Swift Heavy Ion-Induced Decomposition and Phase Transformation in Nanocrystalline SnO 2
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Swift Heavy Ion-Induced Decomposition and Phase Transformation in Nanocrystalline SnO2 Alex B. Cusick1, Maik Lang2, Fuxiang Zhang3, Jiaming Zhang4, Christina Trautmann5, Rodney C. Ewing1,3,4 1
Materials Science & Engineering, University of Michigan, Ann Arbor, MI 48109, USA Nuclear Engineering, University of Tennessee, Knoxville, TN 37996, USA 3 Earth & Environmental Sciences, University of Michigan, Ann Arbor, MI 48109, USA 4 Geological & Environmental Sciences, Stanford University, Stanford, CA 94305, USA 5 GSI Helmholtz Center for Heavy Ion Research, D-64291 Darmstadt, Germany 2
ABSTRACT A chemical decomposition and related phase transformation have been observed in 2.2 GeV Au irradiated SnO2 nanopowder. X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM) were used to characterize the transformation from tetragonal SnO2 (P42/mnm) into tetragonal SnO (P4/nmm). Rietveld refinement of the XRD data determined the structures and proportion of these phases up to a fluence of 2.4×1013 ions/cm2. The initially intense diffraction maxima corresponding to SnO2 gradually decrease in intensity with an increase in fluence. At a fluence of approximately 3.9×1012 ions/cm2, diffraction maxima corresponding to SnO become clearly evident and increase in intensity as fluence increases. Both Raman and TEM analyses confirm the transformation from tetragonal SnO2 to SnO. The XRD refinement results are consistent with a multiple-impact model of transformation, confirmed by TEM as no single tracks were observed. Previous swift heavy ion irradiations of SnO2 have led only to changes in grain size, degrees of crystallinity, and the formation of “holes”. The inconsistency in results is discussed in depth. The proposed mechanism for the currently observed transformation is the interrelation of defect accumulation and thermal-spike mechanisms. The formation of SnO, apparent O loss from the transformation regions, and associated Sn reduction are discussed in terms of thermodynamic, kinetic, and thermal-spike model considerations. 197
1. INTRODUCTION SnO2 (known as tin(IV) oxide, tin dioxide, stannic oxide, and in its mineral form, cassiterite) has many applications in materials engineering due to its bulk and surface properties, making it useful as a transparent conducting oxide, an oxidation catalyst, and a solid state gas sensor.[1] In ambient conditions, SnO2 exhibits a tetragonal structure (P42/mnm) known as rutile. The unit cell parameters are: a = b = 4.737 Å and c = 3.186 Å; Z = 2.[2] Irradiations with swift heavy ions have been conducted on nanoscale SnO2 powders using approximately 950 MeV lead beams (4.6 MeV/u ).[3] In this study, Hemon et al. observed that the resulting ion tracks were in fact holes, where impinging ions had apparently removed material forming cylinders through the samples with diameters of 4 nm ± 0.4 nm. This was attributed to the vaporization of the material induced by the high temperatures reached in the tracks. The theoretical diameter of tracks resulting from vaporized mat
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