Multicomponent equiatomic rare earth oxides

Abstract

Multicomponent rare earth oxide (REO) nanocrystalline powders containing up to seven equiatomic rare earth elements were successfully synthesized in a single-phase CaF 2-type (Fm-3 m) structure. The addition of more than six elements resulted in the formation of a secondary phase. Annealing at 1000°C for 1 h led to the formation of a single-phase (Ia-3) even in the 7-component system. In the absence of cerium (Ce 4+), secondary phases were observed irrespective of the number of cations or the extent of thermal treatment indicating that cerium cations played a crucial role in stabilizing the multicomponent REOs into a phase pure structure. IMPACT STATEMENT Multicomponent equiatomic rare earth oxides pioneer a new group of materials that crystallize into a single-phase structure with the dominant role of a single element instead of entropy.

Solid‐state synthesis of multicomponent equiatomic rare‐earth oxides

“…At 1450°C only the (222) reflection of the C‐type structure is observed, revealing a complete reaction between the five binary oxides. Djenadic produced this same composition with a single fluorite phase using a reactor at 1150°C, which indicates that the ideal synthesis temperature for a composition might differ between synthesis methods. It is important to notice that our samples were cooled to room temperature with a slow cooling rate (3.3°C/min), therefore 1450°C is the temperature necessary to fully dissolve all the binary oxides into a single C‐type structure and make it stable at room temperature.…”

“…Both the synthesis method and the type of cations in any given composition influence the final crystal structure. The presence of Ce alone in these sintered ceramics is not a determining factor for the formation of a single fluorite phase, as was the case in ceramics synthesized using the nebulized spray pyrolysis method; however, the relative concentration of rare‐earths in the 4+ oxidation state is important. In an oxidizing atmosphere, both Ce and Pr contribute to the formation of a cubic phase in all the compositions studied; whereas in a reducing or inert atmosphere, Ce alone favors a cubic structure.…”

“…These were all sintered in air at 1500°C for 10 hours with a slow cooling rate of 3.3°C/min. For the sake of simplicity and consistency, the samples have been named in agreement with Djenadic's labeling style: according to the cations present and not to their stoichiometry.…”

“…Djenadic explored multicomponent equiatomic rare‐earth oxide systems containing different combinations of Ce, Gd, La, Nd, Pr, Sm, Y oxides using the nebulized spray pyrolysis method. Only the cerium‐containing compositions had a single fluorite structure, indicating that cerium plays a crucial role in the phase stabilization.…”

“…We explored the compositional space with oxides of Ce, Gd, La, Nd, Pr, Sm, and Y. This system has already been proven to produce single‐phase ceramics via nebulized spray pyrolysis . The similarity in ionic radii was originally used as a selection criterion for elements; Table shows the typical coordination number and ionic radii of the rare‐earth elements correlated with the four possible crystal structures of the rare‐earth binary oxides.…”