New Insights in Actinide Borate Chemistry

The use of molten boric acid as a reactive flux for synthesizing actinide borates has been developed in the past three years providing access to a remarkable array of exotic materials with both unusual structures and unprecedented properties. [ThB5O6(OH)6][BO(OH)2]'¢2.5H2O (NDTB-1) possesses a cationic supertetrahedral structure and displays remarkable anion exchange properties with high selectivity for TcO4−. In addition, NDTB-1 was determined to show real utilities to solve the 'Technetium Problem'. Uranyl borates form noncentrosymmetric structures with extraordinarily rich topological relationships. Neptunium borates are often mixed-valent and yield rare examples of compounds with one metal in three different oxidation states. Plutonium borates display new coordination chemistry for trivalent actinides. Finally, americium and curium borates show dramatic departures from plutonium borates, and form the first trivalent actinide compounds that do not have lanthanide analogues. There are scant examples of families of actinides compounds that extend past plutonium to examine the bonding of later actinides. There are several grand challenges that this work addresses. The foremost of these challenges is the development of structure-property relationships in transuranium materials. A deep understanding of the materials chemistry of actinides will likely lead to the development of advanced waste forms for radionuclides present in nuclear waste that prevent their transport in the environment. This work may have also uncovered the solubility-limiting phases of actinides in some repositories such as the Waste Isolation Pilot Plant (WIPP), and allows for measurements on the stability of these materials. The core of the this dissertation is focused on the crystal chemistry and the structure-property relationship of actinide borates that derived from the molten boric acid flux reactions including thorium borate (Chapter 3), uranium borates (Chapter 4-7,12,14), neptunium borates (Chapter 9-14), plutonium borates (Chapter 9,12,16,17), americium borate (Chapter 17), and curium borate (chapter 18). During the course of these studies, several important side results were also serendipitously discovered. These includes the anion exchange studies of NDTB-1 (Chapter 3), studies of iodine incorporation into uranyl borates (Chapter 8), the discovery of new cation-cation interaction bonding mode for Np(V) (Chapter 15), and investigations of novel interstitial incorporation of actinides into natural materials (Chapter 19).