1. Could you explain the significance of your article to the non-specialist?

Oxides having the fluorite structure are important for solid oxide fuel cells (SOFCs), catalysts, thermal barrier coatings, and nuclear reactor fuels and waste forms. Their ionic conductivity depends on how oxygen vacancies and cations are distributed and/or clustered. High temperature oxide melt solution calorimetry, in which the samples are dissolved into a molten oxide solvent and the heat of solution related to the energetics of the solid phase, has been used to obtain the thermodynamics of formation of a series of materials containing zirconium, hafnium, and cerium doped with different rare earths..Strongly negative heats of formation from binary oxides in the Zr and Hf systems contrast with slightly positive ones in Ce systems. These differences correlate with observed differences in the location of the oxygen vacancies (mainly next to Zr and Hf versus next to Ce) and the size difference between the trivalent and tetravalent ions. The experimental data corroborate and benchmark a number of computational predictions of the thermodynamic properties and provide insight into which materials may be the best solid electrolytes. 

 

2. What has motivated you to conduct this work?

The diversity of applications and complexity of structures and properties of these materials are fascinating and provide an opportunity to link macroscopic thermodynamics with microscopic (molecular-level) details of structure and bonding. The solution calorimetric techniques have just recently been perfected for these very refractory materials, making it possible to measure their thermodynamic properties directly for the first time.

 

3. Where do you see this work developing in the future?

The calorimetric techniques will be applied to nanophase materials in the same systems. Small particles, nanocomposites, and thin films are finding increasing applications in both SOFCs and catalysts. We already have preliminary evidence that the thermodynamic properties of nanophase yttria stabilized zirconia are different from that of the bulk phase, as are the transport properties. We wish to explore whether these differences are due to surface.interfacial energies, to differences in the structure of defect clusters, and/or to preferential segregation of one component to the grain boundaries and surfaces

 

4. Are there any particular challenges facing future research in this area?

Preparation and characterization of materials with reproducible structural states remains challenging. Calorimetric techniques need to be developed for thin film materials. The role of water, always present in fine grained materials, must be understood. The computational techniques (software and hardware) need to be developed further to deal with much larger systems so that large clusters and disordered systems can be described.