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Correlation between structural and electrical properties in (1-x) AgPO3·xMX2 glasses (M = Pb2+, Hg2+; X = I-, Br-, Cl-) from Raman spectroscopy and ionic conductivity measurements
Solid State Ionics 21 (1986) 131-138 North-Holland, Amsterdam
CORRELATION BETWEEN STRUCTURAL AND ELECTRICAL PROPERTIES IN ( l - x ) AgPO3-xMX2 GLASSES (M = Pb 2+, Hg2+; X = I - , Br-, CI-) FROM RAMAN SPECTROSCOPY AND IONIC CONDUCTIVITY MEASUREMENTS J.P. MALUGANI, R. MERCIER and M. TACHEZ Laboratoire d'Electrochimie des Solides UA 436, Universit~ de Franche-Comt~, 25030 Besan¢on Cddex, France
Received 18 December 1985
The ionic conductivity of (1 -x) AgPO3-xMX2 (M = Pb, I-Igand X = I, Br, CI) was measured as a function of temperature from 25°C to Tg. The influence of the nature of the divalent cation is of primary importance. The conductivity of the Pb glass increases with x in the vitreous domain while the conductivity of the Hg glass increases with x until x = 0.2 then becomes nearly constant. The conductivity of 0.81 AgPO3-0.19PbI2 is close to that of 0.5 AgPO3-0.5Agl while that of 0.5 AgPO3-0.5HgI2 is much lower (NI03). Raman scattering spectra were recorded from 5 to 350 era-1 . No modifications of the (PO3)** chain skeleton occur when adding PbX2 or HgX2. On the other hand, AgX characteristic hands appear for the Pb glasses, while HgX2 molecular unit bands appear for the Hg glasses (when x > 0.2). Explanations of such behaviour are given in terms of creation or non-creation of "AgX clusters" or "AgX, HgX2 clusters" in the AgPO3 host glass, by exchanging M2+ by Ag+ around the halide anions.
1. Introduction The systematic study of both electrical and structural properties o f ( l - x ) AgPO3 .xAgX homogeneous glasses allows us to distinguish the parameters pertinent to conductivity [1,2]. Both the nature and the concentration of the X halide influence the silver conduction which can be described by a percolation model based on microscopic AgX dusters dispersed between the vitreous network made of (PO3)nn chains. The clustered Ag+ cations are tetrahedrally coordinated by X - anions as in a-AgI high temperature superionic structures [3]. Recent Raman and EXAFS investigations [4,5] on A g 2 0 - B 2 0 3 - A g I glasses lead us to consider the model of "amorphous clusters" of AgI dispersed in the host Ag20-nB203 glass. This model has been developed in our laboratory and by Minami et al. [6]. The vitreous support is built with a compact framework of oxide anions and the silver halide is dissolved inside the duster form. No attempts to understand the microscopic characteristics of these dusters have been reported yet. 0 167-2738/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishin~ Division)
The idea of the existence of such microdomains have become more and more realistic since we published a quasielastic neutron scattering study on AgPO3-AgI glass [7] and found unambiguously that there are heterogeneities in AgPO3-Agl, with a mean gyration radius of 20 A; while pure AgPO3 glass can be considered as a perfect single phase system [8]. The silver ion diffusion mechanism is governed by an activation energy very close to that of (x-AgI [9]. The high ionic conductivity of this glass (10-2 ~ - 1 cm-1 at 293 K) should be correlated to the presence of AgI clusters. Similar high conductivities are obtained when adding some divalent metallic halides, other than silver halide, in AgPO3 glass [10]. The divalent cations do not contribute to the transport mechanism. Thus, it is the iodide concentration rather than that of the silver which is the main parameter of conductivity evolution. The aim of this paper, is to confixm the correlation between conductivity and AgX cluster concentration by studying the electrical and structural features in a large composition range for ( l - x ) AgPO3-xMX 2 glasses with X = I, Br or C1 and M = Pb or Hg.
132
J.P. Malugani et al./Structural and electrical properties in (1-xj AgP03" xMX 2 glasses
2. Experimental The preparation of these glasses has already been described in a previous paper [10]. The melting of crystallised AgPO 3 and metallic halide mixtures is obtained between 400 and 7500C in silica tubes under primary vacuum. The melted mixture is then quenched at 25°C. Possible crystallised phases were detected by X-ray diffraction. The glass transition temperatures were measured by DTA with a heating rate of 10*C rain -1 . Electrical measurements were performed on cylindrical pellets made with the bulk glass (diameter: 10 mm, thickness: 2.5 mm). They were covered on both sides with metallic electrodes by means of evaporation for Ag electrodes or by cathodic sputtering for Pt electrodes. The electrical contact was maintained by means of springs on platinum discs. The measurement techniques (complex impedance and Wagner's polarization) were previously described by Malugani et al. [ 11 ]. All measurements were performed after having maintained the cell under a flow of pure dried nitro. gen. Symmetrical M/glass/M cells were used to determine total electrical conductivity, M = Pt for AgPO 3 HgX2 glasse~ and M = Ag for AgPO3-PbX 2 glasses. Electronic conductivity measurements using the blocking electrode method (Pt/glass/Ag) were carried out on the samples with the highest concentration in divalent metallic halides. Raman scattering spectra were recorded by means of a T 800 Coderg spectrometer with one of the two laser beams available (X = 514.5 nm:Ar + or X = 647.1 nm :Kr). This instrument is able to record stokes and antistokes peaks very close to the exciting line: as close as 5 cm -1 with a resolution function better than 2 cm -1 . The low-frequency spectra (Av-< 300 cm -1) shown in this paper are the reduced Ir spectra obtained from Ivv spectra by the transformation Ir = ¢oIvv/1 + n where n is the Bose-Einstein term [12].
3. Experimental
results
3.1. Vitrification ranges
The vitreous domains of (l-x) AgPO 3 .xPbX 2 systems range from x = 0 to x = 0.19 (PbI2); O.16(PbBr2);
_
6,,i"~.
.
~c~ o
_
100
I
I
•1
.2
I
I
I
.4
.5
x
Fig. 1. Variation of Tg with MX2 content for AgPO3-MX2 glasses. The dotted fines is the variation of T8 with AgX content for AgPO3-AgX glasses.
0.12 (PCI2). X-ray diffraction was used to detect the presence of crystallised compounds like fl-AgI, AgBr or AgCI, in each binary system, outside the upper limits of the vitreous domains. In the case of mercury halides, the solubility limits are larger: from x = 0 to x = 0.50 (HgI2); 0.30 (HgBr2); 0.20 (HgC12). Beyond these x values, a mixture of fl-Ag2HgI4 and fl-HgI2 was detected in the xAgPO 3 - ( l - x ) HgI 2 system, while AgBr and AgC1 were detected for the bromide and chloride systems. The glass transition temperature (Tg) is plotted in fig. 1 against the composition. The Tg decrease depends on the halide concentration and the nature of the bivalent metal. Two kinds of variations are observed: (i) an almost linear decrease of T8 with HgX 2 concentration similar to that observed for the AgPO 3AgX system (ii) a sharp decrease with PbX 2. The lowest glass transition is about the same for all systems (90°C) even for the AgPO 3-AgX system. The evolution of the Tg does not depend on the nature of the halide. It is not the steric effect of the dissolved halide but the cation-polyanion ( P O 3 ~ interactions which seem to be the main factor governing the polyanions mobility.
ZP. Malugani et al./Structural and electrical properties in. (1 -:x) AgPOa.xMX2 glasses
3.2. Electrical measurements 3.2.1. Total electrical conductivity The total conductivity can be described by the well known Arrhenius law o = o0 exp - Ec]RT for all the samples in the 293 K-Tg temperature range, the formula symbols having their usual meaning. o, o 0 and E c parameters are strongly dependent on the composition. Their variations are plotted in figs. 2 and 3 versus the MX 2 molar fraction and can be compared to the homologous AgPO 3 -AgX system. The conductivity isotherms (293 K)'(fig. 2) show a dependence on both the concentration and the nature of the halide, as was previously described for other systems [1,13,14] : (i) as the halide concentration increases, conductivity is enhanced up to the highest value that corre. sponds to the saturated glass. At higher concentrations, a crystallised phase appears leading to a decrease in conductivity. (ii) the usual a I > oBr > OC1 series is found for the same concentration of PbX 2 or HgX 2 in the AgPO 3 host glass.
133
However, the influence of the nature of the bivalent cation (Hg 2+ or Pb 2+) on the conductivity behavior is rather dramatic. In the (1 -x) AgPO3.xPbX 2 glasses, a weak variation in PbX 2 concentration leads to a large variation in conductivity: the maximum conductivity of the richer glass (x = 0.19) is about 10 -2 ~ - 1 cm-1 at 25°C, i.e. the same value obtained with AgPO3-Agl glass. On the other hand, for the (l-x) AgPO3.xHgI 2 system, the maximum conductivity value is 3 X 10-5 ~2-1 cm -1 for x = 0.5: a concentration of iodide equal to that in AgPO 3-AgI richer glass. If we consider the mutual evolutions of the o 0 and E c parameters (fig. 3), two main differences are evident between PbX 2 and HgX2 glasses: (1) in the case Of lead halides, the activation energy decreases strongly when the halide concentration increases (12.7 kcal mole -1 -+ 5 kcal mole -1 for PbI2) , where. as the decrease in E c versus HgX2 concentration is rather small; (2) the evolution of the pre-exponential factor % is quite different depending on the nature
3
O Pb 12 A PbBr 2
/
u
PbC =
~D 2 _o
/ / /
°
/
/
Agl / / Ag Br / / / / AgCI
0
/
4 / /
/
/
/
/
I
I
I
•1
.2
I
1
I
12
/ / ,,,sc~o---e-~-
r/
I
'7o
-
-°10
..0..
-
~8
6 I
I
.1
.2
I
.3 x [MX,]
I
I
.4
Fig. 2. Composition dependence of the conductivity logarithm in the AgPO3 -MX 2 system and the AgPO3-AgX system.
l
X [MX,]
I
l
.4
.5
Fig. 3. Composition dependence of the activation energy and preexponential factor.
134
J.P. Malugani et al./Structural and electn'cal properties in ( l - x ) AgPO 3.xMX2 glasses
of M 2+. The variation of the a 0 term is not the pre. dominant parameter to the conductivity contribution.
1 Pb 12 ff /
3.2.2. Determination o f the conduction type
_~
The electronic transport numbers, te, were measured exclusively for the richer glasses: 0.81 AgPO 30.19 PbI 2 and 0.50 AgPO3-0.50 HgI 2. At 50°C, the electronic conductivities are respectively 3 X 10 -9 and 5 X 10 -7 ~2 cm -1 leading to the following electronic transport numbers: 3 X 10 -7 and 5 X 10 -3, Thus, we can admit that the nature of conduction here is essentially ionic and due to Ag+ cations.
.,~
2 PbBr2
/~
0
I
3.3. Raman spectroscopy The most significant reduced spectra: 0.9 AgPO 30.1 PbX 2 and 0.7 AgPO3-0.3 HgX2 are plotted together in figs. 4 and 5. AgPO 3 - A g X spectra are ad. ded for comparison. The characteristic spectra of the dissolved halides range between 5 cm -1 and 350 cm -1 . Indeed, no modifications were detected after 350 cm - I when adding MX 2 to the host AgPO 3 glass; that means that the polymeric phosphate framework is unchanged irrespective of the concentration of the halide. The two main polyanion bands keep their profile and no shift appears (1142 cm -1 for ~sPO2 and 650 cm -1 for ~,sPOP). Their intensity ratio (1.2) is kept at a constant value, so the halides are not inserted into the polyanion structure.
3.3.1. AgPO3-PbX 2 glasses Typical examples of corrected spectra are plotted in fig. 4. They consist essentially of two broad bands: the first at 55 cm -1 is relative to the AgPO 3 glass (2) and is not dependent on the nature of the halide, the maximum frequency of the second is centered at 115 cm -1 (PbI2), 135 cm -1 (PbBr2) and 190 cm -1 (PbC12) and depends on X. The main features of these spectra are quite similar to their homologous AgPO 3 AgX for the same X/PO 3 ratio. The discrepancies between the intensities of the normalized bands and that at 1142 cm -1 do not seem to be significant considering the experimental precision. The root of this second band should be related to the presence of AgX species in the (1-x) AgPO 3. xPbX 2 glasses as was already demonstrated in a previous paper on ( l - x ) A g P O 3.xAgX glasses (2). This band cannot be assigned to PbX 2 species. Therefore, these glasses seem to be constituted
\\
Agl /
3 Pb CI2
200
i
I
T [cm1 ]
I
100
Fig. 4. Reduced low-frequency Raman spectra of 0.9 AgPO3-0.1 PbX 2 glasses and of 0.7 AgPO3-0.3 AgX glasses for comparison,
by AgX microdomains dispersed between the polyanions of the vitreous network.
3.3.2. AgPO3-HgX 2 glasses The Raman spectra of these compounds (fig. 5) are clearly different from those of AgPO 3 - P b X 2 or AgPO 3-AgX. New strong sharp lines appear respectively at 140, 190 and 290 cm -1 for HgI2, HgBr 2 and HgC12 glasses. For mercury iodide and mercury bromide concentrations lower than 0.3, a shoulder is noticed at 170 cm -1 (HgI2) and 225 cm -1 (HgBr2).
Z B
.3 Hgl 2 ---.3 HgBr2
1
..... .2 HgCI2
\
I II
/', 300
\ .
200
.
.
.
.
.
.
.
100
Fig. 5. Reduced low-frequency Raman spectra of AgPO3 HgX2 glasses.
J.P. Malugani et al./Structural and electrical properties in (1 -x)AgPO a.xMX 2 glasses
In this composition range, the profile of these spectra is close to that of the high temperature phase t~-Ag2HgI4 : i.e. a strong line at 122 cm -1 (usHgI2-) with a shoulder at 142 cm -1 which is not defined [15]. This analogy is probably fortuitous because of the higher frequencies observed in the HgI 2 glass spectrum. In order to compare our spectra with those of the yellow high temperature phase a.HgI2, of the HgBr 2 crystallised phase and of the solid solution HglBr, in fig. 6 we plotted these three spectra with the 0.7 AgPO3-0.3 HgX2 (X = I and Br) and the 0.7 AgPO 3.0.3 HgBrI spectra. HgBr2, HgI 2 and HgBrI are built with linear HgX2 molecular units [16,17]. Their spectra were previously described with respect to the symmetry of the normal modes. The bands of the glasses are centered at the same frequencies as those of the Alg stretching modes of the corresponding HgX2 phases (141 cm -1 : HgI 2 and 188 cm -1 : HgBr2). The main lines of the mixed HglBr compound (139 cm -1 : HgI2; 155 cm -1 : HglBr and 181 cm -1 : HgBr2) correspond exactly
Hgl2 glass-- ] ~ a
135
to the three bands of the 0.7 AgPO 3-0.3 HglBr glass, with the same wave numbers and the same relative intensifies. The glass spectrum seems to be the envelope of the crystallised one. No satisfying interpretation can be given to explain the origin of the shoulders at 170 cm -1 (HgI 2 glass) and 225 cm -1 (HgBr2 glass). It cannot be assigned to HgI4 stretching modes nor to antisymmetric Uas X - H g - X modes that should occur at higher frequencies. This shoulder is not expected to be related to the same HgX 2 species as the main bands (141 and 188 cm -1) are. Increase in HgX2 concentration only enhances the intensity of the main band and not that of its shoulder. This behaviour is clearly demonstrated in fig. 7 where the intensity variations of these two bands are plotted versus the concentration for ( l - x ) AgPO 3.xHgI 2 glasses. It is interesting to note that the 140 cm -1 band intensity is proportional to the iodide concentration for x = 0.2 (i.e. I/PO 3 = 0.5), while the 170 cm -1 shoulder intensity is only slightly affected by the increase in concentration. This linear relation between intensity and iodide concentration still reinforces our proposed interpretation. The ( l - x ) AgPO 3-xHgX 2 glas-
"
.---°
^4 "'" " - ~ 1
= ~;
c
c = .D
HglBr g l a s s -
G
HglBr cryst.---°
~3
d
m
o .=
2i HgBr~glassHgBr 2 cryst.'---
I
1 ;z
2 0 0 ~[cm-'] 1OO Fig: 6. Comparison between Raman spectra of 0.7 Agt'O30.3 ttgX2glasses (X2 = 12, IBr and Br2) and ttgX2 crystallized phases.
lo [,]/[Po,]
2.o
Fig. 7. Composition dependence of log o2so C and intensity variations of v (140 cm -1) and v (170 em -1) versus molar ratio [I]/[PO 3 ] for AgPO 3-HgI 2 glasses.
136
J.P. Malugani et aL /Structuml and electrical properties in (1 , x ) AgPO s .xMX 2 glasses
ses (x > 0.2) are built with molecular HgX 2 units, similar to the high temperature ot-Hgl2 phase, dispersed between the phosphate chains of the network.
O
AgPO3 - Pbl 2
• [Ag PO 3 - Pb(PO3) 2]- Agl /
4. Discussion
. . . . AgPO 3- Agl
The above analysis of Raman spectra of AgPO 3 MX 2 glasses leads us to distinguish two kinds of glasses depending on the similarity between their spectra and those of ( l - x ) AgPO3-xAgX glasses. (i) AgPO 3 - P b X 2 glasses are constituted by a-AgI microdomains inserted between the polyanions as in AgI glasses. (ii) AgPO3-HgX 2 glasses are built with HgX2 molecular units dispersed in the host glass (at least for high HgX2 concentrations). As we suggested in a previous paper [10], these AgX clusters are the result of a coordination exchange between Ag+ and Pb 2+ cations. The Pb 2+ cations are linked preferentially to the PO 3 anions while Ag+ cations move towards X - anions to build AgX clusters between the mixed network: (Ag I _ 2yPby)PO3 ; these AgX microdomains are similar to those observed in ( l - x ) AgPO 3-xAgX glasses [2]. The same cation substitution was observed in solid phase reactions by grinding Ag4P207 , AgI and KBr mixtures to make sample pellets for IR spectroscopy [18]. When the electrical behaviour of these two kinds of glasses is taken into account, it is very likely that the AgX cluster concentration is the main factor rather than the whole halide concentration. The conductivity of the AgPO 3 - P b X 2 glasses would then be correlated to the AgX cluster concentration. In this case, the variations of conductivity should result from two antagonistic contributions: (i) the variation of the composition of the host glass from AgPO 3 to Ag(1_ 2y)Pby (PO3) by substituting all Pb 2+ cations by two equivalent Ag+ cations. (ii) the formation of a-AgX clusters resulting from the exchange reaction. The intensity of each of these two effects can be estimated by considering the variations of the conductivity of the following glasses: Ag(1 _ 2y)PbyPO3 mixed host glass (for different y values) and (1-x)(Ag(1_ 2 y ) P b y P O 3 ) . x A g X (withy kept constant) to determine the AgX effect on conductivity. The more significant results obtained with PbI 2
/
[] A g P O 3- Pb(PO3)2
/,
/
•
/
/, /
Io
_o
i IAo,
,"-(effect
exchange between '-I 2Ag" and 1Pb ÷
?
.1
.2
[,]/[,] +[PO,]
.3
Fig. 8. log o2Soc variations versus molar ratio [I]/[I] + [PO3 ] for (1 -x) AgPO3-xPbI2 : (l-x) Ago.875Pbo.125(PO3) l.125.xAgI: (l_x) AgPO3.xAgI: and (1-2y)AgPO3-yPb(PO3)2 : glasses. The exchange between 2 yAg+ and yPb2* leads to a decrease in conductivity (log o2sOc from -6.5 to -7.8 when y = 0.1) while the "AgI effect" involves an increase in conductivity (log a2sOc from -7.8 to -4.8 when x = 0.2). The intersection of the two solid lines corresponds to the following formulae: 0.9 AgPO3-0.1 PbI2 or [0.875 AgPO3-0.125 Pb(PO3)2 ]-0.2 AgI.
glasses are collected in fig. 8: the two solid lines correspond to the conductivity variations versus the molar fraction (I/I + PO3) for ( l - x ) AgPO 3 -xPbl 2 glasses and ( l - x ) [0.875 AgPO3-0.125 Pb(PO3) 2] .xAgI glasses. The intersection of these 2 isotherms corresponds to the same formula but the samples come from two different series of glasses. This common point shows the contribution of the two antagonistic effects resuiting from the total cation exchange process: the decrease in conductivity by substituting 0.2 Ag+ ions by 0.1 Pb 2+ ions in the host glass (log o from -6.5 to -7.8); and the increase in conductivity (o X 103) when 0.2 "AgI clusters" are formed. The evolution of the isotherm related to the mixed
J.P. Malugani et aL /Structural and electrical properties in ( 1 - x ) AgPO s. xMX 2 glasses
host glass leads to the following remarks: (i) the linear correlation between log a and the iodide fraction may be compared to the linear evolution observed in the "homogeneous" AgPO 3-AgI glasses; (ii) the slopes of these straight lines are different in each case; (ill) these linear evolutions show that no major modifications in the host glass (i.e. AgPO 3 or Ag 1 _ 2yPbyPO3) are involved and correspond to the "linear" increase of the cluster concentration. Substitution of Pb 2+ cations linked to PO 3 groups by Ag+ cations from AgI is therefore highly unrealistic. These ideas are in agreement with the expected total exchange Ag+ (PO3) ** Pb(X) in AgPO3-PbI 2 mixtures. Thus, the conductivity of these glasses can be correlated to the concentration of "o~-AgX" dusters, as in AgPO 3-AgX glasseS. One can therefore understand why the electrical properties (a, o0, Ee) of doped glasses AgPO 3-PbX 2 or AgPO 3-AgX are very close. This interpretation cannot be used to justify weak variations in conductivity in the AgPO 3-HgX 2 system. Indeed, conclusions from Raman spectra are in contradiction with a total exchange between the Ag+ cations bound to PO-3 polyanions and the Hg2+ cations of the dissolved halides. Simple dissolution of HgX 2 species inside AgPO 3 glass cannot be retained when considering fig. 7. The variation of the intensity of the 140 cm -1 Raman line does not seem to be compatible with a simple dissolution of HgI 2 inside AgPO 3 network. For low concentrations of HgI 2 (x < 0.20), only a slight increase of the HgI2 band intensity occurs; for 0.2 < x < 0.5 a slight increase in HgI2 concentration involves a large enhancement of the HgI 2 band intensity. In this range, this intensity increases linearly with the HgI2 fraction. If a simple dissolution of HgI 2 occurred, a linear dependence between the 140 cm-1 peak intensity and the HgI 2 fraction would be expected in the whole vitreous range. It seems that for low Hgl 2 concentration, a disappearance of "free HgI2 species" occurs. One may imagine that HgI2 is "consumed" by a partial exchange with Ag+ cations to form mixed dusters (AgI, HgI2) which should increase the conductivity, as it is observed in fig. 2, up to 0.2 Hgl 2. On the
137
other hand, for higher HgI 2 concentrations, the conductivity does not increase further because HgI2 me. lecular units are simply dispersed in AgPO 3 (fig. 7: 140 cm -1 line), which does not involve a conductivity enhancement (fig. 2). The 170 cm -1 band could be related to this new "conducting phase" (mixed AgI, HgI 2 clusters). A similar observation can be done on HgBr2 glasses in which the shoulder at 225 cm -1 could correspond to a "conducting phase". It should be remembered that for Ag2HgI4 [13] and other relevant compounds, the/3 --, a transition gives a new mode appearing as a shoulder on the high frequency side of the main band (140 cm -1 : shoulder of the 122 cm -1 line) which is assigned to HgI4 symmetric stretching mode. No satisfying explanations were given as to the origin of such band. The values of the a 0 preexponential terms of the HgI 2 glasses tend towards 3 X 103 I2 -1 cm -1 , a value close to that of Ag2Hgl 4 given by Suchow et al. [19], while the values of a 0 for PbI 2 glasses are of the same order of magnitude as AgPO 3-AgI glasses. This fact could be related to the existence of such mixed domains (AgI, HgI2). Identification of the nature of species which contribute to form the microdomains will be undertaken by neutron diffraction.
5. Conclusion This study of the electrical behaviour of mixed mono and divalent cation glasses (1-x)AgPO 3 .xMX 2 (M = Pb, Hg) dearly shows that ionic conductivity is essentially dependent on the "a-AgX dusters" concentration and is ensured by Ag+ hopping diffusion. Total Ag+ ~ Pb 2+ exchange occurs in PbX 2 glasses leading to the presence of "AgX dusters" dispersed in the network; conductivity is directly related to AgX concentration and may be compared to that of the AgPO 3-AgX glass system. The melting of AgPO 3 and HgX2 mixtures gives more complicated glasses. For high HgX2 concentrations, HgX2 molecular species are dispersed in the glass so that no conductivity enhancement is observed. For dilute HgX2 glasses, the situation is not very dear. Conductivity enhancement could be involved by the presence of mixed "AgI, HgI2 dusters".
13 8
J.P. Malugani et al./Struetural and electrical properties in (l-x)AgPOa.xMX2 glasses
References [ 1] LP. Malugani, A. Wasniewski,M. Doreau and G. Robert, Mater. Res. Bull. 13 (1978) 427. [2] J.P. Malugani and R. Mercier, Solid State Ionics 13 (1984) 293. [3] R.L Cava, F. Reidinger and BJ. Wuensh, J. Solid State Chem. 31 (1980) 69. [4] A. Fontana, G. Mariotto, E. Cazzanelli, G. Carini, M. Cutroni and M. Federico, Phys. Rev. Letters 43A (1983) 209. [5] G. Dalba, A. Fontana, P. Fomasini, G. Mariotto, M.R. MasuUo and F. Rocca, Solid State Ionics 9/10 (1983) 597. [6] T. Minami, K. Imazawa and M. Tanaka, J. Non-Cryst. Solids 42 (1980) 469. [7] M. Tachez, R. Mercier, LP. Malugani and AJ. Dianoux, Solid State Ionics 20 (1986) 93. [8] M. Tachez, J.P. Malugani, R. Mercier and P. Chieux, Annu. Rep. 84 (Institut Lane Langevin, Grenoble, France).
[9] P.C. Alien and D. Lazarus, Phys. Rev. B17 (1978) 1913. [ 10] LP. Malugani, A. Wasniewski,M. Doreau, G. Robert and R. Mercier, Mater. Res. Bull. 13 (1978) 1009. [ 11 ] J.P. Malugani, A. Wasniewski,M. Doreau and G. Robert, C.R. Acad. Sci. (Paris) 284 (1977) 99. [ 12] R. Shuker and R.W. Gammon, Phys. Rev. 25 (1970) 222. [13] A. Levasseur, J.C. Brethous, J.M. Reau, M. Couzi and P. Hagenmuller, Solid State Ionics 1 (1980) 177. [14] G. Robert, LP. Malugani and A. Saida, Solid State Ionics 3/4 (1981) 311. [ 15 ] D.R. Greig, G.C. Joy and D.F. Schriver, J. Chem. Phys. 67 (1977) 3189. [ 16] S. Nakashima, H. Mishima and H. Tai, J. Phys. Chem. Solids 35 (1974) 531. [ 17] A. Givan and A. Loewenschuss, J. Chem. Phys. 64 (1976) 1967. [ 18] T. Minami, T. Katsuda and M. Tanaka, J. Phys. Chem. 38 (1979) 1306. [ 19] L. Suchow and G.R. Pond, J. Am. Chem. Soc. 75 (1953) 5242.
CORRELATION BETWEEN STRUCTURAL AND ELECTRICAL PROPERTIES IN ( l - x ) AgPO3-xMX2 GLASSES (M = Pb 2+, Hg2+; X = I - , Br-, CI-) FROM RAMAN SPECTROSCOPY AND IONIC CONDUCTIVITY MEASUREMENTS J.P. MALUGANI, R. MERCIER and M. TACHEZ Laboratoire d'Electrochimie des Solides UA 436, Universit~ de Franche-Comt~, 25030 Besan¢on Cddex, France
Received 18 December 1985
The ionic conductivity of (1 -x) AgPO3-xMX2 (M = Pb, I-Igand X = I, Br, CI) was measured as a function of temperature from 25°C to Tg. The influence of the nature of the divalent cation is of primary importance. The conductivity of the Pb glass increases with x in the vitreous domain while the conductivity of the Hg glass increases with x until x = 0.2 then becomes nearly constant. The conductivity of 0.81 AgPO3-0.19PbI2 is close to that of 0.5 AgPO3-0.5Agl while that of 0.5 AgPO3-0.5HgI2 is much lower (NI03). Raman scattering spectra were recorded from 5 to 350 era-1 . No modifications of the (PO3)** chain skeleton occur when adding PbX2 or HgX2. On the other hand, AgX characteristic hands appear for the Pb glasses, while HgX2 molecular unit bands appear for the Hg glasses (when x > 0.2). Explanations of such behaviour are given in terms of creation or non-creation of "AgX clusters" or "AgX, HgX2 clusters" in the AgPO3 host glass, by exchanging M2+ by Ag+ around the halide anions.
1. Introduction The systematic study of both electrical and structural properties o f ( l - x ) AgPO3 .xAgX homogeneous glasses allows us to distinguish the parameters pertinent to conductivity [1,2]. Both the nature and the concentration of the X halide influence the silver conduction which can be described by a percolation model based on microscopic AgX dusters dispersed between the vitreous network made of (PO3)nn chains. The clustered Ag+ cations are tetrahedrally coordinated by X - anions as in a-AgI high temperature superionic structures [3]. Recent Raman and EXAFS investigations [4,5] on A g 2 0 - B 2 0 3 - A g I glasses lead us to consider the model of "amorphous clusters" of AgI dispersed in the host Ag20-nB203 glass. This model has been developed in our laboratory and by Minami et al. [6]. The vitreous support is built with a compact framework of oxide anions and the silver halide is dissolved inside the duster form. No attempts to understand the microscopic characteristics of these dusters have been reported yet. 0 167-2738/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishin~ Division)
The idea of the existence of such microdomains have become more and more realistic since we published a quasielastic neutron scattering study on AgPO3-AgI glass [7] and found unambiguously that there are heterogeneities in AgPO3-Agl, with a mean gyration radius of 20 A; while pure AgPO3 glass can be considered as a perfect single phase system [8]. The silver ion diffusion mechanism is governed by an activation energy very close to that of (x-AgI [9]. The high ionic conductivity of this glass (10-2 ~ - 1 cm-1 at 293 K) should be correlated to the presence of AgI clusters. Similar high conductivities are obtained when adding some divalent metallic halides, other than silver halide, in AgPO3 glass [10]. The divalent cations do not contribute to the transport mechanism. Thus, it is the iodide concentration rather than that of the silver which is the main parameter of conductivity evolution. The aim of this paper, is to confixm the correlation between conductivity and AgX cluster concentration by studying the electrical and structural features in a large composition range for ( l - x ) AgPO3-xMX 2 glasses with X = I, Br or C1 and M = Pb or Hg.
132
J.P. Malugani et al./Structural and electrical properties in (1-xj AgP03" xMX 2 glasses
2. Experimental The preparation of these glasses has already been described in a previous paper [10]. The melting of crystallised AgPO 3 and metallic halide mixtures is obtained between 400 and 7500C in silica tubes under primary vacuum. The melted mixture is then quenched at 25°C. Possible crystallised phases were detected by X-ray diffraction. The glass transition temperatures were measured by DTA with a heating rate of 10*C rain -1 . Electrical measurements were performed on cylindrical pellets made with the bulk glass (diameter: 10 mm, thickness: 2.5 mm). They were covered on both sides with metallic electrodes by means of evaporation for Ag electrodes or by cathodic sputtering for Pt electrodes. The electrical contact was maintained by means of springs on platinum discs. The measurement techniques (complex impedance and Wagner's polarization) were previously described by Malugani et al. [ 11 ]. All measurements were performed after having maintained the cell under a flow of pure dried nitro. gen. Symmetrical M/glass/M cells were used to determine total electrical conductivity, M = Pt for AgPO 3 HgX2 glasse~ and M = Ag for AgPO3-PbX 2 glasses. Electronic conductivity measurements using the blocking electrode method (Pt/glass/Ag) were carried out on the samples with the highest concentration in divalent metallic halides. Raman scattering spectra were recorded by means of a T 800 Coderg spectrometer with one of the two laser beams available (X = 514.5 nm:Ar + or X = 647.1 nm :Kr). This instrument is able to record stokes and antistokes peaks very close to the exciting line: as close as 5 cm -1 with a resolution function better than 2 cm -1 . The low-frequency spectra (Av-< 300 cm -1) shown in this paper are the reduced Ir spectra obtained from Ivv spectra by the transformation Ir = ¢oIvv/1 + n where n is the Bose-Einstein term [12].
3. Experimental
results
3.1. Vitrification ranges
The vitreous domains of (l-x) AgPO 3 .xPbX 2 systems range from x = 0 to x = 0.19 (PbI2); O.16(PbBr2);
_
6,,i"~.
.
~c~ o
_
100
I
I
•1
.2
I
I
I
.4
.5
x
Fig. 1. Variation of Tg with MX2 content for AgPO3-MX2 glasses. The dotted fines is the variation of T8 with AgX content for AgPO3-AgX glasses.
0.12 (PCI2). X-ray diffraction was used to detect the presence of crystallised compounds like fl-AgI, AgBr or AgCI, in each binary system, outside the upper limits of the vitreous domains. In the case of mercury halides, the solubility limits are larger: from x = 0 to x = 0.50 (HgI2); 0.30 (HgBr2); 0.20 (HgC12). Beyond these x values, a mixture of fl-Ag2HgI4 and fl-HgI2 was detected in the xAgPO 3 - ( l - x ) HgI 2 system, while AgBr and AgC1 were detected for the bromide and chloride systems. The glass transition temperature (Tg) is plotted in fig. 1 against the composition. The Tg decrease depends on the halide concentration and the nature of the bivalent metal. Two kinds of variations are observed: (i) an almost linear decrease of T8 with HgX 2 concentration similar to that observed for the AgPO 3AgX system (ii) a sharp decrease with PbX 2. The lowest glass transition is about the same for all systems (90°C) even for the AgPO 3-AgX system. The evolution of the Tg does not depend on the nature of the halide. It is not the steric effect of the dissolved halide but the cation-polyanion ( P O 3 ~ interactions which seem to be the main factor governing the polyanions mobility.
ZP. Malugani et al./Structural and electrical properties in. (1 -:x) AgPOa.xMX2 glasses
3.2. Electrical measurements 3.2.1. Total electrical conductivity The total conductivity can be described by the well known Arrhenius law o = o0 exp - Ec]RT for all the samples in the 293 K-Tg temperature range, the formula symbols having their usual meaning. o, o 0 and E c parameters are strongly dependent on the composition. Their variations are plotted in figs. 2 and 3 versus the MX 2 molar fraction and can be compared to the homologous AgPO 3 -AgX system. The conductivity isotherms (293 K)'(fig. 2) show a dependence on both the concentration and the nature of the halide, as was previously described for other systems [1,13,14] : (i) as the halide concentration increases, conductivity is enhanced up to the highest value that corre. sponds to the saturated glass. At higher concentrations, a crystallised phase appears leading to a decrease in conductivity. (ii) the usual a I > oBr > OC1 series is found for the same concentration of PbX 2 or HgX 2 in the AgPO 3 host glass.
133
However, the influence of the nature of the bivalent cation (Hg 2+ or Pb 2+) on the conductivity behavior is rather dramatic. In the (1 -x) AgPO3.xPbX 2 glasses, a weak variation in PbX 2 concentration leads to a large variation in conductivity: the maximum conductivity of the richer glass (x = 0.19) is about 10 -2 ~ - 1 cm-1 at 25°C, i.e. the same value obtained with AgPO3-Agl glass. On the other hand, for the (l-x) AgPO3.xHgI 2 system, the maximum conductivity value is 3 X 10-5 ~2-1 cm -1 for x = 0.5: a concentration of iodide equal to that in AgPO 3-AgI richer glass. If we consider the mutual evolutions of the o 0 and E c parameters (fig. 3), two main differences are evident between PbX 2 and HgX2 glasses: (1) in the case Of lead halides, the activation energy decreases strongly when the halide concentration increases (12.7 kcal mole -1 -+ 5 kcal mole -1 for PbI2) , where. as the decrease in E c versus HgX2 concentration is rather small; (2) the evolution of the pre-exponential factor % is quite different depending on the nature
3
O Pb 12 A PbBr 2
/
u
PbC =
~D 2 _o
/ / /
°
/
/
Agl / / Ag Br / / / / AgCI
0
/
4 / /
/
/
/
/
I
I
I
•1
.2
I
1
I
12
/ / ,,,sc~o---e-~-
r/
I
'7o
-
-°10
..0..
-
~8
6 I
I
.1
.2
I
.3 x [MX,]
I
I
.4
Fig. 2. Composition dependence of the conductivity logarithm in the AgPO3 -MX 2 system and the AgPO3-AgX system.
l
X [MX,]
I
l
.4
.5
Fig. 3. Composition dependence of the activation energy and preexponential factor.
134
J.P. Malugani et al./Structural and electn'cal properties in ( l - x ) AgPO 3.xMX2 glasses
of M 2+. The variation of the a 0 term is not the pre. dominant parameter to the conductivity contribution.
1 Pb 12 ff /
3.2.2. Determination o f the conduction type
_~
The electronic transport numbers, te, were measured exclusively for the richer glasses: 0.81 AgPO 30.19 PbI 2 and 0.50 AgPO3-0.50 HgI 2. At 50°C, the electronic conductivities are respectively 3 X 10 -9 and 5 X 10 -7 ~2 cm -1 leading to the following electronic transport numbers: 3 X 10 -7 and 5 X 10 -3, Thus, we can admit that the nature of conduction here is essentially ionic and due to Ag+ cations.
.,~
2 PbBr2
/~
0
I
3.3. Raman spectroscopy The most significant reduced spectra: 0.9 AgPO 30.1 PbX 2 and 0.7 AgPO3-0.3 HgX2 are plotted together in figs. 4 and 5. AgPO 3 - A g X spectra are ad. ded for comparison. The characteristic spectra of the dissolved halides range between 5 cm -1 and 350 cm -1 . Indeed, no modifications were detected after 350 cm - I when adding MX 2 to the host AgPO 3 glass; that means that the polymeric phosphate framework is unchanged irrespective of the concentration of the halide. The two main polyanion bands keep their profile and no shift appears (1142 cm -1 for ~sPO2 and 650 cm -1 for ~,sPOP). Their intensity ratio (1.2) is kept at a constant value, so the halides are not inserted into the polyanion structure.
3.3.1. AgPO3-PbX 2 glasses Typical examples of corrected spectra are plotted in fig. 4. They consist essentially of two broad bands: the first at 55 cm -1 is relative to the AgPO 3 glass (2) and is not dependent on the nature of the halide, the maximum frequency of the second is centered at 115 cm -1 (PbI2), 135 cm -1 (PbBr2) and 190 cm -1 (PbC12) and depends on X. The main features of these spectra are quite similar to their homologous AgPO 3 AgX for the same X/PO 3 ratio. The discrepancies between the intensities of the normalized bands and that at 1142 cm -1 do not seem to be significant considering the experimental precision. The root of this second band should be related to the presence of AgX species in the (1-x) AgPO 3. xPbX 2 glasses as was already demonstrated in a previous paper on ( l - x ) A g P O 3.xAgX glasses (2). This band cannot be assigned to PbX 2 species. Therefore, these glasses seem to be constituted
\\
Agl /
3 Pb CI2
200
i
I
T [cm1 ]
I
100
Fig. 4. Reduced low-frequency Raman spectra of 0.9 AgPO3-0.1 PbX 2 glasses and of 0.7 AgPO3-0.3 AgX glasses for comparison,
by AgX microdomains dispersed between the polyanions of the vitreous network.
3.3.2. AgPO3-HgX 2 glasses The Raman spectra of these compounds (fig. 5) are clearly different from those of AgPO 3 - P b X 2 or AgPO 3-AgX. New strong sharp lines appear respectively at 140, 190 and 290 cm -1 for HgI2, HgBr 2 and HgC12 glasses. For mercury iodide and mercury bromide concentrations lower than 0.3, a shoulder is noticed at 170 cm -1 (HgI2) and 225 cm -1 (HgBr2).
Z B
.3 Hgl 2 ---.3 HgBr2
1
..... .2 HgCI2
\
I II
/', 300
\ .
200
.
.
.
.
.
.
.
100
Fig. 5. Reduced low-frequency Raman spectra of AgPO3 HgX2 glasses.
J.P. Malugani et al./Structural and electrical properties in (1 -x)AgPO a.xMX 2 glasses
In this composition range, the profile of these spectra is close to that of the high temperature phase t~-Ag2HgI4 : i.e. a strong line at 122 cm -1 (usHgI2-) with a shoulder at 142 cm -1 which is not defined [15]. This analogy is probably fortuitous because of the higher frequencies observed in the HgI 2 glass spectrum. In order to compare our spectra with those of the yellow high temperature phase a.HgI2, of the HgBr 2 crystallised phase and of the solid solution HglBr, in fig. 6 we plotted these three spectra with the 0.7 AgPO3-0.3 HgX2 (X = I and Br) and the 0.7 AgPO 3.0.3 HgBrI spectra. HgBr2, HgI 2 and HgBrI are built with linear HgX2 molecular units [16,17]. Their spectra were previously described with respect to the symmetry of the normal modes. The bands of the glasses are centered at the same frequencies as those of the Alg stretching modes of the corresponding HgX2 phases (141 cm -1 : HgI 2 and 188 cm -1 : HgBr2). The main lines of the mixed HglBr compound (139 cm -1 : HgI2; 155 cm -1 : HglBr and 181 cm -1 : HgBr2) correspond exactly
Hgl2 glass-- ] ~ a
135
to the three bands of the 0.7 AgPO 3-0.3 HglBr glass, with the same wave numbers and the same relative intensifies. The glass spectrum seems to be the envelope of the crystallised one. No satisfying interpretation can be given to explain the origin of the shoulders at 170 cm -1 (HgI 2 glass) and 225 cm -1 (HgBr2 glass). It cannot be assigned to HgI4 stretching modes nor to antisymmetric Uas X - H g - X modes that should occur at higher frequencies. This shoulder is not expected to be related to the same HgX 2 species as the main bands (141 and 188 cm -1) are. Increase in HgX2 concentration only enhances the intensity of the main band and not that of its shoulder. This behaviour is clearly demonstrated in fig. 7 where the intensity variations of these two bands are plotted versus the concentration for ( l - x ) AgPO 3.xHgI 2 glasses. It is interesting to note that the 140 cm -1 band intensity is proportional to the iodide concentration for x = 0.2 (i.e. I/PO 3 = 0.5), while the 170 cm -1 shoulder intensity is only slightly affected by the increase in concentration. This linear relation between intensity and iodide concentration still reinforces our proposed interpretation. The ( l - x ) AgPO 3-xHgX 2 glas-
"
.---°
^4 "'" " - ~ 1
= ~;
c
c = .D
HglBr g l a s s -
G
HglBr cryst.---°
~3
d
m
o .=
2i HgBr~glassHgBr 2 cryst.'---
I
1 ;z
2 0 0 ~[cm-'] 1OO Fig: 6. Comparison between Raman spectra of 0.7 Agt'O30.3 ttgX2glasses (X2 = 12, IBr and Br2) and ttgX2 crystallized phases.
lo [,]/[Po,]
2.o
Fig. 7. Composition dependence of log o2so C and intensity variations of v (140 cm -1) and v (170 em -1) versus molar ratio [I]/[PO 3 ] for AgPO 3-HgI 2 glasses.
136
J.P. Malugani et aL /Structuml and electrical properties in (1 , x ) AgPO s .xMX 2 glasses
ses (x > 0.2) are built with molecular HgX 2 units, similar to the high temperature ot-Hgl2 phase, dispersed between the phosphate chains of the network.
O
AgPO3 - Pbl 2
• [Ag PO 3 - Pb(PO3) 2]- Agl /
4. Discussion
. . . . AgPO 3- Agl
The above analysis of Raman spectra of AgPO 3 MX 2 glasses leads us to distinguish two kinds of glasses depending on the similarity between their spectra and those of ( l - x ) AgPO3-xAgX glasses. (i) AgPO 3 - P b X 2 glasses are constituted by a-AgI microdomains inserted between the polyanions as in AgI glasses. (ii) AgPO3-HgX 2 glasses are built with HgX2 molecular units dispersed in the host glass (at least for high HgX2 concentrations). As we suggested in a previous paper [10], these AgX clusters are the result of a coordination exchange between Ag+ and Pb 2+ cations. The Pb 2+ cations are linked preferentially to the PO 3 anions while Ag+ cations move towards X - anions to build AgX clusters between the mixed network: (Ag I _ 2yPby)PO3 ; these AgX microdomains are similar to those observed in ( l - x ) AgPO 3-xAgX glasses [2]. The same cation substitution was observed in solid phase reactions by grinding Ag4P207 , AgI and KBr mixtures to make sample pellets for IR spectroscopy [18]. When the electrical behaviour of these two kinds of glasses is taken into account, it is very likely that the AgX cluster concentration is the main factor rather than the whole halide concentration. The conductivity of the AgPO 3 - P b X 2 glasses would then be correlated to the AgX cluster concentration. In this case, the variations of conductivity should result from two antagonistic contributions: (i) the variation of the composition of the host glass from AgPO 3 to Ag(1_ 2y)Pby (PO3) by substituting all Pb 2+ cations by two equivalent Ag+ cations. (ii) the formation of a-AgX clusters resulting from the exchange reaction. The intensity of each of these two effects can be estimated by considering the variations of the conductivity of the following glasses: Ag(1 _ 2y)PbyPO3 mixed host glass (for different y values) and (1-x)(Ag(1_ 2 y ) P b y P O 3 ) . x A g X (withy kept constant) to determine the AgX effect on conductivity. The more significant results obtained with PbI 2
/
[] A g P O 3- Pb(PO3)2
/,
/
•
/
/, /
Io
_o
i IAo,
,"-(effect
exchange between '-I 2Ag" and 1Pb ÷
?
.1
.2
[,]/[,] +[PO,]
.3
Fig. 8. log o2Soc variations versus molar ratio [I]/[I] + [PO3 ] for (1 -x) AgPO3-xPbI2 : (l-x) Ago.875Pbo.125(PO3) l.125.xAgI: (l_x) AgPO3.xAgI: and (1-2y)AgPO3-yPb(PO3)2 : glasses. The exchange between 2 yAg+ and yPb2* leads to a decrease in conductivity (log o2sOc from -6.5 to -7.8 when y = 0.1) while the "AgI effect" involves an increase in conductivity (log a2sOc from -7.8 to -4.8 when x = 0.2). The intersection of the two solid lines corresponds to the following formulae: 0.9 AgPO3-0.1 PbI2 or [0.875 AgPO3-0.125 Pb(PO3)2 ]-0.2 AgI.
glasses are collected in fig. 8: the two solid lines correspond to the conductivity variations versus the molar fraction (I/I + PO3) for ( l - x ) AgPO 3 -xPbl 2 glasses and ( l - x ) [0.875 AgPO3-0.125 Pb(PO3) 2] .xAgI glasses. The intersection of these 2 isotherms corresponds to the same formula but the samples come from two different series of glasses. This common point shows the contribution of the two antagonistic effects resuiting from the total cation exchange process: the decrease in conductivity by substituting 0.2 Ag+ ions by 0.1 Pb 2+ ions in the host glass (log o from -6.5 to -7.8); and the increase in conductivity (o X 103) when 0.2 "AgI clusters" are formed. The evolution of the isotherm related to the mixed
J.P. Malugani et aL /Structural and electrical properties in ( 1 - x ) AgPO s. xMX 2 glasses
host glass leads to the following remarks: (i) the linear correlation between log a and the iodide fraction may be compared to the linear evolution observed in the "homogeneous" AgPO 3-AgI glasses; (ii) the slopes of these straight lines are different in each case; (ill) these linear evolutions show that no major modifications in the host glass (i.e. AgPO 3 or Ag 1 _ 2yPbyPO3) are involved and correspond to the "linear" increase of the cluster concentration. Substitution of Pb 2+ cations linked to PO 3 groups by Ag+ cations from AgI is therefore highly unrealistic. These ideas are in agreement with the expected total exchange Ag+ (PO3) ** Pb(X) in AgPO3-PbI 2 mixtures. Thus, the conductivity of these glasses can be correlated to the concentration of "o~-AgX" dusters, as in AgPO 3-AgX glasseS. One can therefore understand why the electrical properties (a, o0, Ee) of doped glasses AgPO 3-PbX 2 or AgPO 3-AgX are very close. This interpretation cannot be used to justify weak variations in conductivity in the AgPO 3-HgX 2 system. Indeed, conclusions from Raman spectra are in contradiction with a total exchange between the Ag+ cations bound to PO-3 polyanions and the Hg2+ cations of the dissolved halides. Simple dissolution of HgX 2 species inside AgPO 3 glass cannot be retained when considering fig. 7. The variation of the intensity of the 140 cm -1 Raman line does not seem to be compatible with a simple dissolution of HgI 2 inside AgPO 3 network. For low concentrations of HgI 2 (x < 0.20), only a slight increase of the HgI2 band intensity occurs; for 0.2 < x < 0.5 a slight increase in HgI2 concentration involves a large enhancement of the HgI 2 band intensity. In this range, this intensity increases linearly with the HgI2 fraction. If a simple dissolution of HgI 2 occurred, a linear dependence between the 140 cm-1 peak intensity and the HgI 2 fraction would be expected in the whole vitreous range. It seems that for low Hgl 2 concentration, a disappearance of "free HgI2 species" occurs. One may imagine that HgI2 is "consumed" by a partial exchange with Ag+ cations to form mixed dusters (AgI, HgI2) which should increase the conductivity, as it is observed in fig. 2, up to 0.2 Hgl 2. On the
137
other hand, for higher HgI 2 concentrations, the conductivity does not increase further because HgI2 me. lecular units are simply dispersed in AgPO 3 (fig. 7: 140 cm -1 line), which does not involve a conductivity enhancement (fig. 2). The 170 cm -1 band could be related to this new "conducting phase" (mixed AgI, HgI 2 clusters). A similar observation can be done on HgBr2 glasses in which the shoulder at 225 cm -1 could correspond to a "conducting phase". It should be remembered that for Ag2HgI4 [13] and other relevant compounds, the/3 --, a transition gives a new mode appearing as a shoulder on the high frequency side of the main band (140 cm -1 : shoulder of the 122 cm -1 line) which is assigned to HgI4 symmetric stretching mode. No satisfying explanations were given as to the origin of such band. The values of the a 0 preexponential terms of the HgI 2 glasses tend towards 3 X 103 I2 -1 cm -1 , a value close to that of Ag2Hgl 4 given by Suchow et al. [19], while the values of a 0 for PbI 2 glasses are of the same order of magnitude as AgPO 3-AgI glasses. This fact could be related to the existence of such mixed domains (AgI, HgI2). Identification of the nature of species which contribute to form the microdomains will be undertaken by neutron diffraction.
5. Conclusion This study of the electrical behaviour of mixed mono and divalent cation glasses (1-x)AgPO 3 .xMX 2 (M = Pb, Hg) dearly shows that ionic conductivity is essentially dependent on the "a-AgX dusters" concentration and is ensured by Ag+ hopping diffusion. Total Ag+ ~ Pb 2+ exchange occurs in PbX 2 glasses leading to the presence of "AgX dusters" dispersed in the network; conductivity is directly related to AgX concentration and may be compared to that of the AgPO 3-AgX glass system. The melting of AgPO 3 and HgX2 mixtures gives more complicated glasses. For high HgX2 concentrations, HgX2 molecular species are dispersed in the glass so that no conductivity enhancement is observed. For dilute HgX2 glasses, the situation is not very dear. Conductivity enhancement could be involved by the presence of mixed "AgI, HgI2 dusters".
13 8
J.P. Malugani et al./Struetural and electrical properties in (l-x)AgPOa.xMX2 glasses
References [ 1] LP. Malugani, A. Wasniewski,M. Doreau and G. Robert, Mater. Res. Bull. 13 (1978) 427. [2] J.P. Malugani and R. Mercier, Solid State Ionics 13 (1984) 293. [3] R.L Cava, F. Reidinger and BJ. Wuensh, J. Solid State Chem. 31 (1980) 69. [4] A. Fontana, G. Mariotto, E. Cazzanelli, G. Carini, M. Cutroni and M. Federico, Phys. Rev. Letters 43A (1983) 209. [5] G. Dalba, A. Fontana, P. Fomasini, G. Mariotto, M.R. MasuUo and F. Rocca, Solid State Ionics 9/10 (1983) 597. [6] T. Minami, K. Imazawa and M. Tanaka, J. Non-Cryst. Solids 42 (1980) 469. [7] M. Tachez, R. Mercier, LP. Malugani and AJ. Dianoux, Solid State Ionics 20 (1986) 93. [8] M. Tachez, J.P. Malugani, R. Mercier and P. Chieux, Annu. Rep. 84 (Institut Lane Langevin, Grenoble, France).
[9] P.C. Alien and D. Lazarus, Phys. Rev. B17 (1978) 1913. [ 10] LP. Malugani, A. Wasniewski,M. Doreau, G. Robert and R. Mercier, Mater. Res. Bull. 13 (1978) 1009. [ 11 ] J.P. Malugani, A. Wasniewski,M. Doreau and G. Robert, C.R. Acad. Sci. (Paris) 284 (1977) 99. [ 12] R. Shuker and R.W. Gammon, Phys. Rev. 25 (1970) 222. [13] A. Levasseur, J.C. Brethous, J.M. Reau, M. Couzi and P. Hagenmuller, Solid State Ionics 1 (1980) 177. [14] G. Robert, LP. Malugani and A. Saida, Solid State Ionics 3/4 (1981) 311. [ 15 ] D.R. Greig, G.C. Joy and D.F. Schriver, J. Chem. Phys. 67 (1977) 3189. [ 16] S. Nakashima, H. Mishima and H. Tai, J. Phys. Chem. Solids 35 (1974) 531. [ 17] A. Givan and A. Loewenschuss, J. Chem. Phys. 64 (1976) 1967. [ 18] T. Minami, T. Katsuda and M. Tanaka, J. Phys. Chem. 38 (1979) 1306. [ 19] L. Suchow and G.R. Pond, J. Am. Chem. Soc. 75 (1953) 5242.