Electron Energy-Loss Near-Edge Structure of Zircon
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1124-Q03-09
Electron Energy-Loss Near-Edge Structure of Zircon Nan Jiang Department of Physics, Arizona State University, Tempe AZ 85287-1504, USA ABSTRACT This work demonstrates that absorption near-edge structures of the O K-edge in zircon are directly related to atom structure, including bond length, bond angle and coordination. Small variations in the structure can induce measurable changes in absorption spectrum. INTRODUCTION Zircon is an extremely durable and resistant material with the added capability of accommodating a large quantity of actinides. It has been proposed as a candidate waste form for the geologic disposal of excess plutonium from dismantled nuclear weapons [1, 2]. Its structure and behavior under radiation have been intensively investigated in many experimental studies [3]. Radiation damage mainly involves a crystalline-to-amorphous transition, which results in a pronounced decrease in intensity of diffraction maxima [4] and a gradual loss of long-range periodicity in high-resolution electron microscopy (HREM) images [5]. Understanding the mechanisms for this crystalline-to-amorphous transition requires support from experimental observations towards structural and compositional changes at nanometer scale or even at atomic scale. Various spectroscopic techniques have been used to observe radiation damage at short-range order, which includes the changes in the nearest neighbor coordination geometries [6] and the polymerizations of Si and Zr polyhedra [7]. However, the currently used techniques have not provided a unique description of the fully-damaged state of zircon [3]. Electron energy-loss spectroscopy (EELS), combining with transmission electron microscopy (TEM), has a great advantage in probing local structure and chemistry around specific species at very high spatial resolution. However, such a powerful technique has not been used in study of the radiation damage in zircon in literature. Lack of direct relation between the observed electron energy-loss near-edge structure (ELNES) and atom structure information could be one among various reasons. In this work, we demonstrate that the O K-edge ELNES of zircon can be directly interpreted in terms of local structure, including distortion of Si tetrahedron, bond lengths of Si – O and Zr – O, and structure beyond the nearest neighbor distance. Previously, very few studies have been carried out on simulations of O K-edge EELS of zircon [8, 9], and no attempt has been made to identify the structural information from the O K-edge ELNES. EXPERIMENT and CALCULATION Synthetic zircon (ZrSiO4) (Aldrich Chemical Company, Inc.) crystals were used in this study. The transmission electron microscope (TEM) specimens were prepared by grinding the samples into powders in acetone, and picking them up using a Cu grid covered with a lacy carbon thin film. The sample was then immediately transferred into and observed in a JEOL 2010 TEM with a field-emission gun operating at 200 keV and a
Gatan’s Enfina parallel electron spectrometer. The EELS spectra were acqu
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