Chemistry Proposal Defense – Xiaodong Zhao, Chemistry PhD Student

About the event

Title: Experimental and Computational Study on Zircon and Perovskite Solid Solutions for Nuclear Applications

Abstract: This proposal focuses on the thermal stability studies of actinide-mixing zircon solid solutions and halide-mixing perovskite solid solutions. Zircon type materials are great candidates for immobilizing actinides and heavy elements from nuclear wastes, and perovskites have promising applications for radiation detection. Experimentally, a non-ideal mixing behavior has been discovered for U-Th silicate solid solution system (USiO4 – ThSiO4) despite the fact the homogenous mixing system could not introduce local disorder to the system, we performed Density Functional Theory (DFT) simulations and revealed that the local covalency difference in the M-O bonds and potential charge redistribution are the driving forces to derive from ideal mixing, rather than structural aspects. We have also explored the thermodynamics and phase transitions of CeSiO4 (surrogate of PuSiO4) under high pressure by DFT. In this project, we performed Density Functional Theory (DFT) studies to calculate structures and enthalpies of formation of certain key f-block zircon endmembers (CeSiO4, ThSiO4, USiO4, PuSiO4), and use CeSiO4 as a case study to compute its pressure-induced transitions, and phonon dispersion curves at different pressures, which can be integrated with synchrotron X-ray diffraction and Raman scattering data. Thermodynamics and high-pressure structures of CeSiO4 systems are highly recommended to predict nuclear waste long-term behaviors, as CeSiO4 is a good analog to the AnSiO4 system. The mix anion lead-free double perovskite CsAgBiX6 (X = Cl, Br, I) is considered as promising inorganic material in the field of high-energy ionization detector material due to its uniquely high sensitivity, long carrier-charge mobility lifetime under high-energy ionization process. However, its development is hindered by the stability issue. We propose to investigate the thermodynamic mixing behavior of various halide ions to determine the competence between enthalpy and entropy contribution. Besides that, photon-electrical property of the solid solution will also be conducted to establish the correlation between the dopant and the radiation detection ability.