Structure Determination by HRTEM and Electron Diffraction
A particular strength of TEM is that structural investigations can be performed on crystallites with sizes in the μm or even sub-μm range only. An example will be discussed in the following.
XRD investigations of a sample with the starting composition ZrNb6W10O47 showed the presence of a pure phase crystallizing in the structure of Nb8W9O47. The TEM investigation however revealed that a few crystallites of a novel structure appeared as well.
Determination of lattice parameters
Electron diffraction investigation showed a monoclinic unit cell. Selected area electron diffraction (SAED) along [010] (Fig. 1a) indicate a superstructure of the ReO3 type (bright spots) with a = 19.0 Å, c = 13.8 Å, β = 93.5°. The length of the perpendicular axis was determined from the distance r of the weak first order Laue zone (FOLZ) ring from the origin in a convergent beam electron diffraction (CBED) pattern (Fig. 1b): c = 3.9 Å. This distance corresponds to the length of the MO6 octahedra or other polyhedra that are stacked along the c-axis. This finding confirms the presence of a quasi 2-dimensional structure that can directly be derived from HRTEM images.
Derivation of structural model from a HRTEM image
The HRTEM image recorded along [010] (Fig. 2) show a regular array of dark and bright patches. Since this image was recorded close to Scherzer defocus, the dark patches correspond to the positions of the polyhedra centers occupied by metal atoms. A structural model was derived based on this knowledge (Fig. 3). It is an intergrowth between the tetragonal tungsten bronze (TTB) and the ReO3 type with alternating parallel slabs. A simulated image calculated with the parameters of this model agrees with the experimental image (inset in Fig. 2).
Investigation of higher order Laue zones
Additional tilting experiments were performed to reveal the reciprocal lattice more completely (Fig. 4). The first order Laue zone (FOLZ) contains additional reflections at the positions h/2,k,l/2 that indicate the presence of a diagonal glide plane (reflection condition: h + l = 2n for h0l). The structural reason for the larger unit cell might be an ordering of the different cations.
Investigation of the Real Structure
Most crystals observed withthis new structure appear to be perfectly ordered. In the crystal area shown in Fig. 5, a fault boundary is present that displaces the unit cells with respect to each other along [100]. The interpretation of this area shows the incorporation of a row of additional corner-sharing octahedra.
A Novel Intergrowth Structure Between ReO3-Type and Tetragonal Tungsten Bronze-Type in the Zr/Nb/W/O System
F. Krumeich, G. Lietke, W. Mader, Acta Crystallogr. B52 (1996) 917 external page DOI