Conductance and spectroscopy of protonic beta and beta″-aluminas

CONDUCTANCE AND SPECTROSCOPY OF PROTONIC BETA AND BETA”-ALUMINAS G. C. FARRINGTON, J. L. BRIANT and General

Electric Corporate

Research and Development, NY 12301, U.S.A.

H. S. STORY* P.O. Box 8, Schenectady,

and W. C. State University

BAILEY

of New York at Albany, (Recelued 1 November

Albany, NY 12222, U.S.A. 1978)

AbMraet- Fiveprotonicbeta and beta”-aluminas: viz hydrated sodium beta-alumina I1.24Na.O. llAl,O,~ hydronium bet&alumina (1.24HrO .l lAIr0, -i.6Hz6), partially dehydrated hydronium’beta-alu*ti& (1.24H,O. llAl,O, - 1.3H,O), hydrogen beta-alumina (1.24H,O. llAl,O,) and hydronium beta”-alumina (0.84H;O -0.8G&* 5Al,& ;2.8H,6) were examined‘by broad band ru&ear magnetic resonance from - 196°C to 200°C. The spectra of hydronium beta-aluminaand hydronium beta”-alumina are consistent with a mixed composition of H,O, l&O+ and H+ species in the conducting plane. Hydrogen hem-alumina and partially dehydrated hydroniurn beta-alumina appear to contain only relatively isolated (26-2.710 protons; no evidence of molecular water or hydronium ions is found. Water molecules intercalated info the conduction plane of sodium beta-alumina do not appear to be in rapid motion, even at 167”C, but are relatively stationary. The onset of motional narrowing in hydronium beta”-alumina oecura at - 40°C but not until +3O”C in bydrcmium beta-alumina. This is consistent with the higher conductivity reported Tar hydronium beta”-alumina, 1O-3-1O-’ (ohm-cm)-’ at 25”C, in comparison to 10-‘“-lO-” (ohm-cm)-‘for hydronium beta-alumina at 25°C.

INTRODUCIION

High

proton

conductivity

in the solid

state

requires

a

large concentration of potentially mobile protons, a large concentration of proton vacancies, and a low energy of proton hopping from occupied to unoawpied sites. Many structures can be identified in which the frrst two requirements are satisfied[l], but few are known in which a low energy pathway for migration is also present. Because protons are speciea intermediate in size between ions and electrons and have a large electron affinity, they do not exist in condensed phases as isolated ions, but rather as solvated ions in liquids (viz I&O*, HpOz) and hydrogen bonded to one or at most two electron donors in solids. Hydrogen bond energies can be relatively large (S-i0 kcal/mole) and contribute to the activation energy for proton hopping from filled to unIil1edproton sites in a solid structure. Most protonic solid electrolytes have conductivitica less than 10-s (ohm-cm)-1 at 25°C and activation energies of lb-20 kcal/mole (0.43-0.86 eV). Two compounds that have been found to have fast proton motion are H,OCICJ&2] and which have ionic conHUOaP04 -4H,O[3,4] ductivities reported to be 3 x 10e4 and 3 x lo-” (ohm-cm)-’ at 25”C, respectively. Proton migration through a series of interconnected H,O+(H,O), complexes, a mechanism analogous to that for proton conductivity in aqueous acids, has been proposed to explain the high conductivities observed. A similar * Permanent liddress: State University

Albany,

Albany,

NY 12222, U.S.A.

of New York at

mechanism has been suggested for beta”-alumina (0.84H,O -O.MMgO - SAl,O, .2.8H,O) which has been reported to have an ionic conductivity between 10m3 and lo-’ (ohm-cm)- 1 at 2YCf51. The conductivity of hydroni& beta-alur&& (1.24H,O - llAlzOJ .2.6H20) has been found to be only 10-Lo-lO-l’ (ohm-cm)-’ at 2S”C[6].despiteits close structural similarity to beta”-alumina. This paper discusses conductivity. stability, and broad band nuclear magnetic resonance (nmr) investigations of hydronium beta and beta”-alumina, hydrogen betaalumina, and hydrated sodium beta-alumina. CRY!EEAL SERUCITJRE The structureS of sodium beta-[?] and sodium beta”- [8] alumina have been reviewed elsewhere. Both compounds are layer structures in which the sodium ions are found in conduction planes spaced 11.2A apart. The structural backbone of each compound consists of closepacked AI-0 “spinei blocks” connected by Al-O-Al columns parallel to the c axis. The Al-O-Al bonds define the relatively open conducting planes, which are bounded by layers of close-packed oxygen ions. Ionic conduction occurs in two dimensions within the conducting planes. A diagram of theconduction plane for sodium betaalumina is shown in Fig. 1. The top close-packed oxygen layer is not shown. Reta- and beta”-alumina diRer in the spatial relationship of the top and bottom oxygen layers. In beta-alumina, the layers are eclipsed, and the conduction plane is a mirror plane. Sodium ions are distributed among three non-equivalent sites, the so-called Reevers-Ross (d), mid-oxygen (m”), and

L. BRIANT’,H.S.

G.C.FARRINGTON,J.

STORY

Table

Acid HCI HCI H&L W304 H,SO, &SO, H,SO,

OXYSEN tCONDlJCTlON FlANEI

OXYLN

socluu

(CLOSE PACKED1

Fig. 1. Beta-alumina conducting plane; d - Beever-Ross site, b ~ anti-Beever-Ross site, mo - mid-oxygen site.

anti-Beevers-Ross (b). The sodium ion occupational probability is highest in the Beevers-Ross (d) site and lowest in the anti-Beevers-Ross site (b) at WC[9]. Site occupational probabilities vary with ion type, however. For example, silver ions occupy the anti&even-Ross site significantly at 25”C[9], and iithium ions are found in the mid-oxygen sites[lO]. In sodium beta”-alumina, in contrast, the close packed oxygen layers are staggered. Ions alternate between sites having three oxygen ions above at distances of 2.69 A and one below at 2.57A and the inverse configuration. In moving through the plane, an ion undulates. occupying positions 0.17 8, above and below the plane. In sodium beta-alumina about 38% of the total of sites d and b are vacant, whereas only 17% are vacant in sodium beta”aiumina. Ionic migration takes place in both structures by a process of sequential ion hopping through the interconnected series of vacant sites. The smallest gap through which an ion must pass in beta”-alumina is the 3-O.&spacing between the two column oxygens adjoining the mid-oxygen site. This contrasts with the corresponding gap between two close packed oxygen ions in the anti-Beevers-Ross site of beta-alumina, which is only 2.OA wide. It has been suggested that beta”-alumina should be a better conductor of large cations than beta-alumina because of the increased space. This appears to be partly confirmed by recent measurementstll] showing the conductivity of potassium to be 1.2 x 10-l at 25°C in beta”-alumina, compared to 9.7 x 10e4 (ohm-cm)-’ reported for beta-alumina at 25”C[ 123.

SAMPLE PREPARATION

AND COMPOSlTlON

Single crystals of sodium beta-alumina were obtained from crushed bricks of Monofrax sodium beta-alumina. Elemental sodium analysis and weight change upon silver ion exchange both are consistent with a composition of 1.24Na,0.1 lAl,O,. Sodium beta”-alumina single crystals were grown from a melt of 35 wt% NazCOJ, 3.2 wt% MgO, and 62 wt% AllO, in a Pt or alpha alumina crucible held at 1660°C for 36 h and furnace cooled to 25’C over 5 h. The crystals grew

AND

W.C.

BAILEY

1. Ion exchange of sodium beta”-alumina dronium beta”-alumina

Temperature

Time

(“C)

(days)

90 90 90

3 7

90 190 190 240

; 3 9 6

to hy-

Exchange (% Na lost) 23 25 71

77 79 89 93

Experiments carried out on mm sized sodium beta”alumina crystals. Extent of exchangedeterminedby sodium analysis on crystals. HCl concentration = 37 wt% (12 N)

H,SO, concentration = 96 wt% (36 N)

as thin platelets, approximately 2 x 2 x 0.2 mm, on the surface of the melt. Sodium analysis and silver exchange indicate an average crystal composition of 0.84Na,O *0.84MgO - SAi,Os. Powder pattern X-ray diffraction was used to verify the crystal structures of both compounds. In preparing the”hydronium”substituted forms, the sodium content ofthecrystais was replaced by protons and associated water of hydration by immersion in aqueous acids followed by extended washing in water. Ion exchange of sodium beta-alumina requires about f4 days in concentrated sulfuric acid at 2WC[13]. Sodium beta”-alumina, in contrast, exchanges much more rapidly. The table presents a summary of the concentrations of sodium remaining in crystalline samples after various ion exchange treatments. Hydrogen beta-alumina was prepared by reducing silver beta-alumina crystals in dry H2 at 500°C For 36 h. The product readily and reversibly absorbs water when cooled below about 225°C. From the amount of water absorbed, it is apparent that the conversion procedure replaced about 40% of the silver ions originally present. In all nmr samples, small crystals rather than fine powders were used to minimize the effects of surface water. SAMPLE

STABILITY

Both hydronium beta and beta”-alumina undergo various reversible and irreversible dehydration reactions at elevated temperatures as summarized in Fig. 2. Several differences among the compounds are evident. At 25-C, the composition of hydronium betaalumina is consistent with the presence of hydronium ions in the structure. that is the ratio of H,O to H+ is about 1. The actual ionic species in the conduction planes are not necessarily hydronium ions, however. Hydronium beta”-alumina, in contrast, contains excess water at 25°C; the H,O/H* ratio is 1.7. Both compounds undergo partial and reversible dehydrations between 2OO”C-3OO”C, hydronium beta”alumina to a composition in which H#/H+ equals 1. At about 45O”C, hydronium beta”-alumina undergoes a second partial dehydration. Although the physical

Conductance and spectroscopy of protonic beta and beta”-alurninas ‘HYCfKWUM’

OEfA ALUMINA

771

presented here suggest that water in hydronium beta alumina compositions above 250°C is bound not as molecular water but as relatively isolated OH groups. IONIC CONDUCIYVITV

Results previously reported for the dc conductivities of hydronium beta and beta”-alumina are summarized O.‘34l$O# l.IH,O A1203+ H&I in Fig. 3. The conductivity of single crystals of __ O.64li$&llW~O I I hydronium beta”-alumina measured with agar elecI 1 I 1 I I 200 BJO 400 500 600 700 ml 900-c trodes has been reported as 10-3-10-5 (ohm-cm)-1 at Fig. 2. Stabilities of hydronium beta and beta”-aluminas. 25”C[4]. This range of observed conductivity is someDouble arrow indicates transition is reversible; single. what broad and may in part reflect the difficulty of arrow indicates irreversibility. jl= 11A1,03; measuring small crystals with agar electrodes while 8” = 0.84MgO 5Al 205. avoiding the formation of liquid films in crystal cracks. In a complex plane analysis of the conductivity of single crystals of hydronium beta”-alumina contacted with sputtered gold electrodes, we have recently found appearance of hydronium beta”-alumina crystals that the apparent dc conductivity varies with the state heated to 450°C is no different from those heated to of crystalline hydration. This may in part explain the only 35O”C, X-ray diffraction results indicate considerable disorder in the higher temperature form, and there range of conductivities previously reported. A detailed is no indication that the 450°C partial dehydration is investigation of the conductivity of hydronium beta”alumina single crystals contacted with sputtered gold reversible. No counterpart of this reaction is observed for hydronium beta-alumina. Both compounds irre- electrodes is in progress. versibly decompose into water and various aluminates above about 750°C. PROTON MAGNETIC RESONANCE The crystal chemical basis for the various partial Proton magnetic resonance line shapes and widths dehydration reactions observed for both hydronium were measured over a range of temperatures in an beta and beta”-alumina is not well understood. The nntr results presented in this paper illumine the iden- attempt to distinguish the various OH-, H,O and tities of the protonic species present in the several H,O* species in the conduction plane, and to incompositions, however. One obvious fact is that a vestigate the motion of the protons as reflected by the specifm fraction of the water that has dilfused into the temperature dependence of the width of the resonance lines. The results are presented in this section. conduction planes during ion exchange in aqueous acids is bound very strongly, so strongly that it is not dmorbed until 50Q°C-700°C. This suggests a specific (a) Hydrated sodium beta-alumina (1.24Naj0. llAlzO, -xH,O) structural mechanism binding water in the molecular Water penetrates the conducting planes of powform or as OH groups. In fact, the nmr results dered sodium beta-alumina (Monofrax) that has been allowed to absorb water from the atmosphere, as is demonstrated by changes in the sodium resonance spectrum[14]. Although not in itself of interest as a hydrogen conductor, this material provides a line 3 BETA ALlJYlMA shape for which a relatively simple interpretation is possible. The result will prove useful for interpretation of the more complex spectra arising from the hydrated aluminas. The spectrum obtained at 22°C is shown, somewhat smoothed, in the lower plot in Fig. 4. Ignoring the narrow (c0.4G) central response, which is probably due to surface water, the line shape is characteristic of interacting proton pairs, presumably protons in Hz0 molecules in the conduction planes. The spectrum due to relatively isolated Hz0 molecules in polycrystalline materials has been calculated by Pake[lS]. In Fig. 4 the broken line in the upper plot is the tabulated distribution function, while the continuous line on the same plot is the absorption line shape obtained by convolution of a Gaussian function, exp( - H2/26’), on the distribution function. This latter prooedure simulates broadening by the magnetic environment of the interacting proton pair. The broken line in the lower curve is the derivative of the calculated absorp Fig. 3. Conductivities of protonic solid electrolytes. A tion line shape and is compared with the experimenhydronium beta”-alumina,B - hydronium beta-alumina, C partially dehydrated hydronium beta-alumina. tal spectrum. Good agreement is obtained when ‘WOKONIUY’

o.a4nfldr 2.3n,o I 100

WITA’ ALUYINA

772

G. C. FARRfNGTON,J. L. BRIAM, H. S. STORY AF.IDW. C. BAILJZY

8

0

8

16

Fig. 4. Top: broken curve - nmr powder pattern for an interacting proton pair, solid curve - powder pattern with a broadening function. Bottom: broken curve - derivative of solid curve above, solid curve - experimental spectrum of protons in hydrated

sodium beta-alumina.

a = 4.8G and S = 1.4G. Here a = 3,@Zr3, where p is the proton magnetic moment and I is the proton-proton separation. Spectra collected at - 196°C and + 167°C are similar in shape to that obtained at 22°C. At - 196”C, LT= 4.6G and J? = 1.9G; at + 167”C, a = 4.7G and /_I= 0.5G. Within the limits of experimental error, a = 4.7 + 0.2G over this range of temperature. This corresponds to a proton-proton separation of 1.65 f 0.02 A. Line narrowing in this compound takes the form of a gradual decrease in the width of the convolution function with increasing temperature, while the intram&cular dipolar width, a, remains essentially constant. This suggests that the line narrowing is due to motion of sodium ions in the environment of relatively fixed Hz0 molecules.

PI

295 K E2’Cl

HYDRONIUY BETA ALUMA (1.24 Hz0 - liAl2O3 * 26 M2O) Fig. 5. Experimental derivative nmr spectra of hydronium beta-alumina at two temperatures.

increased from U”C!, thereis increasing proton motion as indicated by a continuous narrowing of the spectrum. A plot of the peak-to-peak width of the more intense central response as a function of tempertiure is shown in Fig. 6. The satellite responses, if present, are not observable at temperatures above roughly 100°C. The first of the above models is suggested by the similarity between these spectra and those for Monofrax. The satellite features in the 22°C spectrum are consistent with interacting protons in Hz0 with a = 4.5G and 6 = 1.8G, values similar to those found in Monofrax. As the temperature is lowered the

(b) Hydronium beta-alumim (1.24Hz0 - 11A1203 *2.6H20) Proton magnetic resonance spectra for hydronium beta-alumina at - 196 and 22°C are shown, somewhat smoothed, in Fig. 5. At 22”C, the Iine shape is consistent with either of two models: (1) Interacting proton pairs as in H,O in Monoliax beta-alumina along with additional responses due to a second proton species and (2) interacting protons in H@+ ions, each rotating or tunneling about an axis normal to the molecular plane. A third possibility is a combination of these models. As the temperature is decreased to -l%“C, there are continuous changes in the spectrum consistent with decreasing proton motion. As the temperature is

Fig. 6. Line narrowing curves for : A and A’ ~ two samples of hydronium bem”-alumina; B-hydronium beta-alumina; Chydrogen beta-alumina ; D - partiaUy dehydrated hydronium

beta-alumina.

Conductance and spectroscopy of protonic beta and beta”-aluminas spectrum broadens. At - l%OC, the outer features are best fit with a = 4.643 and 6 = 3.1G. This larger b compared with Monofrax rellacts a greater concentration of proton magnetic moments in the conduction plane consistent with the replacement of sodium ions by protons. The second or hydronium ion model is suggested by a similarity of these spectra in shape and width to spectra published by Powles and Gutowsky[l4] for methyl chloroform, CH&CI,. The spectra for methyl chloroform show gradual narrowing from a line shape at - 1%“C that is characteristic of a rigid triangular configuration of protons to one at -96°C that indicates rapid rotation of this system about an axis normal to the plane of the triangle. Distribution functions and absorption line shap have be&n calculated for each of these extremes by Andrew and Bersohn[17]. The distribution function for the rigid triangle is rather complex and is not reproduced here. The distribution function for the rotating triangle is like that for pair interactions with the addition of a singularity at the center. The function in each case is defined by the parameter a. The hydronium beta-alumina spectrum at - 196°C is not that expected for a rigid triangle but is similar in shape and width to one for methyl chloroform at a temperature where the rigid triangle line has begun to narrow. This suggests that the proton species in the alumina are HsO+ ions, and that at -196°C rigid lattice conditions have not yet been obtained. With increasing temperature the line narrows to the 22°C spectrum. This spectrum agrees reasonably with a line shape calculated for a rotating triangle with a = 4.6G and S = l.SG. This agreemerit further suggests the presence of H30+ ions in the lattice. In summary, equally valid arguments can be made for each of two proton models in this compound and, because of this, we cannot rule out the possibility of some combination of the models. (c) Partially dehydrated hydronium beto-alumina . 1.3H,O) and hydrcge~ betu(1.2H,O - llA1203 alumina (1.24H20 . llAl,O,) In partially dehydrated hydronium beta-alumina, the proton spectrum consists of a single line with peakto-peak width of about 2G at -196°C. This line persists up to about 150°C and begins to narrow as shown in Fig. 6. in hydrogen beta-alumina a similar line of 2G width at -196°C narrows to the limit of resolution of the spectrometer at about 150°C as shown in Fig. 6. A resonance line with peak-to-peak width of 2G interpreted in terms of a uniform distribution of H+ ions throughout the conduction plane implies a proton-proton spacing of about 2.7-2.8 .& or one-half the lattice parameter. (d) Hydrunium beta”-alumina (0.84Hz0 -0.84MgO r 5AlsOs . 2.8H20) Proton spectra for hydronium beta”-alumina at - 196 and 22°C are shown in Fig. 7. The line shape at -196°C is like that for hydronium beta-alumina at this temperature (Fig. 2) but slightly narrowed. This line, therefore, is consistent with either of the two models discussed in (b) for hydronium beta-alumina.

A

(0.84

773

295K(22*C)

HYDRONIUY 8d ALUMINA Ho0 . 0.04YgO . 5Al203 . 2.8 Ii201

Fig. 7. nmr spectra of hydroninm beta”-alumina at two temperatures.

As the temperature is raised from - 194 to about -70°C. the central lines narrows slightly from about 42G to about 3.8G ; from -70 to about -5O’C ; the line narrows more sharply from 3.8G to about 2SG. The broad outermost features persist over this temperature range. In the -60 to -40°C temperature range, the outermost features diminish in intensity until at about -40°C only a single. intense line is apparent. This line narrows from about 2G at 45°C to 0.4G at 22°C as shown in Fig. 6. An estimate of activation energy from the line narrowing data wouid piace it in the 2 kcal/mole (0.1 eV) range. At 22°C the beta and beta” spectra are very different. Whereas beta yields a structured line shape indicative of (H,O,H+) and/or HsO+ proton species in the lattice, beta” yields an intense, narrow (-vO.4G) Ldrentzian derivative line shape suggestive of fast proton motion throughout the lattice. CONCLUSIONS

The nmr results show that water intercalates into the conducting planes of sodium beta-alumina as simple water molecules. No evidence for rapid water or proton migration or dissociation into hydroxyl ions and protons is observed. In hydronium beta-alumina the stoichiometry corresponds to the replacement of each sodium ion by one proton and one water molecule, producing a composition suggesting the presence of h$dronium ions in the conducting plane. But, it is not possible on the basis of stoichiometry alone to conclude that the actual ionic species present are hydronium ions as opposed to water molecules and more isolated protons. On this point, the nmr evidence is also unclear. It is not possible to unequivocally decide between a hydronium model compared to a water/proton model from the rzrnr results. Some insight into the nature of the species present in the conducting plane of hydronium beta-alumina can be obtained by considering the changes in the nmr

774

G. C.

FARRINGTON, 1. L. BRIANT,H. S. STORYANDW. C. BAILEY

that occur when hydronium beta-alumina undergoes partial dehydration between 200 and 250°C. The nmr spectrum of partially dehydrated hydronium beta-alumina consists of a single structureless absorption line with none of the structure resulting from proton-proton dipole interactions seen in hydronium beta-alumina. The fact that the linewidtb is constant from 140°C to - 196°C suggests that the narrowness of the line does not redt from rapid proton motion, but rather reflects a structure in which protons are relatively isolated from each other. Although previous interpretations of Raman data[18] have concluded that protons occur as hydronium ions located in the Beevers-Ross sites ofthis compound, we find no evidence of hydronium ions or molecular water. The nmr observations suggest that the water content of partially dehydrated hydronium beta alumina is dissociated into OH groups that are tightly bound, perhaps by a specific defect mechanism, into the conducting plane. This is consistent with the stability of the compound, which undergoes only slight additional water loss from 200 to 75O”C, at which it irreversibly decomposes. Protons in the structure are probably associated with column oxygens, by analogy to the structure of deuterium beta-alumina[lP], or with close packed oxygens. In contrast, in hydronium beta-alumina a fraction of the proton content exists as water molecules, hydronium ions, or both. The nmr spectrum of hydrogen beta-alumina, like that of partiaily dehydrated hydronium beta-alumina, is consistent with the presence of relatively isolated protons in the structure. No evidence of molecular water or of hydronium ions is seen. Indications of rapid proton motion are only found at temperatures greater than 140°C. The nmr behavior of this compound is consistent with the results of England et al[lP] who found in a neutron diffraction study of deuterium beta-alumina that deuterons are bound to column oxygens at 4.5 K but are disordered at 550°C. At liquid nitrogen temperature the nmr spectrum of hydronium beta”-alumina resembles that of hydronium beta-alumina ; strong dipohr coupling among the protons is apparent. As with hydronium beta-alumina, it is not possible to definitively state that all protons exist as hydronium ions or as water molecules and isolated protons. Rather, we suggest there is an equilibrium among these three species, spectrum

similar to the equilibrium proposed for hydronium beta-alumina. Line narrowing for hydronium beta”alumina begins in the same temperature range observed for lithium ions in Li+-Na+ beta-alumina[20] and much lower in temperature than in hydronium beta-alumina. This is consistent with fast long range proton motion and the high ionic conductivity observed for this compound.

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Reu. 75, 21 (1975).

2. A. Potier and D. Row&et, J. Chim. phys. 70,873 (1973). 3. M. G. Shilton and A. T. Howe, Mater. Res. Bull. 12,701 (1977).

4. G. C. Farrington and J. L. Bciant, Mater. Res. Bttfl. 13,

763 (1978). 5. P. E. Chiids, T. K. Halstead, A. T. Howe, and M. G. Sbilton, Muter. Res. Bail. 13, 609 (1978). 6. G. C. Farrington. J. L. Briant, M. W. Breiter and W. L. Roth, J. Solid St. Chem. 24, 311 (1978). 7. W. L. Roth, F. Reidinger and S. LaPlaca, Superionic Comiuctors[Edited bv G. D. Mahan and W. L. Roth). I_o. _ 223, PIem& Press, fiew York (1976). 8. M. Bettman and C. R. Peters, J. .phvs. _ Chem.. Irhaca 73. 1774 (1969). 9. W. L. Roth, J. Solid St. Chm. 4, 60 (1972). Acta 22, 10. G. C. Farrington and W. L. Roth, Efecectrochim. 767 (1977). II. J. L. Brianl and G. C. Farrineton. Extended Abstracts No. 51. The Electrochemical !&c&y 78-2. 12. M. S. Whittingbam and R. A. Huggins, in Solid Srute Chemistry(Edited by R. S. Roth and S. J. Schneider,Jr.), p. 139, NBS Special Publication 364, U.S. Government -tinting Off&, Washington, D.C. (1972). G. C. Farrington, W. L. Roth and J. L. 13. M. W. B&a, Duf@, A4ater. Res. Bull. 12, 895 (1977). 14. D. Kline, H. S. Story and W. L. Roth, 1. &ma. Phys. 57, 5180 (1972). 15. 0. E. Pake, J. hem. Phys 16, 327 (1948). 16. J. G. Powles and H. S. Gutowsky, J. them. Phys. 21,1695 (1953). 17. E. R. Andrew and R. Bersohn, J. &em. Phys. 18, 159 (1950). 18. Ph. Colomban, G. Lucazeau, R. Mercier and A. Novak, J. them. Phys. 47, 5244 (1977). 19. W. A. England A. J. Jacobson and B. C. Tolield, J. chm. Sot. Chem. Communs 895 (1976). 20. R. R. Dubin and P. A. CaAbelia, Electrochim.Acto 24, 775 (1979).