Ca doping of nanosize Ce-Zr and Ce-Tb solid solutions: Structural and electronic effects

Ceria-based ternary oxides are novel materials with potential use in many areas of chemistry, physics, and materials science. Synchrotron-based time-resolved X-ray diffraction (TR-XRD), X-ray absorption near-edge spectroscopy (XANES), Raman and infrared spectroscopies (RS and IR), and density-functional (DF) calculations were used to study the structural and electronic properties of Ce-M-Ca (M = Zr, Tb) oxide nanoparticles. The nanoparticles were synthesized following novel microemulsion and citrate methods and had sizes in the range of 3-6 nm. The atoms in these nanoparticles adopted a Fluorite-type structure and exhibited cell parameters with deviations with respect to the values predicted by Vegard's rule for ideal solid solutions. The simultaneous presence of Zr/Tb and Ca creates strain in the Fluorite-type lattice which correlates with the presence and number of oxygen vacancies through the Ce-M-Ca samples. The oxygen vacancy and cation distributions of the nanoparticles are strongly affected by the preparation method. The XANES/density-functional theory study indicates that Ce-M-Ca solid solutions display distinctive electronic properties. In the Ce-Zr-Ca system, the Zr(4d) splitting into t _2g and e _g orbitals is affected by the presence of Ca, leaving the Ce orbitals mostly unaffected. This is a consequence of the local environment of the Zr cations, which is modified from a monoclinic-like to a tetragonal (t″) symmetry as the Ca content rises. In Ce-Tb-Ca solid solutions, there is a progressive stabilization of the fully oxidized Tb state (Tb ⁴⁺) as the Ca content increases, mainly to minimize the strain of the structure. In both systems, Ca ²⁺ ↔ O ^2- ↔ M ⁿ⁺ interactions play a major role in the structural and electronic properties and are critical to interpret the thermal behavior of the materials.