PHYSICAL REVIEW B 94, 224109 (2016) Key role of the short-range order on the response of the titanate pyrochlore Y 2 Ti 2 O 7 to irradiation G. Sattonnay, 1 , * S. Cammelli, 2 D. Menut, 3 N. Sellami, 4 C. Grygiel, 5 I. Monnet, 5 J. L. B´ echade, 6 J. P. Crocombette, 6 A. Chartier, 7 A. Souli´ e, 6 R. T´ etot, 4 C. Legros, 4 P. Simon, 8 S. Miro, 6 and L. Thom´ e 1 1 Universit´ e Paris-Sud, Centre de Sciences Nucl´ eaires et de Sciences de la Mati` ere (CSNSM), Centre national de la recherche scientifique (CNRS), IN2P3, Bˆ at. 108, F-91405 Orsay, France 2 Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, BP 48, F-91192 Gif-sur-Yvette Cedex, France 3 Commissariat ` a l’´ energie atomique et aux ´ energies alternatives (CEA), La Direction de l’´ energie nucl´ eaire (DEN), Service de Recherches M´ etallurgiques Appliqu´ ees, F-91191 Gif-sur-Yvette, France 4 Universit´ e Paris-Sud, Institut de Chimie Mol´ eculaire et des Mat´ eriaux d’Orsay (ICMMO), SP2M, B ˆ at. 410, F-91405 Orsay, France 5 Centre de Recherche sur les Ions, les Mat´ eriaux et la Photonique (CIMAP), CEA, CNRS, Universit´ e de Caen, BP 5133, F-14070 Caen Cedex 5, France 6 CEA, DEN, Service de Recherches de M´ etallurgie Physique, Laboratoire JANNUS, F-91191 Gif-sur-Yvette, France 7 CEA, DEN, Service de la Corrosion et du Comportement des Mat´ eriaux dans leur Environnement, Laboratoire de Mod´ elisation, de Thermodynamique et de Thermochimie, F-91191 Gif-sur-Yvette, France 8 Conditions Extrˆ emes et Mat´ eriaux: Haute Temp´ erature et Irradiation (CEMHTI), CNRS, UPR 3079, F-45071 Orl´ eans Cedex 2, France (Received 11 July 2016; revised manuscript received 27 October 2016; published 30 December 2016) Ordering and disordering processes in complex oxides strongly influence their physicochemical properties when they are submitted to severe conditions, such as high temperature, high pressure, or irradiation. This paper examines the role played by the local atomic order on the structural stability of Y 2 Ti 2 O 7 pyrochlore submitted to ion irradiation by combining experimental and atomistic computation studies. X-ray absorption spectroscopy at the Ti K edge, molecular dynamics simulations, and calculations using a tight-binding variable-charge model show that the short-range order around Ti atoms in Y 2 Ti 2 O 7 is strongly modified by irradiation. Strong local distortions around Ti defects occur due to a decrease of the Ti coordination number. These local atomic rearrangements trigger the overall amorphization of the compound. These results show that the local short-range order influences the long-range structural stability of complex oxides, thus providing a key feature for the control of the functional properties of these materials. DOI: 10.1103/PhysRevB.94.224109 I. INTRODUCTION Due to their outstanding properties, complex oxides such as pyrochlores have potential technological applications in various domains, such as catalysis, ionic conductors, nuclear waste forms, or thermal barrier coatings [1]. They also attract considerable attention in modern condensed-matter physics [2,3], and they are extensively studied due to their spin ice properties [4–6]. Among the ternary metallic oxide family, A 2 B 2 O 7 compounds (where A and B are metals) are fas- cinating materials for both fundamental and applied research. Actually, the extreme variety of the pyrochlore chemistry leads to a remarkable versatility of the physicochemical properties of these compounds. More than 450 compositions were reported since they present a wide variety of chemical substitution at the A and B sites, provided ionic radius and charge neutrality criteria are satisfied. Pyrochlores are superstructures (Fd 3m space group) of the fluorite structure (Fm 3m space group) with two cation sites [A atoms at 16d (0.5,0.5,0.5), B atoms at 16c (0,0,0)] and two anions sites [O1 at 48f (x ,0.125,0.125) and O2 at 8b (0.375,0.375,0.375)] using Wyckoff notation. The anion sublattice can be completed by adding missing oxygen atoms in the 8a site to form the fluorite structure. Many properties of this oxide family are closely related to the atomic arrangement in the different cation sublattices. Indeed, the cation disordering process in pyrochlores governs their structural and functional properties [7]. There is thus great * gael.sattonnay@u-psud.fr interest for understanding the order-disorder transformations in these compounds, particularly because unconventional ordering can appear at different length scales [8]. For instance, it has been recently shown that the Ho 2 Zr 2 O 7 disordered pyrochlore exhibits a fluorite-type structure at a long scale, but a local structure constituted of weberite domains was evidenced at a short scale [8]. This behavior is not restricted to pyrochlores, since similar disordered structures also appear in other oxides, for instance, in spinel [8]. This new insight suggests that complex oxides exhibit intricate disorder [9] and that disordering mechanisms are more complex than it was previously believed. The local atomic structure in complex oxides can be differ- ent from the long-range structure, and this phenomenon may affect the disordering mechanisms and, more generally, the phase transformations occurring in these materials. Thus, the short-range atomic arrangement could also play a role in the re- sponse of complex oxides submitted to severe conditions, such as high temperature, high pressure, or irradiation. It is thus cru- cial to understand the mechanisms of structural modifications (phase transformations, disordering, amorphization) in com- plex oxides in order to: (i) improve the knowledge of the funda- mental aspects of phase transformations in these compounds; (ii) help develop new applications involving these materials. Previous work showed that the stability of the pyrochlore phase submitted to intense irradiation or high pressure depends on the sample composition. For both kinds of solicitation, titanate pyrochlores (e.g. Gd 2 Ti 2 O 7 ) are amorphized, whereas zirconate pyrochlores (e.g. Gd 2 Zr 2 O 7 ) remain crystalline and 2469-9950/2016/94(22)/224109(8) 224109-1 ©2016 American Physical Society