The structure of aperiodic, metamict (Ca, Th)ZrTi 2 O 7 (zirconolite): An EXAFS study of the Zr, Th, and U sites
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Rodney C. Ewing Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131-1116
Gordon E. Brown, Jr. Department of Geological & Environmental Sciences and Stanford Synchrotron Radiation Laboratory, Stanford University, Stanford, California 94305-2115 (Received 22 June 1992; accepted 18 March 1993)
The structural environments of Zr, Th, and U in aperiodic (metamict) (Ca, Th)ZrTi2O7 were examined using Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Samples are aperiodic due to a radiation-induced transformation caused by alpha-decay event damage. In the aperiodic samples, Zr is mainly 7-coordinated [d(Zr-O) « 2.14-2.17 ± 0.02 A]; whereas, Th is mainly 8-coordinated [d(Th-O) = 2.40-2.41 ± 0.03 A]. Nearly identical bond lengths 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 ( Z r , T h ) - 0 distances. In one metamict sample with —1.9 wt. % U 3 O 8 , U is essentially tetravalent. The absence of higher oxidation states (U6+) is consistent with the lack of evidence for alteration (samples are over 500 million years old). The reduced medium-range order around Zr, Th, and U is related to the increase of alpha-decay event damage and precludes decomposition of zirconolite into simple oxides of Zr, Th, or U. Comparison with other metamict (Zr, Th, U)-bearing phases (e.g., ZrSiO 4 and ThSiO4) suggests that Zr4+, Th 4+ , and U4+ prefer 7-, 8-, and 6-coordinated sites, respectively, in aperiodic phases at ambient temperatures and pressures. Examination of the structure of crystalline (Ca, Th)ZrTi2O7 demonstrates that M - O - M angles (M = Ca, Ti, Zr, and Th) are relatively small («100-120° for edge-sharing polyhedra). A limited relaxation of the constraints of periodicity around M cations caused by radiation damage (e.g., tilting of polyhedra) dramatically affects the distribution of these angles. This type of structural relaxation may be the mechanism by which long-range periodicity is lost and medium-range order is reduced with increasing radiation damage, while the major cations retain their nearest-neighbor environments. This relaxation may also help explain the lattice expansion observed in zirkelites when they undergo radiation damage.
I. INTRODUCTION Radiation-induced defects in complex ceramics are an important consideration in evaluating the longterm behavior of individual phases that occur in proposed crystalline, high-level nuclear waste forms. Zirconolite/zirkelite (ideally CaZrTi2O7) is an important phase in Synroc, a crystalline titanate phase assemblage that has been proposed as a nuclear waste form.1"3 Zirconolite is the principal host for actinide elements (e.g., U, Th, Cu, Am, and Np)4-5 and, consequently, will be subject to alpha-decay event damage associated with the radioactive decay of the actinides. We report here J. Mater. Res., Vol. 8, No. 8, Aug 1993
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