Effects of electronic energy loss on the behavior of Nd 2 Zr 2 O 7 pyrochlore irradiated with swift heavy ions G. Sattonnay a,⇑ , L. Thomé b , I. Monnet c , C. Grygiel c , C. Legros a a Université Paris-Sud, ICMMO/LEMHE, Bât. 410, F-91405 Orsay, France b CSNSM, CNRS, IN2P3, Université Paris-Sud, Bât. 108, F-91405 Orsay, France c CIMAP, CEA, CNRS, Université de Caen, BP 5133, F-14070 Caen Cedex 5, France article info Article history: Received 12 August 2011 Received in revised form 28 October 2011 Available online 25 November 2011 Keywords: Ion irradiation Pyrochlores X-ray diffraction High resolution transmission electron microscopy abstract Nd 2 Zr 2 O 7 pyrochlore was irradiated with swift heavy ions in order to investigate the effects of electronic energy loss on the structural changes induced in this material. XRD and TEM results show that both pyrochlore–fluorite phase transformation and amorphization occur in Nd 2 Zr 2 O 7 . Ion tracks are thus com- posed of a mixture of anion-deficient fluorite phase and amorphous zones. A new phenomenological model (heterogeneous track overlap model – HTOM), which assumes a direct impact mechanism coupled with a single track overlap process to fit the transformation build-ups, is proposed to estimate the areas of the various track regions. The variation of electronic energy loss mainly influences the structure of ion tracks (and weakly their diameter), and essentially concerns the amount of amorphous phase versus the amount of fluorite counterpart. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Pyrochlores with the A 2 B 2 O 7 stoichiometry provide a large array of structural and physical properties related to their various chem- ical compositions. In particular, due to the capability to incorporate actinides (e.g. U, Np, Th, Pu, Am and Cm), the A 2 3+ B 2 4+ O 7 series is of great interest in the field of nuclear waste management [1]. Pyrochlores are ordered superstructures with two cation sites, one-eighth fewer anions and a lattice constant twice that of their parent fluorite-like (AO 2 ) arrangement. The stability of the pyroch- lore structure is governed by the ratio of the ionic radii of A and B cations (r A /r B ) which extends from 1.46 for Gd 2 Zr 2 O 7 to 1.78 for Sm 2 Ti 2 O 7 [1,2]. Recently, due to the high radiation resistance of some compo- sitions, pyrochlores were considered as potential matrices for the immobilization or transmutation of actinides produced in nuclear power plants [1]. Although the crystalline-to-amorphous phase transformation induced by low-energy ion irradiation in pyroch- lores has drawn much attention in the past [1,3–10], information concerning the damage due to electronic excitation arising from irradiation with swift ions (100 MeV range) is still incomplete owing to the small number of experiments devoted to this topic [11–19]. One of the main results is that the susceptibility to radiation-induced amorphization exhibits a systematic decrease with increasing Zr content, as it has also been observed for irradiations performed with low-energy ions. Whereas Gd 2 Ti 2 O 7 is readily amorphized, the end member Gd 2 Zr 2 O 7 is transformed into a radiation resistant anion-deficient fluorite structure upon irradiation at room temperature [4–6,8,9,12–17]. From these re- sults, it was concluded that zirconates are more radiation resis- tant than titanates. Nevertheless, recent studies showed that some zirconates (Nd 2 Zr 2 O 7 , Sm 2 Zr 2 O 7 and Eu 2 Zr 2 O 7 ) can be amor- phized by high electronic excitations [19]: the tracks created in the wake of swift ions most often consist of a mixture of anion- deficient fluorite structure and amorphous phases. The track structure (but not their diameter) and the relative amount of amorphous and fluorite phases are strongly dependent on the chemical composition of pyrochlores. The percentage of amor- phous phase created in ion tracks for zirconates increases also as the difference between the melting and the order–disorder transition temperatures (T melting  T O/D ) increases. This paper reports a comparative study of the damage created by swift heavy ions in Nd 2 Zr 2 O 7 in order to investigate the ef- fects of electronic excitation (S e ) on the kinetic of phase transi- tions induced in ion tracks. X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) were implemented to determine the structural changes induced by irradiation, as well as to monitor the damage build-up. A new model (called heterogeneous track overlap model – HTOM, see [19]) was applied to interpret the phase transformation build- ups observed experimentally and to determine the cross sections corresponding to the different phase transformations induced by irradiation. 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.11.017 ⇑ Corresponding author. Tel.: +33 1 69 15 70 37. E-mail address: gael.sattonnay@u-psud.fr (G. Sattonnay). Nuclear Instruments and Methods in Physics Research B 286 (2012) 254–257 Contents lists available at SciVerse ScienceDirect Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb