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Hydronium beta″ alumina: A fast proton conductor
Mat. Res. Bull. Vol. 13, pp. 763-773, 1978. P e r g a m o n P r e s s , Inc. Printed in the United States.
HYDRONIUM
General
BETA"
ALUMINA:
A FAST PROTON
CONDUCTOR
G.C. Farrington and J.L. Briant Electric Corporate Research and D e v e l o p m e n t P.O. Box 8 Schenectady, NY 12301
(Received June 7, 1978; Communicated by R. C. DeVries) ABSTRACT Single crystals of sodium beta" alumina (0.84NapO'0.84MgO-5 AI203) undergo rapid ion exchange in c o n c e n t r a t e d sulfuric acid to produce "hydronium" beta alumina (0.84) H20-0.84MgO-5AI203-2.8H20). H y d r o n i u m beta" alumina undergoes a partial, reversible d e h y d r a t i o n between 250-300°C and irreversibly decomposes into alpha alumina and w a t e r above 700°C. The c o n d u c t i v i t y of h y d r o n i u m beta" alumina has been m e a s u r e d with blockinqland= nonblocking electrodes and is 5 x 10 -3 (ohm cm) at 25°C. The high c o n d u c t i v i t y is interpreted on the basis of a two dimensional liquid model. Introduction Protonic conduction in organic and inorganic solids has been e x t e n s i v e l y explored (i). Typical protonic conductors include inorganic salts and acids, small organic molecules, and biological polymers. Most have c o n d u c t i v i t i e s less than about 10 -7 (ohm cm)-l_at 25°C, as shown for KH2PO 4 (2) and Li2so4-H20 (3) in Figure ±. A c o n d u c t i v i t y of as low as 10-15 (ohm cm) -± has been reported for benzoic acid at 25°C (4). The few exceptions are perchloric acid monohydrate, w h i c h has a c o n d u c t i v i t y at 25°C reported to be 3 x 10 -4 (ohm cm)-i (5) and HUO2PO4"4H20 recently found to have a c o n d u c t i v i t y of 5 x 10 -3 at 25°C (6). The d i s c o v e r y of the high ionic c o n d u c t i v i t y of sodium beta alumina (ca. 1.24Na20-IIAI203) stimulated the i n v e s t i g a t i o n of fast ionic c o n d u c t i v i t y in many inorganic s o l i d structures. Beta alumina itself is a p a r t i c u l a r l y versatile solid electrolyte, since its entire m o b i l e sodium content can be replaced by a variety of m o n o v a l e n t cations, including Li +, K+, Ag + and, most importantly, H +, H3 O+, and NH4 + (7). I n v e s t i g a t i o n s of the 753
764
G.C. FARRINGTON,
I0"1
I
J
i0-2
I
I
et I
al.
Vol. 13. No. 8
I
I
-r~No BETAALUMINA ~ ~ H(HzO)xBETA'ALUMINA
_
i0-3
10-4
_
i0-5
T
I0.6
_
/~
H30C104
H30 BETAALUMINA
i
io -7 I--
10-8
/Z
p-
=o I° 9 r~ Z O
o
__ -
i0-10 i0-H
~/l ! "Li]O4"IHI-LiSO4"IH20 I I ~__ 10-12 10-13 10"14 -I00 0 I00 200 300 400 500 600 TEMPERATURE (°C) FIG.
1
Conductivities of various inorganic proton conducting solid electrolytes.
preparation (8,9), stability (i0), and c o n d u c t i v i t y (ll) of h y d r o n i u m beta alumina have found that M o n o f r a x single crystals of sodium beta alumina exposed to concentrated sulfuric acid at about 290°C for i0 days undergo ion exchange to produce a composition that corresponds in its stoichiometry to complete replacement of sodium by h y d r o n i u m ions. The resulting "hydronium" beta alumina (I.24H20"IIAI203-2.6H20) has a conductivity of about 10-11 (ohm cm) -I at 25°C, nine orders of m a g n i t u d e lower than that of sodium beta alumina at the same temperature. H y d r o n i u m beta alumina undergoes a partial and reversible d e h y d r a t i o n between 180-250°C in which about half of the h y d r o n i u m ions lose one water molecule of hydration. The partially dehydrated form (I.24H20-IIAI203"I.3H20) also has a very low conductivity and above 700°C irreversibly decomposes to various hydrated aluminas and water.
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HYDRONIUM B E T A " ALUMINA
765
This paper p r e s e n t s the results of an i n v e s t i g a t i o n into the preparation, stability, and c o n d u c t i v i t y of h y d r o n i u m beta" alumina ( 0 . 8 4 H 2 0 - 0 . 8 4 M g O - 5 A I 2 0 3 - 2 . 8 H 2 0 ) . This compound is particularly i n t r i g u i n g since, despite its s t r u c t u r a l s i m i l a r i t y to h y d r o n i u m beta alumina, we have found and p r e v i o u s l y reported (12) that its ionic c o n d u c t i v i t y at 25°C is about 10 -2 (ohm cm) -I. S o d i u m Beta and Beta"
Alumina Crystal
Structures
S o d i u m beta" a l u m i n a occurs as a ternary phase with an ideal formula of N a 2 0 - M g O - 5 A I 2 0 3 or N a 2 0 - L i 2 0 - 5 A I 2 0 3 , in which MgO or Li20 are i n c o r p o r a t e d to "stabilize" the structure (13). As synthesized, sodium beta" alumina is n o n - s t o i c h i o m e t r i c and has a typical c o m p o s i t i o n of 0 . 8 4 N a 2 0 - 0 . 8 4 M g O - 5 A I 2 0 3 . It is closely related in s t r u c t u r e to sodium beta alumina w h i c h is also nonstoichiometric, having a c h a r a c t e r i s t i c c o m p o s i t i o n of 1.24Na20IIAI203. As w i t h sodium beta alumina, it is possible to replace the sodium c o n t e n t of beta" a l u m i n a by Li+, K +, NH4 +, and H+(H20) x, as well as w i t h other m o n o v a l e n t and d i v a l e n t cations. Both sodium b e t a and beta" aluminas are layer structures in w h i c h the sodium ions are found in c o n d u c t i n g planes spaced 11.2 A apart. The s t r u c t u r a l b a c k b o n e of each compound consists of c l o s e - p a c k e d A I - O "spinel blocks" c o n n e c t e d by A I - O - A I columns p a r a l l e l to the c axis. The A I - O - A I bonds define the r e l a t i v e l y open c o n d u c t i n g planes, w h i c h are boundeH by layers of closepacked o x y g e n ions. Ionic c o n d u c t i o n occurs in two d i m e n s i o n s w i t h i n the c o n d u c t i n g planes. A m o r e complete d e s c r i p t i o n of the s t r u c t u r e s of beta and beta" a l u m i n a can be found e l s e w h e r e (13,14,15). A d i a g r a m of the c o n d u c t i o n plane in sodium beta alumina is shown in Figure 2. The plane is a mirror plane; only one close packed o x y g e n layer is shown. The spacing b e t w e e n the oxygen layers varies w i t h ionic s u b s t i t u t i o n and, for example, is 4.76 A in sodium beta alumina and 5.11 A in r u b i d i u m beta alumina. Sodium ions are d i s t r i b u t e d among three n o n - e q u i v a l e n t sites, the so-called Beevers-Ross, mid-oxygen, and a n t i - B e e v e r s - R o s s sites. Figure 2 r e p r e s e n t s s t o i c h i o m e t r i c beta a l u m i n a w i t h sodium o c c u p a t i o n of each B e e v e r s - R o s s site. Actuallyl nons t o i c h i o m e t r i c b e t a alumina has an excess of sodium ions, w h i c h occur as m i d - o x y g e n / m i d - o x y g e n ion pairs. D i f f u s i o n takes place as the result of s e q u e n t i a l ion h o p p i n g through the i n t e r c o n n e c t e d sequence of ion sites. A sodium ion comes closest to n e i g h b o r i n g oxygens in the a n t i - B e e v e r s - R o s s site, in w h i c h it moves through a gap of 2.0 A b e t w e e n a pair of o x y g e n ions. P a s s a g e through the a n t i - B e e v e r s - R o s s site represents the e n e r g e t i c a l l y most d i f f i c u l t step in ion migration. The structure of the s t o i c h i o m e t r i c sodium beta" a l u m i n a c o n d u c t i n g plane is shown in Figure 3. All sodium sites are shown filled, w h e r e a s about 16% are v a c a n t in actual samples. The
766
G.C.
Q OXYGEN (CONDUCTIONPLANE)
FARRINGTON, et al.
Q
®
OXYGEN (CLOSE PACKED) FIG.
Vol. 13, No. 8
SODIUM
2
C o n d u c t i o n p l a n e of s o d i u m b e t a alumina. Sodium ions are s h o w n in p r e f e r r e d B e e v e r s - R o s s (d) sites. A d d i t i o n a l m o b i l e ion sites are the mid o x y g e n (m.o.) and a n t i - B e e v e r s - R o s s (b).
OXYGEN (CONDUCTION PLANE}
/ ~ ~ ~ FIG.
OXYGEN J (CLOSE PACKED)
3
C o n d u c t i o n p l a n e of s o d i u m beta" alumina. S o d i u m ions are s h o w n in the p r e f e r r e d sites, w h i c h are s e p a r a t e d by the m i d - o x y g e n (m.o.) positions.
SODIUM
Vol. 13, No. 8
HYDRONIUM BETA" ALUMINA
767
structure differs from that of sodium beta alumina in several s i g n i f i c a n t ways. The c o n d u c t i n g plane is not a m i r r o r plane, and all sodium sites are c r y s t a l l o g r a p h i c a l l y equivalent. Ions a l t e r n a t e b e t w e e n sites having three oxygen ions above at distances of 2.69 A and one b e l o w at 2.57 A and the inverse configuration. In each site, a sodium ion is 3.25 A from each of three oxygen ions w i t h i n the c o n d u c t i o n plane. In moving through the plane, the ion undulates, o c c u p y i n g positions 0.17 A above and below the m i d p o i n t of the plane. The smallest gap through which an ion m u s t pass is the 3.0 A spacing b e t w e e n the two column oxygens a d j o i n i n g the m i d - o x y g e n site. This contrasts with the c o r r e s p o n d i n g gap b e t w e e n two c l o s e - p a c k e d oxygen ions in the a n t i - B e e v e r s - R o s s site of beta alumina w h i c h is only 2.0 A wide. Crystal P r e p a r a t i o n
and C o m p o s i t i o n
Single crystals of sodium beta" alumina were grown from a m e l t of 35 wt. % Na2CO3, 3.2 wt. % MgO, and 62 wt. % AI20 3 by h e a t i n g the m i x t u r e in a c o v e r e d Pt or alpha alumina crucible to 1660°C for 24 hours and c o o l i n g to 25°C over 12 hours. As Na20 vaporized, the crystals grew as thin p l a t e l e t s about 2x2x0.2mm on the surface of the melt. They had average compositions of 0 . 8 4 N a 2 0 " 0 . 8 M g O - 5 A I 2 0 3, as d e t e r m i n e d by sodium and m a g n e s i u m analysis. P o w d e r p a t t e r n x-ray d i f f r a c t i o n was used to v e r i f y the crystal structures. Ion exchange to the h y d r o n i u m form was carried out by immersing the crystals in c o n c e n t r a t e d sulfuric acid at 240°C for several days, followed by w a s h i n g in water and air drying. S o d i u m analysis after exchange found less than 5% of the original sodium c o n t e n t r e m a i n i n g in the samples. P r e l i m i n a r y t h e r m o g r a v i m e t r i c analysis (TGA) in dry N 2 at a sweep rate of l°C/min, on single crystals of h y d r o n i u m beta" alumina reveals a 0.5% w e i g h t loss at 70°C and an invariant c o m p o s i t i o n from 100°C to 200°C. B e t w e e n 250°C and 280°C a p a r t i a l r e v e r s i b l e d e h y d r a t i o n with a 3.0% w e i g h t loss occurs p r o d u c i n g a new c o m p o s i t i o n w h i c h is stable until about 700°C, above w h i c h complete d e c o m p o s i t i o n takes place. These reactions are s u m m a r i z e d in E q u a t i o n i. 0.84H20-0.84MgO-5AI203.2.8H20 ÷ 0.84H20.0.8 4MgO-5AI203.1.9H20+0.9H20 275°C
[I]
+ > 700°C 3.6H20+0 .84~/0+5A1203
H y d r o n i u m beta alumina u n d e r g o e s which are s u m m a r i z e d in E q u a t i o n
analogous 2.
dehydration
reactions
1.24H20-IIAI203-2.6H20 ++ 1.24H20"IIAI203"I.3~hO+I.3H~O ÷ IIAI203+3.8H20 225oc ->700°C
[2]
768
G . C . FARRINGTON, et al.
Conductivity
Vol. 13, No. 8
Measurements
The c o n d u c t i v i t i e s of six h y d r o n i u m beta" alumina crystals were measured, five with n o n - b l o c k i n g agar gel or p o l y e t h y l e n e oxide ion exchange m e m b r a n e electrodes, and one with gold blocking electrodes. I m p e d a n c e s were m e a s u r e d with a H e w l e t t - P a c k a r d 3040 A N e t w o r k Analyzer. With n o n - b l o c k i n g electrodes the impedance of each crystal was found to be i n v a r i a n t w i t h i n 10% from 102 to 105 Hz with a phase shift of less than 10 ° . M e a s u r e m e n t s with blocking electrodes w e r e carried out from 102 to 107 Hz and found to approach a straight line intersecting the real axis on a complex impedance plot at high frequencies. The real i n t e r c e p t was taken as the sample resistance. Samples were m o u n t e d for analysis using several d i f f e r e n t techniques. To p r e v e n t liquid e l e c t r o l y t e from the agar or p o l y e t h y l e n e oxide e l e c t r o d e s from creeping around the samples, the crystals were cast in epoxy resin w h i c h was then ground away to expose the crystal edges. Samples were also m e a s u r e d m o u n t e d in p a r a f f i n and w i t h o u t mounting. For b l o c k i n g electrode experiments, the crystals were m o u n t e d w i t h insulating v a r n i s h on alpha alumina discs, and gold e l e c t r o d e s were sputtered onto the edges. A number of e x p e r i m e n t s were carried out to verify the m e a s u r e m e n t procedures. To insure that liquid films from the electrodes did not c o n t r i b u t e to the conductivities, h y d r o n i u m beta alumina crystals, which are similar in physical c h a r a c t e r i s tics and surface p r o p e r t i e s to h y d r o n i u m beta" alumina, w e r e m e a s u r e d with and w i t h o u t casting in epoxy. No c o n d u c t i v i t y was observed, as expected from the low c o n d u c t i v i t y reported for that compound, i0 -II (ohm cm)-i at 25°C. Also, a small M o n o f r a x sodium beta alumina crystal was m o u n t e d with v a r n i s h on an alpha alumina disc and c o n t a c t e d with sputtered gold electrodes. At 25°C its conductivity, as d e t e r m i n e d from the e x t r a p o l a t i o n of a complex impedance plot, was found to be 1 x 10-2 (ohm cm)-l, in good a g r e e m e n t with the value of 1.4 x 10-2 (ohm cm)-i p r e v i o u s l y reported (16). Results
and D i s c u s s i o n
Our e x p e r i m e n t a l results using n o n - b l o c k i n g electrodes, s u m m a r i z e d in the Table, i n d i c a t e an average ionic c o n d u c t i v i t y of h y d r o n i u m beta" alumina of 5 x 10-3 (ohm cm)-i at 25°C. This agrees w i t h the m e a s u r e m e n t obtained with b l o c k i n g electrodes: 8.3 x 10-3 (ohm cm) -I at 25°C. Several previous reports of the partial ion exchange of sodium beta" alumina are e x p l a i n e d by our findings. Th~ry and B r i a n c o n (17) noted that sodium beta" alumina readily hydrolyzes in water at 100°C. The h y d r o l y z e d p r o d u c t d e c o m p o s e s at 1200°C to AI203 and NaAI508, as would be expected for p a r t i a l l y exchanged h y d r o n i u m - s o d i u m beta" alumina. B e t t m a n and Peters (13) exposed sodium beta" alumina to aqueous HCI at 25°C and found that many of the s o d i u m ions exchange with protons and an unknown q u a n t i t y of water molecules. Similar exchange is not observed for sodium beta alumina at this temperature. Both of these
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reports alumina
HYDRONIUM B E T A " ALUMINA
769
indicate that h y d r o n i u m s u b s t i t u t i o n into the beta" s t r u c t u r e proceeds much more rapidly than in beta a l u m i n a
The c o n d u c t i v i t y of h y d r o n i u m beta" alumina is c o m p a r e d with that of several other i n o r g a n i c protonic conductors in Figure i. H y d r o n i u m beta" alumina has a c o n d u c t i v i t y c o m p a r a b l e to sodium beta and beta" alumina at 25°C, an e x t r a o r d i n a r i l y high value for a proton conductor. Two protonic compounds w h i c h approach its c o n d u c t i v i t y are p e r c h l o r i c acid m o n o h y d r a t e and UO2H20(PO4) 3. P e r c h l o r i c acid m o n o h y d r a t e exists in a monoclinic c r y s t a l l i n e form below 30°C, c o n s i s t i n g of h y d r o g e n bonded layers of p e r c h l o r a t e and h y d r o n i u m ions, and in an o r t h o r h o m b i c structure b e t w e e n -30°C and 47°C, at w h i c h point it dissolves in its water of hydration. The higher t e m p e r a t u r e phase is highly d i s o r d e r e d and has a reported c o n d u c t i v i t y of 3 x 10 -4 at 25°C w i t h an a c t i v a t i o n energy of 5.6 k c a l / m o l e (5). Recently it was reported (6) that H U O 2 P O 4 " 4 H 2 0 has a c o n d u c t i v i t y of 5 x 10-3 (ohm cm) -I at 25°C which increases with t e m p e r a t u r e with an a c t i v a t i o n energy of 7.2 k c a l / m o l e until about 80°C at which d e h y d r a t i o n begins and the c o n d u q t i v i t y decreases. These compounds are exceptions, however, to the g e n e r a l l y low cond u c t i v i t y of protonic solids. The most striking aspect of the high ionic c o n d u c t i v i t y of h y d r o n i u m beta" a l u m i n a is that it presents such a contrast to the extremely low c o n d u c t i v i t y of h y d r o n i u m beta alumina. As m e n t i o n e d previously, the compounds have similar structures and c o m p a r a b l e c o n d u c t i v i t i e s in their sodium forms, 1.4 x 10 -2 (ohm cm)-i for sodium beta a l u m i n a and 1.0 x I0 -I (ohm cm)-i for sodium beta" alumina at 25°C. One s i g n i f i c a n t d i f f e r e n c e b e t w e e n the c o m p o s i t i o n s of h y d r o n i u m beta and beta" alumina is in the water contents. During ion exchange each sodium ion is replaced by a proton and a q u a n t i t y of water. Hydronium beta alumina i n c o r p o r a t e s one water m o l e c u l e for each p r o t o n present. H y d r o n i u m beta" alumina contains 1.7 m o l e c u l e s of water for each proton. These observations, combined with a c o n s i d e r a t i o n of the structure of each compound, lead to a possible e x p l a n a t i o n for the large d i f f e r e n c e in their conductivities. The c o n d u c t i n g plane of sodium beta alumina is shown in Figure 2. In h y d r o n i u m beta alumina, each sodium ion has been replaced by a single p r o t o n and a water molecule. A l t h o u g h the required s t o i c h i o m e t r y can be satisfied by a structure c o n s i s t i n g of separate protons and water molecules, recent infrared spectroscopy and wide line NMR results (18) suggest that h y d r o n i u m ions are indeed present as d i s t i n c t species. From examining the h y d r o n i u m beta alumina structure, it is a p p a r e n t that more w a t e r m i g h t be absorbed into the conducting plane than that required for the h y d r a t i o n of each proton This extra w a t e r could o c c u p y m i d - o x y g e n sites. However, there is no evidence in single crystals that extra h y d r a t i o n takes place, ffydronium ions in the B e e v e r s - R o s s or m i d - o x y g e n sites are t h e r e f o r e isolated from each other and h y d r o g e n b o n d e d to n e i g h b o r i n g s t r u c t u r a l oxygen ions. C o n d u c t i v i t y in h y d r o n i u m
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G . C . FARRINGTON, et al.
Vol. 13, No. 8
beta a l u m i n a involves h y d r o n i u m ion m i g r a t i o n or proton hopping among s t r u c t u r a l o x y g e n ions. Each process is impeded by the formation of h y d r o g e n bonds b e t w e e n m o b i l e ions and the conducting plane structure. H y d r o g e n b o n d i n g increases the a c t i v a t i o n energy of hopping and decreases the conductivity. In h y d r o n i u m beta" alumina, in contrast, extra h y d r a t i o n does occur. The greater water c o n t e n t of the structure can be related to its high c o n d u c t i v i t y by c o n s i d e r i n g the sodium beta" alumina c o n d u c t i n g plane shown in Figures 3 and 4. The column oxygen ions are shown by large circles; the close packed oxygens above and below the c o n d u c t i n g plane are indicated by + and - signs c o r r e s p o n d i n g to their p o s i t i o n s relative to the plane. The close packed oxygen layers are not eclipsed in beta" alumina as they are in beta alumina. Both sodium sites are e q u i v a l e n t and each would be occupied in s t o i c h i o m e t r i c beta" alumina. S o d i u m beta" alumina is n o n - s t o i c h i o m e t r i c , however, and has a d e f i c i t of sodium ions and a c o r r e s p o n d i n g number of vacancies in the conducting plane. The region in Figure 4 has a statistical o c c u p a t i o n of 1.68 sodium ions in the sodium beta" alumina used in this study, a m o b i l e ion content 35% higher than in M o n o f r a x sodium beta alumina. B e t t m a n and Peters (12) suggested in their d i s c u s s i o n of the s t r u c t u r e of sodium beta" alumina that sodium ions m i g h t be placed either in the p o s i t i o n s shown in Figure 4 or in the large number of m i d - o x y g e n sites, shown as dashed circles. Ions are not found in the m i d - o x y g e n sites in the sodium isomorph, but o c c u p a t i o n of the m i d - o x y g e n sites by water and h y d r o n i u m ions is i m p o r t a n t in r a t i o n a l i z i n g the properties of h y d r o n i u m beta" alumina. We propose that all of the m i d - o x y g e n sites in h y d r o n i u m beta" alumina are occupied by either h y d r o n i u m ions or water molecules. The c o n d u c t i n g plane structure is e s s e n t i a l l y that of a two dimensional, c l o s e - p a c k e d liquid having a high conc e n t r a t i o n of charge carriers. Ionic c o n d u c t i o n occurs by proton exchange and m i g r a t i o n through the c o n d u c t i o n plane. Proton transport is expected to be much more rapid than oxygen transport. T h i s m e c h a n i s m is analogous to the Grotthus m e c h a n i s m (18) for c o n d u c t i o n in aqueous acids in which protons are transfere~ from h y d r o n i u m ions to water m o l e c u l e s through hydrated h y d r o n i u m (H904 +) intermediates. The proton exchange itself is e x t r e m e l y rapid, o c c u r r i n g as tunneling b e t w e e n the two molecules. The rate d e t e r m i n i n g step, however, is thermally a c t i v a t e d H20/H30+ rotation to move the m o l e c u l e s into position for the proton transfer. The p r o p o s e d c o n d u c t i o n model is supported by the amount of water that is found in h y d r o n i u m beta" alumina. Thermogravimetric analysis indicates that the fully hydrated form of h y d r o n i u m beta" alumina contains 12 wt. % water. If all of the sodium ions in beta" alumina were replaced by h y d r o n i u m ions, which were placed in m i d - o x y g e n sites, and if enough a d d i t i o n a l water m o l e c u l e s were added to fill all of the remaining vacant midoxygen sites, the total water content p r e d i c t e d for the compound would be 11.3 wt. %. The p r e d i c t e d c o m p o s i t i o n agrees closely
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HYDRONIUM B E T A " ALUMINA
771
TABLE H3 O+ Beta"
Crystal
Alumina
Conductivity
Conductivity
(~ cm)
-i
-3 8.7 x 10 1.8xi0-2 2.5 x 10 -2 1.7 x 10 -2
A
at 25°C
Mounting Epoxy
B
6.7 x 10 -3 1.7 x 10 -2 3.6 x 10 -3 2.3xi0-3
Epoxy
C
2.3 x 10 -3
Epoxy
D
4.0 x 10 -3 8.3xi0-3
Epoxy
E
5.3 x 10 -3 4.8xi0-3 6.7 x 10 -3 4.8 x 10-3 9.5xi0-3
Paraffin
Average
Conductivity
= 5 x 10 -3
(~ cm) -I
/
FIG.
4
D e t a i l of the s o d i u m beta" a l u m i n a c o n d u c t i o n p l a n e . C l o s e p a c k e d o x y g e n s a b o v e and b e l o w the p l a n e are i n d i c a t e d by + and - signs, r e s p e c t i v e l y . T h e l a r g e c i r c l e s are c o l u m n o x y g e n s , s m a l l c i r c l e s are s o d i u m ions in the p r e f e r r e d sites, and d a s h e d s m a l l c i r c l e s are a l t e r n a t e s i t e s for s o d i u m o c c u p a t i o n .
772
G.C. FARRINGTON, et al.
Vol. 13, No. 8
with the 12 wt. % observed. The water content can be somewhat artificially separated into three categories: water of c o n s t i t u t i o n (2.5%) arising from the replacement of each sodium ion originally present with a proton, water of coordination (4.9%) required for hydrating each proton, and lattice water (3.9%) which fills each vacant m i d - o x y g e n site. The water of constitution and c o o r d i n a t i o n will be most strongly bound in the structure. The additional lattice water should be more weakly bound and presumably accounts for the water reversibly desorbed (3.0%) between 250-400°C. Critical unanswered questions about the chemistry and conductivity of h y d r o n i u m beta" alumina include how its conductivity and chemical c o m p o s i t i o n change with increasing temperature, as well as whether the model proposed here for its high conductivity is valid. Since h y d r o n i u m beta" alumina does not undergo a large dehydration until about 250-275°C, its conductivity may be high at least until that range of temperature. Experiments examining conductivity and c o m p o s i t i o n as a function of temperature are in progress. Infrared spectroscopy and wide-line NMR analyses are also being carried out. References 1. L. Glasser,
Chem.
Rev.,
7__5, 21
2. L.B.
Harris
and C.J.
3. J.M.
Thomas
and T.A Clarke,
Vella,
4. D.D. Eley, A.S. Fawcett, 64, 1513 (1968). 5. A. Potier 6. M.G.
and A.T.
J. Appl.
Phys.,
37,
Trans
Faraday
Soc.,
and M.R.
Willis,
Trans.
and D. Rousselet,
Shilton
(1975).
Howe,
J. Chim. Mat.
Phys.,
Res.
4294 65,
2718
Faraday
7_~0, 873
Bull.,
(1966).
12,
(1969) Soc.,
(1973).
701
(1977).
7. J.T. Kummer, Beta Alumina Electrolytes, Prog. In Solid State Chemistry, Vol. I, H. Reiss and J.O. McCaldin, eds., Pergamon, New York (1972), p. 150. 8. H. Saalfeld, Keram. Ges., 9. M.W. Mat.
H. Matthies, and S.K. 4_55, 212 (1968).
Datta,
Breiter, G.C. Farrington, W.L. Res. BulL, 12, 895 (1977).
10. W.L. Roth, M.W. Breiter, Chem., 2_~4, 321 (1978). ii.
G.C. Farrington, J.L. J. Solid State Chem.,
12.
G.C. F a r r i n g t o n and J.L. chem. Soc. Atlanta Mtg.,
and G.C.
Roth,
Ber.
and J.L.
Farrington,
Briant, M.W. Breiter, 2_~4, 31] (1978). Briant, October
Deutsche Duffy,
J. Solid State and W.L.
Roth,
Extended Abstracts, Electro1977, 71-2, 822 (1977).
Vol. 13, No. 8
13. M. Bettman
HYDRONIUM B E T A " ALUMINA
and C.R.
Peters,
J. Phys.
14. C.R. Peters, M. Bettman, J.W. Cryst., 827, 1826 (1971).
Moore,
Chem.,
773
73,
and M.D.
1774
Glick,
(1969). Acta
15. W.L. Roth, F. Reidinger, and S.J. LaPlaca, in Superionic Conductors, G.D. Mahan and W.L. Roth, eds., Plenum, New York, 1977, p. 223. 16. M.S. W h i t t i n g h a m (1971).
and R.A.
Huggins,
J. Chem.
Phys.,
17. J. Thery and D. Briancon, Compt. Rend., 254, 2782 Rev. Hautes Temp. Refractaires, I, 221 (1964).
54,
414
(1962) ;
18. G.C. Farrington, J.L. Briant, H.S. Story, and W. Bailey, International Meeting on Solid Electrolytes, St. Andrews, Scotland, September 1978, to be presented. 19. O.J. Samilov, Die Stuktur w a s s e r i g e r E l e k t r o l y t l o s u n g e n die H y d r a t a t i o n von Ionen, Leipzig, 1961.
und
HYDRONIUM
General
BETA"
ALUMINA:
A FAST PROTON
CONDUCTOR
G.C. Farrington and J.L. Briant Electric Corporate Research and D e v e l o p m e n t P.O. Box 8 Schenectady, NY 12301
(Received June 7, 1978; Communicated by R. C. DeVries) ABSTRACT Single crystals of sodium beta" alumina (0.84NapO'0.84MgO-5 AI203) undergo rapid ion exchange in c o n c e n t r a t e d sulfuric acid to produce "hydronium" beta alumina (0.84) H20-0.84MgO-5AI203-2.8H20). H y d r o n i u m beta" alumina undergoes a partial, reversible d e h y d r a t i o n between 250-300°C and irreversibly decomposes into alpha alumina and w a t e r above 700°C. The c o n d u c t i v i t y of h y d r o n i u m beta" alumina has been m e a s u r e d with blockinqland= nonblocking electrodes and is 5 x 10 -3 (ohm cm) at 25°C. The high c o n d u c t i v i t y is interpreted on the basis of a two dimensional liquid model. Introduction Protonic conduction in organic and inorganic solids has been e x t e n s i v e l y explored (i). Typical protonic conductors include inorganic salts and acids, small organic molecules, and biological polymers. Most have c o n d u c t i v i t i e s less than about 10 -7 (ohm cm)-l_at 25°C, as shown for KH2PO 4 (2) and Li2so4-H20 (3) in Figure ±. A c o n d u c t i v i t y of as low as 10-15 (ohm cm) -± has been reported for benzoic acid at 25°C (4). The few exceptions are perchloric acid monohydrate, w h i c h has a c o n d u c t i v i t y at 25°C reported to be 3 x 10 -4 (ohm cm)-i (5) and HUO2PO4"4H20 recently found to have a c o n d u c t i v i t y of 5 x 10 -3 at 25°C (6). The d i s c o v e r y of the high ionic c o n d u c t i v i t y of sodium beta alumina (ca. 1.24Na20-IIAI203) stimulated the i n v e s t i g a t i o n of fast ionic c o n d u c t i v i t y in many inorganic s o l i d structures. Beta alumina itself is a p a r t i c u l a r l y versatile solid electrolyte, since its entire m o b i l e sodium content can be replaced by a variety of m o n o v a l e n t cations, including Li +, K+, Ag + and, most importantly, H +, H3 O+, and NH4 + (7). I n v e s t i g a t i o n s of the 753
764
G.C. FARRINGTON,
I0"1
I
J
i0-2
I
I
et I
al.
Vol. 13. No. 8
I
I
-r~No BETAALUMINA ~ ~ H(HzO)xBETA'ALUMINA
_
i0-3
10-4
_
i0-5
T
I0.6
_
/~
H30C104
H30 BETAALUMINA
i
io -7 I--
10-8
/Z
p-
=o I° 9 r~ Z O
o
__ -
i0-10 i0-H
~/l ! "Li]O4"IHI-LiSO4"IH20 I I ~__ 10-12 10-13 10"14 -I00 0 I00 200 300 400 500 600 TEMPERATURE (°C) FIG.
1
Conductivities of various inorganic proton conducting solid electrolytes.
preparation (8,9), stability (i0), and c o n d u c t i v i t y (ll) of h y d r o n i u m beta alumina have found that M o n o f r a x single crystals of sodium beta alumina exposed to concentrated sulfuric acid at about 290°C for i0 days undergo ion exchange to produce a composition that corresponds in its stoichiometry to complete replacement of sodium by h y d r o n i u m ions. The resulting "hydronium" beta alumina (I.24H20"IIAI203-2.6H20) has a conductivity of about 10-11 (ohm cm) -I at 25°C, nine orders of m a g n i t u d e lower than that of sodium beta alumina at the same temperature. H y d r o n i u m beta alumina undergoes a partial and reversible d e h y d r a t i o n between 180-250°C in which about half of the h y d r o n i u m ions lose one water molecule of hydration. The partially dehydrated form (I.24H20-IIAI203"I.3H20) also has a very low conductivity and above 700°C irreversibly decomposes to various hydrated aluminas and water.
Vol. 13, No. 8
HYDRONIUM B E T A " ALUMINA
765
This paper p r e s e n t s the results of an i n v e s t i g a t i o n into the preparation, stability, and c o n d u c t i v i t y of h y d r o n i u m beta" alumina ( 0 . 8 4 H 2 0 - 0 . 8 4 M g O - 5 A I 2 0 3 - 2 . 8 H 2 0 ) . This compound is particularly i n t r i g u i n g since, despite its s t r u c t u r a l s i m i l a r i t y to h y d r o n i u m beta alumina, we have found and p r e v i o u s l y reported (12) that its ionic c o n d u c t i v i t y at 25°C is about 10 -2 (ohm cm) -I. S o d i u m Beta and Beta"
Alumina Crystal
Structures
S o d i u m beta" a l u m i n a occurs as a ternary phase with an ideal formula of N a 2 0 - M g O - 5 A I 2 0 3 or N a 2 0 - L i 2 0 - 5 A I 2 0 3 , in which MgO or Li20 are i n c o r p o r a t e d to "stabilize" the structure (13). As synthesized, sodium beta" alumina is n o n - s t o i c h i o m e t r i c and has a typical c o m p o s i t i o n of 0 . 8 4 N a 2 0 - 0 . 8 4 M g O - 5 A I 2 0 3 . It is closely related in s t r u c t u r e to sodium beta alumina w h i c h is also nonstoichiometric, having a c h a r a c t e r i s t i c c o m p o s i t i o n of 1.24Na20IIAI203. As w i t h sodium beta alumina, it is possible to replace the sodium c o n t e n t of beta" a l u m i n a by Li+, K +, NH4 +, and H+(H20) x, as well as w i t h other m o n o v a l e n t and d i v a l e n t cations. Both sodium b e t a and beta" aluminas are layer structures in w h i c h the sodium ions are found in c o n d u c t i n g planes spaced 11.2 A apart. The s t r u c t u r a l b a c k b o n e of each compound consists of c l o s e - p a c k e d A I - O "spinel blocks" c o n n e c t e d by A I - O - A I columns p a r a l l e l to the c axis. The A I - O - A I bonds define the r e l a t i v e l y open c o n d u c t i n g planes, w h i c h are boundeH by layers of closepacked o x y g e n ions. Ionic c o n d u c t i o n occurs in two d i m e n s i o n s w i t h i n the c o n d u c t i n g planes. A m o r e complete d e s c r i p t i o n of the s t r u c t u r e s of beta and beta" a l u m i n a can be found e l s e w h e r e (13,14,15). A d i a g r a m of the c o n d u c t i o n plane in sodium beta alumina is shown in Figure 2. The plane is a mirror plane; only one close packed o x y g e n layer is shown. The spacing b e t w e e n the oxygen layers varies w i t h ionic s u b s t i t u t i o n and, for example, is 4.76 A in sodium beta alumina and 5.11 A in r u b i d i u m beta alumina. Sodium ions are d i s t r i b u t e d among three n o n - e q u i v a l e n t sites, the so-called Beevers-Ross, mid-oxygen, and a n t i - B e e v e r s - R o s s sites. Figure 2 r e p r e s e n t s s t o i c h i o m e t r i c beta a l u m i n a w i t h sodium o c c u p a t i o n of each B e e v e r s - R o s s site. Actuallyl nons t o i c h i o m e t r i c b e t a alumina has an excess of sodium ions, w h i c h occur as m i d - o x y g e n / m i d - o x y g e n ion pairs. D i f f u s i o n takes place as the result of s e q u e n t i a l ion h o p p i n g through the i n t e r c o n n e c t e d sequence of ion sites. A sodium ion comes closest to n e i g h b o r i n g oxygens in the a n t i - B e e v e r s - R o s s site, in w h i c h it moves through a gap of 2.0 A b e t w e e n a pair of o x y g e n ions. P a s s a g e through the a n t i - B e e v e r s - R o s s site represents the e n e r g e t i c a l l y most d i f f i c u l t step in ion migration. The structure of the s t o i c h i o m e t r i c sodium beta" a l u m i n a c o n d u c t i n g plane is shown in Figure 3. All sodium sites are shown filled, w h e r e a s about 16% are v a c a n t in actual samples. The
766
G.C.
Q OXYGEN (CONDUCTIONPLANE)
FARRINGTON, et al.
Q
®
OXYGEN (CLOSE PACKED) FIG.
Vol. 13, No. 8
SODIUM
2
C o n d u c t i o n p l a n e of s o d i u m b e t a alumina. Sodium ions are s h o w n in p r e f e r r e d B e e v e r s - R o s s (d) sites. A d d i t i o n a l m o b i l e ion sites are the mid o x y g e n (m.o.) and a n t i - B e e v e r s - R o s s (b).
OXYGEN (CONDUCTION PLANE}
/ ~ ~ ~ FIG.
OXYGEN J (CLOSE PACKED)
3
C o n d u c t i o n p l a n e of s o d i u m beta" alumina. S o d i u m ions are s h o w n in the p r e f e r r e d sites, w h i c h are s e p a r a t e d by the m i d - o x y g e n (m.o.) positions.
SODIUM
Vol. 13, No. 8
HYDRONIUM BETA" ALUMINA
767
structure differs from that of sodium beta alumina in several s i g n i f i c a n t ways. The c o n d u c t i n g plane is not a m i r r o r plane, and all sodium sites are c r y s t a l l o g r a p h i c a l l y equivalent. Ions a l t e r n a t e b e t w e e n sites having three oxygen ions above at distances of 2.69 A and one b e l o w at 2.57 A and the inverse configuration. In each site, a sodium ion is 3.25 A from each of three oxygen ions w i t h i n the c o n d u c t i o n plane. In moving through the plane, the ion undulates, o c c u p y i n g positions 0.17 A above and below the m i d p o i n t of the plane. The smallest gap through which an ion m u s t pass is the 3.0 A spacing b e t w e e n the two column oxygens a d j o i n i n g the m i d - o x y g e n site. This contrasts with the c o r r e s p o n d i n g gap b e t w e e n two c l o s e - p a c k e d oxygen ions in the a n t i - B e e v e r s - R o s s site of beta alumina w h i c h is only 2.0 A wide. Crystal P r e p a r a t i o n
and C o m p o s i t i o n
Single crystals of sodium beta" alumina were grown from a m e l t of 35 wt. % Na2CO3, 3.2 wt. % MgO, and 62 wt. % AI20 3 by h e a t i n g the m i x t u r e in a c o v e r e d Pt or alpha alumina crucible to 1660°C for 24 hours and c o o l i n g to 25°C over 12 hours. As Na20 vaporized, the crystals grew as thin p l a t e l e t s about 2x2x0.2mm on the surface of the melt. They had average compositions of 0 . 8 4 N a 2 0 " 0 . 8 M g O - 5 A I 2 0 3, as d e t e r m i n e d by sodium and m a g n e s i u m analysis. P o w d e r p a t t e r n x-ray d i f f r a c t i o n was used to v e r i f y the crystal structures. Ion exchange to the h y d r o n i u m form was carried out by immersing the crystals in c o n c e n t r a t e d sulfuric acid at 240°C for several days, followed by w a s h i n g in water and air drying. S o d i u m analysis after exchange found less than 5% of the original sodium c o n t e n t r e m a i n i n g in the samples. P r e l i m i n a r y t h e r m o g r a v i m e t r i c analysis (TGA) in dry N 2 at a sweep rate of l°C/min, on single crystals of h y d r o n i u m beta" alumina reveals a 0.5% w e i g h t loss at 70°C and an invariant c o m p o s i t i o n from 100°C to 200°C. B e t w e e n 250°C and 280°C a p a r t i a l r e v e r s i b l e d e h y d r a t i o n with a 3.0% w e i g h t loss occurs p r o d u c i n g a new c o m p o s i t i o n w h i c h is stable until about 700°C, above w h i c h complete d e c o m p o s i t i o n takes place. These reactions are s u m m a r i z e d in E q u a t i o n i. 0.84H20-0.84MgO-5AI203.2.8H20 ÷ 0.84H20.0.8 4MgO-5AI203.1.9H20+0.9H20 275°C
[I]
+ > 700°C 3.6H20+0 .84~/0+5A1203
H y d r o n i u m beta alumina u n d e r g o e s which are s u m m a r i z e d in E q u a t i o n
analogous 2.
dehydration
reactions
1.24H20-IIAI203-2.6H20 ++ 1.24H20"IIAI203"I.3~hO+I.3H~O ÷ IIAI203+3.8H20 225oc ->700°C
[2]
768
G . C . FARRINGTON, et al.
Conductivity
Vol. 13, No. 8
Measurements
The c o n d u c t i v i t i e s of six h y d r o n i u m beta" alumina crystals were measured, five with n o n - b l o c k i n g agar gel or p o l y e t h y l e n e oxide ion exchange m e m b r a n e electrodes, and one with gold blocking electrodes. I m p e d a n c e s were m e a s u r e d with a H e w l e t t - P a c k a r d 3040 A N e t w o r k Analyzer. With n o n - b l o c k i n g electrodes the impedance of each crystal was found to be i n v a r i a n t w i t h i n 10% from 102 to 105 Hz with a phase shift of less than 10 ° . M e a s u r e m e n t s with blocking electrodes w e r e carried out from 102 to 107 Hz and found to approach a straight line intersecting the real axis on a complex impedance plot at high frequencies. The real i n t e r c e p t was taken as the sample resistance. Samples were m o u n t e d for analysis using several d i f f e r e n t techniques. To p r e v e n t liquid e l e c t r o l y t e from the agar or p o l y e t h y l e n e oxide e l e c t r o d e s from creeping around the samples, the crystals were cast in epoxy resin w h i c h was then ground away to expose the crystal edges. Samples were also m e a s u r e d m o u n t e d in p a r a f f i n and w i t h o u t mounting. For b l o c k i n g electrode experiments, the crystals were m o u n t e d w i t h insulating v a r n i s h on alpha alumina discs, and gold e l e c t r o d e s were sputtered onto the edges. A number of e x p e r i m e n t s were carried out to verify the m e a s u r e m e n t procedures. To insure that liquid films from the electrodes did not c o n t r i b u t e to the conductivities, h y d r o n i u m beta alumina crystals, which are similar in physical c h a r a c t e r i s tics and surface p r o p e r t i e s to h y d r o n i u m beta" alumina, w e r e m e a s u r e d with and w i t h o u t casting in epoxy. No c o n d u c t i v i t y was observed, as expected from the low c o n d u c t i v i t y reported for that compound, i0 -II (ohm cm)-i at 25°C. Also, a small M o n o f r a x sodium beta alumina crystal was m o u n t e d with v a r n i s h on an alpha alumina disc and c o n t a c t e d with sputtered gold electrodes. At 25°C its conductivity, as d e t e r m i n e d from the e x t r a p o l a t i o n of a complex impedance plot, was found to be 1 x 10-2 (ohm cm)-l, in good a g r e e m e n t with the value of 1.4 x 10-2 (ohm cm)-i p r e v i o u s l y reported (16). Results
and D i s c u s s i o n
Our e x p e r i m e n t a l results using n o n - b l o c k i n g electrodes, s u m m a r i z e d in the Table, i n d i c a t e an average ionic c o n d u c t i v i t y of h y d r o n i u m beta" alumina of 5 x 10-3 (ohm cm)-i at 25°C. This agrees w i t h the m e a s u r e m e n t obtained with b l o c k i n g electrodes: 8.3 x 10-3 (ohm cm) -I at 25°C. Several previous reports of the partial ion exchange of sodium beta" alumina are e x p l a i n e d by our findings. Th~ry and B r i a n c o n (17) noted that sodium beta" alumina readily hydrolyzes in water at 100°C. The h y d r o l y z e d p r o d u c t d e c o m p o s e s at 1200°C to AI203 and NaAI508, as would be expected for p a r t i a l l y exchanged h y d r o n i u m - s o d i u m beta" alumina. B e t t m a n and Peters (13) exposed sodium beta" alumina to aqueous HCI at 25°C and found that many of the s o d i u m ions exchange with protons and an unknown q u a n t i t y of water molecules. Similar exchange is not observed for sodium beta alumina at this temperature. Both of these
Vol. 13, No. 8
reports alumina
HYDRONIUM B E T A " ALUMINA
769
indicate that h y d r o n i u m s u b s t i t u t i o n into the beta" s t r u c t u r e proceeds much more rapidly than in beta a l u m i n a
The c o n d u c t i v i t y of h y d r o n i u m beta" alumina is c o m p a r e d with that of several other i n o r g a n i c protonic conductors in Figure i. H y d r o n i u m beta" alumina has a c o n d u c t i v i t y c o m p a r a b l e to sodium beta and beta" alumina at 25°C, an e x t r a o r d i n a r i l y high value for a proton conductor. Two protonic compounds w h i c h approach its c o n d u c t i v i t y are p e r c h l o r i c acid m o n o h y d r a t e and UO2H20(PO4) 3. P e r c h l o r i c acid m o n o h y d r a t e exists in a monoclinic c r y s t a l l i n e form below 30°C, c o n s i s t i n g of h y d r o g e n bonded layers of p e r c h l o r a t e and h y d r o n i u m ions, and in an o r t h o r h o m b i c structure b e t w e e n -30°C and 47°C, at w h i c h point it dissolves in its water of hydration. The higher t e m p e r a t u r e phase is highly d i s o r d e r e d and has a reported c o n d u c t i v i t y of 3 x 10 -4 at 25°C w i t h an a c t i v a t i o n energy of 5.6 k c a l / m o l e (5). Recently it was reported (6) that H U O 2 P O 4 " 4 H 2 0 has a c o n d u c t i v i t y of 5 x 10-3 (ohm cm) -I at 25°C which increases with t e m p e r a t u r e with an a c t i v a t i o n energy of 7.2 k c a l / m o l e until about 80°C at which d e h y d r a t i o n begins and the c o n d u q t i v i t y decreases. These compounds are exceptions, however, to the g e n e r a l l y low cond u c t i v i t y of protonic solids. The most striking aspect of the high ionic c o n d u c t i v i t y of h y d r o n i u m beta" a l u m i n a is that it presents such a contrast to the extremely low c o n d u c t i v i t y of h y d r o n i u m beta alumina. As m e n t i o n e d previously, the compounds have similar structures and c o m p a r a b l e c o n d u c t i v i t i e s in their sodium forms, 1.4 x 10 -2 (ohm cm)-i for sodium beta a l u m i n a and 1.0 x I0 -I (ohm cm)-i for sodium beta" alumina at 25°C. One s i g n i f i c a n t d i f f e r e n c e b e t w e e n the c o m p o s i t i o n s of h y d r o n i u m beta and beta" alumina is in the water contents. During ion exchange each sodium ion is replaced by a proton and a q u a n t i t y of water. Hydronium beta alumina i n c o r p o r a t e s one water m o l e c u l e for each p r o t o n present. H y d r o n i u m beta" alumina contains 1.7 m o l e c u l e s of water for each proton. These observations, combined with a c o n s i d e r a t i o n of the structure of each compound, lead to a possible e x p l a n a t i o n for the large d i f f e r e n c e in their conductivities. The c o n d u c t i n g plane of sodium beta alumina is shown in Figure 2. In h y d r o n i u m beta alumina, each sodium ion has been replaced by a single p r o t o n and a water molecule. A l t h o u g h the required s t o i c h i o m e t r y can be satisfied by a structure c o n s i s t i n g of separate protons and water molecules, recent infrared spectroscopy and wide line NMR results (18) suggest that h y d r o n i u m ions are indeed present as d i s t i n c t species. From examining the h y d r o n i u m beta alumina structure, it is a p p a r e n t that more w a t e r m i g h t be absorbed into the conducting plane than that required for the h y d r a t i o n of each proton This extra w a t e r could o c c u p y m i d - o x y g e n sites. However, there is no evidence in single crystals that extra h y d r a t i o n takes place, ffydronium ions in the B e e v e r s - R o s s or m i d - o x y g e n sites are t h e r e f o r e isolated from each other and h y d r o g e n b o n d e d to n e i g h b o r i n g s t r u c t u r a l oxygen ions. C o n d u c t i v i t y in h y d r o n i u m
770
G . C . FARRINGTON, et al.
Vol. 13, No. 8
beta a l u m i n a involves h y d r o n i u m ion m i g r a t i o n or proton hopping among s t r u c t u r a l o x y g e n ions. Each process is impeded by the formation of h y d r o g e n bonds b e t w e e n m o b i l e ions and the conducting plane structure. H y d r o g e n b o n d i n g increases the a c t i v a t i o n energy of hopping and decreases the conductivity. In h y d r o n i u m beta" alumina, in contrast, extra h y d r a t i o n does occur. The greater water c o n t e n t of the structure can be related to its high c o n d u c t i v i t y by c o n s i d e r i n g the sodium beta" alumina c o n d u c t i n g plane shown in Figures 3 and 4. The column oxygen ions are shown by large circles; the close packed oxygens above and below the c o n d u c t i n g plane are indicated by + and - signs c o r r e s p o n d i n g to their p o s i t i o n s relative to the plane. The close packed oxygen layers are not eclipsed in beta" alumina as they are in beta alumina. Both sodium sites are e q u i v a l e n t and each would be occupied in s t o i c h i o m e t r i c beta" alumina. S o d i u m beta" alumina is n o n - s t o i c h i o m e t r i c , however, and has a d e f i c i t of sodium ions and a c o r r e s p o n d i n g number of vacancies in the conducting plane. The region in Figure 4 has a statistical o c c u p a t i o n of 1.68 sodium ions in the sodium beta" alumina used in this study, a m o b i l e ion content 35% higher than in M o n o f r a x sodium beta alumina. B e t t m a n and Peters (12) suggested in their d i s c u s s i o n of the s t r u c t u r e of sodium beta" alumina that sodium ions m i g h t be placed either in the p o s i t i o n s shown in Figure 4 or in the large number of m i d - o x y g e n sites, shown as dashed circles. Ions are not found in the m i d - o x y g e n sites in the sodium isomorph, but o c c u p a t i o n of the m i d - o x y g e n sites by water and h y d r o n i u m ions is i m p o r t a n t in r a t i o n a l i z i n g the properties of h y d r o n i u m beta" alumina. We propose that all of the m i d - o x y g e n sites in h y d r o n i u m beta" alumina are occupied by either h y d r o n i u m ions or water molecules. The c o n d u c t i n g plane structure is e s s e n t i a l l y that of a two dimensional, c l o s e - p a c k e d liquid having a high conc e n t r a t i o n of charge carriers. Ionic c o n d u c t i o n occurs by proton exchange and m i g r a t i o n through the c o n d u c t i o n plane. Proton transport is expected to be much more rapid than oxygen transport. T h i s m e c h a n i s m is analogous to the Grotthus m e c h a n i s m (18) for c o n d u c t i o n in aqueous acids in which protons are transfere~ from h y d r o n i u m ions to water m o l e c u l e s through hydrated h y d r o n i u m (H904 +) intermediates. The proton exchange itself is e x t r e m e l y rapid, o c c u r r i n g as tunneling b e t w e e n the two molecules. The rate d e t e r m i n i n g step, however, is thermally a c t i v a t e d H20/H30+ rotation to move the m o l e c u l e s into position for the proton transfer. The p r o p o s e d c o n d u c t i o n model is supported by the amount of water that is found in h y d r o n i u m beta" alumina. Thermogravimetric analysis indicates that the fully hydrated form of h y d r o n i u m beta" alumina contains 12 wt. % water. If all of the sodium ions in beta" alumina were replaced by h y d r o n i u m ions, which were placed in m i d - o x y g e n sites, and if enough a d d i t i o n a l water m o l e c u l e s were added to fill all of the remaining vacant midoxygen sites, the total water content p r e d i c t e d for the compound would be 11.3 wt. %. The p r e d i c t e d c o m p o s i t i o n agrees closely
Vol. 13, No. 8
HYDRONIUM B E T A " ALUMINA
771
TABLE H3 O+ Beta"
Crystal
Alumina
Conductivity
Conductivity
(~ cm)
-i
-3 8.7 x 10 1.8xi0-2 2.5 x 10 -2 1.7 x 10 -2
A
at 25°C
Mounting Epoxy
B
6.7 x 10 -3 1.7 x 10 -2 3.6 x 10 -3 2.3xi0-3
Epoxy
C
2.3 x 10 -3
Epoxy
D
4.0 x 10 -3 8.3xi0-3
Epoxy
E
5.3 x 10 -3 4.8xi0-3 6.7 x 10 -3 4.8 x 10-3 9.5xi0-3
Paraffin
Average
Conductivity
= 5 x 10 -3
(~ cm) -I
/
FIG.
4
D e t a i l of the s o d i u m beta" a l u m i n a c o n d u c t i o n p l a n e . C l o s e p a c k e d o x y g e n s a b o v e and b e l o w the p l a n e are i n d i c a t e d by + and - signs, r e s p e c t i v e l y . T h e l a r g e c i r c l e s are c o l u m n o x y g e n s , s m a l l c i r c l e s are s o d i u m ions in the p r e f e r r e d sites, and d a s h e d s m a l l c i r c l e s are a l t e r n a t e s i t e s for s o d i u m o c c u p a t i o n .
772
G.C. FARRINGTON, et al.
Vol. 13, No. 8
with the 12 wt. % observed. The water content can be somewhat artificially separated into three categories: water of c o n s t i t u t i o n (2.5%) arising from the replacement of each sodium ion originally present with a proton, water of coordination (4.9%) required for hydrating each proton, and lattice water (3.9%) which fills each vacant m i d - o x y g e n site. The water of constitution and c o o r d i n a t i o n will be most strongly bound in the structure. The additional lattice water should be more weakly bound and presumably accounts for the water reversibly desorbed (3.0%) between 250-400°C. Critical unanswered questions about the chemistry and conductivity of h y d r o n i u m beta" alumina include how its conductivity and chemical c o m p o s i t i o n change with increasing temperature, as well as whether the model proposed here for its high conductivity is valid. Since h y d r o n i u m beta" alumina does not undergo a large dehydration until about 250-275°C, its conductivity may be high at least until that range of temperature. Experiments examining conductivity and c o m p o s i t i o n as a function of temperature are in progress. Infrared spectroscopy and wide-line NMR analyses are also being carried out. References 1. L. Glasser,
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Rev.,
7__5, 21
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13. M. Bettman
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und