Type of presentation: Poster

MS-5-P-2837 Real structure of (Sb1/3Zn2/3)GaO3(ZnO)3 revealed by ED and HAADF-STEM

Garling J.1, Hühne S. M.1, Schmid H.1 2, Assenmacher W.1, Mader W.1
1Institut für Anorganische Chemie, Universität Bonn, Römerstr. 164, 53117 Bonn, Germany, 2Now with JEOL (Germany) GmbH, Oskar-von-Miller-Straße 1, 85386 Eching, Germany
assenmacher@uni-bonn.de

A high temperature (1350 °C) route in sealed Pt tubes was used for the synthesis of pure powders of (Sb1/3Zn2/3)GaO3(ZnO)3. Single-crystal X-Ray diffraction of crystals grown from K2MoO4 flux revealed the space group R-3m (a = 3.2366(3) Å, c = 41.793(8) Å) and the structural characteristics as known from other members with general formula ABO3(ZnO)m [1]. (Sb1/3Zn2/3)GaO3(ZnO)3 consists of an alternate stacking of [(Sb1/3Zn2/3)O2]- and [(GaZn3)O4]+ units corresponding to CdI2 and wurtzite structure type motifs, respectively. Inversions of the ZnO4 tetrahedra occur at the octahedral layers and halfway in the wurtzite type. There is no indication for a cation-ordering on octahedral sites from X-ray data, but the Sb1/3Zn2/3 occupation of the octahedral is confirmed by refinement of the occupation factor.
Surprisingly, electron diffraction and HRTEM show an ordering of the cations within the [(Sb1/3Zn2/3)O2]- octahedral layer by presence of superstructure reflexions (Fig 2) and contrast modulations of the cation columns [2]. This cation ordering can be described by a model deduced earlier for single defect layers in ZnO doped with Sb [3]. The description of the ordered structure succeeds in space group P3112 (a = 5.60 Å, c = 42.02 Å). The observed streaks parallel to 000l (Fig 2c) can be described by a statistic displacement of well ordered octahedral layers among themselves.
To confirm the deduced structure model HAADF-STEM imaging was carried out on a JEOL JEM-ARM 200CF . The present setup provides sub-Å resolution capability in HAADF STEM imaging, whereas BF and particularly ABF with increased sensitivity for light elements enable the elucidation of true atomic structures [4]. The QSTEM software [5] was used for simulation of HAADF images (Fig 3).
HAADF imaging reveals the periodic order of Sb and Zn in the octahedral layers. The intensity ratio of the Zn and Sb columns can be measured to 0.42. This corresponds to an exponent of 1.7 (Int(Zn)/Int(Sb)=301.7/511.7= 0.41), which is in agreement with theoretical predictions [6,7]. In some areas the stacking of the octahedral layers corresponds well with the structure model in P3112. However, the stacking of larger regions does not correlate to the ABC stacking, i.e. stacking disorder occurs. This leads to streaks in SAED patterns (Fig 2).

References
[1] N. Kimizuka, T. Mohri, M. Nakamura, J. Solid State Chem. 81 (1989), p. 70-77.
[2] J. Garling, Diploma Thesis, Uni Bonn (2012).
[3] A. Recnik, N. Daneu, T. Walther, W. Mader, J. Am. Ceram. Soc. 84 (2001), p. 2657-68.
[4] H. Schmid, E. Okunishi, W. Mader, Ultramicroscopy 127 (2013), p. 76-84.
[5] C. Koch, PhD Thesis, Arizona State University (2002).
[6] R.F. Klie, Y. Zhu, Micron 36 (2005), p. 219.
[7] P.D. Nellist, S. J. Pennycook, Ultramicroscopy 78 (1999), p. 111.

We thank E. Arzt for continuous support through INM and G. Schnakenburg for single crystal X-Ray data collection.

Fig. 1: Crystal structure of (Sb1/3Zn2/3)GaO3(ZnO)3 in P3112, drawn as ball and stick model and as polyhedron representation, respectively.

Fig. 2: Electron diffraction patterns of (Sb1/3Zn2/3)GaO3(ZnO)3 in principal orientations and calculated patterns using the structure model in P3112. Diffuse scattering in (c) is caused by stacking disorder of the octahedral planes.

Fig. 3: HAADF image (a), simulated HAADF image (b) and structure model (c) of (Sb1/3Zn2/3)GaO3(ZnO)3 in a-axis orientation.