THE STRUCTURE OF APERIODIC, METAMICT (CA,TH)ZRTI2O7 (ZIRCONOLITE) - AN EXAFS STUDY OF THE ZR-SITES, TH-SITES AND U-SITES

The structural environments of Zr, Th, and U in aperiodic (metamict) ( Ca,Th)ZrTi2O7 were examined using Extended X-ray Absorption Fine Struc ture (EXAFS) spectroscopy. Samples are aperiodic due to a radiation-in duced transformation caused by alpha-decay event damage. In the aperio dic samples, Zr is mainly 7-coordinated [d(Zr-O) almost-equal-to 2.14- 2.17 +/- 0.02 angstrom]; whereas, Th is mainly 8-coordinated [d(Th-O) almost-equal-to 2.40-2.41 +/- 0.03 angstrom]. Nearly identical bond le ngths and coordination numbers for these elements were determined for an annealed, crystalline sample. The radiation-induced transition from the periodic to the aperiodic state is characterized by a significant broadening of the distribution of (Zr,Th)-O distances. In one metamic t sample with almost-equal-to 1.9 wt. % U3O8, U is essentially tetrava lent. The absence of higher oxidation states (U6+) is consistent with the lack of evidence for alteration (samples are over 500 million year s old). The reduced medium-range order around Zr, Th, and U is related to the increase of alpha-decay event damage and precludes decompositi on of zirconolite into simple oxides of Zr, Th, or U. Comparison with other metamict (Zr, Th, U)-bearing phases (e.g., ZrSiO4 and ThSiO4) su ggests that Zr4+, Th4+, and U4+ prefer 7-, 8-, and 6-coordinated sites , respectively, in aperiodic phases at ambient temperatures and pressu res. Examination of the structure of crystalline (Ca, Th)ZrTi2O7 demon strates that M-0-M angles (M = Ca, Ti, Zr, and Th) are relatively smal l (almost-equal-to 100-120-degrees for edge-sharing polyhedra). A limi ted relaxation of the constraints of periodicity around M cations caus ed by radiation damage (e.g., tilting of polyhedra) dramatically affec ts the distribution of these angles. This type of structural relaxatio n may be the mechanism by which long-range periodicity is lost and med ium-range order is reduced with increasing radiation damage, while the major cations retain their nearest-neighbor environments. This relaxa tion may also help explain the lattice expansion observed in zirkelite s when they undergo radiation damage.