Structural tuning of high entropy (LaPrEuGdYb)2Zr2O7+ δ for uranium immobilization at A-site and B-site

Abstract

High-entropy pyrochlore can immobilize multiple radionuclides simultaneously, and thus is a highly promising binder material. In this study, we synthesized high-entropy binder materials (LaPrEuGdYb)₂Zr₂O_7+δ to solidify uranium, and compared the structure variation of solidification forms for uranium at A-site ((LaPrEuGdYb)_yU_x)₂Zr₂O_7+δ) and B-site (LaPrEuGdYb)_0.4(Zr_xU_1-x)₂O_7+δ). The uranium solidification forms were manufactured via high-temperature vacuum sintering with sol-gel spray pyrolysis pretreatment. Uranium solubility limit (x = 0.25) at B-site was higher than that at A-site (x = 0.20), as demonstrated by X-ray diffraction and Raman spectroscopy. However, the chemical stability of uranium solidification form at A-site was better than that at B-site after a 42 d static leaching experiment. Restrained by the uranyl radius and valence, uranium was easy to occupy the B-site of (LaPrEuGdYb)₂Zr₂O_7+δ via the regulation of oxygen vacancies. Accordingly, more oxygen vacancies were found for B-site occupation. The structural adjustment capability of (LaPrEuGdYb)₂Zr₂O_7+δ were performed by the oxygen vacancies which would further affect the cation valence and radius, and hence solubility limit. For example, the ratio of U^6 + to U⁴⁺ increased with higher uranium content for A-site doped solidification form, while this ratio had no significant change for B-site doped solidification form.