Chemistry Final Defense – Shinhyo Bang
About the event
Speaker: Shinhyo Bang
Group: Dr. Xiaofeng Guo
Title: The Role of Cation Polyhedra in the Thermodynamics of Actinide-Bearing Zircon and Related Framework
Abstract: Zircon is a mineral deeply intertwined with human history, evolving from a valued gemstone, to a geological time capsule, and to a cornerstone of modern nuclear science. Its exceptional chemical and physical durability allows it to sequester long-lived actinides within its rigid crystal structure, offering a potentially stable alternative, or an unexpected crystallization product, to the currently favored borosilicate glass used for nuclear waste immobilization. Earlier chapters of this dissertation explore the thermodynamics of actinide-bearing zircon, focusing on the solid-solution systems (uranothorite and chernobylite). For uranothorite, the high pressure phase transition was investigated as the endmembers adopt different high-pressure structures, making the system particularly intriguing. Using high-pressure X-ray diffraction combined with Gibbs energy modeling, this study reveals a breakdown of the traditional “chemical pressure” concept. Instead, non-ideal mixing energetics, rather than simple ionic radius trends, govern the pressure-induced thorite-to-huttonite phase transition and invert the predicted phase stability. For chernobylite, the first comprehensive thermodynamic profile of the solid-solution was established. High temperature oxide melt calorimetry reveals both destabilizing and stabilizing effects associated with uranium incorporation. These results lead to the proposal of a “polyhedral anchoring” mechanism, in which ZrO8 polyhedra energetically stabilize uranium incorporation within the lattice. This polyhedral structural framework was further extended topologically to garnet structures, where the one-dimensional tetrahedral linkage characteristic of zircon expands into a fully three dimensional network. X-ray Absorption Fine Structure analysis of uranium-doped yttrium aluminum garnet shows that dopant-induced local anisotropic strain is accommodated through neighboring flexible yttrium sites. In situ high-temperature neutron diffraction demonstrates that the framework remains structurally stable up to 1000 ℃, while revealing increasing anisotropy in the local polyhedral geometry with temperature. In conclusion, this work demonstrates that polyhedral mixing energetics, connectivity, and rigidity collectively govern the thermodynamic behavior of these materials, whether through the one-dimensional connectivity of silicate chains in zircon-type structures or the three-dimensional networks of aluminate garnets. This polyhedral perspective provides a unifying framework for understanding both natural minerals and engineered crystalline materials.