Phase transformation and chemical decomposition of nanocrystalline SnO2 under heavy ion irradiation

A crystalline-to-crystalline phase transformation, including chemical decomposition, has been observed in SnO2 nanopowder irradiated by 2.2 GeV 197Au ions. X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM) were used to characterize the transformation from tetragonal SnO2 (P42/mnm) to tetragonal SnO (P4/nmm), with trace quantities of β-Sn (I41/amd). At a fluence of approximately 2.0 × 1012 ions/cm2, diffraction maxima corresponding to SnO became clearly evident and increased in intensity as fluence increased. The proportion of SnO, as determined by Rietveld refinement of XRD data, reached 23.1 ± 0.8% at the maximum fluence investigated of 2.4 × 1013 ions/cm2. Raman spectra show high photoluminescence (PL) intensity before and during initial SnO formation, indicating the importance of oxygen vacancies in the transformation process. Small-angle X-ray scattering (SAXS) analysis provided evidence of ion tracks, but no tracks were observed using high-resolution TEM (HRTEM). The transformation likely occurs through a multiple-impact mechanism, based on the accumulation of O vacancies, defect ordering, and partially localized Sn reduction.This work was supported by the Office of Basic Energy Sciences of the US-DOE under Grant DE-FG02-97ER45656 (RCE, ML, ABC and JZ); US-DOE under Contract DE-AC02-10886 (FZ); NSF COMPRES under Grant EAR01-35554. Part of the research was performed at the SAXS/WAXS beamline at the Australian Synchrotron. PK acknowledges the Australian Research Council for financial support. This research used resources of the National Synchrotron Light Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-AC02-98CH10886