Publication date: 10th April 2024
The fluorite structure is an extremely flexible structure, as it permits both anion vacancies and interstitials. This has had led to many applications in ionic conductivity and catalysis, and applies to a host of fluorites based on oxides, oxyfluorides, fluorides, hydrides, and oxyhydrides. We have previously reported on some lanthanide oxyhydrides based on the fluorite structure [1], and their ionic conductivities have been reported [2]. Separately, recently many high entropy oxides of the fluorite [3], bixbyite [3], and pyrochlore structure [4] have also been reported. Although these three structure types are related by their anion vacancy amounts, less characterization has been made on the precise anion content and coordination sites.
We will present our results on 2 high-entropy lanthanide oxides and oxyhydrides, (La0.2Ce0.2Pr0.2Nd0.2Y0.2)H1.5O0.75, (La0.2Ce0.2Pr0.2Nd0.2Y0.2)O1.5+𝛿. Based on a careful examination of X-ray and neutron diffraction data, we find that the high entropy effects of the lanthanide cations cause some interesting structural features not seen in the single-element analogues. For (La0.2Ce0.2Pr0.2Nd0.2Y0.2)H1.5O0.75, we see hydride exclusively occupying a certain interstitial site. The site location, and its selectivity is not reported in other known single-element LnH3-2xOx analogues. A low activation energy for hydride conductivity is observed, perhaps due to these interstitial sites. We also find that for corresponding high entropy distorted/oxygen-deficient fluorite LnO1.6, there are signs of local cation order between the 2 lanthanide sites based on size. Further studies on control over oxygen cotent, and its effect on structure, oxide ionic conductivity, and oxidation catalysis are underway.
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