Nanostructures in calcia stabilized hafnia thin films observed by PAC as a function of

M.C. CARACOCHE, J.A. MARTINEZ, P.C. RIVAS, M.A. TAYLOR, A.F. PASQUEVICH, S. BAROLIN, O. DE SANCTIS

Foz do Iguazu

Conferencia; XIV International Conference on Hyperfine Interactions and XVIII International Symposium on Nuclear Quadrupole Interactions; 2007

Stabilized hafnia thin films obtained by doping HfO2 with 14 and 20 mole % CaO and calcination at 500ºC have been investigated via PAC as a function of temperature up to 1000oC and XRD as a function of annealing temperature up to 1300ºC. XRD results allow to state that the as-prepared materials consist of a substitutional cubic hafnia solid solution. However, small amounts of two hyperfine interactions are determined in addition to the one depicting cubicstabilized hafnia. They are assumed to correspond to the ordered phases CaHf4O9 (fi1) and Ca6Hf19O44 (fi2)which exist as microdomains in grain boundaries and/or at the surface. This phase content is more compatible with the CaO-ZrO2 equilibrium phase diagram than with the CaO-HfO2 one. As temperature increases f1 seems to be more stable than fi2 and traces of m-HfO2 irreversibly appear at 1000ºC. In turn, some CaO segregation is observed inthe diffractograms obtained upon cooling from high temperatures. Though the cubic solid solution is initially more abundant in the as-prepared 20% CaO doped sample, at 1000ºC both concentrations investigated exhibit a maximum of 85% for the stabilized polymorph. Subtle differences between XRD and PAC results indicate that stability depends on the previous thermal history. The activation energies determined suggest that anionic conductivity hardens asdopant concentration increases from 14 to 20% CaO. A comparison between results obtained for the films and for two powder samples of identical compositions shows that in the powders the cubic solid solution is locally more disordered, is less stable and its stabilization is clearly dopant concentration dependent.