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Combined homo-hetero doping for enhancement of ionic conductivity
Solid State Ionics 25 (1987) 2 17-22 1 North-Holland, Amsterdam
COMBINED HOMO-HETERO
DOPING FOR ENHANCEMENT OF IONIC CONDUCT
R. AKKA and K.T. JACOB Departmentof Metallurgyand MaterialsResearch Laboratory,Indian Instituteof Science, Bangalore560012, India Received 12 August 1987; accepted for publication 25 August 1987
Electrical conductivity of N&.ooSCao.995F1.99, and Yo.ooeCao.992Fz.ms samples containing 2 mol% CeOz_x as the dispersed phase has been measured in the range 630 to 1030 K using an ac bridge at 1 kHz. The presence of the dispersed phase enhanced the conductivity of CaFz homogeneously doped with NaF. However, heterogeneous doping with Ce02_x decreased the conductivity of YFrdoped CaF*. This suggests that preferential adsorption of F- ions to the CeO 2_x interface and the consequent creation of a space charge region with increased fluorine vacancies near the interface is the primary mechanism responsible for the enhancement of the conductivity of CaFz heterogeneously doped with Ce@_,. By the synergetic effect of homogeneous and h@terogeneous doping, the conductivity of polyctystalline CaFz can be enhanced by 4.5 orders of magnitude at 630 K.
1. Introduction Mechanisms for enhancing ionic conductivity of solids especially at lower temperatures are important for the development of fuel cells and solid state sensors. Solid electrolytes have been extensively used for the measurement of thermodynamic and kinetic properties of metal and ceramic systems. %ce most of the oxide and fluoride solid electrolytes operate at temperatures in excess of 800 K, measurements have been confined to stable equilibria. If the conductivity of the solid electrolyte can be substantially enhanced, it will be possible to extend measurements to metastable equilibria at lower temperatures. Gillbs energies of potentialiy important metastable phases such as metallic and ceramic glasses and quasicrystals can then be determined experimentally. The ionic conductivity of a solid electrolyte may be increased by homogeneous doping [ 1-3 ] or heterogeneous doping [ 4- 131. Homogeneous doping with an aliovalent ion introduces current carrying vacancy or interstitial defect species to maintain electrical neutrality in the solid. The increased concentration of charge carriers increases the ionic conductivity. Heterogeneous doping involves the dispersion of a “chemically inert” second phase in the matrix of an ionic conductor. The observed enhancements of ionic conductivities of these dis0 167-2738/87/S 03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
persed solid electrolyte systems are larger at lower temperatures [ 5-8,111. The conductivity decreases with particle size of the heterogeneous dopant for fixed concentration, and passes through a maximum as the dopant concentration is varied at constant particle size [ 4,7,12,13 ]I. A number of theoretical models have been proposed to account for the conductivity behaviour of these heterogeneously doped solid electrolytes [ 5,8- lo,14 1. The defect species which is involved in ionic conduction of the matrix may be repelled by or attracted to the interface between the dispersoid and the matrix, resulting in the formation of a space charge region at the interface [5,8-l 01. Thus, in an anti-Frenkel solid, if the anions are attracted to the interface, the vacancy concentration in the space charge region will be increased relative to the bulk value. Repulsion of anions by the inert phase will increase the concentration of interstitials in the space charge region. The space charge region is thus associated with an increased concentration of defects. The additional contribution to t total conductivity from the space charge region accounts for the enhancement in ionic conductivity The quantitative aspects of the various theoretica models and their application to experimental results have been reviewed recently [ 151. Calcium fluoride is a model solid electrolyte for mechanistic investigations. High purity single and
218
R. Akila,K.T. Jacob/Homo-hetero dopingfor enhancement of ionicconductivity
polycrystalline CaFz is readily available. Its conductivity is well characterized in pure and doped ustals [l-3,11-13]. The defect structure [ 16,171 and migration enthalpies for both interstitials and vacancies are available [ 18,191. Calcium fluoride has a larger electrolytic conduction domain than other solid electrolytes [ 201. The melting point and sintering temperatures are relatively low compared t0 oxide solid electrolytes. In pure CaFz, conductivity is due to fluorine vacancy migration at low temperatures. The activation energy for vacancy migration is 0.5 1 eV [ 181, whereas for interstitial migration a value of 0.92 eV has been reported [ 191. Ure [ I! hw shown that addition of NaF to the lattice of CaFz results in the creation of fluorine ion vacancies. The conductivity increases with concentration of NaF upto 0.5 mol%. Doping CaFz with YF3 introduces fluorine interstitials [ l-31. Extensive doping studies with YF3 [ 2,3] indicate that the conductivity of CaF, increases with dopant concentration upto 35 mol%. Vaidehi et al. [ 111 have shown that addition of 2-4 mol% CeOz_, enhances the conductivity of CaFz by approximately three orders of magnitude at 636 K. The oxide phase was present as a fine dispersion primarily located along grain boundaries. Addition of 2-4 mol% A&O3enhanced the conductivity by approximately two orders of magnitude at 636 K. It has been shown by electrochemical studies that conduction in these two phase mixtures is purely ionic [ 111. Fujitsu et al. [ 121 have shown that conductivity of CaF2 heterogeneously doped with A1203 has a maximum value when the concentration of Al& is 10 mol”h for a particle size of 0.3 pm. The maximum increase in conductivity is by approximately one order of magnitude. However, Khandkar et al. [ 131 obtained a maximum enhancement in conductivity by two orders of magnitude with the addition of 5 mol% A&OS.Dispersion of 5 mol% ZrOz increased the conductivity by approximately an order of magnitude [ 131. Vaidehi et al. [ 111 have quantitatively accounfed for the increase in conductivity using the model of Maier [ 8 1. Fr- ,a an analysis of the act; vation energy for conduction, Vaidehi et al. [ 113 have suggested that enhanced conduc;ron is clue to increased concentration of vacancies in the space charge region. The use of both homogeneous and heterogeneous
doping mechanisms simultaneously for enhancing ionic conductivity, using different additives for each, has been explored in this study. Both NaF and YF3 have been used as the homogeneous dopants since the charge compensating defect species introduced in the fluoride lattice are different in the two cases. Cerium dioxide has been chosen as the heterogeneous dopant since it has the maximum effect on ionic conductivity.
2. Experimental aspects 2. I. Materials Anhydrous CaF2,YF, and NaF powders of 99.99% purity obtained commercially were used. Cerium dioxide (CeO& of 99.9% purity was obtained by precipitation from solution followed by calcination in air at 1400 K. CaF2 was mixed with NaF or YF3 powder in proportionate amounts and ground under acetone in an agate mortar. Pellets of pure CaF2 and CaFz doped with NaF or YF3 with and without the addition of 2 mol% CeOz_x, were made from the powders by double end compression in a steel die. The average particle size of CeOz._, was 0.0 1 pm. Argon gas was dehydrated by passing through magnesium perchlorate and anhydrous phosphorus pentoxide and subsequently deoxidised by passing over copper turnings at 750 K and titanium at 1150 K. The purified argon gas atmosphere was used for sintering the pellets at 1300 K for 180 ks. X-ray and SEM studies were used to confinn that no new phases were formed between CaFz and CeOz_,. The oxide dispersion was mainly located along grain boundaries. The average grain size of the sintered pellets is shown in table 1. The grain size of the samples with the dispersed phas c is approximately half that for similar compositions without second phase particles.
The electrodes for conductivity measurements were prepared by coating both surfaces of the sintered pellet with platinum paste (Engelhardt T 1150). The pellet was then heated in air for 600 s at 1050 K an subsequently in vacuum at 1000 E: for 11 ks. The platinized surface was etched in boiling nitric acid.
R. Akila,K.T. Jacob/Homo-hetero dopingfor enhancement of ionicconductivity Table 1 Average grain size of the sintered ( polycrystalline) samples used for conductivity measurements. Specimen No.
Specimen composition
Average grain size (pm)
1 2 3 4
CaF2
37 53 54 23
~O.OOS~&l992~2.008 Wm~Cao.99JC995 yO.OO8c%992F2.008
+
2 mol% Ce02_x Nao.oosCao.~~sF, .99s 2 mol% Ce02+
5
30
+
Platinum leads spot welded to porous Pt foils were spring loaded against the pellet using a set of alumina rods and slabs. The cell was placed in a closed end quartz tube mounted in a vertical furnace heated by resistance winding. The temperature of the furnace was controlled to within I!I1 K with a Thyristor temperature controller. An earthed stainless steel sheet was wrapped around the quartz tube to eliminate induced currents. The temperature was measured with a Pt/Pt-13% Rh thermocouple. The conductivity measurement w-as perfo_rmed with an argon flow rate of 5 ml s-l, at a fixed frequency of 1 ki-k in the temperature range 630 to 1030 K. The conductivities of the following polycrystalline samples were measured: (a) pure CaF2 , (b) CaF, doped homogeneously with 0.8 mol% YF3
CaF, + 2 mol% Ce02 _-xand CaF2 + 4 mol% Ce02 _ X are from an earlier study [ 111. For the Y0.008Ca0.992Pz.008 sample, the increase in conductivity is by an order of magnitude at 630 K and by a factor of six at 1030 K, in agreement with the results of Ure [ 11. Addition of 2 mol% Ce02_X decreases the conductivity by a factor of N 1.5 compared to the homogeneously doped sample. Doping with 0.5 mol% NaF increases the conductivity of pure CaF, by more than three orders of magnitude at 630 IL The present results for the homogeneous sample are slightly higher that the results of Ure [ 11. By the addition of 2 mol% CeG2+ the conductivity is further increased by a factor of three relative to Nh .oosCh.995F,.995.The total enhancement in conductivity of CaF2 at 630 K by use cfboth homogeneous ( NaF) and heterogeneous ( Ce02 J doping is by four orders of magnitude. The addition of the second phase Ce02-, to pure
d& I 10
1 Y ‘i E 7; 10-l kb
(Yo.oosCao.9&.oos)
CaF, doped homogeneously with 0.5 mol% NaF
(c)
?19
lo-2
(Nao.oosCao.99sF1.995) (d)
Y0.008C%.992F2.008
(e)
Na0.005
+2
md%
10-;
CeO2-,
Cao.99sF,.99jj-2 mol% Ce02-,
. lo-&
The measured conductivities of the polycrystalline samples are plotted in fig. 1. The conductivities of
I.-------
0.8
’
12
1 .) ‘Os/
T,
I
14
I
,
16
K-’
Fig. 1. Variation of the conductivity with the reciprocal of absolute temperature (1) polycrystalline CaL (2) Yo.~8C~.~~~F~.~8. + 2 moi% Ce( 3) NaO.Pl, rCao.99sFI .995T(4) YO.,&aO 99zF2.008 0 2_.n (5) N~.m5C~.995F,.ygS+2mol% Ce@-, pnd (6) CaF,+2 (or4) mol%CeOz_, [ll].
220
R. Akila,K. T. Jacob/Homo-hetero dopingfor enhancement of ionicconductivity
CaF2 increases the conductivity by approximately three orders of magnitude at 630 K. The conductivity is independent of the concentration of the CeOZ_, dispersoid between 2 and 4 mol%.
4. Discussion The replacement of Ca*+ by Y3+ introduces negatively charged fluorine interstitials as the charge compensating species [ 11. Neutron diffraction experiments of Cheetham et al. [ 171 and ionic conductivity and thermal depolarization studies of Jacobs and Ong [ 211 have shown that the Y3+ ions and their associated F; interstitials are clustered to some extent. At low Y3+ concentrations the postulated dominant defect is the neutral 2 : 2 : 2 cluster which can be viewed as a planar dimer formed from 3+-Fr pairs, and stabilized by the fortwo neutral YS mation of two anion vacancies and two anion interstitials by the relaxation of two nearest neighbour [ 0011 lattice F ions in the ( 111) direction. The conductivity is due to mobile interstitial fluoride ions (Fi ) in equilibrium with the complex or formed by its ionization. This increase in the concentration of the F, increases ionic conductivity. The activation energy calculated from the extrinsic branch of the conductivity curve for Yo.oosCao.992F2.008 is 0.82 eV. This is close to the reported value of 0.92 eV [ 19 j for interstitial migration. Addition of CeO,_,, having a small deviation from stoichiometry, will have two effects on the conductivity of CaF2. If there is a finite solubility of CeO,,, in CaF2, the nonstoichiometric nature of the oxide will introduce some fluorine vacancies in the bulk. The neutralisation of the vacancies with the fluorine interstitials will reduce the concentration of charge carriers and thereby decrease the conductivity. A more significant effect is probably due to the attraction of F- ions to the surface of CeO,_ x particles which results in lower concentration of interstitial ions in the CTWP -r--- charup -----0region. In the case of doping v .rk NaF, uorine vacancies are introduced into the host CaFz matrix [ 11. The slope of the conchuctivity plot for NaOOOsC~995F,995 is 0.46 eV, which is close to the migration enthalpy of 0.5 1 eV associated with vacancy migration [ 18 ] . A small but finite solubility of nonsroichior,c;rc
Ce02-x would increase fluorine vacancy concentration in the bulk crystal, whereas the adsorption of fluorine ions to the surface of Ce02_X particles will enhance the vacancy concentration in the space charge region. If there is a small solubility of Ce02_X in CaF2, some Ce3+ ions would be present along with Ce4+ ions. Electron hopping between these ions is not expected to be significant since the distance between ions of variable valence would be relatively large at small concentrations. This is supported by electrochemical cell measurements using pure CaF2 containing Ce02 _-xdispersoids [ 111. The effects of homogeneous and heterogeneous doping on conductivity are not additive. The conductivity probably reaches a maximum value with increase in vacancy concentration both in the bulk crystal and in the space charge region. Increase in vacancy concentration beyond an optimum value may not significantly improve ionic conductivity. The ability to elucidate the nature of the interaction between conducting species in the matrix and surface of the dispersed phase by combined doping studies is an important feature that has emerged from this study. If fluorine ions were repelled by Ce02_, particles and enhanced conductivity in the space charge regions was due to fluorine interstitials, conductivity of Yo.oosCao.992F2.008 would have been enhanced and that of Nao.oosCao.99sF1.995 would have been decreased by the presence of CeOz_,. 5. Conclusions Wse of combined homogeneous and heterogeneous doping can be used synergetically to enhance the conductivity of ionic solids. The nature of the defects introduced in the bulk crystal by homogeneous doping and in the space charge region by the heterogeneous phase must be the same or similar to produce synergetic effects. If the defects produced by the two- donine _-- ~ ---Q nrtwe~~ec = - - - -L--- zmnihilsate - ____--______ wwh _I___ nthm WV .v.v_-_-, .a APcrease in conductivity may be expected.
The authors acknowledge the assistance of Thelma Pinto and Mr. AX Narayana in the preparatian of the manuscript.
R. Akila, K. T. Jacob/Homo-hetero doping for enhancement of ionic conductivity
References [ 1 ] R.W. Ure Jr., J. Chem. Phys. 26 (1957) 1363. [ 21 L.E. Nagel and M.O’Keeffe, in: Fast ion transpon in solids, ed. W. Van Gool (North-Holland, Amsterdam and American Elsevier, New York, 1973) p. 165. [ 3 ] J.M. R&auand J. Portier, in: Solid electrolytes, eds. P. Hagenmuller and W. Van Go01 (Academic Press, New York, 1978) p. 313. [ 41 C.C. Liang, J. Electrochem. Sot. 120 (1973) 1289. [ 51 T. Jow and J.B. Wagner Jr., J. Electrochem. Sot. 126 (1979) 1963. [ 61 K. Shahi and J.B. Wagner Jr., J. Solid State Chem. 42 ( 1982) 107. [7] K. Shahi and J.B. Wagner Jr., J. Phys. Chem. So!ids 43 (1982) 713. [ 8 ] J. Maier, J. Phys. Chem. Solids 46 ( 1985) 309. [9] J. Maier, Phys. Status Solidi (b) 123 (1984) KS9. [ lo] J. Maier, Mat. Res. Bull. 20 (1985) 383. [ 111 N. Vaidehi, R. Akila, AK. Shukla and K.T. Jacob, Mat. Res. Bull. 21 (1986) 909.
221
[ 121 S. Fujitsu, M. Miyayama, K. Koumoto, H. Yanagida and T. Kanazawa, J. Mater. Sci. 20 (1985) 2103. [ 131 A. Khandkar, V.B. Tare and J.B. Wagner Jr., RCL. fnim. Miner. 23 (1986) 274. [ 141 A.M. Stoneham, E.Wade and 3.A. Kitner, Mat. Res. Bulk 14 (1979) 661. [ 151 A.K. Shukla, N. Vaidehi and K.T. Jacob, in: Proc. Indian Acad. Sci. (Chem. Sci.) 96 (1986) 533. [ 16 ] AK. Cheetham, B.E.F. Fender, D. Steele, R.I. Taylor and B.T.M. Williams, Solid State Commun. 8 ( 1970) 17 I. [ 171 AK. Cheetham, B.E.F. Fender and M.J. Cooper, J. Phys. C 4 (1971) 3107. [ 181 W. Bollmann and R. Reimann, Phys. Status Solidi (a) 11 (1972) 367. [ 191 W. Bollmann and H. Henniger, Phys. Status Solidi (a) 16 (!9?3! 18?. (201 J.W. Hinze and J.W. Patterson, J. Elxtrochem.
Sot. 120 (1973) 96. [ 2 1 ] P.W.M. Jacobs and S.H. Ong, J. Phys. Chem. Solids 4 f (1980) 431.
COMBINED HOMO-HETERO
DOPING FOR ENHANCEMENT OF IONIC CONDUCT
R. AKKA and K.T. JACOB Departmentof Metallurgyand MaterialsResearch Laboratory,Indian Instituteof Science, Bangalore560012, India Received 12 August 1987; accepted for publication 25 August 1987
Electrical conductivity of N&.ooSCao.995F1.99, and Yo.ooeCao.992Fz.ms samples containing 2 mol% CeOz_x as the dispersed phase has been measured in the range 630 to 1030 K using an ac bridge at 1 kHz. The presence of the dispersed phase enhanced the conductivity of CaFz homogeneously doped with NaF. However, heterogeneous doping with Ce02_x decreased the conductivity of YFrdoped CaF*. This suggests that preferential adsorption of F- ions to the CeO 2_x interface and the consequent creation of a space charge region with increased fluorine vacancies near the interface is the primary mechanism responsible for the enhancement of the conductivity of CaFz heterogeneously doped with Ce@_,. By the synergetic effect of homogeneous and h@terogeneous doping, the conductivity of polyctystalline CaFz can be enhanced by 4.5 orders of magnitude at 630 K.
1. Introduction Mechanisms for enhancing ionic conductivity of solids especially at lower temperatures are important for the development of fuel cells and solid state sensors. Solid electrolytes have been extensively used for the measurement of thermodynamic and kinetic properties of metal and ceramic systems. %ce most of the oxide and fluoride solid electrolytes operate at temperatures in excess of 800 K, measurements have been confined to stable equilibria. If the conductivity of the solid electrolyte can be substantially enhanced, it will be possible to extend measurements to metastable equilibria at lower temperatures. Gillbs energies of potentialiy important metastable phases such as metallic and ceramic glasses and quasicrystals can then be determined experimentally. The ionic conductivity of a solid electrolyte may be increased by homogeneous doping [ 1-3 ] or heterogeneous doping [ 4- 131. Homogeneous doping with an aliovalent ion introduces current carrying vacancy or interstitial defect species to maintain electrical neutrality in the solid. The increased concentration of charge carriers increases the ionic conductivity. Heterogeneous doping involves the dispersion of a “chemically inert” second phase in the matrix of an ionic conductor. The observed enhancements of ionic conductivities of these dis0 167-2738/87/S 03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
persed solid electrolyte systems are larger at lower temperatures [ 5-8,111. The conductivity decreases with particle size of the heterogeneous dopant for fixed concentration, and passes through a maximum as the dopant concentration is varied at constant particle size [ 4,7,12,13 ]I. A number of theoretical models have been proposed to account for the conductivity behaviour of these heterogeneously doped solid electrolytes [ 5,8- lo,14 1. The defect species which is involved in ionic conduction of the matrix may be repelled by or attracted to the interface between the dispersoid and the matrix, resulting in the formation of a space charge region at the interface [5,8-l 01. Thus, in an anti-Frenkel solid, if the anions are attracted to the interface, the vacancy concentration in the space charge region will be increased relative to the bulk value. Repulsion of anions by the inert phase will increase the concentration of interstitials in the space charge region. The space charge region is thus associated with an increased concentration of defects. The additional contribution to t total conductivity from the space charge region accounts for the enhancement in ionic conductivity The quantitative aspects of the various theoretica models and their application to experimental results have been reviewed recently [ 151. Calcium fluoride is a model solid electrolyte for mechanistic investigations. High purity single and
218
R. Akila,K.T. Jacob/Homo-hetero dopingfor enhancement of ionicconductivity
polycrystalline CaFz is readily available. Its conductivity is well characterized in pure and doped ustals [l-3,11-13]. The defect structure [ 16,171 and migration enthalpies for both interstitials and vacancies are available [ 18,191. Calcium fluoride has a larger electrolytic conduction domain than other solid electrolytes [ 201. The melting point and sintering temperatures are relatively low compared t0 oxide solid electrolytes. In pure CaFz, conductivity is due to fluorine vacancy migration at low temperatures. The activation energy for vacancy migration is 0.5 1 eV [ 181, whereas for interstitial migration a value of 0.92 eV has been reported [ 191. Ure [ I! hw shown that addition of NaF to the lattice of CaFz results in the creation of fluorine ion vacancies. The conductivity increases with concentration of NaF upto 0.5 mol%. Doping CaFz with YF3 introduces fluorine interstitials [ l-31. Extensive doping studies with YF3 [ 2,3] indicate that the conductivity of CaF, increases with dopant concentration upto 35 mol%. Vaidehi et al. [ 111 have shown that addition of 2-4 mol% CeOz_, enhances the conductivity of CaFz by approximately three orders of magnitude at 636 K. The oxide phase was present as a fine dispersion primarily located along grain boundaries. Addition of 2-4 mol% A&O3enhanced the conductivity by approximately two orders of magnitude at 636 K. It has been shown by electrochemical studies that conduction in these two phase mixtures is purely ionic [ 111. Fujitsu et al. [ 121 have shown that conductivity of CaF2 heterogeneously doped with A1203 has a maximum value when the concentration of Al& is 10 mol”h for a particle size of 0.3 pm. The maximum increase in conductivity is by approximately one order of magnitude. However, Khandkar et al. [ 131 obtained a maximum enhancement in conductivity by two orders of magnitude with the addition of 5 mol% A&OS.Dispersion of 5 mol% ZrOz increased the conductivity by approximately an order of magnitude [ 131. Vaidehi et al. [ 111 have quantitatively accounfed for the increase in conductivity using the model of Maier [ 8 1. Fr- ,a an analysis of the act; vation energy for conduction, Vaidehi et al. [ 113 have suggested that enhanced conduc;ron is clue to increased concentration of vacancies in the space charge region. The use of both homogeneous and heterogeneous
doping mechanisms simultaneously for enhancing ionic conductivity, using different additives for each, has been explored in this study. Both NaF and YF3 have been used as the homogeneous dopants since the charge compensating defect species introduced in the fluoride lattice are different in the two cases. Cerium dioxide has been chosen as the heterogeneous dopant since it has the maximum effect on ionic conductivity.
2. Experimental aspects 2. I. Materials Anhydrous CaF2,YF, and NaF powders of 99.99% purity obtained commercially were used. Cerium dioxide (CeO& of 99.9% purity was obtained by precipitation from solution followed by calcination in air at 1400 K. CaF2 was mixed with NaF or YF3 powder in proportionate amounts and ground under acetone in an agate mortar. Pellets of pure CaF2 and CaFz doped with NaF or YF3 with and without the addition of 2 mol% CeOz_x, were made from the powders by double end compression in a steel die. The average particle size of CeOz._, was 0.0 1 pm. Argon gas was dehydrated by passing through magnesium perchlorate and anhydrous phosphorus pentoxide and subsequently deoxidised by passing over copper turnings at 750 K and titanium at 1150 K. The purified argon gas atmosphere was used for sintering the pellets at 1300 K for 180 ks. X-ray and SEM studies were used to confinn that no new phases were formed between CaFz and CeOz_,. The oxide dispersion was mainly located along grain boundaries. The average grain size of the sintered pellets is shown in table 1. The grain size of the samples with the dispersed phas c is approximately half that for similar compositions without second phase particles.
The electrodes for conductivity measurements were prepared by coating both surfaces of the sintered pellet with platinum paste (Engelhardt T 1150). The pellet was then heated in air for 600 s at 1050 K an subsequently in vacuum at 1000 E: for 11 ks. The platinized surface was etched in boiling nitric acid.
R. Akila,K.T. Jacob/Homo-hetero dopingfor enhancement of ionicconductivity Table 1 Average grain size of the sintered ( polycrystalline) samples used for conductivity measurements. Specimen No.
Specimen composition
Average grain size (pm)
1 2 3 4
CaF2
37 53 54 23
~O.OOS~&l992~2.008 Wm~Cao.99JC995 yO.OO8c%992F2.008
+
2 mol% Ce02_x Nao.oosCao.~~sF, .99s 2 mol% Ce02+
5
30
+
Platinum leads spot welded to porous Pt foils were spring loaded against the pellet using a set of alumina rods and slabs. The cell was placed in a closed end quartz tube mounted in a vertical furnace heated by resistance winding. The temperature of the furnace was controlled to within I!I1 K with a Thyristor temperature controller. An earthed stainless steel sheet was wrapped around the quartz tube to eliminate induced currents. The temperature was measured with a Pt/Pt-13% Rh thermocouple. The conductivity measurement w-as perfo_rmed with an argon flow rate of 5 ml s-l, at a fixed frequency of 1 ki-k in the temperature range 630 to 1030 K. The conductivities of the following polycrystalline samples were measured: (a) pure CaF2 , (b) CaF, doped homogeneously with 0.8 mol% YF3
CaF, + 2 mol% Ce02 _-xand CaF2 + 4 mol% Ce02 _ X are from an earlier study [ 111. For the Y0.008Ca0.992Pz.008 sample, the increase in conductivity is by an order of magnitude at 630 K and by a factor of six at 1030 K, in agreement with the results of Ure [ 11. Addition of 2 mol% Ce02_X decreases the conductivity by a factor of N 1.5 compared to the homogeneously doped sample. Doping with 0.5 mol% NaF increases the conductivity of pure CaF, by more than three orders of magnitude at 630 IL The present results for the homogeneous sample are slightly higher that the results of Ure [ 11. By the addition of 2 mol% CeG2+ the conductivity is further increased by a factor of three relative to Nh .oosCh.995F,.995.The total enhancement in conductivity of CaF2 at 630 K by use cfboth homogeneous ( NaF) and heterogeneous ( Ce02 J doping is by four orders of magnitude. The addition of the second phase Ce02-, to pure
d& I 10
1 Y ‘i E 7; 10-l kb
(Yo.oosCao.9&.oos)
CaF, doped homogeneously with 0.5 mol% NaF
(c)
?19
lo-2
(Nao.oosCao.99sF1.995) (d)
Y0.008C%.992F2.008
(e)
Na0.005
+2
md%
10-;
CeO2-,
Cao.99sF,.99jj-2 mol% Ce02-,
. lo-&
The measured conductivities of the polycrystalline samples are plotted in fig. 1. The conductivities of
I.-------
0.8
’
12
1 .) ‘Os/
T,
I
14
I
,
16
K-’
Fig. 1. Variation of the conductivity with the reciprocal of absolute temperature (1) polycrystalline CaL (2) Yo.~8C~.~~~F~.~8. + 2 moi% Ce( 3) NaO.Pl, rCao.99sFI .995T(4) YO.,&aO 99zF2.008 0 2_.n (5) N~.m5C~.995F,.ygS+2mol% Ce@-, pnd (6) CaF,+2 (or4) mol%CeOz_, [ll].
220
R. Akila,K. T. Jacob/Homo-hetero dopingfor enhancement of ionicconductivity
CaF2 increases the conductivity by approximately three orders of magnitude at 630 K. The conductivity is independent of the concentration of the CeOZ_, dispersoid between 2 and 4 mol%.
4. Discussion The replacement of Ca*+ by Y3+ introduces negatively charged fluorine interstitials as the charge compensating species [ 11. Neutron diffraction experiments of Cheetham et al. [ 171 and ionic conductivity and thermal depolarization studies of Jacobs and Ong [ 211 have shown that the Y3+ ions and their associated F; interstitials are clustered to some extent. At low Y3+ concentrations the postulated dominant defect is the neutral 2 : 2 : 2 cluster which can be viewed as a planar dimer formed from 3+-Fr pairs, and stabilized by the fortwo neutral YS mation of two anion vacancies and two anion interstitials by the relaxation of two nearest neighbour [ 0011 lattice F ions in the ( 111) direction. The conductivity is due to mobile interstitial fluoride ions (Fi ) in equilibrium with the complex or formed by its ionization. This increase in the concentration of the F, increases ionic conductivity. The activation energy calculated from the extrinsic branch of the conductivity curve for Yo.oosCao.992F2.008 is 0.82 eV. This is close to the reported value of 0.92 eV [ 19 j for interstitial migration. Addition of CeO,_,, having a small deviation from stoichiometry, will have two effects on the conductivity of CaF2. If there is a finite solubility of CeO,,, in CaF2, the nonstoichiometric nature of the oxide will introduce some fluorine vacancies in the bulk. The neutralisation of the vacancies with the fluorine interstitials will reduce the concentration of charge carriers and thereby decrease the conductivity. A more significant effect is probably due to the attraction of F- ions to the surface of CeO,_ x particles which results in lower concentration of interstitial ions in the CTWP -r--- charup -----0region. In the case of doping v .rk NaF, uorine vacancies are introduced into the host CaFz matrix [ 11. The slope of the conchuctivity plot for NaOOOsC~995F,995 is 0.46 eV, which is close to the migration enthalpy of 0.5 1 eV associated with vacancy migration [ 18 ] . A small but finite solubility of nonsroichior,c;rc
Ce02-x would increase fluorine vacancy concentration in the bulk crystal, whereas the adsorption of fluorine ions to the surface of Ce02_X particles will enhance the vacancy concentration in the space charge region. If there is a small solubility of Ce02_X in CaF2, some Ce3+ ions would be present along with Ce4+ ions. Electron hopping between these ions is not expected to be significant since the distance between ions of variable valence would be relatively large at small concentrations. This is supported by electrochemical cell measurements using pure CaF2 containing Ce02 _-xdispersoids [ 111. The effects of homogeneous and heterogeneous doping on conductivity are not additive. The conductivity probably reaches a maximum value with increase in vacancy concentration both in the bulk crystal and in the space charge region. Increase in vacancy concentration beyond an optimum value may not significantly improve ionic conductivity. The ability to elucidate the nature of the interaction between conducting species in the matrix and surface of the dispersed phase by combined doping studies is an important feature that has emerged from this study. If fluorine ions were repelled by Ce02_, particles and enhanced conductivity in the space charge regions was due to fluorine interstitials, conductivity of Yo.oosCao.992F2.008 would have been enhanced and that of Nao.oosCao.99sF1.995 would have been decreased by the presence of CeOz_,. 5. Conclusions Wse of combined homogeneous and heterogeneous doping can be used synergetically to enhance the conductivity of ionic solids. The nature of the defects introduced in the bulk crystal by homogeneous doping and in the space charge region by the heterogeneous phase must be the same or similar to produce synergetic effects. If the defects produced by the two- donine _-- ~ ---Q nrtwe~~ec = - - - -L--- zmnihilsate - ____--______ wwh _I___ nthm WV .v.v_-_-, .a APcrease in conductivity may be expected.
The authors acknowledge the assistance of Thelma Pinto and Mr. AX Narayana in the preparatian of the manuscript.
R. Akila, K. T. Jacob/Homo-hetero doping for enhancement of ionic conductivity
References [ 1 ] R.W. Ure Jr., J. Chem. Phys. 26 (1957) 1363. [ 21 L.E. Nagel and M.O’Keeffe, in: Fast ion transpon in solids, ed. W. Van Gool (North-Holland, Amsterdam and American Elsevier, New York, 1973) p. 165. [ 3 ] J.M. R&auand J. Portier, in: Solid electrolytes, eds. P. Hagenmuller and W. Van Go01 (Academic Press, New York, 1978) p. 313. [ 41 C.C. Liang, J. Electrochem. Sot. 120 (1973) 1289. [ 51 T. Jow and J.B. Wagner Jr., J. Electrochem. Sot. 126 (1979) 1963. [ 61 K. Shahi and J.B. Wagner Jr., J. Solid State Chem. 42 ( 1982) 107. [7] K. Shahi and J.B. Wagner Jr., J. Phys. Chem. So!ids 43 (1982) 713. [ 8 ] J. Maier, J. Phys. Chem. Solids 46 ( 1985) 309. [9] J. Maier, Phys. Status Solidi (b) 123 (1984) KS9. [ lo] J. Maier, Mat. Res. Bull. 20 (1985) 383. [ 111 N. Vaidehi, R. Akila, AK. Shukla and K.T. Jacob, Mat. Res. Bull. 21 (1986) 909.
221
[ 121 S. Fujitsu, M. Miyayama, K. Koumoto, H. Yanagida and T. Kanazawa, J. Mater. Sci. 20 (1985) 2103. [ 131 A. Khandkar, V.B. Tare and J.B. Wagner Jr., RCL. fnim. Miner. 23 (1986) 274. [ 141 A.M. Stoneham, E.Wade and 3.A. Kitner, Mat. Res. Bulk 14 (1979) 661. [ 151 A.K. Shukla, N. Vaidehi and K.T. Jacob, in: Proc. Indian Acad. Sci. (Chem. Sci.) 96 (1986) 533. [ 16 ] AK. Cheetham, B.E.F. Fender, D. Steele, R.I. Taylor and B.T.M. Williams, Solid State Commun. 8 ( 1970) 17 I. [ 171 AK. Cheetham, B.E.F. Fender and M.J. Cooper, J. Phys. C 4 (1971) 3107. [ 181 W. Bollmann and R. Reimann, Phys. Status Solidi (a) 11 (1972) 367. [ 191 W. Bollmann and H. Henniger, Phys. Status Solidi (a) 16 (!9?3! 18?. (201 J.W. Hinze and J.W. Patterson, J. Elxtrochem.
Sot. 120 (1973) 96. [ 2 1 ] P.W.M. Jacobs and S.H. Ong, J. Phys. Chem. Solids 4 f (1980) 431.