J. Phys.: Condens. Matter 12 (2000) 2401–2412. Printed in the UK PII: S0953-8984(00)09915-X Amorphization in zircon: evidence for direct impact damage S R´ ıos†§, E K H Salje†, M Zhang† and R C Ewing‡ † Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK ‡ Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109-2104, USA Received 26 November 1999, in final form 2 February 2000 Abstract. X-ray diffraction has been used to characterize the amorphous phase present in a series of radiation-damaged natural zircons with radiation doses ranging from 0.06 to 16 × 10 18 α-decay events g -1 . The fraction of amorphous material present in each of the samples studied has been determined, and its dependence on the radiation dose has been calibrated. Direct determination of the amorphous fraction confirms that amorphization in natural zircon occurs as a consequence of the direct impact within cascades caused by α-recoil nuclei. These results are not consistent with the commonly accepted double-overlap model of damage accumulation. The volume swelling of amorphous regions changes as a function of dose. Thus, the density of amorphous regions depends on the degree of damage up to a certain point (i.e. 8 × 10 18 α-decay events g -1 ), unlike in previous models for which a constant value independent of the radiation dose was assumed. 1. Introduction Naturally occurring phases such as zircon (ZrSiO 4 ), titanite (CaTiSiO 5 ), the apatites (Ca 10 (PO 4 ) 6 (OH, Cl, F) 2 ) and pyrochlores (A 1,...,m B 2 O 6 (O, OH, F) 1,...,n ·pH 2 with A = Na, Ca, RE, ... and B = Nb, Ta, . . . ) are known to undergo amorphization as a consequence of the α-decay of radionuclide impurities (typically 238 U, 235 U and 232 Th and their decay products) (Holland and Gottfried 1955, Lumpkin and Ewing 1988, Ewing 1994, Weber et al 1994). Due to the properties (mainly chemical durability) that some of these materials offer as waste forms for the immobilization of high-level waste and plutonium (Weber et al 1998, Ewing 1999), natural materials and their synthetic equivalents have been extensively studied. In order to elucidate the amorphization process, the effect of natural radioactive impurities has been simulated by heavy-ion bombardment (e.g. Kr + , Xe + ) (Wang and Ewing 1992a, Meldrum et al 1996) or by actinide doping ( 238 Pu, 244 Cm) (Weber 1990, Weber et al 1994, 1997b). During the amorphization process in natural zircon three different stages may be observed with increasing dose, depending on the age of the sample and/or the content of radioactive impurities (Holland and Gottfried 1955, Murakami et al 1986, 1991). At low degrees of damage the material is essentially crystalline, α-particles produce isolated defects and α-recoil nuclei produce a few isolated amorphous regions. At this first stage, sharp Bragg maxima are observed that decrease in intensity with increasing radiation dose. The crystalline matrix is expanded (as shown by the unit-cell swelling (Holland and Gottfried 1955, Murakami et al 1991)) as a consequence of the shear deformation produced by the localized defects created by α-particles § Author to whom any correspondence should be addressed. 0953-8984/00/112401+12$30.00 © 2000 IOP Publishing Ltd 2401