A high temperature lattice parameter and dilatometric study of nonstoichiometric ceria and calcia-doped ceria
Abstract
The defect structure of nonstoichiometric cerium dioxide has been investigated in the composition range from CeO$\sb2$ to CeO$\sb{1.8}$ at 800 and 900$\sp\circ$C using the combined techniques of high temperature dilatometric length change and x-ray lattice parameter measurements. The oxygen partial pressure associated with the nonstoichiometric composition was controlled by passing different ratios of H$\sb2$/He gas mixtures through a constant temperature water bubbler and was monitored constantly with a yttria stabilized zirconia (YSZ) oxygen sensor. The difference between the percent expansion for the dilatometric and x-ray lattice parameter measurements was almost zero, which indicated that the predominant defects of the nonstoichiometric cerium dioxide are oxygen vacancies. The percent expansion of pure cerium dioxide and 10 mole% calcia doped cerium dioxide were determined using both x-ray lattice parameter and dilatometric measurements. The addition of 10 mole% calcia increased the percent expansion of cerium dioxide, however, the difference between the expansion of the calcia doped ceria and that of the ceria itself decreased with decreasing oxygen partial pressure, and finally approached zero at large departures from stoichiometry. This may result from the calcium ions being substitutionally incorporated on cerium sites of the cerium dioxide crystal structure and the production of doubly ionized oxygen vacancies. These extrinsic oxygen vacancies are thought to be predominant when the cerium dioxide is nearly stoichiometric, while nonstoichiometric oxygen vacancies are thought to be the predominant defects at large departures from stoichiometry. Finally, the miscibility gap found for pure cerium dioxide was not observed for 10 mole% calcia doped cerium dioxide at 660$\sp\circ$C, which suggests that the miscibility gap was suppressed by the addition of calcia.
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