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.
|
Fig.
1: SAED (a) and CBED (b) pattern along [010].
|
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 (program EMS)
agrees with the experimental image (inset in Fig. 2). |
Fig.
2: HRTEM image along [010]. Slabs of TTB units (outlined white)
and of ReO3 type occur alternately. The inset (lower
right side) shows a image simulated with the parameters
of the derived structural model.
Fig.
3: Structural model of ZrnNb8-2nW12+nO56
projected onto the ac plane. TTB-type subcell are shaded, pentagonal
tunnels occupied with metal-oxygen strings marked by a black
circle.
|
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. |
Fig.
4: SAED pattern of a crystal tilted away from the direction
[010] by a few degree. The FOLZ (enlarged in the inset) shows more reflections
than
the
ZOLZ. |
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. |
 
Fig.
5: HRTEM image (left side) of a planar defect and a structural
model for the defect area. The additional octahedra are white.
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 DOI
|
