Elsevier

Solid State Ionics

Volume 176, Issues 39–40, December 2005, Pages 2957-2961
Solid State Ionics

Proton conductivity of Ca-doped Tb2O3

https://doi.org/10.1016/j.ssi.2005.09.030Get rights and content

Abstract

The electrical conductivity of 2 mol% CaO-doped Tb2O3 has been characterized as a function of the oxygen pressure and water vapor pressure from 300 to 1200 °C with two- and four-point electrode setups. EMF-measurements under reducing conditions have, furthermore, been applied to establish the contribution from the different charge carriers. Tb2O3 is a p-type semiconductor. Protons are the major ionic charge carrier under reducing conditions constituting 2–35% of the total conductivity, increasing with decreasing temperature. The conductivity under wet hydrogen has been modeled assuming that protons and oxygen vacancies compensate the acceptor (Ca). On this basis, the enthalpy and entropy of hydration, and the enthalpy and pre-exponential of proton mobility have been determined. These parameters are compared to corresponding data for the other rare-earth sesquioxides.

Introduction

The conductivity of a number of the rare-earth sesquioxides (Ln2O3 where Ln = Nd, Sm, Gd, Dy, Er, Y) doped with Ca has been systematically investigated by Larring and Norby [1], [2], [3] as a function of oxygen pressure and water vapor pressure in the temperature range 500–1200 °C. It was observed that the oxides exhibit a considerable contribution of proton conductivity below ∼ 1000 °C. Thermodynamic constants and transport coefficients were extracted from the conductivity measurements and measurements of the EMF resulting from gradients in the oxygen, hydrogen or water vapor pressure. Empirical trends between the derived physicochemical constants and physical properties of the oxides were, furthermore, sought.

The previous comprehensive study did not include the oxides with oxidizable cations, Pr and Tb [1], [2], [3]. Tb(III) is stable in a reasonably large and accessible oxygen pressure range between 500 and 1200 °C and is, therefore, most interesting with respect to comparison of the electrical properties with the other rare-earth oxides. The present investigation addresses the defect chemistry and transport properties of Tb1.98Ca0.02O 3. Characterization of the electrical properties has been performed by means of the two- and four-point electrical conductivity techniques as a function of the oxygen pressure and water vapor pressure and EMF measurements in wet hydrogen in the temperature range 300–1200 °C. Modeling of the conductivity behavior enabled comparison of thermodynamic and transport properties with corresponding data for the other rare-earth oxides.

Section snippets

Literature

Studies of properties of the oxides in the Ln-series are interesting since they, despite their similarities, can be divided into three major structural classes. These classes are simply referred to as the A-, B- and C-type rare earth oxides where the A-type is hexagonal, B-type monoclinic and the C-type cubic. The fact that the rare earth oxides count as many as 16 and that one, to a certain extent can play with the transitions between the different structure types by mixing the cations, yields

Experimental

The synthesis of Ca-doped Tb2O3 followed a solid-state route. TbOx was reduced to Tb2O3 through annealing in wet hydrogen at 1000 °C. Nominal amounts of Tb2O3 and CaCO3 were co-milled in an agate planetary ball-mill in isopropanol followed by drying, pressing to pellets and calcinations for ∼ 20 h under reducing conditions at 1100 °C. The calcined pellets were crushed down, milled and calcined over again until powder X-ray diffraction showed a single-phase pattern. The final powder was

Results

The electrical properties of Tb1.98Ca0.02O3 were characterized within its stability range and Fig. 1 shows the total conductivity as a function of the inverse absolute temperature in wet and dry Ar. There is no sign of any phase transition in the temperature region represented here. The conductivity is higher under dry conditions and the difference compared to wet Ar increases with decreasing temperature. The dependence of the total conductivity on the oxygen pressure and on the water vapor

Discussion

To interpret the behavior of the conductivity of Ca-doped Tb2O3 one may use the approach of Larring and Norby [1], [2], [3] as a starting point. In this respect one should compare the variations in the conductivity with changes in the reaction conditions for different Ln2O3 sesquioxides with the data set obtained here for Tb2O3. An example of such a comparison is given in Fig. 5 where the temperature dependence of the proton conductivity for one A, one B and one C-type Ln2O3 is compared to the

Conclusions

Tb1.98Ca0.02O 3 is a p-type semiconductor under the present experimental window. Protons comprise the major ionic charge carrier. In wet hydrogen, protonic conductivity comprises between 2% and 35% of the total conductivity, increasing with decreasing temperature. The absolute values for the proton conductivity in Tb2O3 are in the same order of magnitude as for the other C-type Ln2O3 sesquioxides. The conductivity in wet hydrogen has been modeled according to a point defect scheme where the

Acknowledgement

This investigation has to a large extent been financed by the EU commission under contract number G1RD-CT-2001-00651, “CERHYSEP”.

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Present address: SINTEF Materials and Chemistry, P.O. Box 124. NO-0314 Oslo, Norway.

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