Critical role of annealing atmosphere on solid solution formation between PrO2-δ and ThO2

Abstract

Taking cognizance of the existence of fluorite structure for various stochiometric and non-stochiometric compositions of praseodymia, solid solution formation of the type Pr1-xThxO2-δ has been investigated employing epoxide mediated gel synthesis. The xerogels were calcined under three different atmospheres (air, oxygen and argon) followed by characterizing them by powder X-ray diffraction, Raman, UV–visible spectroscopy techniques. A complete miscibility of Pr and Th with oxygen possessing fluorite structure was found for calcined xerogels in air or flowing oxygen atmosphere from PrO2-δ to ThO2, where as only up to 70 mol% of thorium could be substituted for praseodymium in PrO2-δ when the samples were annealed in argon. Combining the results from lattice parameter trend and XPS analysis of PrO2-δ and Pr0.50Th0.50O2-δ, Th4+ has been found to substitute for Pr4+ in the samples obtained in air or flowing oxygen. For samples, prepared under flowing argon (reducing) atmosphere, reduction of Pr4+ to Pr3+ might have occurred in addition to the replacement of Pr4+ with Th4+. These experimental conditions resulted in samples with brown, black and brownish yellow colors with their estimated band gap values in the range 1.29–2.07 eV as analysed from their UV–visible spectra.