Solid State Ionics 58 (1992) 359-362 North-Holland

SOLID STATE IONICS

Effect of aliovalent-cation substitution on the oxygen-ion conductivity of Bi,V,O, 1 *

Vandana Sharma, AK. Shukla ’ and J. Gopalakrishnan Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560012, India

Received 10 July 1992; accepted for publication 24 August 1992

Up to 10 atom.% of Li+, Zn2+, Al’+, Ti4+ and Ge4+ readily substitute for vanadium in Bi4V201, stabilizing a/r structures. The y-phases are good oxygen-ion conductors, the Ti-substituted y-phase, Bi4V1,sTi0.2010.9, being the best ionic conductor among all the substituted bismuth vanadates investigated. It is conjectured that a high ionic potential of the dopant cation facilitates oxygen- ion mobility in the disordered phase of the substituted bismuth vanadate.

1. Introduction

Stabilized zirconias are well-known solid electro- lytes exhibiting high oxygen-ion conductivity above 700’ C [ 11. Technologically, there is a need for su- perior oxygen-ion electrolytes operative at lower temperatures [ 2 1. To this end, various crystal-chem- ical strategies have been adopted in the literature [ 2 1. One of the strategies has been to engineer three-di- mensional oxide materials possessing considerable number of disordered oxygen-ion vacancies in the oxygen sublattice, typical example being perovskite- related brownmillerite Ba&O, [ 3 1. Recently, a new oxide of the formula Bi4VZOIL belonging to the Au- rivillius layered-perovskite family has been reported to be a good oxygen-ion conductor in its high tem- perature y-phase [ 41. Subsequently, it has been shown that Cu-substituted Bi4V201 1 is the best known oxygen-ion conductor (500-700 K) possessing ox- ygen ion transference number near unity [ 5 1. More recently, Ni-substituted bismuth vanadate has also been reported to have a similar value for oxygen-ion transference number although its oxygen-ion con- ductivity is lower than its Cu-substituted analogue

[61.

* Contribution No. 866 from the solid State and Structural Chemistry Unit.

’ To whom correspondence should be addresed.

We considered it important to investigate the ef- fect of substitution of various aliovalent cations on the oxygen-ion conductivity of Bi,V,O, i. For this purpose, we have chosen Li+, Zn2+, A13+, Ti4+ and Ge4+ (along with CL?+), all of which are expected to substitute for V5+ in Bi4VZOI ,. Indeed we find that single-phase materials with the structure of a/y- Bi4V201 1 are formed up to 10% substitution with all the cations investigated. Significantly, the 10% Ti- substituted bismuth vanadate stabilizing in the -y- structure, exhibits the highest oxygen-ion conductiv- ity. We report the results of these investigations in this communication.

2. Experimental

Lithium-, copper-, zinc-, aluminium-, titanium-, and germanium-substituted bismuth vanadates with 5% to 10% of the dopant concentrations were pre- pared by heating the appropriate amounts of the constituent oxides at 923 K in air with intermittent cooling and grinding. These compounds have been characterized by X-ray diffraction (XRD) patterns obtained on a Jeol JDX-8P Diffractometer using Cu Ku radiation. The comounds were pelletized and sintered in air at 973 K before coating the surfaces of the pellets with gold employing a Polaron E-5000 Coating Unit. Impedance data on sintered pellets of

0167-2738/92/$05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.

360 V. Sharma et al. /Aliovalent-cation substitution

various substituted vanadates were obtained in the temperature range 500-900 K employing a 4194-A HP Impedance-Gain Phase Analyser in the fre- quency range 100 Hz-l 5 MHz. The plots of the re- sistive component of impedance, a versus bw, where b is the reactive component of impedance and w is the angular frequency, have been used for the de- termination of equivalent circuit resistance (R) and capacitance (C) values in parallel configuration. This was found imperative as a Nyquist semicircle could not be fitted through the experimental data points. A plot of a versus bo is a straight line of slope -RC

and intercept R [ 7 1, according to the equation:

a=R-RCbw. (1)

Conductivities of various specimens in the temper- ature range 500-900 K have been computed from corresponding R values thus obtained.

3. Results and discussion

X-ray diffraction patterns of bismuth vanadate substituted with 5% and 10% of lithium, copper, zinc, aluminium, titanium and germanium are given in

fig. 1. The lattice parameters derived from these XRD patterns are given in table 1. It is found that bismuth vanadates substituted with 5% of lithium, copper,

zinc, aluminium, titanium and germanium stabilize with the a-Bi4V20,, structure that contains an or- dered array of Bi202 layers and V03,500.s layers [ 5 1. On increasing the dopant concentration to 1 O%, lith- ium, copper, zinc and titanium substituted bismuth vanadates stabilize in the y-Bi4V201, structure where the oxygen vacancies in the VO&!O.s layers are dis- ordered [ 51, but their aluminium and germanium substituted analogues retain the a-Bi,V,O, 1 structure.

The data on the temperature dependence of the oxygen-ion conductivities (a) for the various sub- stituted bismuth vanadates are given in fig. 2. It is found that conductivity values for all these materials are between 10-4-101 L2-’ cm-’ in the temperature range (500-900 K) of the study. Interestingly, the titanium-substituted bismuth vanadates, which have minimum number of oxygen-ion vacancies, exhibit higher oxygen-ion conductivities than any of their substituted analogues. The conductivity is even higher than the copper substituted y-phase which is

Biqh 9 Gee 1010.95

0’4V1 9AlolOlo 9

l( 1 2” 3” 40 50 60 10 20 30 40 5c 60

26 28

Fig. 1. X-ray powder diffraction patterns of various substituted

bismuth vanadates.

reportedly the best known oxygen-ion conductor [ 5 1. It could, therefore, be inferred that the concentration of oxygen-ion vacancies alone does not dictate the oxygen-ion conductivity in the substituted y-bis- muth vanadates.

Small, highly charged cations exert a greater effect in polarizing the anions than the large and/or singly charged cations. This is often expressed by the ionic potential (o) of the cation [ 8 1, which is obtained by dividing the ionic charge (Z) (corresponding to its valence) by its radius (Y) taken in Angstrom units (A). These arbitrary units of potential are adequate to take into account the effect of charge and size of the ionic constituents in the investigated com- pounds, The ionic potentials for the various cations substituted in bismuth vanadate are given in table 2. Among the cations investigated, substituted Ge4+ has the maximum ionic potential and hence the maxi- mum power to polarize oxygen-ions. One would, therefore, expect a high oxygen-ion conductivity for germanium-substituted bismuth vanadates. The ger-

1_

V. Sharma et al. /Aliovalent-cation substitution 361

Table 1 Unit cell parameters and ionic conductivities of aliovalent-cation substituted Bi4V201 1.

a

(A)

5.543 16.57 3.914

16.50 3.92

16.48 3.93

16.51 16.46 16.58 16.52 16.60 3.934

b

(A)

5.615 5.568

5.568

5.576

5.568 5.552 5.59 5.56 5.611

C Structure

(A) type

15.321 a 15.26 a 15.39 Y 15.59 a 15.34 Y 15.13 a 15.38 Y 15.19 a 15.19 a 15.00 a 14.99 a 15.39 a 15.32 Y

o500 K (n-1 cm-‘)

o.1ox1o-6 0.25x 1O-6 0.10x low 0.30x 1o-6 0.40x 1o-6 0.60x 1O-6 0.40x 10-G 0.30x 1o-6 0.14x 1o-6 0.60x lo-’ 0.30x 1o-6 0.20x 1o-5

‘) After ref. [lo].

T(K) T(K)

900600 700 600 503 900 800 700 600 500

r’ 1 1 I 11” / I o &“I sL’o~% B o B’I.“I 94 1010 9

l ~~4"18L'O2~106 l Bi4Vl d10.20109

II I J I, / IO 12 14 16 18 20 10 12 14 16 18 20

F(K&) yL(K-1)

Fig. 2. Temperature dependence of oxygen-ion conductivities for various substituted bismuth vanadates.

manium-substituted bismuth vanadates however stabilize in the a-Bi,V,O,, structure that does not facilitate high oxygen-ion mobility. A similar situa- tion exists for the aluminium-substituted bismuth vanadates.

Table 2 Ionic potential of various aliovalent cations employed for substi- tution in Bi4VZOI ,.

Cation Charge, Z Radius ‘I, r Ionic potential, o

(A) (arbitrary units)

Li+ 1 0.76 1.32 cua+ 2 0.73 2.74 Zn2+ 2 0.74 2.70 A13+ 3 0.535 5.61 Ti4+ 4 0.605 6.61 Ge4+ 4 0.530 7.55

a) Octahedral effective ionic radii taken from ref. [ 111.

The conductivities of 10% substituted y-phases vary in the order: Ti4+ > Cu2+ -Zn’+ >Li+. Inter-

estingly, the ionic potential of these cations also var- ies in the same order (table 2). The ionic potentials

of Cu2+ and Zn2+ are nearly the same and accord- ingly the conductivities of Cu- and Zn-substituted vanadates are similar. The Li-substituted y-phase ex- hibits lowest conductivity. Thus it appears that the ionic potential of the substituting cation, rather than the actual concentration of anion vacancies, deter- mines the oxygen-ion conductivity of the material. Possibly, the electrostatic repulsion between filled and vacant sites is optimized to provide smooth pathways for oxygen-ion conduction in the 10% Ti- substituted y-bismuth vanadate. Further investiga-

362

tion to understand required.

4. Conclusions

K Sharma et al. /Aliovalent-cation substitution

the reason for this behaviour is

Substitution of vanadium in Bi4V201 I by various cations such as Li+, Zn*+, A13+, Ti4+ and Ge4+ has been investigated. Up to 10% of these cations sub- stitute in Bi4V20, 1 stabilizing in a/y phases. The y- phases are good oxygen-ion conductors just as the Cu*+-substituted analogue which has already been reported. The 10% Ti4+-substituted phase,

Bi4Vi.8Ti0.2010.9, is the best oxygen-ion conductor among the substituted bismuth vanadates investi- gated. It appears that the ionic potential of the sub- stituting cation, rather than the oxygen vacancy con- centration, is an important factor in determining the oxygen-ion conductivity of these materials.

Acknowledgement

We are grateful to Dr. K.B.R. Varma for providing samples of germanium-doped bismuth vanadate.

Note added in proof

Since submission of this paper, we have become

aware that Goodenough et al. [ 9 ] have reported on Bi4VZ_-xMxOLL_-d with M=Nb, Ta, Ti, Zr, Cu, Mg, Al, In, Ni, and Pd. They noted superior O*--ion con- duction for M =Nb as well as M=Ti and Cu, and they have presented a model for the superior O*-- ion conductivities associated with these particular cations.

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