixed (Ag,Na)-nitrite sodalites has been described elsewhere [6]. The introduction of silver ion into the parent nitrite sodalite was achieved by ion exchange using AgNO 3 in a high pressure bomb and at 130"C for 24 hours. The final reaction product was then washed, dried, and analyzed titrimetrically for its actual silver content using standardized KSCN with NH4Fe(SO4) 2 as an indicator. The structure characterization of the silver substituted sodalites was performed using a Siemens 8-28 D5000 diffractometer. AC impedance were measured using an HP 4284A Precision LCR meter in the frequency region 20 Hz to 1 MHz using pellets of 6 mm diameter [6]. The HP 4284A Precision LCR meter was remotely controlled by a computer via an IEEE-488 interface. Possible impedance contribution from external connection cables and pellet mounting devices were corrected using a built-in "short" and "open" calibration facility. Pellets of the sodalite were sintered at 550 °C before being painted with silver based paint for subsequent AC impedance measurement. As it has been well established that electrical conduction in zeolites occurs bY migration of the cations, with no evidence of electronic conduction [7-8], no further verification was
M.R.M. Jiang, M. 71 Weller / The ionic conductivity of (Ag, Na)-nitrite sodalites
342
conducted on this aspect. Possible volume expansion of the pellets was not monitored and the thickness measurements of the pellets were performed after the sintering stage and before being coated with the silver based paint. In fact, no significant change in pellet thickness was observed after the sintering process.
3. RESULTS AND DISCUSSION Typical AC impedance diagrams obtained using painted silver electrodes for a pellet of nitrite sodalite are shown in Fig. I. They generally consist of two well defined parts, a portion of a high frequency semicircle of depressed nature and a low frequency straight line at less than 45 ° to the real impedance axis. The depressed semicircle, which has one extended intercept at high frequency at the origin and another extended intercept at low frequency to the real axis, is regarded as the bulk impedance of the pellet arising from the ionic conduction of the bulk electrolyte [9]. The intercept of the depressed semicircle to the real impedance axis at low frequency yields the DC impedance of the pellet. The inclined straight line which appears at low frequency is generally regarded as the interfacial impedance of the electrolyte/electrode interface arising from the geometric microstructure of the electrolyte/electrode interface [9-10]. For an (Ag,Na)-nitrite sodalite pellet of constant silver composition, the inclined straight line had a constant angle to the real impedance axis at all temperatures studied (Fig. i), reflecting a consistent electrolyte/electrode interface property of the pellet over the temperature range. For pellets of different silver content, however, there was a decline in
the angle of these straight lines as the degree of silver substitution is increased (Fig. 2), reflecting changes in the electrolyte/electrode interface among the various silver substituted nitrite sodalites. A likely source of these differences would be changes in the dominant mobile species. It can be clearly seen from Figure 2.d that the angle of the line for the highest silver substitution (i.e. x=7.5,) is almost zero, i.e. the line became almost parallel to the real axis. From electrolyte/electrode interfacial view point, this phenomenon represents the transition of interfacial impedance from complete blocking in the case of an Na+/Ag interface to complete non-blocking in the case of an Ag+/Ag interface. This clearly suggests that a gradual transition in ionic conduction mechanism from Na + ion domination for low silver content sodalite pellets to Ag ÷ ion domination for high silver content sodalite pellets occurs. The appearance of only one depressed semicircle at high frequency followed immediately by an inclined straight line at low frequency for all five samples in the temperature range investigated revealed that the effect of intergranular impedance (i.e. the grain boundary effect) on total conductivity measurement for the materials studied is negligible. Since intergranular impedance is negligible for the nitrite sodalite pellets, DC conductivities at a particular temperature can be calculated from DC impedances obtained from the intercept of the low frequency part of the depressed semicircle on the real axis of the impedance diagrams. The sample conductivities are plotted as a function of inverse temperature for all the compounds studied (Fig. 3). For all the samples studied a very good linear relationship exists for Ln(a) vis 1000/T (K) in the temperature range 250 to 550 °C in ambient
463 493 523 553
0
~ ~w~
0
2
~"#~
4
"4,
~
6
8
~,~
10
Zr ( k ~ ) Fig. i. Impedance plots for (Ag,Na)-nitrite sodalite with x-0.4 at various temperatures. Numbers above each curve represent temperatures in °C, at which the curve was obtained.
M.R.M. Jiang, M.T. Weller / The ionic conductivity of (Ag, Na)-nitrite sodalites
343
3
(a)
ii 0
(b)
2 1
'
1
523
i
539 '
2
3
"~'"
,--
0
4
.,....
0
1
3
2
iI
15 11
(c)
4
Zr (k~)
Zr (k-Q)
! (d)
K~ 0.75
o'51
i, 0,5]
485
0,25 385
0
"~ 0
0.5
1
1.5
2
O~ 2.5
Zr (k~)
0
0.5
1
1.5
Zr (kn)
Fig. 2. Impedance plots for (Ag,Na)-nitrite sodalites with x=0 (a), 0.4 (b), 3.3 (c), and 7.5 (d). Numbers in each figure represent temperatures in °C, at which each plot was obtained.
3 -10
........
\
i
-20 1.5
1000/T (1/K)
\
atmosphere, yielding reliable Arrhenius activation energy for conduction process in all the samples• At temperatures below 250 °C, interferences from ambient moisture was experienced due to absorption of water molecules by the sodalite pellets, resulting in upward deviation of the Ln(a) - 1000/T plots. We are currently repeating conductivity measurements for all five samples in a dry argon environment for temperatures below 250 °C. Preliminary results for a Ag3.3Na4.7(AISiO4)6(N02) 2 pellet show a common Ln(a)- 1000/T plot for data obtained either above 250 °C in ambient atmosphere or below 250 =C in the dry argon environment. For the dry pellets, above 250 °C, it can be readily seen that substitution of sodium ion in the nitrite sodalite by silver reduces the thermal activation energy required for ion conduction and also enhances bulk ionic conductivity. The effect of silver concentration on the total ionic conductivity for nitrite sodalites is quite similar to that observed for other
Fig. 3. Arrhenius plots for (Ag,Na)-nitrite sodalites with x=0 ([]), 0.4 (+), 3.3 (~), 5.1 (E) and 7.5 (0). Conductivity was measured in S/cm.
M.R.M. Jiang, M.T. Weller / The tonic conductivity of (Ag, Na)-nitrite sodalites
344
-6
80
7~
~, 7oj
-9'
~"
60
50~
40
0
~,.~...
2
~ -
4
- -
~
ii
6
L
Ag content~ x
Fig. 4. Activation energy for conduction as function of silver ion content per unit cell.
types of 3-D structure electrolyte, e.g. A3M2(P04)3, (where A=Li, Na, Ag, K; M=Cr, Fe.). In this system as well, replacing sodium with silver results in an increased conductivity [ii]. However, the effects of silver substitution for sodium in 5- and B"-aluminas are quite different. Ag+-B-alumina has a lower ionic conductivity and higher activation energy than its sodium analogue [12] while Ag+-B"-alumina has lower ionic conductivity and lower activation energy than its sodium analogue [13-14]. In order to reveal the detailed effect of cation composition, activation energies were obtained for the five nitrite sodalite samples and plotted against their silver content (Fig. 4). It is clear from Fig. 4 that the activation energy for the mixed (Ag,Na)- nitrite sodalite varies monotonically with silver content. Further more, conductivity isotherms shown in Fig. 5 also demonstrate a monotonic variation of the ionic conduction as a function of silver content. Comparing above results to those reported for the mixed (Ag,Na)- 6- and B"aluminas [12-14], it is obvious that in the temperature range studied, the so called "mixed alkali effect", which exists in almost every partially ion exchanged B- or B"- alumina, was not evident in the mixed (Ag,Na)nitrite sodalites. This behaviour of the nitrite sodalite family is in agreement with another member of the zeolite family, zeolite X , for
0
a
2
4
6 Ag content,
8 x
Fig. 5. Ionic conductivity isotherms for (Ag,Na)nitrite sodalites as a function of silver content, x. ~ 500 °C; 0 400 °C; × 300 °C. Conductivity was measured in S/cm.
which no obvious "mixed alkali effect" was reported. Interactions between cations and the framework lattice and between cationic sites were thought to have influenced both the activation energy and the ionic conductivity of this zeolite [15]. Since the cell parameter has been found to be directly related to the degree of silver substitution in the mixed (Ag,Na)- nitrite sodalites [6], the silver composition dependence of both the activation energy and the total ionic conductivity is indicative of a predominantly electrostatic interaction between the cations and the framework lattice. Fig. 6 shows a polygonal representation for a perfect sodalite cage (i.e. the 6-cage) [16]. It is a truncated octahedron consisting of six four-membered windows and eight six-membered windows. A given central cage shares each of its six-membered windows with one of eight other such cages. For an ideal synthetic sodalite, every such cage contains one anion and four cations. The anion resides at the centre of the sodalite cage and surrounded tetrahedrally by the four cations. For the mixed (Ag,Na)- nitrite sodalites, the distribution of the two types of cation around each anion is likely to be statistical rather than stoichiometrical. It is the
M.R.M. Jiang, M.T. Weller / The ionic conductivity of (Ag, Na)-nitrite sodalites
/
345
tion becomes less. It is likely, therefore, that the mobilities for both sodium and silver ions in higher silver content framework are higher than in lower content framework, and in all cases sodium is more mobile than silver. From preceding discussion, it is seen that the total ionic conductivity of the mixed Ag+/Na +- nitrite sodalites is strongly dependent on the size of the mobile cations concerned. Further studies on other sodalites with different cations and anions are in progress to study conductivity as a function of ion size and intereage bottleneck size.
4. ACKNOWLEDGEMENT Fig. 6. A polygonal representation for the sodalite cage with vertices as Si or AI, showing the six 4-membered and eight 6-membered windows. six-membered windows which form continuous channels for the cations to migrate among neighbouring cages [17]. However, to enable a cation to hop from one cage to another through the six-membered windows, it would have to overcome the potential barrier of the six-membered window. The larger the cation the higher the energy required for hopping but the larger the size of the sixmembered window, the lower the potential barrier for hopping. The accepted ionic radii for four co-ordination for Ag + and Na + ions are 1.16 and 1.13 A, respectively. Replacing sodium by silver in the structure results in an expansion of the lattic and therfore also the dimensions of the sixmembered window. It is, therefore, quite reasonable to expect that, with low silver substitution, e.g. at x-0.4, the potential barrier for the smaller Na + ion to hop between cages could be significantly reduced as a consequence of framework expansion and the resultant enlargement of the six-membered windows caused by some incorporation of silver. An inevitable result of this is a significant decrease in the activation energy for sodium ions in the lowest silver'content compound (i.e. x=0.4), from the value 81.6 kJ/mol found for the parent sodalite. Hence, the total ionic conductivity increases significantly for this composition as sodium ions are the dominant mobile ions. However, as more sodium ions are substituted by silver ions, the somewhat lower mobility of the slightly larger Ag ÷ ions becomes the more important factor in contributing to the total ionic conductivity. As a consequence, although the six-membered windows continue to expand as more sodium is replaced by silver, the rate of decrease of the activation energy is lower as more of the conductivity involves the larger Ag + ions hopping between cages. Concomitantly, as more sodium is replaced by silver, the contribution from sodium conduc-
The authors wish to thank the SERC, the Ford Motor company and A.B. Electronics Products for grants in association with this research.
5. REFERENCES [i I P.T. Bolwijn, D.J. Schipper, and C.Z. Van Doorn, J. Appl. Phys. 43 (1972) 132. [2] G.A. Ozin, A. Kuperman and A. Stein, Angew. Chem. Int. Ed. Engl. 28 (1989) 359. [3] M.T. Weller and G. Wong, Solid State lonics 32/33 (1989) 430. [4] M.T. Weller, G. Wong, C.L. Adamson, S.M. Dodd, J.J.B. Roe, J. Chem. Soc., Dalton Trans. (1990) 593. [5] G. Wong, Ph D Thesis, (University of Southampton, 1990). [6] M.T. Weller, S.M. Dodd and M.R.M. Jiang, J. Mater. Chem. accepted for publication. [7] I.R. Beattie and A. Dyer, Trans. Faraday Soc. 53 (1957) 61. [8] D.C. Freeman, Jr. and D.N. Stamires, J. Chem. Phys. 35 (1961) 799. [9] W.I. Archer and R.D. Armstrong, Specialist Periodical Reports, Electrochem. 7 (1980) 157. [I0] J.B. Bates, J.C. Wang and Y.T. Chu, Solid State Ionics 18/19 (1986) 1045. [ii] F. d'Yvoire, M. Pintard-Ser~pel, E. Bretey, M. de la Roch~re, Solid State Ionics 9/10 (1983) 851. [12] C.C. Hunter, M.D. Ingram and A.R. West, Solid State Ionics 8 (1983) 55. [13] M. Wasiucionek, J. Garbarczyk and W. Jakubowski, Solid State lonics 14 (1984) 113. [14] M. Maly-Schreiber, P. Linhardt and M.W. Breiter, Solid State Ionics 23 (1987) 131. [15] E.K. Andersen, I.G.K. Andersen, J. Metcalf-Johansen, K.E. Simonsen and E. Skou, Solid State lonies 28-30 (1988) 249. [16] E.M. Flanigen, Reviews in Mineralogy, 4 (1981) 19. [17] R.M. Barter and D.E.W. Vaughan, J. Phys. Chem..Solids. 32 (1971) 731.
The ionic conductivity of (Ag, Na)-nitrite sodalites M.R.M. Jiang and M.T. Weller Department of Chemistry, University of Southampton, Southampton, S09 5NH, UK Received 7 January 1991; accepted for publication 25 March 1991
Ionic conductivity data for mixed Ag/Na-nitrite sodalites, AgxNas_x(AISiO4)6 (NO2)2, have been determined over the temperature range 250 to 550°C in ambient atmosphere using an ac impedance method. The activation energy for conduction is reported for five samples with silver contents ranging from x = 0 to 7.5 and was found to be a monotonic function of silver content, with a m i n i m u m at the highest silver content. Conductivity isotherms for the mixed A g / N a nitrite sodalites were constructed, showing a direct relationship of total ionic conductivity as a function of Silver content, with fastest ionic motion at the highest silver content. A transition of conduction m e c h a n i s m from Na + ion predominance to Ag + ion predominance with increasing x was confirmed from variations in the interfacial impedance characteristics. Ionic conductivity of nitrite sodalites is strongly dependent upon the ionic radius of the mobile cations incorporated.
i. INTRODUCTION Zeolites consist of a three dimensional network of SiO 4 and AIO 4 tetrahedra connected by shared oxygen atoms and charge balanced in the so formed cages by positively charged organic or inorganic ions. Sodalite is a special member of the zeolite family comprising exclusively of close-packed B-cages (or sodalite cages). Applications of this material have included its use as a cathodochromic/photochromic material as coloration takes place upon exposure to daylight or electron beams [ i ] . More recently, other applications, such as use as a fast ionic conductive electrolyte, as a high resolution imaging/printing material and as a high density optical data storage material, have been suggested by Ozin [2]. We have synthesised, in our own course of research, a range of sodalites with different guest inorganic ions occupying the B-cages [36]. One type of material of particular interests to us are the mixed (Ag,Na)- nitrite sodalites of general formula, AgxNaa_x(AiSiO4)8(NO2)2. In an earlier article, we reported the synthesis and characterisation of the sodalite series using Xray and NMR techniques [6]. The purpose of this paper is to study the general ionic conductivity behaviour of the sodalite family and to investigate the effect of silver substitution on the overall ionic conductivity of the nitrite sodalite. Results obtained in this study are compared, wherever possible, to similar work on
Author to w h o m all correspondence should be addressed.
Elsevier Science Publishers B.V. ( North-Holland )
mixed metal ionic conductivity reported for Band B"-alumina, and for other members of the zeolite family.
2. EXPERIMENTAL Full details of the synthetic process for the mixed (Ag,Na)-nitrite sodalites has been described elsewhere [6]. The introduction of silver ion into the parent nitrite sodalite was achieved by ion exchange using AgNO 3 in a high pressure bomb and at 130"C for 24 hours. The final reaction product was then washed, dried, and analyzed titrimetrically for its actual silver content using standardized KSCN with NH4Fe(SO4) 2 as an indicator. The structure characterization of the silver substituted sodalites was performed using a Siemens 8-28 D5000 diffractometer. AC impedance were measured using an HP 4284A Precision LCR meter in the frequency region 20 Hz to 1 MHz using pellets of 6 mm diameter [6]. The HP 4284A Precision LCR meter was remotely controlled by a computer via an IEEE-488 interface. Possible impedance contribution from external connection cables and pellet mounting devices were corrected using a built-in "short" and "open" calibration facility. Pellets of the sodalite were sintered at 550 °C before being painted with silver based paint for subsequent AC impedance measurement. As it has been well established that electrical conduction in zeolites occurs bY migration of the cations, with no evidence of electronic conduction [7-8], no further verification was
M.R.M. Jiang, M. 71 Weller / The ionic conductivity of (Ag, Na)-nitrite sodalites
342
conducted on this aspect. Possible volume expansion of the pellets was not monitored and the thickness measurements of the pellets were performed after the sintering stage and before being coated with the silver based paint. In fact, no significant change in pellet thickness was observed after the sintering process.
3. RESULTS AND DISCUSSION Typical AC impedance diagrams obtained using painted silver electrodes for a pellet of nitrite sodalite are shown in Fig. I. They generally consist of two well defined parts, a portion of a high frequency semicircle of depressed nature and a low frequency straight line at less than 45 ° to the real impedance axis. The depressed semicircle, which has one extended intercept at high frequency at the origin and another extended intercept at low frequency to the real axis, is regarded as the bulk impedance of the pellet arising from the ionic conduction of the bulk electrolyte [9]. The intercept of the depressed semicircle to the real impedance axis at low frequency yields the DC impedance of the pellet. The inclined straight line which appears at low frequency is generally regarded as the interfacial impedance of the electrolyte/electrode interface arising from the geometric microstructure of the electrolyte/electrode interface [9-10]. For an (Ag,Na)-nitrite sodalite pellet of constant silver composition, the inclined straight line had a constant angle to the real impedance axis at all temperatures studied (Fig. i), reflecting a consistent electrolyte/electrode interface property of the pellet over the temperature range. For pellets of different silver content, however, there was a decline in
the angle of these straight lines as the degree of silver substitution is increased (Fig. 2), reflecting changes in the electrolyte/electrode interface among the various silver substituted nitrite sodalites. A likely source of these differences would be changes in the dominant mobile species. It can be clearly seen from Figure 2.d that the angle of the line for the highest silver substitution (i.e. x=7.5,) is almost zero, i.e. the line became almost parallel to the real axis. From electrolyte/electrode interfacial view point, this phenomenon represents the transition of interfacial impedance from complete blocking in the case of an Na+/Ag interface to complete non-blocking in the case of an Ag+/Ag interface. This clearly suggests that a gradual transition in ionic conduction mechanism from Na + ion domination for low silver content sodalite pellets to Ag ÷ ion domination for high silver content sodalite pellets occurs. The appearance of only one depressed semicircle at high frequency followed immediately by an inclined straight line at low frequency for all five samples in the temperature range investigated revealed that the effect of intergranular impedance (i.e. the grain boundary effect) on total conductivity measurement for the materials studied is negligible. Since intergranular impedance is negligible for the nitrite sodalite pellets, DC conductivities at a particular temperature can be calculated from DC impedances obtained from the intercept of the low frequency part of the depressed semicircle on the real axis of the impedance diagrams. The sample conductivities are plotted as a function of inverse temperature for all the compounds studied (Fig. 3). For all the samples studied a very good linear relationship exists for Ln(a) vis 1000/T (K) in the temperature range 250 to 550 °C in ambient
463 493 523 553
0
~ ~w~
0
2
~"#~
4
"4,
~
6
8
~,~
10
Zr ( k ~ ) Fig. i. Impedance plots for (Ag,Na)-nitrite sodalite with x-0.4 at various temperatures. Numbers above each curve represent temperatures in °C, at which the curve was obtained.
M.R.M. Jiang, M.T. Weller / The ionic conductivity of (Ag, Na)-nitrite sodalites
343
3
(a)
ii 0
(b)
2 1
'
1
523
i
539 '
2
3
"~'"
,--
0
4
.,....
0
1
3
2
iI
15 11
(c)
4
Zr (k~)
Zr (k-Q)
! (d)
K~ 0.75
o'51
i, 0,5]
485
0,25 385
0
"~ 0
0.5
1
1.5
2
O~ 2.5
Zr (k~)
0
0.5
1
1.5
Zr (kn)
Fig. 2. Impedance plots for (Ag,Na)-nitrite sodalites with x=0 (a), 0.4 (b), 3.3 (c), and 7.5 (d). Numbers in each figure represent temperatures in °C, at which each plot was obtained.
3 -10
........
\
i
-20 1.5
1000/T (1/K)
\
atmosphere, yielding reliable Arrhenius activation energy for conduction process in all the samples• At temperatures below 250 °C, interferences from ambient moisture was experienced due to absorption of water molecules by the sodalite pellets, resulting in upward deviation of the Ln(a) - 1000/T plots. We are currently repeating conductivity measurements for all five samples in a dry argon environment for temperatures below 250 °C. Preliminary results for a Ag3.3Na4.7(AISiO4)6(N02) 2 pellet show a common Ln(a)- 1000/T plot for data obtained either above 250 °C in ambient atmosphere or below 250 =C in the dry argon environment. For the dry pellets, above 250 °C, it can be readily seen that substitution of sodium ion in the nitrite sodalite by silver reduces the thermal activation energy required for ion conduction and also enhances bulk ionic conductivity. The effect of silver concentration on the total ionic conductivity for nitrite sodalites is quite similar to that observed for other
Fig. 3. Arrhenius plots for (Ag,Na)-nitrite sodalites with x=0 ([]), 0.4 (+), 3.3 (~), 5.1 (E) and 7.5 (0). Conductivity was measured in S/cm.
M.R.M. Jiang, M.T. Weller / The tonic conductivity of (Ag, Na)-nitrite sodalites
344
-6
80
7~
~, 7oj
-9'
~"
60
50~
40
0
~,.~...
2
~ -
4
- -
~
ii
6
L
Ag content~ x
Fig. 4. Activation energy for conduction as function of silver ion content per unit cell.
types of 3-D structure electrolyte, e.g. A3M2(P04)3, (where A=Li, Na, Ag, K; M=Cr, Fe.). In this system as well, replacing sodium with silver results in an increased conductivity [ii]. However, the effects of silver substitution for sodium in 5- and B"-aluminas are quite different. Ag+-B-alumina has a lower ionic conductivity and higher activation energy than its sodium analogue [12] while Ag+-B"-alumina has lower ionic conductivity and lower activation energy than its sodium analogue [13-14]. In order to reveal the detailed effect of cation composition, activation energies were obtained for the five nitrite sodalite samples and plotted against their silver content (Fig. 4). It is clear from Fig. 4 that the activation energy for the mixed (Ag,Na)- nitrite sodalite varies monotonically with silver content. Further more, conductivity isotherms shown in Fig. 5 also demonstrate a monotonic variation of the ionic conduction as a function of silver content. Comparing above results to those reported for the mixed (Ag,Na)- 6- and B"aluminas [12-14], it is obvious that in the temperature range studied, the so called "mixed alkali effect", which exists in almost every partially ion exchanged B- or B"- alumina, was not evident in the mixed (Ag,Na)nitrite sodalites. This behaviour of the nitrite sodalite family is in agreement with another member of the zeolite family, zeolite X , for
0
a
2
4
6 Ag content,
8 x
Fig. 5. Ionic conductivity isotherms for (Ag,Na)nitrite sodalites as a function of silver content, x. ~ 500 °C; 0 400 °C; × 300 °C. Conductivity was measured in S/cm.
which no obvious "mixed alkali effect" was reported. Interactions between cations and the framework lattice and between cationic sites were thought to have influenced both the activation energy and the ionic conductivity of this zeolite [15]. Since the cell parameter has been found to be directly related to the degree of silver substitution in the mixed (Ag,Na)- nitrite sodalites [6], the silver composition dependence of both the activation energy and the total ionic conductivity is indicative of a predominantly electrostatic interaction between the cations and the framework lattice. Fig. 6 shows a polygonal representation for a perfect sodalite cage (i.e. the 6-cage) [16]. It is a truncated octahedron consisting of six four-membered windows and eight six-membered windows. A given central cage shares each of its six-membered windows with one of eight other such cages. For an ideal synthetic sodalite, every such cage contains one anion and four cations. The anion resides at the centre of the sodalite cage and surrounded tetrahedrally by the four cations. For the mixed (Ag,Na)- nitrite sodalites, the distribution of the two types of cation around each anion is likely to be statistical rather than stoichiometrical. It is the
M.R.M. Jiang, M.T. Weller / The ionic conductivity of (Ag, Na)-nitrite sodalites
/
345
tion becomes less. It is likely, therefore, that the mobilities for both sodium and silver ions in higher silver content framework are higher than in lower content framework, and in all cases sodium is more mobile than silver. From preceding discussion, it is seen that the total ionic conductivity of the mixed Ag+/Na +- nitrite sodalites is strongly dependent on the size of the mobile cations concerned. Further studies on other sodalites with different cations and anions are in progress to study conductivity as a function of ion size and intereage bottleneck size.
4. ACKNOWLEDGEMENT Fig. 6. A polygonal representation for the sodalite cage with vertices as Si or AI, showing the six 4-membered and eight 6-membered windows. six-membered windows which form continuous channels for the cations to migrate among neighbouring cages [17]. However, to enable a cation to hop from one cage to another through the six-membered windows, it would have to overcome the potential barrier of the six-membered window. The larger the cation the higher the energy required for hopping but the larger the size of the sixmembered window, the lower the potential barrier for hopping. The accepted ionic radii for four co-ordination for Ag + and Na + ions are 1.16 and 1.13 A, respectively. Replacing sodium by silver in the structure results in an expansion of the lattic and therfore also the dimensions of the sixmembered window. It is, therefore, quite reasonable to expect that, with low silver substitution, e.g. at x-0.4, the potential barrier for the smaller Na + ion to hop between cages could be significantly reduced as a consequence of framework expansion and the resultant enlargement of the six-membered windows caused by some incorporation of silver. An inevitable result of this is a significant decrease in the activation energy for sodium ions in the lowest silver'content compound (i.e. x=0.4), from the value 81.6 kJ/mol found for the parent sodalite. Hence, the total ionic conductivity increases significantly for this composition as sodium ions are the dominant mobile ions. However, as more sodium ions are substituted by silver ions, the somewhat lower mobility of the slightly larger Ag ÷ ions becomes the more important factor in contributing to the total ionic conductivity. As a consequence, although the six-membered windows continue to expand as more sodium is replaced by silver, the rate of decrease of the activation energy is lower as more of the conductivity involves the larger Ag + ions hopping between cages. Concomitantly, as more sodium is replaced by silver, the contribution from sodium conduc-
The authors wish to thank the SERC, the Ford Motor company and A.B. Electronics Products for grants in association with this research.
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