Description and classification of uranium oxide hydrate sheet topologies
Conference
·
OSTI ID:264442
- Univ. of New Mexico, Albuquerque, NM (United States). Dept. of Earth and Planetary Sciences
- Univ. of Manitoba, Winnipeg, Manitoba (Canada). Dept. of Geological Sciences
The uranyl oxide hydrates (UOH) are important corrosion products of uraninite and UO{sub 2} in spent nuclear fuel under oxidizing conditions. However, the systematics of the crystal chemistry, thermodynamic parameters, and solubilities of this mineral group are poorly understood. With the exception of the synthetic UO{sub 2}(OH){sub 2} polymorphs, all UOH crystal structures are based on sheets of edge-sharing 5 and 4-coordinated uranyl dipyramids. This structural similarity suggests that it is possible to develop a model by which to estimate the thermodynamic behavior of UOHs from data on structural endmember phases. Toward this end, a method of quantitatively describing all known UOH sheets has been developed. Only four structural unit chains are required to construct the uranyl oxide hydrate sheets (as well as the structurally similar U{sub 3}O{sub 8} sheets). The H-chain is restricted to {alpha}-UO{sub 2}(OH){sub 2} and is made up of hexagonally coordinated uranyl ions sharing opposing edges. The arrowhead chain composed of pentagonal dipyramids sharing edges and alternating with trigonal vacancies is present in all other UOH sheets. These arrowhead chains are directed and can occur in both an {Up_arrow} and {Down_arrow} sense within a single sheet. The P-chain consists of edge-sharing pentagonal dipyramids forming a zigzag chain. The P-chain is flanked on both sides by arrowhead chains of the same sense. The remaining structural unit is a discontinuous chain of rhombic dipyramids. This R-chain is produced when nested adjacent {Up_arrow} and {Down_arrow} arrowhead chains are translated by a diagonal shift. This chain occurs in sheets which contain only 4-coordinate uranyl ion and those containing both 4- and 5-coordinate uranyl ions.
- DOE Contract Number:
- FG03-95ER14540
- OSTI ID:
- 264442
- Report Number(s):
- CONF-951155--; ISBN 1-55899-315-0
- Country of Publication:
- United States
- Language:
- English
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