Electrical conductivity of the system Y2O3CeO2

J. fhys.Chm. .%/ids Vol.46.No. IO.pi. 1173-l178.1985 Printed in GreatBritain.

0022-3697/U $3.00 + .oO 0 1985 Pergamon Plas Ltd.

ELECTRICAL CONDUCTIVITY SYSTEM Y203-Ce02

OF THE

KEU HONG KIM,? JONG Ho JUN and JAE SHI CHOI Department of Chemistry, Yonsei University, Seoul 120, Korea (Received 6 April 1984; accepted in revised form 28 March 1985) Abstract-The electrical conductivity of the system Y203-Ce02 was measured in the temperature range 500- 1100°C and PO, range IO-‘-IO-’ atm. Possible defect models were suggested on the basis of conductivity data, which were investigakd as a function of temperature and of PO,. The observed activation energies were 0.40 eV and 1.79 eV in the low- and high-temperature regions, respectively. The observed conductivity dependences on Pal were o K Pg in the temperature range 500-75O’C and c cc Pg.’ at temperatures from 750-I 100°C. It is suggested that the system Y203-Ce02 shows a mixed ionic plus hole conduction due to an 0: defect and an electronic hole conduction due to a V+ defect in the low- and high-temperature regions, respectively.

INTRODUCTION Y203 exhibits small deviations

from stoichiometry at room temperature and atmospheric pressure. However, a substoichiometric oxide with composition Y0,.491 may be prepared by arc-melting the sesquioxide and metal in an inert gas atmosphere [ 11. Oxygen diffusion in Yz03 was studied by Wirkus ez al. [2, 31. They measured the weight gain of the substoichiometric YO,.,w as a function of time during oxidation of Y01.4u at IOOO-1500°C in air [2]. Assuming that the rate-controlling step involved diffision of oxygen vacancies, the estimated self-diffusion coefficient [2] was reported to be D = 7.24 exp(-58,6OO/RT) cm2/ sec. Yttrium self-diffusion in Y2O3 has been reported by Berard and Wilder [4, 51. They measured yttrium self-diffusion using a tracer sectioning technique using specimens of two different densities. Bulk diffusion predominated, and the diffusion coefficient was found to be D = 2.41 X 10e4 exp(-43,9OO/RT) cm2/sec in the less dense and vacuum-annealed sample 141. However, the diffusion coefficient was D = 1.65 X Joe2 exp(-69,2OO/RT) cm2/sec for an air-annealed sample having 98.2% of the theoretical density [5]. They suggested that the larger diffusion coefficient in the less dense specimen may be due to an appreciable contribution of surface diffusion along the walls of the internal pores. Tallan and Vest [6] found that Y2O3 is a pure electronic conductor in the temperature range 12001600°C and P, range 10-‘7-10-’ atm. It was concluded that the most probable point defects responsible for the conductivity between 1200” and 1600°C and Po,‘s above about lob9 atm were fully-ionized yttrium vacancies. They also observed from polarization measurements that the ionic contribution to the electrical conductivity increased with decreasing temperatures below 900°C. At 800°C and a PO2of lo-l5 atm, the t Present address: Department of Chemistry, Purdue University, West Lafayette, IN 47907, U.S.A.

transference number for ions was 0.15, and increased to 0.3 at 700°C and the same PO,. From the transference data, they concluded that mixed ionic and electronic conductivity exists in pure Y2O3 below 900°C. This mixed ionic and electronic conduction was also found from emf measurements [7] at 825°C and Po2’s of 10e2’ to 10e4 atm. Noddack et al. [8] reported that the ionic transport number was less than 0.01 at 600°C and PO2 of 2 X 10m6torr, and thus concluded that Y2O3 was an electronic semiconductor. Rao et al. [9] estimated from the difference between ad, and a., that the ionic conductivity in Y2O3 was about 30% at -7OO”C, which agreed with the ionic transference number of 0.3 obtained by Tallan and Vest [6]. The electrical conductivity of yttrium sesquioxides doped with 2.4 mol% GO2 and 5.0 mol% Ce02 has been measured in the temperature range 500-I 100°C under oxygen partial pressures of lo-’ to 10-l atm in order to determine possible defect structures in this oxide. EXPERIMENTAL SECI-ION

Sample preparation Y2O3 (N.B.S. 99.999%) and Ce4 (N.B.S. 99.99%) powders were weighed, mixed in varying proportions, ball-milled for several hours in C2H30H. and then dried at 25O’C. The powder mixtures were made into pellets under a pressure of 49.03 MPa in vacuum. Pellets of Y2O3 doped with 2.4 and 5.0 mol% CeO2 were presintered for 60 hours at 1000°C and annealed for 72 hours at 1350°C under atmospheric pressure in air, and then cooled rapidly to room temperature. The pellets were given a light abrasive polish on one face, turned over and polished until the voids in the interface region of the specimen were fully eliminated. The pellets were cut into rectangular forms with dimensions of 1.4 X 0.9 X 0.6 cm3 and 1.3 X 0.9 x 0.7 cm), respectively. Pycnometric densities of the

1173

1174

K. H. KIM elal.

ature until equilibrium with the oxygen phase was achieved, indicated by a constant conductivity.

pellets showed 3-5% pore volume. X-ray superstructure lines showed that the system Y203-Ce02 was uniformly homogeneous, and spectroscopic analysis of the above prepared specimen showed 9 ppm of total impurities such as Fe, Co, Ni, Cu, Sm, Ho, and Zr.

Estabhshment

Conductivity measurement Measurements of electrical conductivity were carried out according to the Valdes’ technique [IO], which was briefly described in the previous articles [ 1I, 121. This technique has also been employed to measure the electrical conductivity of other oxide semiconductors; for example, cu-Fe203/ar-Fe203:Cd [ 13-161, La203:Cd [ 171, Hz-Reduced TiOz [ 181 and SrTi03 :Ni/CO-Reduced SrTiOs:Ni [ 191. A schematic diagram of the conductivity cell, the cell geometry, and electrical circuit was described previously [Ill. Details have also been described for the vacuum system (201, instruments [21], a four-probe model [ 11, 121 and the conductivity calculation procedure [ 11, 121. The current through the sample was maintained from IO-” to IO-’ A by means of a rheostat; also, the potential across the inner two probes was maintained between 0.2 and I.8 V. The potential difference was measured by a Leeds and Northrup 7554 K-4 potentiometer, and the current through the sample was measured by a Keithley 610 C electrometer. The measurements of electrical conductivity were performed over a cycle in the temperature range 500-l 100°C under Pm’s from IO-‘-IO-’ atm, starting from the low temperature end, proceeding toward the high temperature end, and then back again to the lower end. The sample was held at each temper-

I

lloo”c

I

RESULTS AND DISCUSSION Conductivities were measured as a function of temperature and oxygen partial pressure. Figure 1 shows that the conductivity increases with increasing temperature and PO, for the 2.4 mol% CeQ-YzOs system. A discontinuity appears in the log u vs. 103/ T plot around 750°C. A similar discontinuity is seen in the 5.0 mol% Ce02-Yz03 system, as shown in Fig. 3. These. inflection points indicate that the conduction mechanism in the low-temperature region could be

I 800°C

I

looo"c

9oo"c

qf PO,

The various oxygen partial pressures were established using pure oxygen or a mixture of 0.001% oxygen in nitrogen obtained from Matheson Gas Products. The quartz sample container was evacuated to a pressure of 5 X 10T6 torr by a diffusion pump [22] at room temperature, and then the temperature of the sample container was increased up to 500°C. A mixture of oxygen and nitrogen, or pure oxygen, was introduced into the sample container, which was then evacuated again to a pressure of 5 X 10e6 torr. The introduction and evacuation of gas at 5OO’C were performed two or three times, and the total pressure controlled with 0.001% 02-N2 in order to establish the required Po2. The pressures of the oxygen-nitrogen mixture and the evacuated sample container were read on a thermocouple gauge, pirani gauge, and an ultra-high vacuum ionization gauge, respectively.

I 7oo"c

I 600°C

I 5oo"c

-1

Po2 -2

tatm)

1 : 1 x 10-l 3 : 1 x 10'3 5 : 1 x 10-5 7 : 1 x 10-7

-6

0.7

0.8

0.9

1.0

1.1

1000/T Fig.

1. Conductivity isobars for 2.4 molgb Ce02-Y20,

1.2

1.3

(K-l)

system under various oxygen partial pressures.

Electrical conductivity of the system Y~03-Ce02

1175

600% _-.-:___----:=.-

5oooc

+!.Y -a-

-5

-6

-7

-3

-4 Log Po2

-1

-2

(atm)

Fig. 2. Conductivity isotherms for 2.4 mol% CeOz-Y203 system at various temperatures.

from that in the high-temperature region at the same PO, values. The conductivity isotherms of 2.4 and 5.0 mol% CeOr-YrOr systems are shown in Figs. 2 and 4. It is seen that the conductivity dependences on Po, are approximately fixed, regardless of the dopant level of Ce02 in YrOr. This indicates that the conduction mechanism is the same for the two different samples. different

I

lloo"c

I

I

looo"c

9oooc

The n values in u a Pg are listed in Tables 1 and 2 at the various temperatures investigated. The average values of the activation energies in the low- and high-temperature regions (from Figs. 1 and 3) are 0.40 eV and 1.79 eV, respectively. It is suggested that the higher activation energy is composed of energies for the formation of defects, defect ionization, and electron-hole mobility in the high

I

I

I

800°C

7oo"c

600°C

I 5oooc

l-

PO2

orl I

aI

(atm)

: 1 x 10-2 4 : 1 x 10-4 2

6 :

1 x

lo+

-l-

5 D -2g il -3-

-4t

6.7

0.8

0.9

1.1

1.0

1000/T

1.2

1.3

(K-l)

Fig. 3. Conductivity isobars for 5.0 molW Ce02-Y203 system under various oxygen partial pressures. PCS

46:10-E

K. H. KIM etal.

1176

O.+ I

3 I

-1-

z D g -2;1

-3 -

-4 t

I

I -7

I

1

I

I

I

I

-6

-5

-4

-3

-2

-1

Log PO2

(atm)

Fig. 4. Conductivity isotherms for 5.0 molW CeO*-Y203 system at various temperatures.

temperature region. In the low temperature region, the activation energy of 0.40 eV is believed to be the sum of migration energies of an electron hole and an ion, which are formed in the valence band and interstitial site by doping of the impurity. The conductivities of the present CeOrdoped Yz03 samples are significantly higher than those for pure YzOs. Either the effects of the considerable amount of impurity dopant (up to 5.0 mol%) or the variation in the ionic contribution to the electrical conductivity may be associated with the magnitude of the conductivity and activation energy. The low activation energy of 0.40 eV in the temperature range 500-750°C suggests that there is extrinsic conduction in the system YzOs-Ce02. Moreover, the activation energy of 1.79 eV in the temperature range 750-l IOO’C is lower than that of 1.94 eV obtained by Tallan and Vest [6] in the 1200-16OO’C temperature range and the same Po, range IO-‘-lo-’ atm. This lower activation energy and the higher conductivities may originate from the doping of the impurities; an oxygen

interstitial defect with two negative charges and an electron hole will be produced and fixed by the considerable amount of doped CeOz. These species may act as additional carriers and increase the conductivity. Since the different slope values of l/6 and l/5.3 in the log u vs. log PO, plots are interpreted as due to different defect models, the error in the slope measurement is critical. A least-squares analysis of the 36 low-temperature data points at a PO2of IO-’ atm gave a slope of -2.010 f 0.001. In this case, the small deviation involved may be the possible error due to scatter from the least-squares line. As shown in Figs. I and 3, the representative conductivity data points (one point from four or five data points) obtained from two samples at 500-750°C lie on a good straight line. A high-temperature slope of -8.997 f 0.001 was also obtained in this manner [6]. The data points obtained from two samples at 750I 1OO’C also show a good straight line. Log u vs. log PO, plots (Figs. 2 and 4) were drawn with the data

Table I. Electrical conductivity dependences on Pe for the

Table 2. Electrical conductivity dependences on Pal for the 5.0 molW CeO,-Y,O* svstem at various temccratures

2.4 mol% CeO*-Y201 system at various temperatures Temperatures (“C) 500 600 700 800 900 1000 II00

n (u cc P@) 6.0 6.0 6.0 5.3 5.3 5.3 5.3

Temneraturea (“C)

n (0 cc PZ)

500 600 700 800 900 1000 II00

6.0 5.9 6.0 5.3 5.3 5.3 5.2

Electrical conductivity of the system Y203-Ce02 obtained from the log r~ vs. l/T plots. It is generally believed that this log u vs. log PO, plot is insensitive to a small change of slope. However, our test plot of u vs. Pg [6] at 500°C was consistent with the l/6 slope of log u vs. log Po,, showing a straight line for the data for u vs. Pg:. a (YPgz.’ was found with a test plot of u vs. Pg at IooO”C. This plot was satisfied with a slope of l/5.3 for a log u vs. log PO, plot, indicating that the data for u vs. Pg:.’ lie on a good, straight line. It is noted that the proportionalities u a Pgz.’ and u a Pg.3 in the study of defects in Smz03 [23] were found using the same test plot of u vs. Pg: as that developed by Tallan and Vest [6] in their study on pure YzO,.

Suggestion of possible defect at 500-750°C As shown in Tables 1 and 2, the Po, dependence of conductivity is l/6 in the temperature range 500750°C. The increase in conductivity with increasing PO, shows that the system Y203-Ce02 is a p-type in this temperature region. Bratton [24] and Duwez et al. [25] studied the defect structure of Yz09-ZrOz solid solutions. They reported that the solid solubility of ZtQ in Y203 amounted to 18 mol% at 1375’C. The pycnometric density was compared with the Xray density of sintered specimens of solid solutions containing up to 10 mol% ZtQ [24]. It was concluded from this that the dominant chemical defect in solid solutions of Y203 containing ZrOt was the oxygen interstitial. The lower activation energy of 0.40 eV in the 50075O’C temperature range suggests that there is extrinsic conduction in 2.4 and 5.0 mol% CeOz-doped YzOj. It is assumed that oxygen interstitials are produced by the incorporation of CeOz in YzO1. This oxygen interstitial may be represented by the following equilibrium YI 112 02 (g) =q+2/r*,

(1)

where q is an oxygen interstitial defect with two effectively negative charges, and h - represents an electron hole. In eqn (I), the oxygen interstitial concentration should be fixed by doped Ce. In this situation, considering the equilibrium constant in eqn (I), K, = J?[O;]P&‘~ and taking [O;] as constant, it is found that p = K’,Pg. Here, taking p as the electron hole concentration and assuming that only the electron hole acts as a carrier in this temperature range, the electrical conductivity dependence on Po, is u a p = K;Pgz.

(2)

This interpretation disagrees with the experimentallyobserved dependence of u a Pgf. It is likely that, in the low-temperature range, the system Yz03-CeOz is a mixed semiconductor (ionic + hole). The combination of a P& independent ionic conductivity and a PQ dependent electron hole conductivity (a a p

1177

= K’,Pg:) could give rise to a lower PO, dependence (a a P&t) for the total measured electrical conductivity. It is suggested that the electrical conduction of YzOs doped with 2.4 and 5.0 mol% Ce02 may occur via a mixed carrier, i.e., q + h-, which is produced by doped Ce.

Suggestion of possible defect at 750-l 100°C Tables I and 2 show that the conductivity dependence on PO2is l/5.3 in the temperature range 7501100°C. The u a Pg.’ indicates that the defect structure in this high-temperature region may be different from that in the low-temperature region investigated. Tallan and Vest [6] studied the electrical conductivity as a function of PO2 at temperatures ranging from 1200- 1600°C. They concluded that fully ionized yttrium vacancies (I’+) were formed at the above temperatures and Po;s of 10-9-10-’ atm in undoped Y203. They confirmed electron holes as carriers and v’+ as the most probable point defect from the good agreement between the experimentally-observed conductivity dependence of Po, of u = 1.3 X 10’ P$‘I:(’ exp(--1.94/kT) and the theoretically interpreted u a P&‘” with the simplified electroneutrality condition p = 3[vI$]. It is obvious that the system Yz03-Ce02 has essentially intrinsic behavior in this temperature range. The increasing conductivity with increasing PO, indicates that the system conserves a p-type character, as shown in Figs. 2 and 4. It is assumed that the fully ionized yttrium vacancy [6] may be formed due to the diffusion of oxygen. This defect formation may be written & 112 02 (9) = 2/3 v;’ + 2h. + OO.

(3)

In eqn (3), it is interpreted that p = K;Pg.3 with the electrical neutrality condition [6], p = 3[Vy]. The electrical conductivity dependence on PO, may be represented by u

a

p = K;Po2l/5.3 ,

(4)

with only VT defects and hole carriers. This is consistent with the experimental result, i.e. u a Pgz,“. It should be noted that the major ionic defect (0;) would not change from W to VC and might be conserved upon exceeding 75O’C. In this situation, a more correct electroneutrality condition has to be considered, i.e. p + [Ce\] = 2[q]. However, it is more likely that the Y203-Ce02 solid solution is in the intrinsic region at the higher temperature. The experimentally-observed Pm dependence of l/5.3 in this intrisic range could be due to the fact that the contribution from [Ce;] would diminish with an increase in Po2. It is, therefore, suggested from the observed and interpreted PO2 dependences that the possible predominant defect structure of the system Y203-Ce02

K. H. KIM el al.

1178 may be v’;’ and if so, the electron carrier.

hole is a main

Acknoti~iedgmenr--The

authors are grateful to the Korean Science and Engineering Foundation for financial support and to Professor Ki Hyun Yoon for helpful discussions. REFERENCES

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