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An investigation of some modified AgI solid electrolytes containing AsO3−4, Cr2O2−7 and Mo2O2−7 anions
AN INVESTIGATION OF SOME MODIFIED AgI SOLID ELECTROLYTES CONTAINING AsO:-, Cr,O;Mo,O$ANIONS*
and
M. LAZZARI Centro Studio Processi Elettrodici de1 C.N.R., Polytechnic of Milan, Italy and B. SCROSATI lnstituto di Chimica Fisica ed Elettrochimica, University of Rome, Italy and C. A. VINC~T Department of Chemistry, University of St. Andrews, Scotland (Received 24 November 1975; in ,final form 16 February Abstract-The
dichromate
1976)
conducting phases of three modified silver iodide salid electrolytes, containing arsenate, and dimolybdate anions respectively, have been studied. With a view to establishing their
stabilities, conductivities have been measured after various heat treatments, etc and structural changes have been investigated using DTA and X-ray techniques. A phase diagram is suggested for the A@-
Ag,AsO, system. We conclude that some of the conducting phases are in a metastable condition at room temperature,
but the decomposition into poorly conductive materials may be extremely slow at moderate temperatures, especially for the arsenate electrolyte. There is some evidence to suggest that the arsenate and dichromate electrolytes behave as supercooled liquids, but the dimolybdate electrolyte remains crystalline and shows a number of anomalous properties.
1NTRODUCTION
it would seem of interest to define the range of stability of the solid conductors of Table 1. This has been attempted thermodynamically, with the determination of the phase diagrams and electrochemically, with conductivity studies over a wide temperature range.
In previous papersClL3-j some of the electrochemical properties of silver solid conductors of the silver iodide-silver oxyacid salt type found in OUT laboratories, have been described. These properties, summarized in Table 1, show the electrolytes to be of interest for practical applications since their conductivity is reasonably high and almost purely ionic in character. Furthermore some of the new materials show good stability at room temperature in iodine and moisture: this property is significant since it may be related to the shelf life of solid-state power sources which are usually based on a silver-iodine couple. There have been indications, however, that the electrolytes undergo decomposition reactions into poorly conductive compounds at moderately high temperatures. In view of their possible practical applications,
EXPERIMENTAL Silver iodide, arsenate, dichromate and dimolybdate were prepared using the procedures previously described[ l-31. The electrolytes were formed by melting, under vacuum or inert gas, intimate mixtures of weighed amounts of silver iodide and the appropriate silver salt. The melt was subsequently cooled or quenched in one of four ways: (a) slow cooling under an inert gas stream at l”C/min, (b) ‘spontaneous’ cooling to room temperature under an inert gas stream, (c) quenching in a Dewar flask containing a mixture of solid and liquid chIoroform, {d) quenching in Iiquid nitrogen. To achieve a maximum rate of cooling in
* This paper was presented at the 148th Meeting of the Electrochemical Society, Dallas, TX, U.S.A., 5-9 October
the latter medium, the melt was extruded in droplets of dia. < 1 mm into a 1 m column of liquid nitrogen.
1975.
Table
1. Electrochemical
Agl-Ag,AsO, AgI-Ag,Cr,O, AgI-Ag,Mo,O,
properties
at
25°C of some laboratories[l-31
solid
Electrolyte approximate composilion (m/o AgI)
Total conductivity* @cm)-’
Electronic conductivity (n cm)-’
80 85 75
0.004 0.017 0.008
1o-8 lo-’ lo-*
* Values corrected for electrode/electrolyte 7 At temperatures higher than 50°C.
contact resistances. 51
Ag
electrolytes
Ag transport No. 1.01 f 0.03 1.01 + 0.003 0.99 f 0.03
examined
in
our
Activation energy (kcal/mole) 4
52
M.
LAZZARI, B. SCROSAII AND C. A. VINCENT
25.
20
I 20
0
I 40
I 60 TIME,
I 60
I 100
days
Fig. 1. Stability of the arsenate, dichromate and dimolybdate electrolytes in the presence of moisture at 25°C.
The resulting solid electrolytes were ground to a fine powder, and then investigated directly or after periods of annealing at various temperatures. X-ray powder diffraction patterns were obtained with CuK, radiation at 30 kV and 20 mA (JEDL diffractiometer, Model JDX43s). Differential thermal analysis studies were carried out using a differential calorimeter (Perkin Elmer, Model DSC IB). For conductance measurements, samples were pressed at about 2500 kg cm-’ into pellets with a geometric surface area of 1.26cm2. These pellets were sandwiched between two electrodes formed by mixing silver powder with the sample in a weight ratio of 1:2 and pressing into pellets also at 2500 kgcmm2. The resistance of the resulting symmetrical cells was then measured using an ac bridge (Wayne Kerr) operating at 1.592 kHz. For temperatures above ambient, the cells were maintained at the desired temperature with an accuracy of + 0.2 K using a recirculating thcrmostatic bath. Low temperature resistance measurements were performed by introducing the cell into a cryostat in which a stable temperature gradient was maintained from - 100 to 250K. Temperatures were measured with an accuracy of f 2 K by measuring the voltage between thermocouple junctions strapped to the cells and similar junctions maintained in a triple-point cell. RESULTS Silver
iodide-silver
AND
arsenate
DISCUSSION
system
The investigation of the system silver iodide-silver arsenate was considered of priority since the intermediate compound, at 80mole “/, (m/o) Agl, has a number of important properties. Among these, there is the excellent stability at room temperature, even in the presence of high moisture content. Figure 1 shows the time dependence of the resistance of a cell of the type [email protected],AsO,iAg, kept open in an environment
The resistance, after an initial increase of loo/,, remains practically constant for several months. Another interesting property of the A&Ag,AsO, system is indicated in Fig. 2, where conductivity-composition plots are shown at 25°C. Curve A refers to results previously reported[l], related to samples obtained by spontaneous cooling to room temperature of the respective melted mixtures. Curve B, on the other hand, refers to samples quenched in liquid nitrogen. The maxima appear at the same composition, ie 80m/o AgI, but the conductivity of the quenched samples is significantly larger. The evaluation of the conductivity at various temperatures, in the particular case of quenched samples of SOm/o AgI, is shown in Fig. 3, where the resistance of syrnmetrical cells of type I is plotted as a function of
(1)
of about 70% humidity.
100
60
60
40
A61 m/b Fig. 2. Composition dependence of the specific conductivity in the system Agl-Ag,AsO, at 25°C for samples quenched under a nitrogen stream(A) and in liquid 1. ,_>
Modified AgI solid electrolytes containing AsO:-, Cr,O:-
and Mo,Of-
53
anions
until a constant resistance value was reached. The conductivity increases monotonically up to about lOO”C, then falls and remains at a lower level in the following cooling run. These results indicated that near 100°C an irreversible decomposition into poorly conductive compounds takes place. Similar behaviour has been reported by Takahashi et a&4], for the silver iodide-silver phosphate system, where an intermediate compound at 80m/o AgI decomposes at 79°C according to the reaction
15
Ag,I,P04+
Fig. 3. Resistance
of cell [I] vs electrolyte various temperatures.
thickness
at
sample thickness. The contribution of the electrodeelectrolyte interfacial resistance, which in cells having a complete solid state configuration may be significant, has been obtained by extrapolation to the origin. Taking into account this contribution (1.15R) the conductivity of the quenched electrolyte is found to 0.012 (n cm-’ at 20°C. A possible explanation for the significantly lower conductance of the slowly cooled material (Table 1) is that the latter contains a higher proportion of non-conductive phase or phases. The temperature dependence of the conducti-
vity of the 4AgI.Ag,AsO, electrolyte was examined and the result for samples quenched in liquid nitrogen is shown in Fig. 4. The points reported were obtained by maintaining cells of type I at each temperature _
-3
I 2.6
I 2s
T, ‘C
50
80
100
1
I 3.0
I
2 AgI + Ag,I,PO,.
(2)
In this case also the decomposition produces poorly conductive compounds, one of which is silver iodide. On the other hand in an Arrhenius plot over a restricted temperature range (eg from 20 to 90°C) the points obtained on the heating process were coincident with those of the cooling process, while samples kept at 110°C showed a decay in conductivity of half an order of magnitude within 15 h. With the aim of clarifying the solid state reactions connected with the observed conductometric behaviour and defining the range of stability of the electrolyte, a systematic study, based on X-rays on differential thermal analyses, was undertaken on various samples at different compositions and submitted to different thermal treatments. (a) XOm/oAgI. The X-ray powder patterns of a sample obtained with the spontaneous cooling from the melt (method b) show a number of characteristic diffractions which are different from those of the starting materials. These diffractions, reported in Table 2, may be then related to a phase, at SOm/o AgI, which will be here indicated as phase 1. It has to be noted, however, that the spectra show a limited number of peaks over a rather broad base line thus indicating a limited degree of crystallinity for the material. The degree of crystallinity is further reduced when the samples are quenched rapidly as discussed below. A sample of the electrolyte, after being annealed at 120°C for ten days, was submitted to X-ray analysis. This spectrum showed the peaks of silver iodide
1 3.2
I
0
20
I
I
3.4
I 3.6
-10
I
I 3.8
1 OS/T, ‘K-’ Fig.
4. Temperature
dependence of the specific conductivity of 0.85 Ag14.15 Ag,Cr*O, electrolytes.
the
0.8 AgI-0.2 Ag,AsO,
and
M. LAZZARI, B. SCROSATI AND C. A. VINCENT
54
Table 2. X-ray diffraction potider patterns in the system AgI-Ag,AsO, Silver iodide (y-phase) 28 23.7 39.2 46.3
IA” lbo’ 60 30
Phase 1 (80 m/o AgI) 20 24.7 30.8 32.8 36.3 37.7
I/10 55 50 100 50 45
Phase II (72 m/o Agl) 21 25.6 27.6 30.7 31.2 32.3 33.3 36.6 39.8 41.5
phase I -
x AgI + y phase II
phase II + z AgI + j phase TTI
Phase III (66 m/o Agl) 21 30.9 31.6 33.4 33.9 44.1
IA0 j9 :: 64 100 64 39 47 36
together with others different from those of phase T and therefore to be associated to another phase, here indicated as phase II. The diffraction powder patterns of the latter are reported in Table 2. This result supports the conductivity evidence that the electrolyte decomposes irreversibly at < 120°C. To further characterize the system, a fast quenched sample was annealed at 150°C for 15 days and then cooled to room temperature. The X-ray spectrum of this sample showed reflections and new patterns different either from those of phase I and from those of phase II. This new compound has been indicated as phase III and the related patterns are reported in Table 2. Phases I and II seem therefore to be unstable at 150°C and convert irreversibly to phase III. This conclusion was further confirmed by melting a mixture of AgI and Ag,AsO, (with SOm/o AgI) and cooling it very slowly (method a) to room temperature. The X-ray spectrum of the solidified product was identical to that of the sample annealed at 15O”C, thus indicating that the slow cooling procedure forms AgI and phase III at about lSo”C, which, at lower temperatures, do not transform into either I or II. This result therefore accounts for the observed decrease in conductivity of the 4AgI.Ag,AsO, electrolyte when the rate of cooling from the melt is decreased. On the basis of the above results, in the system Agl-Ag,AsO, the following two solid state reactions may be postulated (31
(CuK, radiation)
I/IO kl ;: 100 37
Silver Arsenate 20 29.1 32.7 35.9 53.9 56.1
IL
is’
100
55
20 30
This sample was then annealed at 120°C for 5 days and then cooled slowly to room temperature. The X-ray analysis showed that the annealing had caused the disappearance of phase I, while the patterns of phase II became stronger and those of phase III remained unchanged Finally a prolonged annealing at 170°C produced a material which contains phase III and AgI. On the basis of these results, it is thcrefore possible to attribute tentatively this composition to phase II. (d) 66 m/o AgI. The X-ray spectrum of a quenched sample showed reflections of Ag,AsO,, phase III and Agl. Annealing at 170°C resulted in the disappearance of Ag,AsO,. This composition may be therefore associated with phase III. The DTA spectrum, reported in Fig. 5, shows the unidentified peak at 21&22O”C, the peak of the eutectic at 245°C and a large peak at 3OO”C, which is the peritectic peak. (e) SOm/o AgI. The X-ray analysis for quenched samples gave appreciable reflections of Ag,AsO, only. For slowly cooled samples (method a) the peaks of phase III and of AgI also appeared. The DTA of quenched samples (see Fig. 5) gave only the peak of peritectic melting at 3oO”C, and a very small eutectic peak at 245°C. The above data were used to trace the possible phase diagram for the system, shown in Fig. 6. This phase diagram closely resembles that of the Agl-Ag,PO, system, investigated by Takahashi et al[4]_ Here again, the conductive phase, Ag,I,PO,, is formed by fast cooling from the melt. In such a
(4)
which are characterized by kinetics which become appreciable at temperatures higher than 100 and 150°C respectively. Due, it is believed, to the slowness of these transformations, attempts to detect them by differential thermal analysis were unsuccessful. The DTA spectrum of a quenched sample, reported in Fig. 5, does show a peak at 15O”C, probably ascribable to silver iodide (Ir- TVtransition) resulting from decomposition, a peak at 220°C which remains unexplained and a large peak at 250°C which may be associated to the melting of the eutectic (AgI and phase III). (b) 9Om/o AgI. The X-ray analyses of quenched samples showed reflections of silver iodide (very intense) and of phase I. (c) 72m/o AgI. The X-my spectrum of a quenched sample showed patterns of all phases I, II and III.
I
I
I
I
50
150
250
350
Fig.
T , ‘C
5. Differential thermal analysis in GgI-Ag,AsO,.
the
system
Modified AgI solid electrolytes containing AsO:-,
-’ ,
0
; I
0
Cr,O:-
and MO@-
anions
55
I 1,
II
,
Ill
,
,
,
,
,
100
so
m/o Ag,AsO, Fig. 6. Phase diagram of the system A&Ag,AsO,.
phase
Fig. 8. Resistance of quenched samples of O.SAgla.2 Ag,AsO, and 0.85 AgI-0.15 Ag,Cr,O, at low temperatures.The open and filled circles refer to two independent measurements.
Fig. 7. X-ray diffraction patterns of AgI-Ag,As04 (80 m/o AgI) samples quenched under a nitrogen stream(A), in solid-liquid chloroform(B) and in liquid nitrogen(C). Curve D refers to a quenched sample after annealing at 120°C.
and produces a metastable highly conducting solid structure. One might indeed consider specific interactions between the two parent components AgI and Ag,As04 in the liquid state. Such a configuration might then be quenched in the solid, which could therefore be regarded as a supercooled liquid. X-ray patterns of 80m/o material quenched at different rates are shown in Fig. 7. The vitreous character of the sample formed by spraying droplets of molten electrolyte into a liquid nitrogen column is illustrated by the complete absence of diffraction peaks (see pattern C). This glassy nature of the material would support the idea of a supercooled liquid, and it would be very interesting to investigate the silver ion conductance at different compositions of melt. However we have at this time no firm evidence that Ag,I,AsO, is a single phase and confirmation would require detailed structural studies. The variation of resistance with temperature of an 80 m/o quenched sample at low temperatures is shown in Fig. 8. It is interesting to note that there is no evidence of a phase transition in this temperature range, such as that reported by Owens and Argue[6] for RbAg,I, at - 155°C. On the other hand there is a change in slope at about - 150°C. This behaviour is exactly analogous to that found by Ingram et al (personal communication) for LiSi04 systems at much higher temperatures, and shown by these workers to be frequency dependent. At high frequencies or low temperatures a parallel capacitatively coupled conductive admittance, perhaps due to grain surface conduction or to interparticle impedance, becomes significant compared with the normal bulk conductance and hence lowers the overall impedance of the system.
diagram, on cooling below the eutectic point the material should solidify with the formation of AgI and a non-conductive Ag,I,PO, phase. On further cooling to the electrolyte decomposition point, 79°C (lOO’C), it would then be necessary for these phases to combine to form the single conducting phase Ag,I,PO, (Ag,I.+AsO,). As pointed out by Owens[S], it is difficult to accept the formation of a single phase compound by this sequence of reactions. It is perhaps more reasonable to suppose that the quenching procedure minimizes the demixing at the eutectic point
A
B
C
20’
I
I
I
I
I
I
40”
30’ 28
(Cu Ka)
M. LAZZARI, B. SCRCF.ATI AND C. A. VINCENT
56
1
I
Fig. 10. X-ray diffraction patterns of AgI-Ag,Cr,O, (85&o AgI) samples quenched under a nitrogen
stream(A) and in liquid nitrogen (B]. Curve C refers to a quenched sample after annealing at 70°C.
Fig. 9. Compositnm
dependence of the specific conductivity in the systems AgI-Ag,Cr,O, and AgI-Ag,Mo30, at 25°C.
Silver iodide-silver
dichromate
system
The conductivity composition curve for this system, for samples obtained by cooling from the melt by method (b) is given in Fig. 9 and shows a maximum conductance of 0.017 fi2- ‘cm-’ at approx. 85 m/o of AgI. Provided that they were maintained in a completely dry atmosphere, the resistance of cells containing this electrolyte at room temperature was found to remain virtually constant for a long period (Fig. 1). On the other hand, cells kept in an environment of circa 70% humidity suffered a progressive increase in resistance, suggesting a water-catalysed decomposition or phase transformation of even more serious proportions than that associated with RbAg,I,[7]. Measurement of the conductivity as a function of temperature also indicated that the material spontaneously underwent a phase transition or reaction at relatively low temperatures. In Fig.4 the values of log CTreported are the relatively stationary values reached after equilibrating the cell for 8-12 h at each temperature. The fall in conductance at temperatures above 40°C should be compared with the behaviour in the arsenate system at temperatures above 100°C. At temperatures down to liquid nitrogen this material behaves in the same way as 80m/o Agl/Ag,AsO, showing a similar kink in the log 0 vs l/T plot and no phase transition (Fig. 8). The electrolyte was further investigated by X-rays and differential thermal analysis. The X-ray diffraction pattern of slowly cooled Urn/o material (Fig. 10) could not be resolved except for two weak AgI reflections. Electrolyte quenched by method (c) was even more amorphous in nature. However the pattern derived from a sample aged in a (moist) laboratory atmosphere for several days showed well resolved Agl reflections together with other peaks associated with one or more unknown crystalline phases. An identical pattern was found for a sample annealed at 70°C for
3 days, thus confirming that the water catalysed and temperature activated decomposition processes were the same transformation. In the DTA results (Fig. ll), curve A refers to the heating of a mechanical mixture of 85 m/o AgI/Ag,CrzO, and shows a peak at about 1SO”C, corresponding to the Agl fl---) CItransition and a double peak at about 165°C. In the subsequent cooling trace, B, the solidification peak is first seen, followed by a just discernible peak for AgI or-p transition, suggesting that the silver iodide has been mainly combined. The heating scan of a sample of newly formed electrolyte is shown on trace C while that of a sample annealed at 70°C is given on trace D. Thus it would appear that the conducting phase formed in this system is again thermodynamically unstable and tends to revert to AgI and other poorly conductive phases. The decomposition reaction is slow up to circa 40°C but is catalysed by water. Silver iodidesilver
dimolybdate
system
While the two systems discussed above have great similarities in their behaviour, the properties of the
:_rr “-1 I 50
I
I
100
150
I
200
1 . ‘C Fig. 11. Differential thermal analysis of AgI-Ag,Cr,O, (85 m/o AgI) samples under various conditions. Curves A and 3 refer to the heating and the cooling of a mechanical
mixture, respectively. Curve C is related to a newly prepared sample and curve D to a sample annealed at 70°C.
Modified Agl solid electrolytes containing AsO:-, Cr,O:-
;.4 Fig. 12. Temperature
2.6
a6
2.6
32
57
anions
a4
10%/T , OK-’ dependence of the specific conductivity of the 0.75AgI-0.25Ag,Mo,O, electrolyte.
silver iodide-silver dimolybdate electrolyte show some interesting contrasts. The conductivity--composition plot (Fig. 9) shows a maximum at 75 m/o AgI, but the value of the maximum conductance. at room temperature, viz 0.008 a- 'cm- ‘, is lower than that commonly found in electrolytes of this type. Further, this value is rather insensitive to the cooling procedure used to form the electrolyte from the melt. Like the arsenate, the dimolybdate electrolyte is stable in moisture: the cons&t resistance of a cell containing this electrolyte over a period of months is shown in Fig. 1. The complex temperature variation of the resistance of the 75 m/o electrolyte is perhaps the most interesting feature of this material. In Fig. 12 is shown the log 0 vs l/T plot for two cycles between 20 and 120°C for slowly cooled samples (method a). A reversible change of slope from a high temperature of 5.0 kcal mole- ’ to a low temperature value of 10.8 kcal mole- 1 occurs at approx. 56°C indicating a “subtle structural transition” of the type discovered by Geller and Owens[S] in (C5H,NH)Ag,16. In the latter case a very small heat effect was detected due to the transition; here we have not seen any discontinuity in DTA traces over this temperature range. The most obvious difference in the X-ray diffraction spectra between the dimolybdate electrolyte and the other two electrolytes of the present study is the presence of well defined diffraction lines in the fast quenched material (Fig. 13). It may be noted that the traces for the quenched and slowly cooled samples are very similar, and one may therefore conclude that the 75 m/o silver iodidc+silver dimolybdate does not form a vitreous
and Mo,O:-
trolyte is stable in the presence of moisture while the dichromate is rapidly decomposed. The arsenate is stable to nearly 100°C while the dichromate is irreversibly transformed to a less conducting material at around 40°C. For the 8Om/o AgI/Ag,AsO, and 85m/o A@/ Ag,Cr,O, systems we suggest that rapid cooling from the melt retains the material in the form of a supercooled liquid. The decomposition reaction then has the form of a devitrification followed by separation into stable crystalline phases. It may be noted the
*;liJ-\jrl-
phase.
CONCLUSIONS
The first two systems studied in this investigation are character&d by conducting phases which although thermodynamically unstable, remain unchanged in a metastable state with varying degrees of tolerance to ambient conditions. The arsenate elec-
20’
306
40” 20
(Cu Ku)
Fig. 13. X-ray diffraction patterns of AgI-AgzMo,O, (75m/o AgI) samples quenched under a nitrogen stream (A) and in liquid nitrogen(B). Curve C refers to a quenched sample after annealing at 180°C.
58
M. LAZZARI, B. SCRCBATIAND C. A. VINCENT
phases have already been suggested by Kunze[9] for the AgI/Ag,SeO, system. Thermodynamically unstable phases have also been reported for AgI/Ag,MoO, aid AgI/Ag,CrO,[lO]. In the case of the dichromate electrolyte the onset of the vitreous character occurs at high& quenching temperatures-a phenomenon which may be related to the much lower temperature at which the decomposition reaction becomes appreciable. Further study of these two systems is necessary, especially on the properties of the binary liquid phases and on the types of interaction which may take place between the various components in these situations where the formation of true compounds remains doubtful. It is unlikely that the transport mechanism for silver ions is identical to that in the MAgJ, class of compounds. Unfortunately structural studies of these oxyacid systems are likely to prove difficult because of their amorphous nature. It may be of great interest to identify the precise role of water in the decomposition of the dichromate electrolyte. The role of water in the decomposition of RbAg,I,[7], which implies a chemical reaction, is not well understood either. As pointed out above, the properties of the electrolyte found in the AgI/Ag,Mo,O, deviate markedly from the common pattern shown by the majority of silver iodide-silver oxyacid salt systems. In particular the structure remains crystalline even with fast quenching. Taking into account the relatively low value of the conductance and the peculiar behaviour of the temperature dependence of the conductivity, it might be suggested that this system is located near some limit of the stability range of this class of electrolyte. As in another class of silver iodide modified vitreous
electrolytes, group[lI],
the silver iodide-silver the size of the minority
alkylammonium anion may play
part in stabilising disordered structures characterised by networks of channels through which
an important
the silver ion can move.
Further
work
is in progress
in our laboratories to investigate whether there is a critical size for the minority substituting ion and to establish a phase diagram for this system. Acknowledgements-We are grateful to D. M. Finlayson and W. G. Picken for their help with the low temperature
conductance measurements, and we thank the Science Research Council and the Consiglio Nazionale delle Riccrche (CNR-Roma) for financial support.
REFERENCES 1. B. Scrosati, F. Papaleo, G. Pistoia and M. Lazzari, J. el~ctrochem. sot. 122. 339 (1975). J. appl. Electro2. B. Scrosati, A. Ricci anh M. iazz&, them. in press. 3. M. Lazzari. A. Ricci. B. Rivolta and B. Scrosati. submitted for publication. 4. T. Takahashi. S. Ikeda and 0. Yamamoto, J. rlectrothem. sot. 115, 477 (1972). 5. B. B. Owens, Fasr Ion Transport In Solids (Edited by W. van Goal), p. 593. North Holland, Amsterdam (1973). 6. B. B. Owens and G. R. Argue, Science 157. 308 (1967). 7. L. E. Top01 and B. B. Owens, J. phys. Chem. 72, 2106 (1968). 8. S. Geller and B. B. Owens. Phvsics Chem. Solids 33. 1241 (1972). (Edited by W. 9. D. Kunze, Fast Ion Transport in Soiids van Go011 t). 405. North Holland. Amsterdam 11973). 10. G. Chiodkfii, A. Magistris and A. Schiraldi, Eiec&chim. Acta 19, 655 (1974). J I. B. B. Owens, Advances in Electrochemistry and Electrochemical Engineering, Vol. 8, Chap. 1. Wiley, New York (1971).
and
M. LAZZARI Centro Studio Processi Elettrodici de1 C.N.R., Polytechnic of Milan, Italy and B. SCROSATI lnstituto di Chimica Fisica ed Elettrochimica, University of Rome, Italy and C. A. VINC~T Department of Chemistry, University of St. Andrews, Scotland (Received 24 November 1975; in ,final form 16 February Abstract-The
dichromate
1976)
conducting phases of three modified silver iodide salid electrolytes, containing arsenate, and dimolybdate anions respectively, have been studied. With a view to establishing their
stabilities, conductivities have been measured after various heat treatments, etc and structural changes have been investigated using DTA and X-ray techniques. A phase diagram is suggested for the A@-
Ag,AsO, system. We conclude that some of the conducting phases are in a metastable condition at room temperature,
but the decomposition into poorly conductive materials may be extremely slow at moderate temperatures, especially for the arsenate electrolyte. There is some evidence to suggest that the arsenate and dichromate electrolytes behave as supercooled liquids, but the dimolybdate electrolyte remains crystalline and shows a number of anomalous properties.
1NTRODUCTION
it would seem of interest to define the range of stability of the solid conductors of Table 1. This has been attempted thermodynamically, with the determination of the phase diagrams and electrochemically, with conductivity studies over a wide temperature range.
In previous papersClL3-j some of the electrochemical properties of silver solid conductors of the silver iodide-silver oxyacid salt type found in OUT laboratories, have been described. These properties, summarized in Table 1, show the electrolytes to be of interest for practical applications since their conductivity is reasonably high and almost purely ionic in character. Furthermore some of the new materials show good stability at room temperature in iodine and moisture: this property is significant since it may be related to the shelf life of solid-state power sources which are usually based on a silver-iodine couple. There have been indications, however, that the electrolytes undergo decomposition reactions into poorly conductive compounds at moderately high temperatures. In view of their possible practical applications,
EXPERIMENTAL Silver iodide, arsenate, dichromate and dimolybdate were prepared using the procedures previously described[ l-31. The electrolytes were formed by melting, under vacuum or inert gas, intimate mixtures of weighed amounts of silver iodide and the appropriate silver salt. The melt was subsequently cooled or quenched in one of four ways: (a) slow cooling under an inert gas stream at l”C/min, (b) ‘spontaneous’ cooling to room temperature under an inert gas stream, (c) quenching in a Dewar flask containing a mixture of solid and liquid chIoroform, {d) quenching in Iiquid nitrogen. To achieve a maximum rate of cooling in
* This paper was presented at the 148th Meeting of the Electrochemical Society, Dallas, TX, U.S.A., 5-9 October
the latter medium, the melt was extruded in droplets of dia. < 1 mm into a 1 m column of liquid nitrogen.
1975.
Table
1. Electrochemical
Agl-Ag,AsO, AgI-Ag,Cr,O, AgI-Ag,Mo,O,
properties
at
25°C of some laboratories[l-31
solid
Electrolyte approximate composilion (m/o AgI)
Total conductivity* @cm)-’
Electronic conductivity (n cm)-’
80 85 75
0.004 0.017 0.008
1o-8 lo-’ lo-*
* Values corrected for electrode/electrolyte 7 At temperatures higher than 50°C.
contact resistances. 51
Ag
electrolytes
Ag transport No. 1.01 f 0.03 1.01 + 0.003 0.99 f 0.03
examined
in
our
Activation energy (kcal/mole) 4
52
M.
LAZZARI, B. SCROSAII AND C. A. VINCENT
25.
20
I 20
0
I 40
I 60 TIME,
I 60
I 100
days
Fig. 1. Stability of the arsenate, dichromate and dimolybdate electrolytes in the presence of moisture at 25°C.
The resulting solid electrolytes were ground to a fine powder, and then investigated directly or after periods of annealing at various temperatures. X-ray powder diffraction patterns were obtained with CuK, radiation at 30 kV and 20 mA (JEDL diffractiometer, Model JDX43s). Differential thermal analysis studies were carried out using a differential calorimeter (Perkin Elmer, Model DSC IB). For conductance measurements, samples were pressed at about 2500 kg cm-’ into pellets with a geometric surface area of 1.26cm2. These pellets were sandwiched between two electrodes formed by mixing silver powder with the sample in a weight ratio of 1:2 and pressing into pellets also at 2500 kgcmm2. The resistance of the resulting symmetrical cells was then measured using an ac bridge (Wayne Kerr) operating at 1.592 kHz. For temperatures above ambient, the cells were maintained at the desired temperature with an accuracy of + 0.2 K using a recirculating thcrmostatic bath. Low temperature resistance measurements were performed by introducing the cell into a cryostat in which a stable temperature gradient was maintained from - 100 to 250K. Temperatures were measured with an accuracy of f 2 K by measuring the voltage between thermocouple junctions strapped to the cells and similar junctions maintained in a triple-point cell. RESULTS Silver
iodide-silver
AND
arsenate
DISCUSSION
system
The investigation of the system silver iodide-silver arsenate was considered of priority since the intermediate compound, at 80mole “/, (m/o) Agl, has a number of important properties. Among these, there is the excellent stability at room temperature, even in the presence of high moisture content. Figure 1 shows the time dependence of the resistance of a cell of the type [email protected],AsO,iAg, kept open in an environment
The resistance, after an initial increase of loo/,, remains practically constant for several months. Another interesting property of the A&Ag,AsO, system is indicated in Fig. 2, where conductivity-composition plots are shown at 25°C. Curve A refers to results previously reported[l], related to samples obtained by spontaneous cooling to room temperature of the respective melted mixtures. Curve B, on the other hand, refers to samples quenched in liquid nitrogen. The maxima appear at the same composition, ie 80m/o AgI, but the conductivity of the quenched samples is significantly larger. The evaluation of the conductivity at various temperatures, in the particular case of quenched samples of SOm/o AgI, is shown in Fig. 3, where the resistance of syrnmetrical cells of type I is plotted as a function of
(1)
of about 70% humidity.
100
60
60
40
A61 m/b Fig. 2. Composition dependence of the specific conductivity in the system Agl-Ag,AsO, at 25°C for samples quenched under a nitrogen stream(A) and in liquid 1. ,_>
Modified AgI solid electrolytes containing AsO:-, Cr,O:-
and Mo,Of-
53
anions
until a constant resistance value was reached. The conductivity increases monotonically up to about lOO”C, then falls and remains at a lower level in the following cooling run. These results indicated that near 100°C an irreversible decomposition into poorly conductive compounds takes place. Similar behaviour has been reported by Takahashi et a&4], for the silver iodide-silver phosphate system, where an intermediate compound at 80m/o AgI decomposes at 79°C according to the reaction
15
Ag,I,P04+
Fig. 3. Resistance
of cell [I] vs electrolyte various temperatures.
thickness
at
sample thickness. The contribution of the electrodeelectrolyte interfacial resistance, which in cells having a complete solid state configuration may be significant, has been obtained by extrapolation to the origin. Taking into account this contribution (1.15R) the conductivity of the quenched electrolyte is found to 0.012 (n cm-’ at 20°C. A possible explanation for the significantly lower conductance of the slowly cooled material (Table 1) is that the latter contains a higher proportion of non-conductive phase or phases. The temperature dependence of the conducti-
vity of the 4AgI.Ag,AsO, electrolyte was examined and the result for samples quenched in liquid nitrogen is shown in Fig. 4. The points reported were obtained by maintaining cells of type I at each temperature _
-3
I 2.6
I 2s
T, ‘C
50
80
100
1
I 3.0
I
2 AgI + Ag,I,PO,.
(2)
In this case also the decomposition produces poorly conductive compounds, one of which is silver iodide. On the other hand in an Arrhenius plot over a restricted temperature range (eg from 20 to 90°C) the points obtained on the heating process were coincident with those of the cooling process, while samples kept at 110°C showed a decay in conductivity of half an order of magnitude within 15 h. With the aim of clarifying the solid state reactions connected with the observed conductometric behaviour and defining the range of stability of the electrolyte, a systematic study, based on X-rays on differential thermal analyses, was undertaken on various samples at different compositions and submitted to different thermal treatments. (a) XOm/oAgI. The X-ray powder patterns of a sample obtained with the spontaneous cooling from the melt (method b) show a number of characteristic diffractions which are different from those of the starting materials. These diffractions, reported in Table 2, may be then related to a phase, at SOm/o AgI, which will be here indicated as phase 1. It has to be noted, however, that the spectra show a limited number of peaks over a rather broad base line thus indicating a limited degree of crystallinity for the material. The degree of crystallinity is further reduced when the samples are quenched rapidly as discussed below. A sample of the electrolyte, after being annealed at 120°C for ten days, was submitted to X-ray analysis. This spectrum showed the peaks of silver iodide
1 3.2
I
0
20
I
I
3.4
I 3.6
-10
I
I 3.8
1 OS/T, ‘K-’ Fig.
4. Temperature
dependence of the specific conductivity of 0.85 Ag14.15 Ag,Cr*O, electrolytes.
the
0.8 AgI-0.2 Ag,AsO,
and
M. LAZZARI, B. SCROSATI AND C. A. VINCENT
54
Table 2. X-ray diffraction potider patterns in the system AgI-Ag,AsO, Silver iodide (y-phase) 28 23.7 39.2 46.3
IA” lbo’ 60 30
Phase 1 (80 m/o AgI) 20 24.7 30.8 32.8 36.3 37.7
I/10 55 50 100 50 45
Phase II (72 m/o Agl) 21 25.6 27.6 30.7 31.2 32.3 33.3 36.6 39.8 41.5
phase I -
x AgI + y phase II
phase II + z AgI + j phase TTI
Phase III (66 m/o Agl) 21 30.9 31.6 33.4 33.9 44.1
IA0 j9 :: 64 100 64 39 47 36
together with others different from those of phase T and therefore to be associated to another phase, here indicated as phase II. The diffraction powder patterns of the latter are reported in Table 2. This result supports the conductivity evidence that the electrolyte decomposes irreversibly at < 120°C. To further characterize the system, a fast quenched sample was annealed at 150°C for 15 days and then cooled to room temperature. The X-ray spectrum of this sample showed reflections and new patterns different either from those of phase I and from those of phase II. This new compound has been indicated as phase III and the related patterns are reported in Table 2. Phases I and II seem therefore to be unstable at 150°C and convert irreversibly to phase III. This conclusion was further confirmed by melting a mixture of AgI and Ag,AsO, (with SOm/o AgI) and cooling it very slowly (method a) to room temperature. The X-ray spectrum of the solidified product was identical to that of the sample annealed at 15O”C, thus indicating that the slow cooling procedure forms AgI and phase III at about lSo”C, which, at lower temperatures, do not transform into either I or II. This result therefore accounts for the observed decrease in conductivity of the 4AgI.Ag,AsO, electrolyte when the rate of cooling from the melt is decreased. On the basis of the above results, in the system Agl-Ag,AsO, the following two solid state reactions may be postulated (31
(CuK, radiation)
I/IO kl ;: 100 37
Silver Arsenate 20 29.1 32.7 35.9 53.9 56.1
IL
is’
100
55
20 30
This sample was then annealed at 120°C for 5 days and then cooled slowly to room temperature. The X-ray analysis showed that the annealing had caused the disappearance of phase I, while the patterns of phase II became stronger and those of phase III remained unchanged Finally a prolonged annealing at 170°C produced a material which contains phase III and AgI. On the basis of these results, it is thcrefore possible to attribute tentatively this composition to phase II. (d) 66 m/o AgI. The X-ray spectrum of a quenched sample showed reflections of Ag,AsO,, phase III and Agl. Annealing at 170°C resulted in the disappearance of Ag,AsO,. This composition may be therefore associated with phase III. The DTA spectrum, reported in Fig. 5, shows the unidentified peak at 21&22O”C, the peak of the eutectic at 245°C and a large peak at 3OO”C, which is the peritectic peak. (e) SOm/o AgI. The X-ray analysis for quenched samples gave appreciable reflections of Ag,AsO, only. For slowly cooled samples (method a) the peaks of phase III and of AgI also appeared. The DTA of quenched samples (see Fig. 5) gave only the peak of peritectic melting at 3oO”C, and a very small eutectic peak at 245°C. The above data were used to trace the possible phase diagram for the system, shown in Fig. 6. This phase diagram closely resembles that of the Agl-Ag,PO, system, investigated by Takahashi et al[4]_ Here again, the conductive phase, Ag,I,PO,, is formed by fast cooling from the melt. In such a
(4)
which are characterized by kinetics which become appreciable at temperatures higher than 100 and 150°C respectively. Due, it is believed, to the slowness of these transformations, attempts to detect them by differential thermal analysis were unsuccessful. The DTA spectrum of a quenched sample, reported in Fig. 5, does show a peak at 15O”C, probably ascribable to silver iodide (Ir- TVtransition) resulting from decomposition, a peak at 220°C which remains unexplained and a large peak at 250°C which may be associated to the melting of the eutectic (AgI and phase III). (b) 9Om/o AgI. The X-ray analyses of quenched samples showed reflections of silver iodide (very intense) and of phase I. (c) 72m/o AgI. The X-my spectrum of a quenched sample showed patterns of all phases I, II and III.
I
I
I
I
50
150
250
350
Fig.
T , ‘C
5. Differential thermal analysis in GgI-Ag,AsO,.
the
system
Modified AgI solid electrolytes containing AsO:-,
-’ ,
0
; I
0
Cr,O:-
and MO@-
anions
55
I 1,
II
,
Ill
,
,
,
,
,
100
so
m/o Ag,AsO, Fig. 6. Phase diagram of the system A&Ag,AsO,.
phase
Fig. 8. Resistance of quenched samples of O.SAgla.2 Ag,AsO, and 0.85 AgI-0.15 Ag,Cr,O, at low temperatures.The open and filled circles refer to two independent measurements.
Fig. 7. X-ray diffraction patterns of AgI-Ag,As04 (80 m/o AgI) samples quenched under a nitrogen stream(A), in solid-liquid chloroform(B) and in liquid nitrogen(C). Curve D refers to a quenched sample after annealing at 120°C.
and produces a metastable highly conducting solid structure. One might indeed consider specific interactions between the two parent components AgI and Ag,As04 in the liquid state. Such a configuration might then be quenched in the solid, which could therefore be regarded as a supercooled liquid. X-ray patterns of 80m/o material quenched at different rates are shown in Fig. 7. The vitreous character of the sample formed by spraying droplets of molten electrolyte into a liquid nitrogen column is illustrated by the complete absence of diffraction peaks (see pattern C). This glassy nature of the material would support the idea of a supercooled liquid, and it would be very interesting to investigate the silver ion conductance at different compositions of melt. However we have at this time no firm evidence that Ag,I,AsO, is a single phase and confirmation would require detailed structural studies. The variation of resistance with temperature of an 80 m/o quenched sample at low temperatures is shown in Fig. 8. It is interesting to note that there is no evidence of a phase transition in this temperature range, such as that reported by Owens and Argue[6] for RbAg,I, at - 155°C. On the other hand there is a change in slope at about - 150°C. This behaviour is exactly analogous to that found by Ingram et al (personal communication) for LiSi04 systems at much higher temperatures, and shown by these workers to be frequency dependent. At high frequencies or low temperatures a parallel capacitatively coupled conductive admittance, perhaps due to grain surface conduction or to interparticle impedance, becomes significant compared with the normal bulk conductance and hence lowers the overall impedance of the system.
diagram, on cooling below the eutectic point the material should solidify with the formation of AgI and a non-conductive Ag,I,PO, phase. On further cooling to the electrolyte decomposition point, 79°C (lOO’C), it would then be necessary for these phases to combine to form the single conducting phase Ag,I,PO, (Ag,I.+AsO,). As pointed out by Owens[S], it is difficult to accept the formation of a single phase compound by this sequence of reactions. It is perhaps more reasonable to suppose that the quenching procedure minimizes the demixing at the eutectic point
A
B
C
20’
I
I
I
I
I
I
40”
30’ 28
(Cu Ka)
M. LAZZARI, B. SCRCF.ATI AND C. A. VINCENT
56
1
I
Fig. 10. X-ray diffraction patterns of AgI-Ag,Cr,O, (85&o AgI) samples quenched under a nitrogen
stream(A) and in liquid nitrogen (B]. Curve C refers to a quenched sample after annealing at 70°C.
Fig. 9. Compositnm
dependence of the specific conductivity in the systems AgI-Ag,Cr,O, and AgI-Ag,Mo30, at 25°C.
Silver iodide-silver
dichromate
system
The conductivity composition curve for this system, for samples obtained by cooling from the melt by method (b) is given in Fig. 9 and shows a maximum conductance of 0.017 fi2- ‘cm-’ at approx. 85 m/o of AgI. Provided that they were maintained in a completely dry atmosphere, the resistance of cells containing this electrolyte at room temperature was found to remain virtually constant for a long period (Fig. 1). On the other hand, cells kept in an environment of circa 70% humidity suffered a progressive increase in resistance, suggesting a water-catalysed decomposition or phase transformation of even more serious proportions than that associated with RbAg,I,[7]. Measurement of the conductivity as a function of temperature also indicated that the material spontaneously underwent a phase transition or reaction at relatively low temperatures. In Fig.4 the values of log CTreported are the relatively stationary values reached after equilibrating the cell for 8-12 h at each temperature. The fall in conductance at temperatures above 40°C should be compared with the behaviour in the arsenate system at temperatures above 100°C. At temperatures down to liquid nitrogen this material behaves in the same way as 80m/o Agl/Ag,AsO, showing a similar kink in the log 0 vs l/T plot and no phase transition (Fig. 8). The electrolyte was further investigated by X-rays and differential thermal analysis. The X-ray diffraction pattern of slowly cooled Urn/o material (Fig. 10) could not be resolved except for two weak AgI reflections. Electrolyte quenched by method (c) was even more amorphous in nature. However the pattern derived from a sample aged in a (moist) laboratory atmosphere for several days showed well resolved Agl reflections together with other peaks associated with one or more unknown crystalline phases. An identical pattern was found for a sample annealed at 70°C for
3 days, thus confirming that the water catalysed and temperature activated decomposition processes were the same transformation. In the DTA results (Fig. ll), curve A refers to the heating of a mechanical mixture of 85 m/o AgI/Ag,CrzO, and shows a peak at about 1SO”C, corresponding to the Agl fl---) CItransition and a double peak at about 165°C. In the subsequent cooling trace, B, the solidification peak is first seen, followed by a just discernible peak for AgI or-p transition, suggesting that the silver iodide has been mainly combined. The heating scan of a sample of newly formed electrolyte is shown on trace C while that of a sample annealed at 70°C is given on trace D. Thus it would appear that the conducting phase formed in this system is again thermodynamically unstable and tends to revert to AgI and other poorly conductive phases. The decomposition reaction is slow up to circa 40°C but is catalysed by water. Silver iodidesilver
dimolybdate
system
While the two systems discussed above have great similarities in their behaviour, the properties of the
:_rr “-1 I 50
I
I
100
150
I
200
1 . ‘C Fig. 11. Differential thermal analysis of AgI-Ag,Cr,O, (85 m/o AgI) samples under various conditions. Curves A and 3 refer to the heating and the cooling of a mechanical
mixture, respectively. Curve C is related to a newly prepared sample and curve D to a sample annealed at 70°C.
Modified Agl solid electrolytes containing AsO:-, Cr,O:-
;.4 Fig. 12. Temperature
2.6
a6
2.6
32
57
anions
a4
10%/T , OK-’ dependence of the specific conductivity of the 0.75AgI-0.25Ag,Mo,O, electrolyte.
silver iodide-silver dimolybdate electrolyte show some interesting contrasts. The conductivity--composition plot (Fig. 9) shows a maximum at 75 m/o AgI, but the value of the maximum conductance. at room temperature, viz 0.008 a- 'cm- ‘, is lower than that commonly found in electrolytes of this type. Further, this value is rather insensitive to the cooling procedure used to form the electrolyte from the melt. Like the arsenate, the dimolybdate electrolyte is stable in moisture: the cons&t resistance of a cell containing this electrolyte over a period of months is shown in Fig. 1. The complex temperature variation of the resistance of the 75 m/o electrolyte is perhaps the most interesting feature of this material. In Fig. 12 is shown the log 0 vs l/T plot for two cycles between 20 and 120°C for slowly cooled samples (method a). A reversible change of slope from a high temperature of 5.0 kcal mole- ’ to a low temperature value of 10.8 kcal mole- 1 occurs at approx. 56°C indicating a “subtle structural transition” of the type discovered by Geller and Owens[S] in (C5H,NH)Ag,16. In the latter case a very small heat effect was detected due to the transition; here we have not seen any discontinuity in DTA traces over this temperature range. The most obvious difference in the X-ray diffraction spectra between the dimolybdate electrolyte and the other two electrolytes of the present study is the presence of well defined diffraction lines in the fast quenched material (Fig. 13). It may be noted that the traces for the quenched and slowly cooled samples are very similar, and one may therefore conclude that the 75 m/o silver iodidc+silver dimolybdate does not form a vitreous
and Mo,O:-
trolyte is stable in the presence of moisture while the dichromate is rapidly decomposed. The arsenate is stable to nearly 100°C while the dichromate is irreversibly transformed to a less conducting material at around 40°C. For the 8Om/o AgI/Ag,AsO, and 85m/o A@/ Ag,Cr,O, systems we suggest that rapid cooling from the melt retains the material in the form of a supercooled liquid. The decomposition reaction then has the form of a devitrification followed by separation into stable crystalline phases. It may be noted the
*;liJ-\jrl-
phase.
CONCLUSIONS
The first two systems studied in this investigation are character&d by conducting phases which although thermodynamically unstable, remain unchanged in a metastable state with varying degrees of tolerance to ambient conditions. The arsenate elec-
20’
306
40” 20
(Cu Ku)
Fig. 13. X-ray diffraction patterns of AgI-AgzMo,O, (75m/o AgI) samples quenched under a nitrogen stream (A) and in liquid nitrogen(B). Curve C refers to a quenched sample after annealing at 180°C.
58
M. LAZZARI, B. SCRCBATIAND C. A. VINCENT
phases have already been suggested by Kunze[9] for the AgI/Ag,SeO, system. Thermodynamically unstable phases have also been reported for AgI/Ag,MoO, aid AgI/Ag,CrO,[lO]. In the case of the dichromate electrolyte the onset of the vitreous character occurs at high& quenching temperatures-a phenomenon which may be related to the much lower temperature at which the decomposition reaction becomes appreciable. Further study of these two systems is necessary, especially on the properties of the binary liquid phases and on the types of interaction which may take place between the various components in these situations where the formation of true compounds remains doubtful. It is unlikely that the transport mechanism for silver ions is identical to that in the MAgJ, class of compounds. Unfortunately structural studies of these oxyacid systems are likely to prove difficult because of their amorphous nature. It may be of great interest to identify the precise role of water in the decomposition of the dichromate electrolyte. The role of water in the decomposition of RbAg,I,[7], which implies a chemical reaction, is not well understood either. As pointed out above, the properties of the electrolyte found in the AgI/Ag,Mo,O, deviate markedly from the common pattern shown by the majority of silver iodide-silver oxyacid salt systems. In particular the structure remains crystalline even with fast quenching. Taking into account the relatively low value of the conductance and the peculiar behaviour of the temperature dependence of the conductivity, it might be suggested that this system is located near some limit of the stability range of this class of electrolyte. As in another class of silver iodide modified vitreous
electrolytes, group[lI],
the silver iodide-silver the size of the minority
alkylammonium anion may play
part in stabilising disordered structures characterised by networks of channels through which
an important
the silver ion can move.
Further
work
is in progress
in our laboratories to investigate whether there is a critical size for the minority substituting ion and to establish a phase diagram for this system. Acknowledgements-We are grateful to D. M. Finlayson and W. G. Picken for their help with the low temperature
conductance measurements, and we thank the Science Research Council and the Consiglio Nazionale delle Riccrche (CNR-Roma) for financial support.
REFERENCES 1. B. Scrosati, F. Papaleo, G. Pistoia and M. Lazzari, J. el~ctrochem. sot. 122. 339 (1975). J. appl. Electro2. B. Scrosati, A. Ricci anh M. iazz&, them. in press. 3. M. Lazzari. A. Ricci. B. Rivolta and B. Scrosati. submitted for publication. 4. T. Takahashi. S. Ikeda and 0. Yamamoto, J. rlectrothem. sot. 115, 477 (1972). 5. B. B. Owens, Fasr Ion Transport In Solids (Edited by W. van Goal), p. 593. North Holland, Amsterdam (1973). 6. B. B. Owens and G. R. Argue, Science 157. 308 (1967). 7. L. E. Top01 and B. B. Owens, J. phys. Chem. 72, 2106 (1968). 8. S. Geller and B. B. Owens. Phvsics Chem. Solids 33. 1241 (1972). (Edited by W. 9. D. Kunze, Fast Ion Transport in Soiids van Go011 t). 405. North Holland. Amsterdam 11973). 10. G. Chiodkfii, A. Magistris and A. Schiraldi, Eiec&chim. Acta 19, 655 (1974). J I. B. B. Owens, Advances in Electrochemistry and Electrochemical Engineering, Vol. 8, Chap. 1. Wiley, New York (1971).