MS-5-P-2639 The modulated structure of Fe₂O₃(ZnO)₇ studied by HAADF-STEM

The existence of phases ABO₃(ZnO)_m in the system ZnO-Fe₂O₃ is known up to a composition of Fe₂O₃(ZnO)₁₂ at 1350 °C [1]. These phases share the structural characteristics of the better known compounds InFeO₃(ZnO)_m [2] which exist in the range m ≥ 1. ABO₃(ZnO)_m compounds consist of wurtzite slabs separated by layers of AO₆ octahedrons fully occupied by A³⁺ cations such as In³⁺. Within the wurtzite layers B³⁺ ions occupy tetrahedral positions. The crystal structures of compounds with low ZnO content were verified by single crystal X-ray diffraction [3], however Fe₂O₃(ZnO)_m compounds show an obvious super-structure in diffraction.
We obtained Fe₂O₃(ZnO)₇ by solid state reaction of ZnO and ZnFe₂O₄ powders at 1600 °C. The reaction product was quenched in water to retain the metastable high temperature phase. XRD proves the layer structure of Fe₂O₃(ZnO)₇, however, EDS measurements show a metal content of 30% Fe and 70% Zn vs. nominal composition of 22% Fe and 78% Zn as derived from the formula. Hence, the correct formula should be written as Fe₂O₃(Zn_1-xFe_xO)₇ with x ≈ 0.1. The Compound crystallizes in the monoclinic system with lattice constants a = 5.566(8) Å, b = 3.234(5) Å, c = 24.00(3) Å and β = 95.35° with possible spacegroups C2, Cm or C2/m. EDS and electron diffraction was conducted on a Philips CM30 with a Noran System 7 EDS system and a twin lens providing large tilt angles of up to 45°. HAADF was conducted on a probe C_s-corrected JEOL ARM200F equipped with a cold FEG.
Figure 1 shows an SAED pattern from Fe₂O₃(ZnO)₇ in [100] orientation where additional superstructure reflections 0klx are clearly visible as satellites to the main reflections. The modulation vector q is about 34 Å measured from electron diffraction patterns. The periodicity of the (00l) reflections corresponds to a 24 Å lattice plane spacing.
A HAADF micrograph in the same orientation is shown in figure 2. Multiple layers of Zn²⁺ cations show a wave like modulation along [010] direction. Also Fe³⁺ octahedral layers show considerably less detail in atomic resolution which is attributed to significant displacements of Fe³⁺ cations in FeO₆-octahedrons. The misfit of the rather small Fe³⁺ cation in octahedral coordination is considered to be the origin of the structure modulation.
These materials offer challenging crystallographic and analytical questions, such as Fe²⁺ distribution in the wurtzite layers, which can be tackled by advanced electron microscopy methods, i.e. quantitative EDS and EELS analyses with high spatial resolution in TEM/STEM [4].

[1] N. Kimizuka et al., J. Solid State Chem. 103 (1993) p. 394
[2] N. Kimizuka et al., J. Solid State Chem. 74 (1988) p. 98
[3] I. Keller et al., Z. Anorg. Allg. Chem. 635 (2009) p. 2065
[4] H. Schmid et al., Ultramicroscopy. 127 (2013) 76-84