On the Effect of Standard Deviation of Cationic Radii on the Transition Temperature in Fluorite-Structured Entropy-Stabilized Oxides (F-ESO)

Abstract

It is confirmed that Fluorite-structured Entropy-Stabilized Oxides (F-ESO) can be obtained with multicomponent (5) equimolar systems based on cerium, zirconium, and other rare earth elements, selected according to the predictor already proposed by the authors. Indeed, in the present study, three different samples owning a standard deviation (SD in the following) of their cationic radii greater than the threshold value (i.e., SD > 0.095 with cationic radii measured in Å) needed to ensure the formation of the single-phase fluorite structure, were prepared via co-precipitation method. After a calcination step at 1500 °C for 1 h, the entropy-driven transition from multiple phases to single-phase fluorite-like structure has been actually confirmed. Thus, with the aim of defining the temperature at which such entropy-driven transition occurred, and identifying possible relation between such temperature and the actual value of SD, the phase evolution of all the prepared samples as a function of temperature (ranging from 800 °C to 1300 °C) was analyzed by in situ High Temperature X-ray Diffraction. An apparent inverse correlation between the standard deviation and the entropy-driven transition temperature has been identified, i.e., the higher the former, the lower the latter. These results, based on the conducted basic structural analysis, provide further support to the SD-based empirical predictor developed by the authors, suggesting that high values of SD could bring additional contribution to the overall entropy of the system, other than the configurational one. Thus, this SD-driven entropy contribution directly increases with the increasing of the standard deviation of the cationic radii of a given F-ESO.

High-temperature elemental segregation induced structure degradation in high-entropy fluorite oxide

“…The formation of single-phase HEFOs has been widely investigated, with theoretical advancements also aiding in the prediction of such phase formations. Spiridigliozzi et al [26] introduced the standard deviation of atomic radii as an indicator for predicting the formation of single-phase HEFOs, positing that greater variance among atomic radii increases the likelihood of the formation of fluorite-structured HEFO. Focused research on the synthesis and application of these oxides has indicated intriguing structural symmetry transition behaviors [25,28].…”

“…In this study, we selected representative HEFO, composed of the commonly used fluorite bases CeO 2 , HfO 2 , and ZrO 2 , along with rare earth oxides such as Gd 2 O 3 and La 2 O 3. This composition results in a significant standard deviation in cationic radii of 0.1624 Å, indicative of high propensity for the formation of a single-phase fluorite structure [26]. We designated this synthesized material as (CeHfZrGdLa)O x (Ce-HEO) and subjected it to prolonged annealing at its intended operational temperature to demonstrate high-temperature stability behavior of HEFO.…”