Radiation Damage in Pyrochlore and Related Compounds
- PDF / 156,212 Bytes
- 10 Pages / 612 x 792 pts (letter) Page_size
- 53 Downloads / 213 Views
Radiation Damage in Pyrochlore and Related Compounds Lumpkin G.R.1,2, Whittle K.R.1, Rios S1, Trachenko K.1, Pruneda M.1, Harvey E.J.1, Redfern S.A.T.1, Smith K.L2, and Zaluzec N.J.3 1 Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK 2 ANSTO Materials, Private Mail Bag 1, Menai 2234, NSW, Australia 3 Materials Science Division, Argonne National Laboratory, Argonne, IL , USA
ABSTRACT The radiation damage properties of synthetic pyrochlore-defect fluorite compounds containing lanthanides on the A-site and Ti, Zr, Sn, and Hf on the B-site have been studied extensively using Kr ion irradiation. Using statistical analysis, we show that the results can be quantified in terms of the critical temperature for amorphization, structural parameters, classical Pauling electronegativity difference, and defect energies. The best current model is able to predict the critical temperature to within about 80 degrees Kelvin. The model indicates that radiation tolerance is correlated with an increase in the X anion coordinate toward the value characteristic of the defect fluorite topology, a smaller unit cell dimension, and lower defect energies. Our analysis also demonstrates that radiation tolerance is promoted by an increase in the Pauling cation-anion electronegativity difference or, in other words, an increase in the ionicity of the chemical bonds. Of the two possible cation sites in ideal pyrochlore, the B-site cation appears to play the major role in bonding. This result is supported, for a subset of pyrochlore compounds, by ab initio calculations, which reveal a correlation between the Mulliken overlap populations of the B-site cation and the critical temperature. INTRODUCTION An uncommon mineral in nature, the pyrochlore structure type (ideal A2B2X6Y compounds) exhibits a range of potentially useful properties including ionic conductivity, electrical conductivity (including superconductivity), and geometrically frustrated magnetism [1]. Pyrochlore is also one of the principal actinide host phases in oxide ceramics designed for the safe disposal of actinide-rich wastes, including weapons Pu [2]. The crystal chemistry and aqueous behaviour of synthetic and natural pyrochlores have been extensively investigated, demonstrating that this structure type is capable of high waste loadings and exhibits excellent chemical durability, including low release rates for the lanthanide and actinide elements. In parallel with the laboratory experiments, studies of natural pyrochlore have largely confirmed the long-term durability of this phase in aqueous fluids [3, 4]. In the context of nuclear waste remediation, a major concern for these structure types is that they will undergo a crystalline to amorphous transformation as a function of the cumulative alpha decay dose over time, which may lead to volume expansion, cracking, and reduced chemical durability. Radiation damage effects in these materials have been studied using actinide doping with short-lived 238Pu or 244Cm, natural samples, and various heavy ion irrad
Data Loading...
