Oxygen diffusion in perovskite-related oxides

221

Materials Chernbt~ and Physics, 35 (1993) 221-224

Oxygen diffusion in perovskite-related J.B. Goodenough,

A. Manthiram

oxides

and J.-F. Kuo

Center for MateriaLs Science and Engineering, ETC 9.102, Universiry of Texas at Austin, Austin, ‘1x 78712-1084 (USA)

Abstract Investigation of the system BaZr,_XIn,O,,l has revealed that above 500 “C Ba21n205 has an oxide-ion conductivity uo analogous to that of Bi20, and the Zr-substituted compounds a uo analogous to that of the stabilized zirconias; at lower temperatures both neutral oxygen and water can be reversibly inserted into or extracted from the oxygen vacancies to give an enhanced ionic conduction.

Introduction Identification of a solid oxide-ion electrolyte capable of supporting an oxide-ion conductivity u, > 10e2 R-’ cm-l at an operating temperature Top~600 “C has remained a frustratingly elusive target. In this paper we report an oxide-ion conduction in several perovskiterelated materials and compare the results with the wellknown oxide-ion conductors with fluorite-related structures. In a cubic oxide, the oxide-ion conductivity is generally given by the expression [l] a0 = (BIT) exp( -E,/kT)

(1)

with E, = At&, + (1/2)AH, where AHi is the energy required 12 of mobile oxide ions: n =no exp( - ~il~ucT)

(2) to create a density

(3)

Oxide-ion conductors contain a fixed cation array; the mobile 02- ions move in the interstitial space of the cation array. We may expect fast 02--ion conduction above an order-disorder transition temperature T, if anion vacancies are ordered on an array of anion interstitial sites at T T,, where AHi = 0 and E, = AHm. For T < T,, AHi=AHg(T)=AHH,-nE

0254-0584/931$6.00

(4)

where the density n of mobile 02- ions has the exponential temperature dependence of eqn. (3). In eqn. (4), AHg is the energy gap induced by ordering that splits the 02--ion potential at the vacancy sites from that at the occupied sites. In the application of eqn. (4), it is customary to assume that E, is a constant, independent of T; but substitution of eqn. (1) into eqn. (2) with n given by eqn. (3) shows that E, decreases from LU&,+ (1/2)AH, at T=O K to AH, at T> T,. The number of charge carriers n may increase discontinuously at T, if the energy E of eqn. (4) is large enough. The observation of fast F--ion conduction in PbF, suggested that there is something special about the fluorite structure, and initial efforts to identify a fast 02--ion electrolyte concentrated on oxides with the fluorite structure [2]. The stoichiometric MO2 oxides with the fluorite structure do not exhibit a transition to fast 02--ion conduction at temperatures of technical interest. Therefore, doping with an aliovalent cation so as to introduce anion vacancies has been explored extensively. However, the dopants act as trapping centers for the anion vacancies they create, so it is necessary to introduce a trapping energy pi = AHH,into eqn. (2) for temperatures T < T,, where T, represents a saturation temperature for freeing of the vacancies from the traps. Moreover, the dopants increase the motional enthalpy AH,_. The magnitude of AH1 contains elastic as well as coulombic components, and clever selection of both dopant and dopant concentration allows minimization of AH, [2]. This strategy has been modestly successful, but a AH,> 0 continues to reduce the effectiveness of this approach; at operating temperature Top> 800 “C, cation mobilities stabilize clustering of the dopants over time, which increases AHt and represents an undesirable ‘aging’ of the ceramics.

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222

The AMO, cubic perovskites contain a CsCl-type cation array that readily accepts oxygen vacancies [3]; it offers a stable cation subarray within which 02- ions may move. However, in a cubic perovskite the smaller M cations are coordinated by only six oxygen near neighbors, whereas in the fluorite structure each cation has eight nearest-neighbor oxide ions. This difference means that oxygen vacancies place some of the M cations of a perovskite in less than sixfold coordination. In this study we show that if an M cation is not stable in fivefold oxygen coordination, water and/or neutral oxygen can readily be inserted below 400 “C into the oxygen vacancies to create OH- and/or O- species that are mobile in the presence of unfilled oxygen vacancies. Moreover, the introduction of OH- ions also gives rise to an H+-ion conductivity a,, and the insertion/ extraction of oxygen and/or water on cycling thermally produces volume changes that cause cracking and disintegration of ceramic membranes. Oxidation of the oxide-ion array does not appear to introduce an important electronic contribution a, to the conductivity.

Brownmillerites We began our investigation of perovskite-related oxides with Ba,In,O, [4]. We correctly assumed that it would be isostructural with brownmillerite, Ca,FeAIOS, since the In3+ ion can be stabilized in tetrahedral as well as octahedral coordination. In this orthorhombic structure, the oxygen vacancies of the AMO,, array are ordered so as to alternate octahedral and tetrahedral coordination of the In3’ ions along a principal axis of the perovskite structure. A first-order order-disorder transition at T,=930 “C gives a log u, versus l/T plot similar to that found for Bi,O,, and the limited data for T> T, indicate that AH,,, can be small enough in the disordered phase to give the desired u, at 600 “C provided T, can be lowered without increasing AHH,. As in the case of Bi,03, substitution of an aliovalent ion for In3’ introduces a AH, and increases AH,,, without lowering T, to any significant extent; however, it does suppress the first-order order-disorder transition and gives a a, versus l/T plot comparable to that obtained for the stabilized zirconias [4]. We next noted that Ca3FezTi0, is reported [5] to have an ordered-vacancy structure in which two planes of corner-shared MO, octahedra alternate with a single plane of corner-shared MO, tetrahedra. Since both Fe3+ and Ti4’ can be reduced easily by the further removal of oxygen, we did not investigate this compound as a possible 02--ion electrolyte. However, we argued that any attempt of the analogous Ba,In,ZrO, compound to order similarly would inevitably encounter disorder

owing to Zr4+ in tetrahedral-site layers pulling oxygen from the octahedral-site layers. A preliminary investigation of the Ba,In,MO, compounds containing M = Zr, Ce or Hf revealed an extraordinarily high ionic conductivity below 400 “C and an instability of the ceramic in the range 400 < T< 600 “C [4]. In this paper we report further investigation of this unusual phenomenon.

The system ‘BaZr, _,In,O,,’ Nominal BaZr, -Jn,Oro.ti samples were prepared by firing the required quantities of intimately mixed BaCO,, MO, and In203 in air at 900 “C for 24 h followed by grinding the product, pelletizing, and refiring at 1350 “C for 40 h. The final product was cooled to room temperature in the furnace. Conductivity and thermogravimetric analysis (TGA) measurements were carried out in both dry and wet N2 or air. The dry gases were prepared by passing the commercial cylinder gases (Big Three Industries, Inc., Austin, TX) of > 99.999% purity through a glass tube containing dry ice and another glass tube containing P,O, to remove any residual water. Wet gases were prepared by passing the cylinder gases through a glass tube containing distilled water. Samples with O
.

LLL_I 1000/T

(I?)

Fig. 1. Arrbenius beating followed by cooling curves of the lattice conductivity in air for nominal BaZr0,Jn,,802,6.

223

111111.1

(4

1000/T

(K-‘1

In order to confirm our hypothesis, we monitored the weight loss on heating and the weight gain on cooling in air, in dry N2, in wet N,, and in cycling between wet and dry N,. Figure 3 shows representative TGA curves for x= 0.5. In air and dry N,, insertion did not have time to go to equilibrium during the cooling run, so there is a further weight gain on heating to 150 “C in the second cycle before the inserted species leaves at higher temperatures. The weight change corresponds to a near filling of the vacancies with water and/or neutral oxygen. In wet Nz, the insertion of H,O appears to be more rapid; the weight has nearly its equilibrium value for a given temperature after the first cycle. A DSC plot for an x=0.5 sample shows a broad endotherm, found during all heating cycles; it corresponds to the desorption of water and/or oxygen. Although water and oxygen are absorbed back into the sample during the cooling cycling, a cooling rate of 10 “C min-l is too fast to give an exothermic peak. Moreover, a slower absorption at lower temperatures (T<250 "C)is consistent with the thermal hysteresis shown in Figs. 1 and 2.

l-----l

2000/T

@)

(K-‘1

Fig. 2. Arrhenius heating followed by cooling curves of the lattice conductivity of BaZr,,In,,s0,75 in (a) dry Nz and (b) wet Nz atm. for the third thermal cycle.

thermal hysteresis in the conductivity data of Fig. 1 would appear to reflect the insertion/extraction of water and/or neutral oxygen on cooling/heating the sample. The insertion of H,O and/or oxygen on cooling does not saturate until temperatures T< 200 “C, whereas on heating the inserted species are retained to higher temperatures. The data of Fig. 2 were taken in order to discriminate between the effects of Hz0 and oxygen insertion. In dry N,, the cylinder nitrogen contains some 02, and this low partial pressure of oxygen is the source of the thermal hysteresis. In wet N,, the ~ntribution to the conductivity from the inserted species is even more apparent, which shows that H,O is also inserted. The thermal hysteresis was not observed above 300 “C in either the x= 1 (Ba,In,OS) or the x=0.1 sample, which indicates it is more prominent with a high concentration of disordered vacancies.

Tenpetature

Temperature

(“Cl

(“C)

Temperature

@I

Tempersture

Conclusions Oxygen-deficient perovskites show promise as oxideion electrolytes competitive with those on the fluorite structure. However, where the smaller cation requires an oxygen coordination of six or more, the aniondeficient perovskites readily absorb water or neutral oxygen into the anion vacancies at lower temperatures. In the system BaZr,_,In,O,_,,, the H,O molecule donates a proton to the oxygen array in the reaction

(“C)

(‘C)

Fig. 3. TGA data at 1 “C min-’ for BaZr OJIn0.50 2.7~in (a) dry and (b) *et cylinder N1: (i) first and (ii) second thermal cycle.

224

(H,O), I- Cl + Oz- -

2OH-

(5)

where q represents an oxygen vacancy. This reaction is expected to introduce not only mobile OH- species, but also the possibility of mobile H’ ions. Insertion of oxygen oxidizes the anion array via the reaction (0,),+2[3+202-

-

40-

(6)

where the O- ions can be expected to form molecuIar clusters such as the peroxide ion (O,)‘- at which the holes introduced into the 02-:2p6 band are trapped. The inserted oxygen and water are lost at higher temperatures, but a residual concentration to 700 “C can contribute to the conductivity, At 500 “C, the conductivity is ionic, with an oxide-ion transport number to close to unity [4], but the insertion/extraction reactions change the volume and tend to crack the ceramic disks on repeated cycling.

Acknowledgements

We gratefully acknowledge the financial support of the Welch Foundation, Houston, TX, and the Texas Advanced Research Program.

References J.B. Goodenough, Proc. R. Sot. London, Ser. A, 393 (1984) 215. J.A. Kilner and B.C.H. Steele, in O.T. Sorensen (ed.), Nonstoichiomebic oxides, Academic Press, New York, 1981, p. 233 and references therein. J.B. Goodenough and J.M. Longo, La~lf-~~~te~ Tabellen Group ZZZf4a, 1970, p. 126. J.B. Goodenough, J.E. Ruiz-Diaz and Y.S. Zhen, Solid State zonics, 44 (1990) 21. J. Rogriguez-Carvajal, M. Vallet-Regi and J.M. GonzalesCalbet, Mater. Res. BulL, 24 (1989) 423.