- Email: [email protected]
Oxygen ion conductivity in BaBi4Ti3MO14.5 (M=Sc, In and Ga)
SOLID STATE
Solid State Ionics 70/71 (1994) 225-228 North-Holland
IONUS
Oxygen ion conductivity in BaBi4Ti3M014.5 (M = SC, In and Ga) Julie K. Thomas, Kurt R. Kendall and Hans-Conrad Department
of Chemistry. Massachusetts
Institute of Technology,
zur Loye ’
Cambridge,
MA 01239, USA
BaBi.,Ti,MO,,.s (M = Ga, In and SC), compounds that consist of intergrowths between the Aurivillius phase Bi,Ti30i2 and brownmillerite-like layers of BaM02,s were synthesized and their oxygen ion conductivities were measured using complex impedance. All three materials display a discontinuity in Arrhenius plots of their ionic conductivity. A phase transition at this discontinuity is confirmed by differential thermal analysis and is consistent with an oxygen vacancy order-disorder transition. For BaBi,Ti,Sc0i4,, there is an observed hysteresis for this phase transition. At 900°C the conductivities are 2.5X 10’ Scm-’ for BaBi4Ti3Sc014,5and 4.9X lo-* S.cm-’ for BaBi4Ti31n0,,.s and 4.5~ lo-’ Scm-’ for BaBi,Ti,GaO,,,,.
1. Introduction
New oxygen ion conductors with high conductivity at low temperatures (600-800” C) are of interest as components of solid oxide fuel cells ( SOFCs). SOFCs represent a clean and efficient source of energy. Currently, however, SOFCs must be operated near 1000 ’ C to achieve sufficient conductivity in the yttria-stabilized zirconia (YSZ) electrolyte [ l-31. This high temperature limits the choices for other components of the cell and, eventually causes deterioration of the electrolyte itself. Oxygen ion conduction generally occurs via a hopping mechanism whereby 02--ions hop from an occupied to a vacant site [ 11. YSZ, like most other studied materials, contains extrinsic vacancies created by doping with aliovalent cations [ 41. Although this strategy frequently produces oxygen ion conduction, it has limitations. The dopant-vacancy pairs interact, and there is an extra energy associated with overcoming this attraction. Consequently, activation energies for doped materials are often high. Only a small percentage of vacancies may be added before this interaction actually decreases the conductivity [ 5-7 1. Also, after extended use at high operating temperatures, the vacancies and dopants may order to form other phases. It is, therefore, useful to look ’ To whom correspondence should be addressed. 0167-2738/93/$06.00
at other strategies for designing oxygen ion conductors. One approach would be to look at materials with intrinsic vacancies, but there are few of these. Among those that are known, however, there are materials which show good conductivity above an order-disorder transition ( Bi203 [ 8 ] and BazInzOs [ 9, lo] ). Altering these materials to increase conductivity or decrease transition temperatures has not been especially successful. The addition of dopants to BiZ03, for instance, will lower the transition temperature, but also decrease the conductivity [ 81. Our approach is to use layered intergrowth structures. In this approach one component should provide structural organization and show some promise as an oxygen ion conductor, while the second component should provide intrinsic vacancies. In addition there must be a possible structural match between the components. We have chosen to use Aurivillius phases ( Bi2A,_ ,Mnn03n+3) as the strucand brownmillerite type component tural ( BazM (III ) 205 ) slabs for the oxygen deficient layers. The Aurivillius phases are layered materials with perovskitic [A,_ ,Mnn03n+, ] 2- regions sandwiched between puckered [ Bi202 ] 2+ sheets (fig. 1) [ 1l- 13 1. Some materials with this structure (Bi2W06 and Bi2V05,5) are oxygen ion conductors [ 14,15 1. The brownmillerite structure can be viewed as an oxygen deficient perovskite structure with vacancies ordering along the [ 1011 direction. In related structures
0 1993 Elsevier Science Publishers B.V. All rights reserved.
226
J.K. Thomas et al. /Oxygen ion conductivity in BaBi4Ti3M0,4 5
2. Experimental The samples were prepared by solid state reactions of BaCO, (Aesar, 99.9%), Ti02 (Aldrich, anatase, 99.9%), Bi203 (Cerac, 99.9%), and In,O, (Cerac, 99.99%), Ga203 (Johnson-Matthey, 99.999%) or Sc203 (Aran Isles, 99.99%). BaBi,Ti,O,, was also made for comparison. The powders were ground together, pressed into a pellet, and heated slowly to 900°C. Samples were ground are reheated until a clean X-ray pattern was obtained. A Rigaku RU-300 diffractometer was used for powder X-ray diffraction measurements. Total heating time was about two weeks. For conductivity measurements the pellets were sintered for two hours at higher temperatures (1lOO’C for M=In and SC, and 1000°C for M = Ga). The conductivity was measured as a function of temperature using complex impedance spectroscopy. Measurements were made in air on pellets coated with Pt ink, using a Solartron 1260 impedance/gain-phase analyzer with Z-plot software. A TA Instruments SDT 2960 simultaneous TGA/DTA was used for differential thermal analysis measurements in air.
3. Results and discussion
Fig. 1. Structure of the Aurivillius phase BaBi4Ti40,5. Within the puckered sheet region (shown as ball and stick) the dark spheres represent Bi’+ and the light spheres 02-. In the perovskitic region the Ti06 octahedra are shown as polyhedra and the dark spheres are a mixture of Ba2+ and BP.
the vacancies may exhibit different orderings [ 16 1. Since the brownmillerite and perovskite structures are so similar, we have been able to substitute brownmillerite related layers for some of the perovskite layers in the structure, thereby introducing oxygen vacancies. We have previously reported preliminary studies on BaBi4Ti31n014.5 and BaBi4Ti3Sc0,,., [ 171, and we have investigated these materials more fully here. We have also synthesized a new member of the series, BaBi,Ti,GaO,,.,.
Powder X-ray diffraction shows that all three new phases can be indexed to a tetragonal unit cell [for BaBi4Ti3GaOL4.5, a=5.447(2) A, c=41.80(2) A; BaBi4TisIn014.5, a=5.663(4) A, c=41.91(3) A; BaBi4Ti3014.5, a=5.682(3) A, c=41.74(3) A] and are structurally similar to the non-oxygen deficient phase BaBi4Ti40i5 (fig. 1). This unit cell shows some distortion from the idealized BaBi4Ti40i5 structure, however. The new a axis is $2 times larger than that of the simple cell. This distortion is seen in many brownmillerite type materials and in distortions of the Aurivillius phases [ 9,181. From powder X-ray diffraction it is not possible to determine if the brownmillerite “layer” is truly a layer or if the M’+ cations are randomly distributed throughout the perovskitic region. It is also not possible to investigate the oxygen vacancy ordering, and, consequently, neutron diffraction experiments are planned. Based on our experience with brownmillerite-perovskite intergrowths it is likely that the cations are randomly
227
dist~buted throu~out the layers [ 19 1. For BaBi,Ti,Ol 5 the Arrhenius plot of conductivity was linear over a temperature range of 250 to 950°C with an activation energy of 0.72 eV and a conductivity of 1.3x 10e4 at 900°C. Arrhenius plots of conductivity for all three phases showed a jump in conductivity. For BaBi4Ti31n014.5 the low temperature activation energy was 1.OOeV, with a transition between 800 and 850’ C to a high temperature form with an Ea of 0.35 eV and a conductivity of 4.9x1O-2 Scm-* at 900°C (fig. 2). For BaBi4Ti3Sc014.J the low temperature activation energy was 0.93 eV, with a transition between 725 and 850°C to the high temperature form, which has an E, of 0.50 eV and a conductivity of 3.5~ lo--’ Scm- l at 900°C (fig. 3). The large transition temperature range for M = SCis due to an observed hysteresis. On cooling, the high conductivity could be stabilized to a temperature about 100°C lower than the onset temperature on heating. This appears to be due to a slow relaxation process, since a sample in the highly conductive phase that was cooled to 800°C and left for several days eventually converted to the low temperature phase. For BaBi4Ti~GaO~4.~the activation energy was 0.95 eV below 800°C and 0.37 eV above 800” C. The conductivity at 900°C was 4.5X 10m2Scm-’ (fig. 4). Di~erential thermal analysis (DTA) shows an endothermic transition for each of the oxygen deficient materials at temperatures close to the observed jump
- __. -3-
-7-
-%--
I
I.2
I.4
1.6
1.8
2
1000rr Fig. 2. The Arrhenius piot of conductivity for BaBi4TisInOt..5 shows a jump in conductivity above 800°C.
-I
-2
z
-3
3j -
-4
x
COOLING
E Sw
-5
-6
-7 0.8
1.2
I
1.4
1.6
1000I T Fig. 3. The Arrhenius plot of conductivity for BaBi,Ti$cOLa,S shows a jump in conductivity above 800°C but shows hysteresis in thermal cycling.
-7
i
-84--0.8
I
I.2
1.4
1.6
1000/T
Fig. 4. The Arrhenius plot of conducti~ty for BaBi4Ti3Ga014.5 shows a jump in conductivity above 800°C.
in conductivity in the Arrhenius plot. For BaBi.+Ti3Sc014,5the transition has an onset temperature of 842 oC and a peak temperature of 849 ’ C. For BaBi4Ti31n014.5the transition occurs at 833°C onset and 838°C peak, and for BaBi4Ti3GaOL4.5at 817°C onset and 827°C peak. Fig. 5 shows the DTA plot of BaBi4Ti3Ga014.5 which is qualitatively similar to those of the other two compounds. BaBi4Ti4015 which shows no jump in conductivity shows no transition in the DTA plot. The observed transition in the oxygen deficient materials is consistent with an oxygen vacancy order-disorder transition. Through further studies including neutron diffraction we hope to elucidate the nature of this transition.
228
J.K. Thomas et al. /Oxygen ion conductivity in BaBi,Tr3M0,4
A
References [ 1] I. Riess, in: Science and Technology of Fast Ion Conductors,
-1.04 200
I 400
600
Temperature
800
1000
(“C)
Fig. 5. Differential thermal analysis data for BaBi,TisGaO,.,~. DTA plots for BaBi4TisSc0,4.s and BaBidTisIn0i4.S are qualitatively similar.
4. Summary In summary, we have found a new class of oxygen ion conductors which have intrinsic oxygen vacancies and undergo order-disorder transitions. Using intergrowth structures we are able to modify these materials and to control the number of oxygen vacancies within them. We are currently exploring the oxygen ion conductivity of other members of this class of materials to find compounds that have oxygen vacancy order-disorder transitions at lower temperatures. This may be achieved by adding a different number of the perovskite or brownmillerite related layers or by starting with a different Aurivillius phase.
Acknowledgement The authors thank the National Science Foundation for support of this research though grant DMR9200688. JKT would like to thank A.D. Little and AT&T Bell Laboratories for support.
eds. H.L. Tuller and M. Balkanski (Plenum Press, New York, 1987) p. 23. [2] R.M. Dell and A. Hooper, in: Solid Electrolytes, eds. P. Hagenmtlller and W. van Gool (Academic Press, New York, 1978) p. 291. [ 31 T. Takahashi, in: Superionic Solids and Solid Electrolytes, eds. A.L. Laskar and S. Chandra (Academic Press, New York, 1989) p. 1. [4] J.A. Kilner and R.J. Brook, Solid State Ionics 6 (1982) 237. [ 51C.R.A. Catlow, in: Superionic Solids and Solid Electrolytes, eds. A.L. Laskar and S. Chandra (Academic Press, New York, 1989) p. 339. [6] A.J. Burggraaf, M.P. Van Dijk, M.J. Verkerk and K.J. de Vries, in: Structure and Conductivity of Some Pyrochlore and Fluorite Type Solid Solutions. eds. R. Metselaar, H.J.M. Heijligers and J. Schoonman, Vol. 3 (Elsevier, Amsterdam, The Netherlands, 1982) p. 185. [ 71 J.A. Kilner and C.D. Waters, Solid State lonics 6 ( 1982) 253. [ 81 T. Takahashi and H. Iwahara, Mat. Res. Bull. 13 ( 1978) 1447. [ 91 J.B. Goodenough, J.E. Ruiz-Diaz and Y.S. Zhen, Solid State Ionics 44 (1990) 21. [ lo] J.B. Goodenough, A. Manthiram, P. Paranthaman and Y.S. Zhen, Solid State Ionics 52 ( 1992) 105. [11]B.Aurivillius,Ark.Kemi2 (1951) 519. [ 121 E.C. Subbarao, J. Am. Ceram. Sot. 45 (1962) 166. [ 13 ] G.N. Subbanna, T.N. Guru Row and C.N.R. Rao, J. Solid State Chem. 86 (1990) 206. [ 141 F. Abraham, M.F. Debreuille-Gresse, G. Mairesse and Nowogrocki, Solid State Ionics 28-30 ( 1988) 529. [ 15 ] V.K. Yanovskii, V.I. Voronkova, Y.E. Roginskaya and Y.N. Venevtsev, Sov. Phys. Solid State 24 ( 1982) 1603. [ 161 M.T. Anderson, J.T. Vaughey and K.R. Poeppelmeier, Chem. Mater. 5 (1993) 151. [ 171 J.K. Thomas and H.-C. zur Loye, JACS (1993) submitted for publication. [ 181 B. Frit and J.P. Mercurio, Alloys Comp. 188 ( 1992) 27. [ 191 J.K. Thomas, W.E. Krause and H.-C. zur Loye, in: Solid State Ionics III, eds. G.-A. Nazri, J.-M. Tarascon and M. Armand, Vol. 293 (MRS, Pittsburgh, 1993).
Solid State Ionics 70/71 (1994) 225-228 North-Holland
IONUS
Oxygen ion conductivity in BaBi4Ti3M014.5 (M = SC, In and Ga) Julie K. Thomas, Kurt R. Kendall and Hans-Conrad Department
of Chemistry. Massachusetts
Institute of Technology,
zur Loye ’
Cambridge,
MA 01239, USA
BaBi.,Ti,MO,,.s (M = Ga, In and SC), compounds that consist of intergrowths between the Aurivillius phase Bi,Ti30i2 and brownmillerite-like layers of BaM02,s were synthesized and their oxygen ion conductivities were measured using complex impedance. All three materials display a discontinuity in Arrhenius plots of their ionic conductivity. A phase transition at this discontinuity is confirmed by differential thermal analysis and is consistent with an oxygen vacancy order-disorder transition. For BaBi,Ti,Sc0i4,, there is an observed hysteresis for this phase transition. At 900°C the conductivities are 2.5X 10’ Scm-’ for BaBi4Ti3Sc014,5and 4.9X lo-* S.cm-’ for BaBi4Ti31n0,,.s and 4.5~ lo-’ Scm-’ for BaBi,Ti,GaO,,,,.
1. Introduction
New oxygen ion conductors with high conductivity at low temperatures (600-800” C) are of interest as components of solid oxide fuel cells ( SOFCs). SOFCs represent a clean and efficient source of energy. Currently, however, SOFCs must be operated near 1000 ’ C to achieve sufficient conductivity in the yttria-stabilized zirconia (YSZ) electrolyte [ l-31. This high temperature limits the choices for other components of the cell and, eventually causes deterioration of the electrolyte itself. Oxygen ion conduction generally occurs via a hopping mechanism whereby 02--ions hop from an occupied to a vacant site [ 11. YSZ, like most other studied materials, contains extrinsic vacancies created by doping with aliovalent cations [ 41. Although this strategy frequently produces oxygen ion conduction, it has limitations. The dopant-vacancy pairs interact, and there is an extra energy associated with overcoming this attraction. Consequently, activation energies for doped materials are often high. Only a small percentage of vacancies may be added before this interaction actually decreases the conductivity [ 5-7 1. Also, after extended use at high operating temperatures, the vacancies and dopants may order to form other phases. It is, therefore, useful to look ’ To whom correspondence should be addressed. 0167-2738/93/$06.00
at other strategies for designing oxygen ion conductors. One approach would be to look at materials with intrinsic vacancies, but there are few of these. Among those that are known, however, there are materials which show good conductivity above an order-disorder transition ( Bi203 [ 8 ] and BazInzOs [ 9, lo] ). Altering these materials to increase conductivity or decrease transition temperatures has not been especially successful. The addition of dopants to BiZ03, for instance, will lower the transition temperature, but also decrease the conductivity [ 81. Our approach is to use layered intergrowth structures. In this approach one component should provide structural organization and show some promise as an oxygen ion conductor, while the second component should provide intrinsic vacancies. In addition there must be a possible structural match between the components. We have chosen to use Aurivillius phases ( Bi2A,_ ,Mnn03n+3) as the strucand brownmillerite type component tural ( BazM (III ) 205 ) slabs for the oxygen deficient layers. The Aurivillius phases are layered materials with perovskitic [A,_ ,Mnn03n+, ] 2- regions sandwiched between puckered [ Bi202 ] 2+ sheets (fig. 1) [ 1l- 13 1. Some materials with this structure (Bi2W06 and Bi2V05,5) are oxygen ion conductors [ 14,15 1. The brownmillerite structure can be viewed as an oxygen deficient perovskite structure with vacancies ordering along the [ 1011 direction. In related structures
0 1993 Elsevier Science Publishers B.V. All rights reserved.
226
J.K. Thomas et al. /Oxygen ion conductivity in BaBi4Ti3M0,4 5
2. Experimental The samples were prepared by solid state reactions of BaCO, (Aesar, 99.9%), Ti02 (Aldrich, anatase, 99.9%), Bi203 (Cerac, 99.9%), and In,O, (Cerac, 99.99%), Ga203 (Johnson-Matthey, 99.999%) or Sc203 (Aran Isles, 99.99%). BaBi,Ti,O,, was also made for comparison. The powders were ground together, pressed into a pellet, and heated slowly to 900°C. Samples were ground are reheated until a clean X-ray pattern was obtained. A Rigaku RU-300 diffractometer was used for powder X-ray diffraction measurements. Total heating time was about two weeks. For conductivity measurements the pellets were sintered for two hours at higher temperatures (1lOO’C for M=In and SC, and 1000°C for M = Ga). The conductivity was measured as a function of temperature using complex impedance spectroscopy. Measurements were made in air on pellets coated with Pt ink, using a Solartron 1260 impedance/gain-phase analyzer with Z-plot software. A TA Instruments SDT 2960 simultaneous TGA/DTA was used for differential thermal analysis measurements in air.
3. Results and discussion
Fig. 1. Structure of the Aurivillius phase BaBi4Ti40,5. Within the puckered sheet region (shown as ball and stick) the dark spheres represent Bi’+ and the light spheres 02-. In the perovskitic region the Ti06 octahedra are shown as polyhedra and the dark spheres are a mixture of Ba2+ and BP.
the vacancies may exhibit different orderings [ 16 1. Since the brownmillerite and perovskite structures are so similar, we have been able to substitute brownmillerite related layers for some of the perovskite layers in the structure, thereby introducing oxygen vacancies. We have previously reported preliminary studies on BaBi4Ti31n014.5 and BaBi4Ti3Sc0,,., [ 171, and we have investigated these materials more fully here. We have also synthesized a new member of the series, BaBi,Ti,GaO,,.,.
Powder X-ray diffraction shows that all three new phases can be indexed to a tetragonal unit cell [for BaBi4Ti3GaOL4.5, a=5.447(2) A, c=41.80(2) A; BaBi4TisIn014.5, a=5.663(4) A, c=41.91(3) A; BaBi4Ti3014.5, a=5.682(3) A, c=41.74(3) A] and are structurally similar to the non-oxygen deficient phase BaBi4Ti40i5 (fig. 1). This unit cell shows some distortion from the idealized BaBi4Ti40i5 structure, however. The new a axis is $2 times larger than that of the simple cell. This distortion is seen in many brownmillerite type materials and in distortions of the Aurivillius phases [ 9,181. From powder X-ray diffraction it is not possible to determine if the brownmillerite “layer” is truly a layer or if the M’+ cations are randomly distributed throughout the perovskitic region. It is also not possible to investigate the oxygen vacancy ordering, and, consequently, neutron diffraction experiments are planned. Based on our experience with brownmillerite-perovskite intergrowths it is likely that the cations are randomly
227
dist~buted throu~out the layers [ 19 1. For BaBi,Ti,Ol 5 the Arrhenius plot of conductivity was linear over a temperature range of 250 to 950°C with an activation energy of 0.72 eV and a conductivity of 1.3x 10e4 at 900°C. Arrhenius plots of conductivity for all three phases showed a jump in conductivity. For BaBi4Ti31n014.5 the low temperature activation energy was 1.OOeV, with a transition between 800 and 850’ C to a high temperature form with an Ea of 0.35 eV and a conductivity of 4.9x1O-2 Scm-* at 900°C (fig. 2). For BaBi4Ti3Sc014.J the low temperature activation energy was 0.93 eV, with a transition between 725 and 850°C to the high temperature form, which has an E, of 0.50 eV and a conductivity of 3.5~ lo--’ Scm- l at 900°C (fig. 3). The large transition temperature range for M = SCis due to an observed hysteresis. On cooling, the high conductivity could be stabilized to a temperature about 100°C lower than the onset temperature on heating. This appears to be due to a slow relaxation process, since a sample in the highly conductive phase that was cooled to 800°C and left for several days eventually converted to the low temperature phase. For BaBi4Ti~GaO~4.~the activation energy was 0.95 eV below 800°C and 0.37 eV above 800” C. The conductivity at 900°C was 4.5X 10m2Scm-’ (fig. 4). Di~erential thermal analysis (DTA) shows an endothermic transition for each of the oxygen deficient materials at temperatures close to the observed jump
- __. -3-
-7-
-%--
I
I.2
I.4
1.6
1.8
2
1000rr Fig. 2. The Arrhenius piot of conductivity for BaBi4TisInOt..5 shows a jump in conductivity above 800°C.
-I
-2
z
-3
3j -
-4
x
COOLING
E Sw
-5
-6
-7 0.8
1.2
I
1.4
1.6
1000I T Fig. 3. The Arrhenius plot of conductivity for BaBi,Ti$cOLa,S shows a jump in conductivity above 800°C but shows hysteresis in thermal cycling.
-7
i
-84--0.8
I
I.2
1.4
1.6
1000/T
Fig. 4. The Arrhenius plot of conducti~ty for BaBi4Ti3Ga014.5 shows a jump in conductivity above 800°C.
in conductivity in the Arrhenius plot. For BaBi.+Ti3Sc014,5the transition has an onset temperature of 842 oC and a peak temperature of 849 ’ C. For BaBi4Ti31n014.5the transition occurs at 833°C onset and 838°C peak, and for BaBi4Ti3GaOL4.5at 817°C onset and 827°C peak. Fig. 5 shows the DTA plot of BaBi4Ti3Ga014.5 which is qualitatively similar to those of the other two compounds. BaBi4Ti4015 which shows no jump in conductivity shows no transition in the DTA plot. The observed transition in the oxygen deficient materials is consistent with an oxygen vacancy order-disorder transition. Through further studies including neutron diffraction we hope to elucidate the nature of this transition.
228
J.K. Thomas et al. /Oxygen ion conductivity in BaBi,Tr3M0,4
A
References [ 1] I. Riess, in: Science and Technology of Fast Ion Conductors,
-1.04 200
I 400
600
Temperature
800
1000
(“C)
Fig. 5. Differential thermal analysis data for BaBi,TisGaO,.,~. DTA plots for BaBi4TisSc0,4.s and BaBidTisIn0i4.S are qualitatively similar.
4. Summary In summary, we have found a new class of oxygen ion conductors which have intrinsic oxygen vacancies and undergo order-disorder transitions. Using intergrowth structures we are able to modify these materials and to control the number of oxygen vacancies within them. We are currently exploring the oxygen ion conductivity of other members of this class of materials to find compounds that have oxygen vacancy order-disorder transitions at lower temperatures. This may be achieved by adding a different number of the perovskite or brownmillerite related layers or by starting with a different Aurivillius phase.
Acknowledgement The authors thank the National Science Foundation for support of this research though grant DMR9200688. JKT would like to thank A.D. Little and AT&T Bell Laboratories for support.
eds. H.L. Tuller and M. Balkanski (Plenum Press, New York, 1987) p. 23. [2] R.M. Dell and A. Hooper, in: Solid Electrolytes, eds. P. Hagenmtlller and W. van Gool (Academic Press, New York, 1978) p. 291. [ 31 T. Takahashi, in: Superionic Solids and Solid Electrolytes, eds. A.L. Laskar and S. Chandra (Academic Press, New York, 1989) p. 1. [4] J.A. Kilner and R.J. Brook, Solid State Ionics 6 (1982) 237. [ 51C.R.A. Catlow, in: Superionic Solids and Solid Electrolytes, eds. A.L. Laskar and S. Chandra (Academic Press, New York, 1989) p. 339. [6] A.J. Burggraaf, M.P. Van Dijk, M.J. Verkerk and K.J. de Vries, in: Structure and Conductivity of Some Pyrochlore and Fluorite Type Solid Solutions. eds. R. Metselaar, H.J.M. Heijligers and J. Schoonman, Vol. 3 (Elsevier, Amsterdam, The Netherlands, 1982) p. 185. [ 71 J.A. Kilner and C.D. Waters, Solid State lonics 6 ( 1982) 253. [ 81 T. Takahashi and H. Iwahara, Mat. Res. Bull. 13 ( 1978) 1447. [ 91 J.B. Goodenough, J.E. Ruiz-Diaz and Y.S. Zhen, Solid State Ionics 44 (1990) 21. [ lo] J.B. Goodenough, A. Manthiram, P. Paranthaman and Y.S. Zhen, Solid State Ionics 52 ( 1992) 105. [11]B.Aurivillius,Ark.Kemi2 (1951) 519. [ 121 E.C. Subbarao, J. Am. Ceram. Sot. 45 (1962) 166. [ 13 ] G.N. Subbanna, T.N. Guru Row and C.N.R. Rao, J. Solid State Chem. 86 (1990) 206. [ 141 F. Abraham, M.F. Debreuille-Gresse, G. Mairesse and Nowogrocki, Solid State Ionics 28-30 ( 1988) 529. [ 15 ] V.K. Yanovskii, V.I. Voronkova, Y.E. Roginskaya and Y.N. Venevtsev, Sov. Phys. Solid State 24 ( 1982) 1603. [ 161 M.T. Anderson, J.T. Vaughey and K.R. Poeppelmeier, Chem. Mater. 5 (1993) 151. [ 171 J.K. Thomas and H.-C. zur Loye, JACS (1993) submitted for publication. [ 181 B. Frit and J.P. Mercurio, Alloys Comp. 188 ( 1992) 27. [ 191 J.K. Thomas, W.E. Krause and H.-C. zur Loye, in: Solid State Ionics III, eds. G.-A. Nazri, J.-M. Tarascon and M. Armand, Vol. 293 (MRS, Pittsburgh, 1993).