Structure solution of intermediate tin oxide, SnO2-x, by electron precession

T. WHITE; M.S. MORENO; P.A. MIDGLEY

Aachen, Alemania

Congreso; 14th European Microscopy Congress, EUREM 2008; 2008

Tin oxide is known to exist in two forms, SnO and SnO2, but the existence of at leastone additional form with a composition in between the two, i.e. SnO2-x, is now generallyaccepted [1-3]. To date, no ab initio structural information has been extracted from thisintermediate compound. A major reason for this is that the compound is usually foundin the form of small (<1 μm), often defective, crystals which are attached to the metallictin particles from which they grow (Figure 1a). Therefore, crystals large enough and ofsufficient quality for single-crystal X-ray diffraction have not been available.Electron diffraction allows the acquisition of single-crystal diffraction patterns fromsuch small particles due to the ability of the electron microscope to form small focussedprobes. The technique of precession electron diffraction [4] was applied to theintermediate tin oxide compound, and a typical diffraction pattern is shown in Figure1b. The diffraction patterns from our specimens of the intermediate structure areconsistent with the cell parameters given by Lawson [2] for Sn3O4.Phasing the intensity data using the Tangent Formula [5] produced electrostaticpotential maps which we interpret as showing the tin atoms as strong peaks, andpossibly some semblance of the oxygen positions in the weaker background ripple. Amap for the [010] projection according to Seko’s choice of unit cell [3] is shown inFigure 2. Although dynamical effects are likely to be influencing the results quitestrongly, the positions of the tin atoms in these maps are in agreement with models forthe structure which are based on an oxygen-deficient supercell of the rutile structure ofSnO2, with small displacements of the tin atoms. In both our reconstructed map andSeko’s proposed structure for Sn3O4, the tin atoms do not lie on a straight line as theywould for an exact supercell of the rutile structure in this projection. A least-squaresrefinement to verify these subtle differences is the subject of ongoing work.