Ceria-based materials are very promising candidates for electrolytes in solid oxide fuel cells (SOFCs) because of their high ionic conductivities. In this work, high temperature oxide melt drop solution calorimetry was employed to study the energetics of selected bulk and nanophase ceria-based materials ((1-x)CeO 2 -xMO 1.5 (M=Y, Gd, and La), xCe 0.8 Y 0.2 O 1.9 -(1-x)Zr 0.8 Y 0.2 O 1.9 , nano-CeO 2 and nano-Ce 0.8 Gd 0.2 O 1.9 ). Thus, the influences of cation radius and particle size on the energetic behavior of ceria-based materials were investigated. The energetic behavior not only illustrates the thermodynamic stability, but also provides information on defect chemistry in these systems and thus can be correlated to the conduction behavior.
The formation enthalpies of the (1-x)CeO 2 -xMO 1.5 solid solutions are slightly positive with different maximum values obtained at different doping levels for different dopants. It is proposed that the destabilizing effect of lattice deformation might be partially compensated by the stabilizing effect of defect association. Because the relatively large Ce 4+ prefers 8-coordination, oxygen vacancies primarily locate nearest neighbor to the dopants when forming defect associates in (1-x)CeO 2 -xMO 1.5 . In contrast, in (1-x)ZrO 2 -XYO 1.5 and (1-x)HfO 2 -xYO 1.5 , substantial stabilization is realized by locating oxygen vacancies next to the relatively small host cations, rendering them 7-coordinate. It is also suggested that the local site distortion is more important than the global lattice deformation on determining the energetic and conduction behavior.
Zr-Ce substitution in xCe 0.8 Y 0.2 O 1.9 -(1-x)Zr 0.8 Y 0.2 O 1.9 was found to result in a slightly positive and asymmetrically varying formation enthalpy (from the corresponding solid solution end-members). A scavenging effect of Zr 4+ on oxygen vacancies might be operative in the relatively stable Ce-rich region, where redistribution of oxygen vacancies from the nearest neighbor sites of Y 3+ to the nearest neighbor sites of Zr 4+ makes the fluorite phase less unstable. Such a scavenging effect might also explain the variation of conductivity with the substitution level.
The surface enthalpy of nano-CeO 2 was estimated to be 1.2 ± 0.1 J/m 2 , which agrees well with the values derived from computer simulations. Nano-Ce 0.8 Gd 0.2 O 1.9 was found to have a surface enthalpy of about 1.7 ± 0.2 J/m 2 implying an increased surface instability upon GdO 1.5 doping.
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