Some properties of mixed electronic-ionic glassy conductors of the AgIAg2OV2O5P2O5 system

.,__ E!l!lii

&a

SOLID STATE

d

__

ELSEYIER

Solid State Ionics 92 (1996)

Some properties

M. Wasiucionek, Institute

IONICS

155-160

of mixed electronic-ionic AgI-Ag,O-V,O,-P,O, J.E. Garbarczyk”,

glassy conductors system

B. Wngtrzewski,

P. Machowski,

of the

W. Jakubowski

of Physics, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland Received

3 December

1995; accepted

18 April 1996

Abstract The mixed electronic-ionic conducting glasses of the AgI-Ag,O-V,O,-P,O, system have been prepared. XRD and DTA analyses have confirmed the amorphous nature of these glasses. DTA analysis was used to determine glass transition and recrystallization temperatures. EPR measurements showed that at the V,O,-rich end of the compositions, the number of V4+ ions, essential for electronic conduction, is considerable. Electrical properties have been studied using complex immittance spectroscopy. Numerical analysis of the admittance spectra showed that the ionic transference number at the AgI-rich end of the compositions is close to one and for the compositions with high V,O, content it is considerably lower. Keywords: Glass; Electronic-ionic

transition;

Silver iodide; Silver phosphate

1. Introduction The rapidly growing interest in mixed electronicionic conductors is stimulated mainly by numerous applications of these materials e.g. as cathodes in electrochemical energy sources [ 11 or smart windows [2]. The attractiveness of these compounds, from the point of view of basic research, lies in the fact that they exhibit properties of both solid electrolytes (ionic component) and of semiconductors (electronic component). It seems that the central problem in the area is to separate the motion of ions and electrons [3,4]. It is also important to find possible correlations between electronic and ionic transport mechanisms [5]. Until now most experimental work on mixed

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electronic-ionic conductors has been done on systems in which one of the components (either ionic or electronic) dominated. Much less work has been devoted to systems in which both electronic and ionic components have comparable effect on the total conductivity. It was our aim to prepare a series of samples of conductivity going from purely ionic to purely electronic without dramatic changes of structure. Such a possibility is offered by glasses which usually can be obtained in quite wide ranges of chemical compositions. From among many possibilities we have chosen the AgI-Ag,O-V,O,--P,O, system. The main arguments for that choice are as follows: V,O, is well known as a glass forming compound exhibiting semiconducting properties, Ag,O is a glass modifier and its content can influence the number of V4+ ions important for electronic conduction, and the number of Ag+ ions, important for ionic conduction. P205 acts as a

156

M. Wasiucionek

et al. I Solid State Ionics 92 (1996) 155-160

network glass former strengthening the moderate glass forming properties of V,O,. AgI is an important dopant increasing the number of mobile Ag+ ions.

2. Experimental of pre-dried V,O,, Appropriate amounts NH,H,PO,, AgNO, and AgI (all reagent grade POCh-Polish Chemicals) were thoroughly mixed in a mortar. Alumina crucibles filled with the powders were placed in an electric furnace, first at 5OO”C, and then heated at 900°C for 5-10 min. The molten mixture was rapidly poured out between two stainless-steel plates. Resulting samples had an average thickness of about 0.5 mm. Several series of samples were prepared. Results of XRD and DTA analysis have generally confirmed glass forming regions published by Minami et al. [6] for ternary AgIAg,O-V,O, and AgI-Ag,O-P,O, systems. The appearance of the prepared glasses varied from black (for high contents of V,O,) to transparent reddish (for AgI-rich compositions). In order to control the amorphous nature of prepared samples and/or to find and identify possible crystalline phases, XRD analysis was done. XRD showed that only compositions lying at the borders or outside the glass formation region contained traces of crystalline phases. Additional confirmation of non-crystallinity of glasses within the glass formation region was given by DTA scans (taken at MOM derivatograph D-15OOQ apparatus in the temperature range 20500°C at S”C/min). The content of V4+ ions was monitored by EPR (at frequency 9.375 MHz and magnetic field of 0.33 T). Measurements of electrical properties were carried out on a fully computerised Solar&on 1260 system. The frequency range extended from 100 mHz-10 MHz, the a.c. signal amplitude was 10 mV and the temperature range 20-150°C. The ion-blocking aluminium electrodes of 5 mm diameter were evaporated onto the two opposite sides of the samples. The numerical analysis of experimental immittance spectra was based on proper electrical equivalent circuits. The non-linear least square method was used to determine the fit parameters of the circuits.

3. Results and discussion For all samples under study, two DTA runs were carried out. In the first one, some features common to all samples could be observed, namely an endothermic shift of a base line followed by an exothermic peak (Fig. 1). Those features represent, correspondingly, glass transition and re-crystallisation. For AgI-rich glasses an endothermic peak due to melting of a recrystallized phase was also observed in the studied range of temperatures. The glass transition (T,) and recrystallization (T,) temperatures are listed in Table 1. After the first run, samples were cooled down to room temperature inside the furnace and then the DTA run repeated. The second trace did not include either glass transition nor recrystallization features, It is easily under-

k i /.....:‘ -6

I

TC

1

100

0

200

300

400

500

t

lo

T /“C Fig.

1.

DTA

trace

of

(AgI)o.o,(Ag&%.,,(v,o,),

the

glass

with

composition

&P@5)o.,v

Table 1 Temperatures of glass transition (T,) and recrystallisation selected glasses of the AgI-Ag,O-V,O,-P,O, system

(T,) for

Symbol

Content (mole ratio) AgI

Ag,O

V,O,

P,O,

(“C)

(“C)

O/18/70/12 O/20/64/16 4/16/64/16 O/30/50/20 30/45/25/O 50/25/25/O

0.00 0.00 0.04 0.00 0.30 0.50

0.18 0.20 0.16 0.30 0.45 0.25

0.70 0.64 0.64 0.50 0.25 0.25

0.12 0.16 0.16 0.20 0.00 0.00

250 280 275 216 103 77

300 340 325 244 >140 115

T,

Tc

M. Wasiucionek

et al. / Solid State Ionics 92 (1996) 155-160

standable if one takes into account that the sample cooled down slowly after the first run is no longer amorphous but polycrystalline. The important factor influencing the electronic component of electrical conductivity of glasses based on V,O, is the concentration of V4+ ions, responsible for a hopping of electrons between V4’ and V5’ centres [7]. The presence of those ions can be monitored e.g. by the EPR technique. The electronic configuration of the V 5+ ion (3~~) is similar to that of noble gases and therefore the resultant electronic spin of such an ion is zero. On the other hand V4+ ions have the electronic structure 3p63d’ and their magnetic moments can interact with an electromagnetic field in the microwave range. As a consequence the intensity of the EPR resonance lines depends only on the amount of V4+ ions. For glasses under study, it has been found that for V,O,-rich samples the resonance lines are clearly visible (Fig. 2) but as the content of V,O, decreases and that of Ag,O increases the resonance lines become stronger. For low contents of V,O, the lines are hardly visible or even absent. The tendency arises from the fact that at high content of V,O, the most of the vanadium ions are in the V5+ state of oxidation. Upon addition of Ag,O the number of V 4+ ions increases compensating the decrease of the total number of vanadium ions. Finally at low content of V,O,, the overall

Fig. 2. EPR spectrum of the glass (Ag,O),,,(V,O,),,,(P,O,),,,.

157

number of vanadium ions is too low to produce a strong EPR line even if the V4+:V5’ ratio is high. The immittance spectra for most of the samples under study and at all temperatures exhibited some common features. In the impedance representation they consisted of two well separated semicircles and in the admittance one they consisted of a semicircle, offset from the origin, and a linear spur at high frequencies (Fig. 3). The proportions between positions of intercepts at the beginning and the end of admittance semicircles (Re Yr and Re Y2 respectively) have exhibited a well marked tendency: namely at high content of V,O, and ,absence of AgI the ratio Re Y, IRe Y2 was close to one and decreased, in a monotone way, to zero with decreasing V,O, and increasing AgI. Those spectra can be interpreted by means of two formally equivalent electrical circuits, known in the literature as the Voigt’s and the Maxwell’s circuits [8]. In the Voigt’s model (Fig. 4a), there are two parallel R, CPE subcircuits in series, one of which (Rb, CPE,) could be interpreted as the bulk contribution and the other (Ret, CPE,,) as the double layer contribution. The parameters of this circuit are: R, = the total electrical resistance of the sample, R,, = charge transfer resistance, CPE, = the constant phase element related to the geometrical capacitance C, of the sample, and CPE,, = the constant phase element related to the double layer capacitance C,,. The admittances of CPE elements are defined here in the following way: YCPEcgj = A,(iw)‘-” and YCPEcd,) = Ad,(iw)l-P, where A,, a, A,, and fi are the fit parameters. The Voigt’s model, generally, does not permit separation of the electronic and ionic components of the total electrical conductivity. In the Maxwell’s model (Fig. 4b) the interpretation of the parameters might be as follows: R,, Ri =. the resistances related to the electronic and ionic components, respectively, CPE, and CPE,, have the same meanings as in the Voigt”s circuit case, although values of the parameters for both models might be different. The assumption that the Al electrodes are practically entirely blocking for Ag+ ions seems to be fully supported. If it would not be fulfilled, the admittance semicircles would be offset (shifted) from the origin of the complex plane for all

158

M. Wasiucionek

et al. I Solid State Ionics 92 (1996) 155-160

-z

e t 2

0 0

A> 0.2

0.4

0.6

0.6

Re(Y)

[lo4

1

12

3.

Admittance

(Ag,O),,,(V,O,),,,(P,o,),,,,

05-

j

-. 1 .4

f-:

- -

“y-G , ____

0.5

0

15

1

RqY)

Sl

[lo-4

S]

-5 1 d

b

Fig.

.

1.2

spectra

for

glasses

of

different

compositions

Cc)(Ag~),,,,(A8,o),,,(v,O,),,,(P,O,),,,

samples under study (particularly for those of high concentration of Ag+ mobile ions). The mentioned offset is observed, however, only for the samples of low content of Ag and high content of V, responsible for the electronic conduction in this system (cf. Fig.

at ca. 65°C: (a) (Ag,O), ,s(V,O,),.,,,(P,O,),,,, and W (AgI),.,,(As,O),,,(V~O~)~.~~(P~O~)~.,~.

R,IR, - C&, k

@)

- 1

(1)

112

where

3). The values of the fit parameters for,botb models, for the samples under study are listed in Table 2 and Table 3. It is worthy to note that corresponding values of the parameters: A,, a, A,, and p in the Voigt’s model are close to those of Maxwell’s model. Furthermore for both circuits the ratio (Ag/ A,,) r (C&J E 10-3-10-4. The resistance characterizing both circuits are interrelated by the following transformation @I

2

(2) If (C, /C,,)*O, the general equations the much simpler relations

R&i Rb=

R,+R,

_

Re 1 i- (R,lRi)

(1) converge to

Ri

= 1 + (R,lR,)

(3)

M. Wasiucionek

et al. I Solid State Ionics 92 (1996) 155-160

Inverse transformations have the following forms: R, = R, + R,, and R, = Rb( 1+ R, IR,,). Taking into account a measuring error, all those relationships are perfectly fulfilled by the experimentally determined resistances (cf. corresponding columns in Table 2 and Table 3). Furthermore, from Eq. (3) and Eq. (4) and from the experimental data it is seen that for the conductor of the predominant electronic component (R, K Ri) the total resistance: R, 2 R, and R,, +Z R, (e.g. data for the sample O/18/70/ 12). On the other hand, for the conductor of the predominant ionic component (R, z=-Ri): R, s Ri and R,, zR, X-R, (e.g. data for the sample 30/45/25/O). It means that for those extreme cases (electronic or ionic conduction), there is practically no difference in values of glass conductivity determined by the two models. For the glasses of evidently mixed conduction (e.g. sample 18/12/56/14) the bulk resistance in the Voigt’s model is equal to the electronic and the ionic resistance connected in parallel in the Maxwell’s model. Possibility and eventual relevance of using the latter model to determine electronic and ionic transference numbers in the glasses of the AgIAg,O-V,O,-P,O, system is being studied by us.

Fig. 4. Two electrical equivalent circuits of the same impedance: (a) the Voigt’s circuit, (b) the Maxwell’s circuit.

and

__!lc_=

Rci=R,fRi

Table 2 The parameters

K

(4)

1 + (R,lR,)’

of the electrical

equivalent

circuit in the Voigt’s model for the samples under study at 55°C

Symbol

R, (W

A, (F/s”)

(Y

K, (W

A,, (F/s’)

O/18/70/12 O/20/64/16 4/16/64/16 18/12/56/14 24/6/56/14 30/45/25/O

9.4 x lo3 2.3 X lo4 2.3 X lo4 1.2x lo4 1.9x lo4 3.5x lo3

5.5x lo-” 2.6X lo-” 2.1 x10-l’ 1.ox1o-10 5.2X lo-” 4.7x lo-”

0.10 0.09 0.09 0.14 0.13 0.08

4.9x IO2 1.4x lo4 4.0x lo4 2.5 X lo4 1.1 x lo6 5.1 x lo6

7.2x 6.4~ 1.4x 3.6X 2.0x 1.9x

Table 3 The parameters Symbol

O/18/70/12 O/20/64/16 4/16/64/16 18/12/56/14 24/6/56/14 3Ol45l25fO

of the electrical R, w 1.0x 3.7x 6.3 X 3.7 x 1.0x 4.3 x

lo4 lo4 lo4 lo4 lo6 lo6

equivalent

159

circuit in the Maxwell’s

A, (F/s”)

(Y

8.0X lo-” 2.3~ 10-l’ 2.1 x10-” 9.7x 10-l’ 7.4x10-” 4.9x lo-”

0.14 0.08 0.09 0.13 0.15 0.08

10-7 lo-’ 1o-7 lo-’ lo-’ lo-’

P

R, +R,, (W

R,(l +RJR,,) (.n)

0.14 0.21 0.11 0.24 0.31 0.17

9.9x lo3 3.7 x lo4 6.3 X lo4 3.7x lo4 1.1x106 5.1 x lo6

1.9x105 6.1X104 3.6X lo4 1.8X lo4 1.9x lo4 3.5 x lo3

model for the samples under study at 55°C

Ri (0)

*,I (F/s’)

2.5 x 10’ 6.3 X lo4 3.6X lo4 1.7x104 2.0x lo4 3.5x 10’

8.2X 5.3 x 5.6X 1.7x 8.0X 1.8 x

P 1o-8 1o-8 lo-* lo-’ 1o-8 1o-7

0.10 0.21 0.11 0.24 0.19 0.17

R,R,I(R,+Ri) (W

R:fCR, +R,) (51)

9.6x lo3 2.3 X lo4 2.3 X lo4 1.2x lo4 1.9x lo4 3.5 x lo3

3.9x 1.4x 4.0x 2.6 x 1.0x 4.3 x

lo2 lo4 lo4 lo4 lo6 lo6

160

M. Wasiucionek

et al. I Solid State tonics 92 (1996) 155-160

Acknowledgments This work has been financially supported by the Polish Committee of Scientific Research under grant no. 3P40703007.

References [l] J.B. Bates, G.R. Grualski, NJ. Dudney, C.F. Luck and X. Yu, Solid State Ionics 70/71 (1994) 619.

Dl S.J. Visco, M. Liu, M.M. Doeff, Y.P. Ma, C. Lampert and L.C. DeJonghe, Solid State Ionics 60 (1993) 175. [31 I. Riess, Solid State Phenom. 39/40 (1994) 89. [41 I. Riess, Solid State Ionics 44 (1991) 199. [51 H.-I Yoo, J.-H. Lee, M. Martin, J. Janek and H. Schmalzried, Solid State Ionics 67 (1994) 317. Fl T. Minami, K. Imazawa and M. Tanaka, J. Non-Cryst. Solids 42 (1980) 469. 171 N.F. Mott, Adv. Phys. 16 (1967) 41. and D.R. Franceschetti, in: Impedance PI J.R. Macdonald Spectroscopy Emphasizing Solid Materials and Systems, ed. J.R. Macdonald (Wiley, New York, 1987) p. 98.