Nano-crystals of cerium-hafnium binary oxide: Their size-dependent structure

Cerium oxide (CeO2, "ceria") and hafniuni oxide (HfO2, "hafnia") were aqueously co-precipitated and subsequently calcined to allow for homogenization. The size of the (1-x)CeO2-xHfO2 crystallites, determined by the Scherrer equation, varied from 140 nm for x = 0 to 15 nm for x = 0.73. For x <= 0.14, only cubic structures are visible in X-ray diffractograms, and the lattice parameters are consistent with the values expected for structurally cubic solid solutions of hafnia in ceria. At x = 0.26, tetragonal and monoclinic phases nucleated with the former not being observed in the bulk phase diagram for ceria-hafnia. Therefore, the solubility limit of the cubic structure is between x = 0.14 and x = 0.26 for 40-61 nm crystallites, the sizes of these respective compositions. More specifically, for the 40 nm crystallites of x = 0.26 (1 - x)CeO2-xHfO2, 15% of the hafnia remains in a structurally cubic solid solution with ceria based on the observed cubic lattice parameter. The compositional domain for the cubic fluorite structure in this study is narrower than other nanostructured (1 - x)CeO2-xHfO2 studies, especially studies with crystallite sizes less than 10 nm, but wider than observed in the bulk and helps to expand the size regime over which the relationship between crystallite size and phase stability is known. The extent of this cubic-structure domain is mainly attributable to the intermediate crystallite size and the roughly zero Ce3+ content as determined by X-ray absorption near edge structure spectroscopy. (C) 2015 Elsevier B.V. All rights reserved.