Structural modifications of Gd 2 Zr 2-x Ti x O 7 pyrochlore induced by swift heavy ions: Disordering and amorphization
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Jie Lian Department of Mechanical, Aerospace & Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180; and Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1005; and Department of Materials Science & Engineering, University of Michigan, Ann Arbor, Michigan 48109-1005
Christina Trautmann Gesellschaft fu¨r Schwerionenforschung (GSI), 64291 Darmstadt, Germany
Zhongwu Wang Cornell High Energy Synchrotron Source, Cornell University, Ithaca, New York 14853 (Received 20 August 2008; accepted 3 November 2008)
The isometric, pyrochlore structure type, A2B2O7, exhibits a wide variety of properties that find application in a large number of different technologies, from electrolytes in solid oxide fuel cells to actinide-bearing compositions that can be used as nuclear waste forms or inert matrix nuclear fuels. Swift xenon ions (1.43 GeV) have been used to systematically modify different compositions in the Gd2Zr2-xTixO7 binary at the nanoscale by radiation-induced phase transitions that include the crystalline-to-amorphous and order-disorder structural transformations. Synchrotron x-ray diffraction, Raman spectroscopy, and transmission electron microscopy provide a complete and consistent description of structural changes induced by the swift heavy ions and demonstrate that the response of pyrochlore depends strongly on chemical composition. The high and dense electronic energy deposition primarily results in amorphization of Ti-rich pyrochlore; whereas the formation of the fully disordered, defect-fluorite structure is the dominant process for Zr-rich pyrochlore.
I. INTRODUCTION
Pyrochlore, A2B2O7 (A = La to Lu, Y, and actinides; B = Ti, Zr, Sn, Hf), is isometric (Fd3 m, Z = 8, a = 0.9– 1.2 nm) and is closely related to the fluorite structure (AX2), except that there are two cation sites and one-eighth fewer anions. The A-site at 16c is eightcoordinated and located within a distorted cubic coordination polyhedron, generally occupied by larger tri- and tetravalent actinides and lanthanides. The B-site at 16d is a distorted octahedron and typically occupied by Ti, Zr, Hf, or Sn. There are two unique oxygen sites: the 48f oxygen is coordinated to two B4+ and two A3+ cations, while the 8b oxygen is in tetrahedral coordination with only A3+ cations. An unoccupied interstitial site, 8a, is surrounded by four B4+ ions, and the “vacancies” at the 8a site are ordered on the anion sublattice.1–3 The oxygen at the 48f site is slightly displaced from that in the a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0151
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J. Mater. Res., Vol. 24, No. 4, Apr 2009 Downloaded: 13 Mar 2015
ideal-fluorite structure toward the B-site cation. The pyrochlore structure is completely described by the cubic lattice parameter, a, and the positional parameter, x48f, of the 48f oxygen. For x48f = 0.3750, the A-site coordination polyhedron is a regular cube, and the B-site polyhedron is distorted to a trigonally
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