J. Phys. Chem. 1987, 91, 5807-5813
LogQ -5.0 -5.81
-6.8
-
I
I
1
I
, ‘tj
(9) Bailey, Jr., F. E.; Koleslee, J. V.Poly(ethy/ene oxide);Academic: New York, 1976.
5807
from the shear modulus data is 7.2 f 0.75. The fact that C,(‘) is less than CIindicates that $’) is less than one. Thus, it suggests that the volume of the polymer jumping unit is greater than that of the solute (Le. M R or its cis isomer). Further, since the size of trans-MR is expected to be larger than that of cis-MR, the ratio of CI(l)to C1(*)is 5514.4 = 1.25, indicating that the molar volume of trans-MR is about 25%greater than that of cis-MR, consistent with the geometric model. It should be emphasized that eq 5 yields the result for C2in agreement with free volume theory, and the ((‘1 parameters are consistent with the size of the trans and cis form of MR, despite the fact that a t a given temperature the diffusion coefficient of trans-MR is greater than that of cis-MR. Thus, the rate of diffusion of small molecules in the polymer host is not determined by the size factor of the diffusant alone. Other factors such as the viscoelastic property of the polymer host and the interaction between the diffusant and the polymer host also play an important role. The fact that cis-MR has a smaller size, yet diffuses at a slower speed, indicates that considerable intermolecular interaction is present between the photoexcited isomer and the polymer host. The free volume theory as developed by Vrentas et al. gives a satisfactory account of the experimental results.
Acknowledgment. We thank the N S F Polymer Program (DMR-8606884) for partial financial support. Acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Registry No. MR, 493-52-7; cis-MR, 108203-16-3; PEG,25322-68-3.
Cation-Network Interactions in Binary Alkali Metal Borate Glasses. A Far-Infrared Study E. I. Kamitsos,* M. A. Karakassides, and G. D. Chryssikost Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, Athens I 16/35, Greece (Received: April I O , 1987)
The far-infrared spectra of compositions probing the glass-formingregions of all five binary alkali metal borate systems xM2041 Q 0.65, M = Li; 0 < x Q 0.40, M = Na; and 0 < x Q 0.35, M = K, Rb, Cs) have been measured and analyzed to systematically study the alkali metal cation-network interactions and their compositional dependence. Band deconvoiution of the measured spectra showed the presence of two distinct distributions of alkali metal cation sites in Li, Na, and K glasses. Similar results have been obtained for rubidium and cesium borate glasses of compositions x > 0.25. One distribution of cation sites has been observed for the lower alkali metal content Rb and Cs glasses. The fractions of cations in the two different network sites have also been evaluated. The squares of the frequencies of the cation-motion bands were found to vary linearly with composition, and exhibit kinks at x N 0.20, for all but the Cs glasses. This behavior was explained on the basis of the network structural changes known to occur at this composition.
- x)B203(0 < x
Introduction The structure of alkali metal borate glasses has been the subject of numerous investigations. Spectroscopic techniques, including NMR,I-’ Raman?’ and i ~ ~ f r a r e dhave , ~ , ~been mainly employed, and proved to be effective probes of the borate glass structure. Thus, very informative data concerning the type of the various boron-oxygen arrangements, and their relative concentrations, have been accumulated over the last two decades.’ Interest in the study of borate glasses has been lately renewed, due to the discovery of glass compositions exhibiting exceptionally high ionic conductivity. Such glasses, known as fast ionic conducting glasses, have technological importance as candidates for On leave from the Chemistry Department, Brown University, Providence, RI 02912.
0022-3654/87/2091-5807$01.50/0
energy storage applications.* Ionic conductivity in solids has been related to ion migration from one site to a neighboring one. Among other factors, the ionic hopping rate between neighboring sites? as well as the activation energy for ionic migration,I0 directly (1) Bray, P. J.; OKeefe, J. G. Phys. Chem. Glasses 1963, 4, 37. (2) Jellison, G. E.; Bray, P. J. J . Non-Cryst. Solids 1978, 29, 187. (3) Bray, P. J. J. Non-Cryst. Solids 1985, 73, 19 and references therein. (4) Brill, T. W. Philips Res. Rep. Suppl. 1976, No. 2. (5) Konijnendijk, W. L.; Stevels, J. M. J. Non-Cryst. Solids 1975, 18, 307. (6) Krogh-Moe, J. Phys. Chem. Glasses 1965,6,46 and references therein. (7) For a review article on borate glass structure see: Griscom, D. L. In Borate Glass: Structure and Applications; Pye, L. D., Frechette, V. D., Kreidl, N. K., Eds.; Plenum: New York 1978. (8) Tuller, H. L.; Button, D.P.; Uhlmann, D. R. J . Non-Cryst. Solids 1980, 40, 93.
0 1987 American Chemical Society
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The Journal of Physical Chemistry, Vol. 91, No. 22, 1987
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depends on the frequency of vibration of the mobile cation. Estimates of this so-calledattempt frequency are frequently made. Thus, Almond and West," and recently Mundy and Jin,12 estimated the effective ion attempt frequency from an analysis of the ac conductivity data of various glasses. The fact that these values appeared quite high for attempt frequencies was attributed to the possibility that an ion can perform more than one jump, depending on the number of available neighboring sites.I2 Values of the attempt frequencies can be directly obtained from far-infrared measurements, requiring no assumptions to be made, or other additional information. Far-infrared spectra of ionic oxide glasses were initially reported by Exarhos and RisenI3 for alkali metal and alkaline earth metal metaphosphate glasses. It was observed that metal cations in glass matrices give characteristic bands in the far-infrared, due to vibrations of cations in their network sites. Subsequently, cation-motion bands were observed in the far-infrared spectra of various other glass systems1e1g and proved to give valuable information for understanding transport and glass transition phenomena in glasses.10,20 The binary alkali metal borate glasses are of special interest, since they form the basis for a great variety of fast ionic conducting 600 500 400 300 200 100 systems.* Understanding the ionic conduction process in multiCM4 component glasses requires good knowledge of the corresponding Figure 1. Far-infrared spectra of xLi20.( 1 - x)B,O, glasses. phenomena in the simpler binary alkali borate glasses.21 The ways by which alkali metal ions interact with the borate network are of importance for ionic transport. Far-infrared spectroscopy x Na2G.(l-x I B 2 0 3 can provide useful information concerning such cation-network 03s interactions. Far-infrared spectra of alkali metal borate glasses have been reported for a limited number of c o m p o ~ i t i o n s . ~ ~ * ~ ~ - ~ ~ Since, such far-infrared data are highly desirable in light of the interest in fast ionic conducting glasses, we report in this work a systematic far-infrared study of all five binary alkali metal borate glasses: xM20.(l - x)B203( M = Li, Na, K, Rb, Cs). Forty-five glass sample compositions, covering the known glass-forming regions, were prepared and their far-infrared spectra were measured and analyzed. Thus, direct information on the cationnetwork interactions, and their compositional dependence, were obtained. The results are discussed in light of the structural characteristics of borate glasses known from other spectroscopic studies7 and related transport properties of these glass systems. I
1
Experimental Section Reagent grade powders of anhydrous B203and dried metal carbonates were used for preparation of the glasses studied in this work. The appropriate amounts of the starting materials were thoroughly mixed and melted in Pt crucibles in an electric furnace. Melting times of ca 15 min, and temperatures in the range 800-1 100 OC (depending on composition), were found adequate to obtain clear and bubble-free melts. Glasses were then prepared
C M-l
Figure 2. Far-infrared spectra of xNa,O.( 1 - x ) B 2 0 3 and x K 2 0 41 x ) B 2 0 3 glasses.
(9) Mcneehin. P.: Hoooer. A. J . Mater. Sci. 1977. 12. 1. P.J.; Risen, W. M., Jr. Solid State Commun.
(10) E&hos,'G. J ; Mil, 1974 79 -, -17, -_
__
(11) Almond, D. P.; West, A. R. Solid State tonics 1983, 9/10, 277. (12) Mundy, J. N.; Jin, G. L. Solid State Ionics 1986, 21, 305. (13) Exarhos, G. J.; Risen, W. M., Jt. Chem. Phys. Lett. 1971, 10, 484. (14) Paeglis, A. U. Ph.D. Thesis, Brown University, 1979. (15) Windisch, C. F. Ph.D. Thesis, Brown University, 1982. (16) Kamitsos, E. I.; Risen, W. M., Jr. J. Non-Cryst. Solids 1984,65, 333. (17) Rao, C. N. R.; Randhawa, H. S.; Reddy, N. V. R.; Chakravarty, D. Speczrochim. Acta, Part A 1975, 31A, 1283. (18) Rao, K. J.; Elliot, S. R. J . Non-Cryst. Solids 1981, 46, 371. (19) Nelson, B. N.; Exarhos, G. J. J . Chem. Phys. 1979, 71, 2739. (20) Exarhos, G . J.; Miller, P. J.; Risen, W. M., Jr. J . Chem. Phys. 1974, 60, 4145. (21) Kamitsos, E. I.; Karakassides, M. A,; Chryssikos, G. D. J . Phys. Chem. 1986, 90,4528. (22) Exarhos,G. J.; Risen, W. M., Jr. SolidStateCommun. 1972, 11, 755. (23) Kamitsos, E. I.; Karakassides, M. A.; Chryssikos, G. D. Solid State Commun. 1986,60, 885. (24) Kamitsos, E. I.; Chryssikos, G . D.; Karakassides, M. A. J . Phys. Chem. 1987, 91, 1067. (25) Button, D. P.; Mason, L. S.; Tuller, H. L.; Uhlmann, D. R. Solid State tonics 1983, 9/10, 585. (26) Kamitsos, E. I.; Karakassides, M. A.; Chryssikos, G. D. Phys. Chem. Glasses, in press.
by quenching the melt between two copper blocks. Glass compositions up to x = 0.35 ( M = K, Rb, Cs) and x = 0.40 (M = Li, Na) were obtained by this method. The glass-forming region of lithium borate glasses can be extended by a faster quenching technique. We have recently demonstrated that this can be achieved by dipping part of the melt containing crucible into cold water, while maintaining a dry nitrogen flow over the melt surface.26 Thus, lithium-containing borate glasses of compositions up to x = 0.65 were obtained. The prepared glasses were used without any further heat treatment. Selected compositions were checked and found to be X-ray amorphous. Glasses containing small or large amounts of alkali metal oxide are relatively hygroscopic. Cesium- and to a lesser extent rubidium-containing glasses were the most hygroscopic. Special care was thus taken t o avoid hydrolysis by storing all glass samples in evacuated desiccators. Samples appropriate for far-IR measurements were prepared by grinding the glasses in a vibrating mill and then dispersing the powder in low-density polyethylene. The mixture of glass/polyethylene was melted between two glass plates at 100 OC to give about 0.5 mm thick plate-shaped samples, containing about 15
Spectra of Binary Alkali Metal Borate Glasses
300
200
100
300
200
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5809
100
C M-’
Figure 3. Far-infrared spectra of xRb2O-(l - x)B203and xCs2O-(l -
x)B20, glasses.
CM’
Examples of deconvolution of far-infrared spectra of 0.30M20.0.70B203glasses. Experimental spectrum, -; simulated spectrum, and component bands, - - -. Figure 4.
e-;
wt % glass. For the hygroscopic samples, the film preparation was done in a dry nitrogen atmosphere. Far-infrared spectra were recorded on a Fourier-Transform Bruker 113V vacuum spectrometer. The Hg source, and the 3.5-, 6-, 12-, and 23-pm Mylar beam splitters were employed to cover effectively the far-IR spectral region down to 20 cm-’. Each spectrum is the result of signal averaging 100 scans at 4 cm-’ resolution. All far-infrared measurements were performed at room temperature.
Results The far-infrared spectra of characteristic compositions, probing the whole glass-forming region of all five binary borate systems, are shown in Figures 1-3. Each spectrum is characterized by a broad band, with both frequency at the absorption maximum and intensity increasing with M 2 0 content. These bands are assigned to vibrations of the alkali metal cations in their equilibrium network sites.” The weak bands at ca 465 and 550 cm-’, present in the spectra of lithium borate glasses with composition x < 0.41, are associated with vibrations of the boron-oxygen network. Weak bands in this region of the spectrum of glassy B2O3 were previously observed and assigned to network deformations. Boroxol ring angle bending and ring breathing vibrations are responsible for these absorpt i o n ~ . ~ Similar ~ ~ ’ features are present in the spectra of all alkali metal borate glasses. As noted above, the measured cation-motion bands are broad, and quite asymmetric mainly for Li, Na, and K glasses. Their large bandwidth can be accounted for by the absence of welldefined equilibrium cation sites in glasses. Instead, a broad distribution of such cation sites is characteristic of the glassy nature. Moreover, coupling of the cation oscillators, in slightly different sites, causes additional broadening of the cation-motion bands.I3 On the other hand, pronounced asymmetry of these bands, mostly in the spectra of Li, Na, and K glasses, may result from a number of different factors. These may include the existence of an asymmetric distribution of cation sites, contributions from the low-frequency “boson” peak, analogous to that observed in the Raman spectra,28and particle size. dependent light scattering in this long-wavelength spectral region.24 Obviously, a better understanding of the origin of this asymmetry requires decon-
volution of these far-IR envelopes and study of the compositional dependence of their component bands. Band deconvolution was performed by utilizing the BANDSIM Pascal program.29 For the deconvolution of the band envelopes, it is necessary to determine the number of bands to be included and give estimates for the values of their frequency at band maximum, bandwidth, and intensity. These are then taken as start values for the subsequent least-squares optimization. Before band deconvolution is performed, a background subtraction from the experimental spectra is necessary. The far-infrared spectrum of glassy B203, measured under the same conditions, was used as a background as previously described.24 Since these far-IR band envelopes, and especially those of Li, Na, and K glasses, are very broad, it is obvious that a progressively better fit would be obtained as the number of component bands increased. We start by assuming the minimum number of components which could describe these envelopes. The component bands are further assumed to have a Gaussian line shape because a better fit was obtained with a Gaussian rather than with a Lorentzian band shape. This will be clearly shown below. Representative deconvoluted spectra are shown in Figure 4 for the alkali metal borate glasses of composition x = 0.30. By using two component bands, a quite satisfactory agreement between simulated and experimental spectra is obtained for Li, Na, and K glasses. However, a third band was required to describe the high-frequency asymmetry of the spectra of Cs and R b glasses. We should recall though at this point that the Cs and Rb glasses, and especially those of high alkali metal oxide content, are the most hygroscopic. Thus this third band is probably the result of hydrolysis which could not be completely avoided during the glass and film preparation procedures. To further demonstrate this point, we present in Figure 5 the spectra of the 0.35 Cs20.0.65B2O3 glass, obtained after exposing the sample to the laboratory atmosphere for 5 min (a) and h (b). An increase of the intensity of this high-frequency third band is evident, suggesting that this feature is not inherent to the glass structure, but rather the result of hydrolysis. Indeed, the mid-infrared spectra of these glasses showed also an increased amount of -OH content, upon exposure to atmospheric conditions. Durig et al.30have observed bands near 140 and 200 cm-’, in the far-infrared spectrum of crystalline boric
(27) Kristiansen, L. A.; Krogh-Moe, J. Phys. Chem. Glasses 1968, 9,96. M.;Pelous, J.; Vacher, R.;Levasseur, A. J . Non-Crys?. Solids 1984, 69, 1.
(29) This is a Fourier-transform IR utility program available from Bruker. (30) Durig, J. R.;Green, W. H.; Marston, A. L. J. Mol. Struct. 1968.2, 19.
(28) LarBsch, J.; Couzi,
5810 The Journal of Physical Chemistry, Vol. 91, No. 22, 1987
Kamitsos et al.
350-
~
32 05 0 :
Tg
>dzOox150-
-
1100-
50 -
3
0.2
0.1
-M
-+/2
03
04
( a u-1/2 )
Figure 7. Cation-motion frequencies versus r d 2 ,where rn is the cation
mass. TABLE I: Frequencies of Cation-Motion Bands of Cations Residing in High (vH) and Low (uL) Energy Sites for xM20.(1 - x)B,03 Glasses
CM-'
Effect of hydrolysis on the far-infrared spectrum of 0.35Csz0.0.65B2O3glass. Spectrum taken after exposing the glass sample to the laboratory atmosphere for 5 min (a) and h (b).
vH, cm-'
Figure 5.
x
Li"
0.05 0.10 0.14 0.17 0.20 0.25 0.30 0.35 0.40 0.46 0.51 0.56 0.65
345 350 360 360 375 382 400 425 440 462 475 480
Na 165 175 180 192 196 213 226 237 253
K 127 137 145 147 150 161 173 185
Rb 84 84 96 99
105 108 122 129
Cs 68 75 82 84 87 94 98 99
Li 195 205 210 215 218 230 240 254 260 275 280 285
vL, cm-I Na K Rb Cs 105 82 110 84 112 87 113 88 114 91 122 94 128 102 68 50 134 110 70 50 146
a For x = 0.05 the lithium cation-motion band was weak and broad and thus no deconvolution was attempted.
Figure 6. Deconvolutionsof far-IR spectra of xMzO.(l - x)BzO, glasses. (b) M (a) M = Li, x = 0.10, one band fit (-.-.-), two bands fit = Rb, x = 0.05, one Gaussian (.-), and one Lorentzian and (c) (e-);
(-e---);
M = Cs,
x = 0.20, one Gaussian
(e-),
and one Lorentzian
(-e-.-).
acid (H,BO,), assigned to lattice translational modes. Boric acid, as well as other borate arrangements containing OH groups, can result from hydrolysis of borate glasses. Thus, this third band at ca 140 cm-' is probably the result of overlapping bands of the various hydrolysis products. Similar results to those shown in Figure 4 were obtained for the rest of the spectra of Li, Na, and K glasses. Thus, simulated spectra with two component bands agree well with the experimental ones. The spectra of the very low alkali metal content glasses (x d 0.10) containing Li, Na, or K appear quite symmetric after background subtraction. Thus, it is tempting to try to fit such spectra with one Gaussian band only. Such an attempt is shown in Figure 6 , for the Li glass of composition x = 0.10. It is observed that a better fit is achieved with two bands rather than one. Besides, a single band fit requires an unrealistically high
bandwidth, compared to that of the high-frequency band, for the rest of the glass compositions. If we ignore the third band due to hydrolysis, the spectra of R b and Cs glasses of composition x > 0.25 could be deconvoluted into two bands, while spectra of glasses with lower cesium or rubidium content could be well described with one Gaussian line. Such representative examples are depicted in Figure 6b,c for two compositions. For the purpose of comparison, fitting with one Lorentzian band is also included, showing clearly that these far-IR bands have Gaussian line shapes. To study the cation dependence of each component band of the deconvoluted far-IR band envelope, we have plotted in Figure 7 the peak frequency versus m-1/2,where m is the cation mass, for representative compositions. The frequencies at peak maxima are designated by v H and vL, for the high- and low-frequency components, respectively. The third band of Cs and R b glasses, due to hydrolysis whereever present, was not taken into account. The linear dependence of both vH and vL on m-ll2 provides strong evidence that not only the high-frequency bands but also the low-frequency bands are originating from vibrations of cations in their network sites. Although the plots in Figure 7 are useful for understanding the origin of the far-IR bands, they should not be taken to imply that m-l/* is the appropriate reduced mass of vibration. The frequencies of the deconvoluted bands, for all cations and compositions studied, are listed in Table I. The results of the deconvolution can be viewed as indicating the presence of two distinct types of distributions of cation sites in Li, Na, and K glasses. Taking into account the nature of the highest frequency band in the spectra of Cs and Rb glasses for
Spectra of Binary Alkali Metal Borate Glasses 21
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5811
Ll
ISOt
! o
0.1
a2
a3
-x-
04
0.6
05
07
Figure 8. Compositional dependence of uH2, where uH is the frequency of the higher energy cation-motion band.
I
0
01
a2
03
01
05
06
X-
Figure 10. Compositional dependence of the bandwidths AuH (a) and AuL (b) of the cation-motion bands.
I
o
I5Ot
1
01
,
1
02
Y
U1
1
04
03
”
a5
I
06
a7
-X
Figure 9. Compositional dependence of uL2, where uL is the frequency of the lower energy cation-motion band.
x > 0.25, we can also infer the existence of two cation-site distributions in such glasses. A single distribution of cation sites is observed for rubidium and cesium borate glasses of composition x < 0.25. Having established the nature of the low-frequency asymmetry of the cation-motion bands, we next investigate the compositional dependence of the two distinct cation-site distributions. Figures 8 and 9 show the compositional dependence of V H ~and vL2. Obviously both vHZ and vL2show a linear dependence on x,with a characteristic change in slope at x = 0.25-0.30 for Li and x 0.20 for the rest of the alkalis. The difference in slope in the two composition regions (Le., x < 0.25 and x > 0.25) is more evident for the data of Li, Na, and K glasses. Nevertheless it is still detectable for rubidium borate glasses but nearly absent for Cs glasses. For the Li glasses, for which the widest compositional region is available, a deviation from linearity is observed in the x-dependence of vH2 and vL2 for compositions x > 0.50. The bandwidths of the high- and low-frequency deconvoluted bands, AuH and AvL, are plotted in Figure 10 versus composition, revealing quite interesting trends. Thus, while AvH is almost independent of composition for Li, Na, and K glasses, it shows a progressive increase with x for Rb and Cs glasses. The bandwidth of the low-frequency band, AvL, shows an increase with composition for Li, Na, and K glasses. The limited data of AYL, for R b and Cs glasses, does not allow any trend to be established for these glasses. The results presented in this section will be discussed on the basis of the borate glass structure, and their implications on ionic transport will be also considered.
Discussion The vibrations of cations in their sites provide a direct probe of the interaction forces operating between them and their network anionic environments. Thus, structural changes affecting the cation-containing oxygen sites will be reflected in the far-IR cation-motion bands. Abrupt changes in physical properties of alkali metal borate glasses have been observed at compositions x = 0.15-0.20’ and became known as the “borate anomaly” effect. Although this Composition range does not coincide with that over which tetracoordinated boron atoms attain their maximum concentration, it certainly reflects structural changes of the borate network upon alkali metal oxide modification. In relation to our far-IR investigation, it appears that the kink observed in the vH2 and uL2 versus x plots, at x = 0.20-0.25 is yet another manifestation of the borate anomaly. This kink is mostly pronounced for Li, Na, and K glasses and tends to disappear for R b and Cs glasses. In fact, this is in good agreement with the variations of the thermal expansion coefficient reported recently by Shelby.31 We have recently shown that, assuming an octahedral-type oxygen site and a Born-Mayer-type potential,24the following relation can be obtained
where v is the cation motion frequency, ro is the cation-oxygen equilibrium distance, p is the reduced mass of the vibration, a is a pseudo-Madelung constant, tois the permittivity of free space, c is the speed of light, and p is the repulsion parameter. qC$ was defined by qcff2= qCqA, where qc and q A are the charges of cation and anionic site, respectively. Obviously, the reduced mass p and the equilibrium internuclear distance ro strongly depend on the nature of the cation. Both p and ro have their lowest value for lithium borate glasses, and thus eq 1 predicts that the slope of u2 versus x would be maximum for such glasses. This expectation is in agreement with the experimental results depicted in Figures 8 and 9. Considering a series of glasses of the same cation, both p and ro are constant to a first approximation. In this case, the compositional behavior of VH’ and U L is~ a direct manifestation of the compositional dependence of qe$, that is, of qA. We recall that increasing the amounts of alkali metal oxide in borate glasses causes the progressive formation of tetraborate, diborate, and finally nonbridging oxygen-containing groups.’ Since such groups contribute to the formation of the network sites containing the (31) Shelby, J. E. J . Am. Ceram. SOC.1983,66,225.
5812 The Journal of Physical Chemistry, Vol. 91, No. 22, 1987
vibrating cation, an increase of the site charge density qA with x is expected. From that point of view, we attribute the observed kink of uH2 and vL2 versus x to an abrupt change of the anionic charge density at x = 0.20-0.25. The distribution curves of the various borate groups' indicate that a number of structural changes occur simultaneously at x 0.20-0.25.31 Thus, boroxol rings disappear, diborate groups start to form, while tetraborate groups are at their maximum concentration. In contrast to tetraborate and diborate groups which are charged, boroxol rings are neutral. Thus, at compositions x C 0.20, where boroxol rings still exist, it is expected that they act as neutral spacers between charged network segments. At x N 0.20-0.25 no more boroxol rings exist, and thus an almost continuous charged network is formed. Such an effect is probably responsible for the abrupt change of the network site charge density q A . For higher x values nonbridging oxygens are also formed, contributing to even higher q A values and consequently to higher cation-motion frequencies. For lithium borate glasses of composition x > 0.50 a departure from linearity is observed in the compositional dependence of uHZ and uLz. This apparent reduction in q A can be attributed to the onset of a considerable degree of covalent interactions between lithium and oxygen atoms.26 This is reasonable, considering the fact that Li,O is one of the oxides thought to act, at certain compositions, as network formers as well.I9 Network-forming properties are associated with the formation of L i 0 4 tetrahedra, characterized by strong covalent interactions. According to Tarte32 Li04 tetrahedra give bands in the 400-550-cm-' region. We have observed a Raman band at ca 440 cm-' in the spectra of Li glasses, with x > 0.50.26The appearance of such a band is consistent with the network-forming properties of LizO, at such compositions. As shown in the Results section, this study suggests the existence of two distributions of cation sites for lithium, sodium, and potassium borate glasses, as well as for borate glasses of Rb and Cs with higher alkali metal content. It should be noted here that phase separation in alkali metal borate glasses, proposed by Shaw and Uhlmann,33 could account for such a diversity of sites. However, recent results of P ~ r a i - K o s h i tand s ~ ~Shelby31have ruled out phase separation in all five borate systems. Thus, the site distributions necessary to account for an interpretation of our data must be envisioned in a homogeneous glassy environment. In relation to this study, we note that Schmidt and collaborators have recently measured the far-IR spectra of various lithium salts dissolved in tetrahydrofuran (THF) and dimethyl sulfoxide (MezS0).35,36 The room temperature spectra showed the characteristic broad far-IR band, originating from localized Li+ motion. However, the low-temperature spectra measured at 110 K where polycrystalline solids are formed revealed splitting to two or three component bands. This was attributed to cation-anion pairing effects, which cause removal of degeneracy by symmetry reduction. This is affected either by a direct participation of the halide anion in the first coordination sphere of Li+ (for THF), or by strong cation-anion interactions (for Me2S0).35,36The two bands, obtained by deconvoluting our far-IR spectra, do not originate from such degeneracy removal effects, but result from two different oxygen environments, for the following reasons. First, the two bands are observed even at room temperature, and second, a well-defined crystalline environment is required for the degeneracy to be removed, which conflicts with the glassy nature of our materials. The results of this investigation, regarding the distributions of alkali metal ions, are interesting in light of the models proposed to account for ionic conduction in the fast ionic conducting glasses. The random site model treats all cations as equivalent assuming
Kamitsos et al.
.
-
(32) Tarte, P. Spectrochim. Acta 1964, 20, 238. (33) Shaw, R. R.; Uhlmann, D. R. J. Am. Ceram. SOC.1968, 51, 317. (34) Porai-Koshits, E. A,; Golubkov, V. V.; Titov, A. P. In Borate Glass: Structure, Properties and Applications; Pye, L . D., Frechette, V. D.,Kreidl, N . K., Eds.; Plenum: New York, 1978. (35) Chang, S.;Severson, M. W.; Schmidt, P. P. J. Phys. Chem. 1985.89, 2892. (36) Chang, S.; Schmidt, P. P.; Severson, M. W. J. Phys. Chem. 1986.90, 1046.
0.1
02
03
:c s
'Rb
0.4
05
0.6
7
-x-
Figure 11. Compositional dependence of the fraction q H of cations residing in high-energy sites.
that all of them contribute to conduction,* while the weak electrolyte mode1,37-39as well as the diffusion path is based on the distinction between two types of cations. Only one kind of them (mobile ions) are assumed to contribute to conductivity. Our results obtained by analyzing the far-IR cation-motion bands of Li, Na, and K glasses suggest the existence of two types of cation distributions. The fraction of cations residing in high and low potential energy wells (sites), q H and qL, respectively, can now ~ *integrated ~ absorbance be evaluated. As previously ~ h o w n , ' ~the of the cation-motion band is given by
where N is the number of cation oscillators. Let N be NH and NL for cations in high- and low-energy sites, respectively. Assuming that for a specific cation I.L and ro are independent of the cation environment, eq 1 and 2 can be combined to give (3) where and ( A ) Hare obtained by deconvoluting the cation motion bands. Then the fractions q H and qL are given by
where /3 = NL/NH. Figure 11 gives the compositional dependence of q H for all alkali metal borate glasses and shows quite interesting trends. Thus, while Na- and K-containing glasses show a similar behavior, described by a decrease in the population of the highenergy sites with x, the opposite trend is exhibited by Li-containing glasses. While the frequency of the cation-motion band is related to the depth of the potential well for a particular site, the bandwidth is associated with the interactions between neighboring sites. Thus, the distribution and interaction energy between the high-energy sites (Au,) appears to be independent of composition for Li, Na, and K glasses, while that for the low-energy sites increases systematically with x. Cations located in such sites are then expected to contribute more to the conduction process. However, based on these far-IR results alone, it is not possible to rule out con(37) Ravaine, D.; Souquet, J. L. Phys. Chem. Glasses 1977, 18, 27. (38) Ingram, M. D.; Moyniham, C . T.; Lesikar, A. V. J . Non-Cryst. Solids 1980, 38/39, 37 1. (39) Monynihan, C . T.; Lesikar, A. V. J . Am. Ceram. Soc. 1981, 64,40. (40) Minami, T. J . Non-Cryst. Solids 1985, 73, 273.
J. Phys. Chem. 1987, 91, 5813-5818 tributions to conductivity from cations in high-energy sites. The random site model of nearly equivalent cation sites probably gives a better description of the behavior of the majority of Rb and Cs glasses studied in this work. Thus, it appears that there is not a uniform picture for the cationsite interaction, for all five alkali metal borate systems. The distribution of the anionic charge over the glass network, and thus the site charge density, is the important factor determining such cationsite interactions. It is obvious that the site charge density qA is a function of the various borate groups present at each composition x. The compositional dependence of such borate groups is usually taken as being independent of the ~ a t i o n .However, ~ this investigation has shown pronounced differences, between the various cations, in the compositional dependence of their cation-motion bands. A better understanding of such differences requires a more detailed knowledge of the possible preference of each cation to specific boron-oxygen arrangements which in fact dictates the distribution of the anionic charge. Such useful information can be obtained from a comparative Raman and mid-infrared study of the five alkali metal borate systems. This work is in progress and results will be published later.
Conclusions Binary alkali metal borate glasses are of principal interest in view of their importance for fast ionic conducting glass systems. Understanding ionic transport phenomena in such glasses requires a good knowledge of the interaction forces between the charge carrying cations and the borate lattice. Far-infrared spectroscopy provides a direct probe of.these cation-network interactions and their compositional dependence. We report in this study far-infrared spectra of glasses covering all five alkali metal borate glass-forming regions. The cation-motion bands have been observed and analyzed to provide better understanding of their compositional and cation
5813
dependence. Band deconvolution, preformed on all measured spectra, revealed the existence of two kinds of distributions of cation sites in Li, Na, and K glasses. This was also observed for Rb and Cs glasses of high alkali metal oxide content (x > 0.25). However, for lower x values, the latter glasses appeared to be well described by one distribution of cation sites. The squares of the frequencies of cation vibrations, located in high and low potential energy sites, were found to vary linearly with composition, with a characteristic change in slope a t x N 0 . 2 0 . 2 5 . This kink was mostly pronounced for Li, Na, and K glasses and was observed to disappear for Cs glasses. The origin of this effect was attributed to a number of combined structural 0.20. Such changes in the borate network occurring at x changes result in an abrupt increase of the anionic charge density, which in turn is reflected by the compositional dependence of the cation-motion frequency squared. The relationship to the borate anomaly was also discussed. The results of this far-IR study, concerning the kinds of distributions of cation sites, were also considered in terms of the existing models for ionic transport in fast ionic conducting glasses. It was demonstrated that it is difficult to use a single model to describe the behavior of cations in all five alkali metal borate glass systems. ,
Acknowledgment. We express our appreciation to Professor C. A. Nicolaides, of N H R F , for support and encouragement throughout this work. Helpful discussions with Professor P. J. Bray are also gratefully acknowledged. Professor P. P. Schmidt is thanked for providing reprints of his work on far-IR studies of the solvation of alkali metal cations. G.D.C. thanks Professor C. A. Nicolaides and Professor W. M. Risen for making his collaboration possible. This project has been financially supported by NHRF. Registry No. Cs20, 20281-00-9; Rb20, 18088-1 1-4.
Influence of Oxldation State, pH, and Counterion on the Conductlvity of Polyaniline Walter W. Focke,l Gary E. Wnek,*2 and Yen Wei3 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 021 39 (Received: February 9, 1987; In Final Form: May 7, 1987)
The resistivity of electrochemicallysynthesized polyaniline films was measured with the films submerged in electrolyte. The resistivity was found to depend on the redox state of the film, the pH of the solution and, to a lesser extent, on the type of anion present. The resistivity at a given pH is low but only inside a narrow potential window. The width of this window decreases with increasing pH and vanishes at pH 6. The walls of the potential window correlate roughly with the formal potentials of the redox processes as determined by cyclic voltammetry. It has been shown that the resistivity of polyaniline depends on its moisture content. For the wet polymer a small degree of protonation is apparently sufficient to cause a decrease in resistivity of more than 6 orders of magnitude. This behavior may be rationalized by assuming that, in the presence of water, the charge transport mechanism involves proton exchange reactions as well as intermolecular electron transport.
Introduction Polyaniline (PAn) is the electroactive polymer obtained by the chemical4or electrochemical5oxidation of aniline in acidic aqueous media. It is now well established that the structure of PAn, in the base form,is that of a para-linked phenyleneaminimine!.6 As (1) Present address: NIMR, CSIR, Box 395, Pretoria OOO1, South Africa. (2) To whom cOrreSpOndence and requests for reprints should be addressed at the Department of Chemistry, Rensselaer Polytechnic Institute, Troy, N Y 12180-3590. (3) Present address: Department of Chemistry, Drexel University, Philadelphia, PA 19104. (4) (a) Willstfitter, R.; Dorogi, S. Chem. Ber. 1909, 42, 2143. (b) Ibid. 1909,42,4118. (c) Green, A. G.;'Wocdhead, A. E. J. Chem. SOC.1910, 97, 2388. (d) Ibid. 1912, 101, 1117. (5) Diaz, A. F.;Logan, J. A. J . Electroanal. Chem. 1980, I I I, 1 1 1.
such the polymer can exist in various oxidation states characterized by the ratio of imine to amine nitrogens. The conductive form of polyaniline is associated with intermediate oxidation states, e.g., emeraldine which features an equal number of imine and amine nitrogens in the free-base form.7 In the neutralized free-base form, the polymer is an insulator. The conductive salt form is obtained upon treatment with simple Brernsted acids.7 A polyradical cation nature has been postulated (6) (a) Lu, F.-L.; Wudl, F.;Nowak, M.; Heeger, A. J. J . Am. Chem. SOC. 1986, 108, 8311. (b) Vachon, D. J.; Angus, R. 0.; Lu, F.-L.; Liu, Z. X.; Shaefer, H.; Wudl, F.;Heeger, A. J. Synfh. Met. 1987, 18, 297. (7) (a) MacDiarmid, A. G.; Chiang, J.-C.; Halpern, M.; Huang, W.-S.; Mu, S.-L.;Somasiri, N . L. D.; Wu, W.; Yangier, S. I. Mol. Cryst. Liq. Cryst. 1985, 121, 173. (b) MacDiarmid, A. G.; Chiang, J.-C.; Huang, W.; Humphrey, B. D.; Somasiri, N.L.D. Mol. Crysf.Liq. Cryst. 1985, 125, 309.
0022-3654/87/2091-5813$01.50/00 1987 American Chemical Society
