The effect of entropy on the structure and aqueous leaching resistance of nano monazite-type phosphates

Abstract

Monazite, with its unique chemical composition and stable crystal structure, shows significant potential for immobilizing high-level radioactive waste. In this study, a series of monazite ceramics were synthesized to study the effect of entropy change on their structure and aqueous durability. The synthesized ceramics exhibit a monoclinic structure (P21/n), with an average grain size below 80 nm. MCC-1 leaching tests reveal that the normalized leaching rate decreases over time but increases with higher entropy. The five-element high-entropy ceramics have an average leaching rate of 4.2 × 10⁻³ g·m⁻2 d⁻1. Notably, the higher leaching rate in high-entropy nanocrystalline monazites is likely attributed to the reduced grain size. The presence of numerous grain boundaries tends to mitigate the precipitation inhibition effect typically associated with high-entropy ceramics. These findings offer insights into the role of entropy and grain size in ceramic materials for radioactive waste immobilization.