SOUP STATE ELSEMER
Solid State lonics 86-88 ( 1996) 459-462
An investigation of superionic conduction in the mixed system Ag 1 _XCu,I-Ag,O-P,O, (0.05 5 x 5 0.25) M.K.P. Seydei, S. Austin Suthanthiraraj” Department
of Energy, University of Madras, Guindy Campus, Madras (500 025, India
with the preparation and characterization of different compositions in the mixed system 80% (2Ag,O . PZ05) (0.05 Ix 5 0.25). Solid samples were synthesized by quenching the appropriate molten mixtures and characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), complex impedance and transport number techniques. The XRD data have indicated the formation of glassy phases mixed with AgI or other
(Ag , _ACuJ)-20%
polycrystalline phases. From the DSC measurements it is clear that a glass transition temperature around 327 K is exhibited by these specimens. The complex impedance studies carried out in the frequency range 65.5 kHz-1 Hz and transport number analysis suggest that the best conducting composition, namely 80% (Ag, ,Cu,, ,I)-20% silver ionic conductivity of 8.64 X IO-’ S cm-’ at room temperature. Keywords:
Superionic conduction; Mixed systems; Glass transition temperature
1. Introduction It is well-known that silver oxysalt matrices such as Ag,O-P,O,, Ag,O-Moo,, Ag,O-B,O, and Ag,O-V,O, in combination with Agl would offer a group of superionic glasses exhibiting Ag+ ionic conductivities of the order of 10-5-10-2 S cm-’ at 298 K [l-4]. Schmidt et al.  examined the mixed system Agl-Cul and found that solid solutions containing low copper content (i.e., 5 25 mol% Cul) would behave like a pure silver ionic conductor lying in a zone in which the crystal structure was that of Agl, favourable for the movement of silver ions. Hence, the present work has been undertaken with the ultimate aim of arriving at a new class of solid electrolytes for ambient conditions by employing *Corresponding author. 0167-2738/96/$15.00 Copyright 01996 Elsevier Science PII SO167-2738(96)00175-O
appropriate AgI-Cul solid solutions as dopant salts into a matrix of Ag,O-P,O,. Earlier investigations on the reciprocal role of Cu+ ions in the Ag,OB,O, matrix carried out in our laboratory have indicated the feasibility of obtaining high conductivity superionic solids in the mixed system CulAg,O-B,O, . The present study deals with the synthesis and characterization of a variety of compositions in the new system 80% (Ag , _,CuJ)-20% (2Ag,O * P20s) where 0.05 5 x 5 0.25.
2. Experimental 2.1. Preparation
Initially, polycrystalline samples of Ag ,_,Cu,I (x = 0.05, 0.1, 0.15, 0.2 and 0.25‘) were prepared B.V.All rights reserved
Seydei. S.A. Suthunthiraruj
I Solid St&e Ionics
using Analar grade AgNO,, CU(NO,)~. 2H,O and KI as starting materials. To start with, aqueous solutions of AgNO, and Cu(NO), .2H,O taken in appropriate amounts were mixed together. To this, an aqueous solution of KI was added in excess and the precipitate thus obtained was washed several times with double-distilled water and then with acetone. It was then filtered and dried in the dark at 373 K for nearly 3 h to obtain Ag, mLCu,I solid solutions for further studies. Commercially available Analar grade reagents of NH,H,PO, and Ag,O were employed as raw materials together with Ag,_,CuxI (x =0.05, 0.1, 0.15, 0.2 and 0.25) specimens, for the preparation of compositions of various the system 80% (Ag , -.,Cu,I)-20% (2Ag,O . P20,). NH,H,PO, was dried at 393 K just prior to use in view of the fact that NH,H,PO, on heating produces P?O, according to the reaction, 2NH,H2P0,“il;(P,0,
+ 3H20 + 3NH,
PzO, thus obtained was kept in a desiccator. Appropriate amounts of P20s, Ag,O and Ag,_,Cu,I were weighed and ground together in an agate mortar for thorough mixing of the ingredients. The above mixture was melted in a Pyrex glass crucible at 773 K before being quenched to low temperature with the aid of an ice bath. 2.2. Powder X-ray diffraction (XRD) and differential scanning calorimetric (DSC) studies The finely powdered samples of various compositions in the mixed system 80% (Ag, _,Cu.,I)20% (2Ag,O*P,O,) for 0.05~~~0.25 were analysed using a Philips X-ray generator unit model PWll40 with CuK, radiation (A= 1.5418 A) in order to identify and confirm the phases present in these samples at room temperature. DSC measurements were carried out on all samples using a Perkin Elmer model DSC7 differential scanning calorimeter along with a Perkin Elmer 3700 data station over the temperature range 3 13-673 K at a heating rate of 10 K/min with platinum as the reference material and aluminium pans as sample containers.
2.3. Complex impedance analysis and ionic transport number measurements The prepared samples were ground into fine powders and pressed with electrodes on either side under a pelletizing pressure of 4000 kg cm-’ to form circular pellets of 12 mm diameter. The electrodes consisted of silver metal powder mixed with the sample in the weight ratio 2:l in order to reduce interfacial resistance and also to maximise the interfacial surface area . Electrical conductivity studies were carried out on all the pellet samples using a Solartron model 1254 four channel frequency response analyser and a Solar&on model 1286 electrochemical interface coupled with a BBC model B + microcomputer over the frequency range 65.5 kHz- 1 Hz. The experimental data obtained in the form of complex impedance plots i.e., plots of the imaginary part, Z”, against the real part Z of the impedance, Z were used for the estimation of the bulk resistance of the different samples. Ionic transport number measurements were made by employing e.m.f. technique. For this purpose, fine powders of each solid sample were pressed together with electrodes on either side under the pelletizing pressure of 4000 kg cm-‘. The negative electrode consisted of a mixture of silver powder and sample material in the weight ratio 2:l whereas the positive electrode was made from reagent grade iodine. The electrochemical cells thus fabricated had the configuration (-) Ag sample (2: l)/sample/I, (+). Silver ionic transport number in the case of individual composition was evaluated from the measured value of the open circuit voltage at room temperature (298 K).
3. Results and discussion 3.1. Structural
and thermal analytical
In Table 1 are presented the results of powder X-ray diffraction analysis carried out at room temperature on the various compositions of the system 80% (Ag, -.,Cu,I)-20% (2Ag,0.P205). Table 1 shows the glassy nature of three compositions corresponding to x =0.05, 0.2 and 0.25 respectively. For x=0.1 there are only three diffraction peaks which
M.K.P. Seydei, S.A. Suthanrhiraraj I Solid State Ionics 86-88 Table 1 Room-temperature powder XRD data for the mixed system 80% (Ag,_,Cu,I)-20% (2Ag,O.P,0,) where x=0.05, 0.1, 0.15, 0.2 and 0.25 respectively x =0.05
Broad and featureless XRD pattern (glassy system)
100 75 43
3.61 2.24 1.91
100 91 66
* Peaks corresponding
Broad and featureless XRD pattern (glassy system)
Table 2 DSC results (2Ag,O.P,0,)
obtained for the system 80% (Ag, _rCuJ)-20% where x=0.05, 0.1, 0.15, 0.2 and 0.25 respectively
Glass transition temperature, Lrs
(R) 0.05 0. I 0.15 0.2 0.25
325 330 327 326 332
421 _ _
may be attributed to yAg1. It is therefore expected that in addition to AgI another glassy phase may also be present in this particular specimen. In the case of the remaining composition corresponding to x = 0.15, the observed peaks at 3.61, 2.24 and 1.91 A are found to be new thus suggesting the formation of another solid phase mixed with a glassy phase. Hence it is clear that it is feasible to obtain glassy phases mixed with AgI or other polycrystalline products as a result of melting Ag , _,Cu,I, Ag,O and P?05 together. In fact, the formation of a crystalline phase consisting of a mixture of y and P-AgI was reported in AgI-rich samples of the ternary glassy system AgI-Ag,O-P,O, . From the above discussion it is obvious that crystalline AgI may be present in the form of dendrites in the case of a typical composition corresponding to x=0.1 in the mixed system 80% (Ag , _ ,&I)-20% ( 2Agz0. P705). The DSC data obtained for the five different compositions of the system 80% (Ag,_1Cu,I)-20% (2Ag,O*P,O,) where x=0.05, 0.1, 0.15, 0.2 and 0.25 respectively are indicated in Table 2. Table 2 shows that a glass transition occurs at 325, 330, 327, 326 and 332 K respectively in these compositions. These glass transition temperature (T,) values are comparable to those reported for silver phosphate superionic glasses [9,10]. Typically, 7’, values were reported to be 321, 332 and 325 K for the three respective compositions viz., 60AgI-28Ag,O12P,O,, 57AgI-28.6Ag,O-14.3P20, and 65AgIG 23.3Agz0-1 1.7P,O,  while that of yet another glassy system 85AgI-lSAg,P,O, was found to be 322 K [l 11.
Table 2 also shows that an endothermic peak appears at 421 K only in the case of the composition corresponding to x =O. 1. This value compares well with the characteristic p to (Y phase transition temperature observed in the case of AgI (-420 K). In other words, the appearance of an endothermic peak at 421 K suggests that AgI may also be present in the multiphase composition corresponding to x= 0.1. These results are found to be in good agreement with the structural data in view of the fact that the formation AgI in the above specimen was revealed during the X-ray analysis itself. 3.2. Electrical number data
and ionic transport
Table 3 presents the room temperature electrical conductivity data obtained for various compositions of the mixed system 80% (Ag, _,Cu,I)-20% (2Ag,O*P,O,) (0.051x50.25). In Table 3 it is seen that the various compositions of this system exhibit electrical conductivities of the order of lo-” S cm * at room temperature and that a maximum conductivity of 8.64X 10m3 S cm-’ is possessed by the composition 80% (Ag,, ,Cu, ,I)-20% (2Ag,O* Table 3 Room-temperature electrical conductivity data for the system 80% (Ag,_,Cu,I)-20% (2Ag,O. P?O,) where x=:0.05, 0.1, 0.15, 0.2 and 0.25 respectively Composition (.r)
Room-temperature conductivity car (S cm-‘)
0.05 0. i 0.15 0.2 0.25
2.42x 8.64~ 3.33r: 2.86r: 3.82>:
IO-’ 10 -’ 10-I IO ’ IO ’
M.K.P. Seydei, S.A. Suthanthiraraj
I Solid Store Ionicv 86-88
Table 4 Results of transport number measurements on the system 80% (Ag,_,Cu,I)-20% (2AgZ0.P,0,) where x=0.05, 0.1. 0.15 and 0.2 respectively Composition (x)
Silver ionic transport number (rAg + 1
0.05 0.1 0.15 0.2 0.25
0.98 0.97 0.96 0.97 0.97
P,Os). Ag’ conducting glasses in the ternary system AgI-Ag,O-P,O, exhibit electrical conductivities of the order of 10m3- 1O-* S cm - ’ at room temperature [l]. The fact that the present conductivity values are large compared to that of AgI or Ag, _.ICu.,I indicates that new highly conducting phases may be formed in these compositions [ 121. The present DSC results appear to suggest that all the specimens in the mixed system 80% (Ag, _,Cu,I)-20% (2Ag,O* P20,) exhibit very low T, values. For these reasons, Arrhenius plots of electrical conductivity data could not be drawn. The results of transport number measurements carried out on the various compositions of the mixed system 80% (Ag, _ ,Cu,I)-20% (2Ag,O*P,O,) (x= 0.05, 0.1, 0.15, 0.2 and 0.25 respectively) at room temperature are summarized in Table 4. Table 4 reveals that the transport number of Ag’ ions in these compositions is nearly unity, as in the case of the ternary glassy system AgI-Ag,O-P,O, . This means that the observed high conductivity values are mainly due to Agf ions.
4. Conclusion The present investigation concerning the preparation and characterization of a variety of compositions
in the mixed system 80% (Ag, _,Cu,I)-20% (2Ag,O.P*0,) has revealed the formation of multiphase superionic solids possessing silver ionic conductivities of the order of lo-’ S cm-’ at room temperature. A glass transition temperature around 327 K is also exhibited by these composite materials.
Acknowledgments The authors are grateful to Dr. R.J. Neat, Applied Electrochemistry Department, Harwell Laboratory, UK for his kind help in collecting DSC and impedance data.
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