Phenomenological model for the formation of heterogeneous tracks in pyrochlores irradiated with swift heavy ions G. Sattonnay a,⇑ , C. Grygiel b , I. Monnet b , C. Legros a , M. Herbst-Ghysel a , L. Thome ´ c a Univ. Paris Sud, ICMMO-LEMHE, Ba ˆt. 410, F-91405 Orsay, France b CIMAP, CEA–CNRS–Universite ´ de Caen, BP 5133, F-14070 Caen Cedex 5, France c CSNSM, CNRS, IN2P3, Univ. Paris-Sud, Ba ˆt. 108, F-91405 Orsay, France Received 1 July 2011; received in revised form 8 September 2011; accepted 9 September 2011 Available online 26 October 2011 Abstract Zirconate and titanate pyrochlores were irradiated with swift heavy ions in order to investigate the effects of the chemical composition on the structural changes induced by high electronic excitation. Both transmission electron microscopy and X-ray diffraction results show that the structural modifications induced by irradiation with 120-MeV U ions are strongly dependent on the sample composition: Gd 2 Ti 2 O 7 is readily amorphized, whereas Gd 2 Zr 2 O 7 is transformed into a radiation-resistant anion-deficient fluorite structure. For Sm 2 Zr 2 O 7 , Eu 2 Zr 2 O 7 and Nd 2 Zr 2 O 7 , more complex behavior is observed, since both pyrochlore–fluorite phase transformation and amorphization occur. A new phenomenological model (the heterogeneous track overlap model, HTOM), which assumes a direct impact mechanism coupled with a single track overlap process, is proposed to describe the formation of heterogeneous track structures consist- ing of mixed anion-deficient fluorite and amorphous domains. The pyrochlore composition mainly influences the structure of ion tracks (and weakly their diameter), and essentially concerns the amount of amorphous phase vs the amount of fluorite counterpart. Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ion irradiation; Pyrochlores; Phase transformations; X-ray diffraction; Transmission electron microscopy 1. Introduction The study of irradiation-induced phase transformations in ceramics is of great interest from both fundamental and technological viewpoints. Several phenomenological mod- els were developed to interpret the damage build-up induced by ion irradiation, and they were more particularly applied to the description of the crystalline-to-amorphous phase transformation induced by displacement cascades at low energy where ballistic collisions (S n ) are dominant [1,2]. Among these models, the Gibbons description [1] was frequently used to interpret the effects of S n induced by low-energy ions. A simplified version of this model (direct-impact process), assuming that one ion impact is suf- ficient to totally transform the matter inside the cylinder surrounding the ion path, was successfully used to describe the damage accumulation in insulators irradiated with swift heavy ions, where electronic excitation (S e ) overwhelms the ion slowing-down (most of metals and semiconductors being insensitive to S e ) [3–5]. Nevertheless, it should be remembered that the validity of this representation relies on the strong assumption that, above a S e threshold, all the volume of an ion track is transformed. Actually, trans- mission electron microscopy (TEM) observations are needed to verify this hypothesis of homogeneous track structure. However, very recently, inhomogeneous track structures have been observed in TEM experiments devoted to the investigation of radiation damage induced by swift heavy ions in a particularly interesting class of insulators: pyrochlore-type oxides with the A 2 B 2 O 7 stoichiometry [4–6]. Ordered A 2 B 2 O 7 pyrochlores belong to the Fd  3m space group, which is a superstructure of the ideal fluorite 1359-6454/$36.00 Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2011.09.017 ⇑ Corresponding author. Tel.: +33 1 69 15 70 37. E-mail address: gael.sattonnay@u-psud.fr (G. Sattonnay). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 60 (2012) 22–34