High-resolution electron microscopy of condensed clusters in In₅Mo₁₈O₂₈

The recently synthesized compound In₅Mo₁₈O₂₈ is a representative of the reduced ternary molybdenum oxides in which oligomeric clusters built from molybdenum octahedra are arranged in layers of composition A_mMo_4n+2O_6n+4, where n denotes the number of fused, trans edge-sharing molybdenum octahedra in the cluster. Known representatives of this class include In₃Mo₁₁O₁₇, In₁₁Mo₄₀O₆₂ (= In₅Mo₁₈O₂₈ x In₆Mo₂₂O₃₄), and Sn₂Mo₁₀O₁₆.

In the case of the indiumoxomolybdates, m = n + 1. According to the structure principles sketched above, In₅Mo₁₈O₂₈ consists of 4-membered oligomeric clusters. Since the layer composition is hardly differing when n is varied around 4, intergrowth layers of longer or shorter oligomeric clusters are expected to occur. High-resolution electron microscopy (HREM) is an excellent means for studying such intergrowth structures as well as other structural defects which may not be realized with X-ray methods but may influence the results of structure refinement from X-ray data.

Small fragments of a well-shaped crystal have been investigated by HREM using a Philips CM30/ST electron microscope operating at 300 kV (point resolution 0.19 nm). At appropriate orientations of the crystal fragments, HREM images reveal monoclinic as well as orthorhombic domains of various sizes. The monoclinic domains show a strong tendency for polysynthetic twinning through the (001) plane ( Figure 1). Although macroscopic twinning may be assumed, it is not observable by HREM of crystal fragments. In both the monoclinic and orthorhombic domains, monolayers and multilayers of longer oligomeric clusters have been found (Figure 2). Since these intergrowth structures can themselves exhibit subdomains showing different stacking sequences, a large variety of structural features arises. The monoclinic modification of In₅Mo₁₈O₂₈ can be indexed in the space group P2₁/a (a = 0.989 nm, b = 0.951 nm, c = 1.323 nm, beta = 79.02^o) and the orthorhombic one in Pnam (a = 0.990 nm, b = 0.952 nm, c = 2.597 nm; rounded-off values from X-ray data). Using these nonstandard settings, prominent zone axes have equivalent indices for both modifications. The two kinds of domains differ primarily in the stacking sequences of their cluster layers (Figures 4 and 3), that is, either ABABA or ABCDA. (In the course of monoclinic stacking the first layer is almost exactly reproduced at every fifth layer.) Stimulated by the HREM findings, it was possible to identify In₅Mo₁₈O₂₈ single crystals of both the monoclinic and orthorhombic polytypic modifications, and full structure analyses have been carried out using X-ray methods.

Figure 1: In the [110] orientation moniclinic and orthorhombic domains can be easily recognized from their stacking sequence. Monclinic domains show polysynthetic twinning through the (001) plane.

Figure 2: In HREM micrographs of the [010] orientation, orthorhombic and monoclinic domains cannot be distinguished. This is illustrated by the projected (a) orthorhombic and (b) monoclinic crystal structures, where molybdenum is marked as red and indium as green. Intergrowth layers with 5-membered clusters are indicated by arrowheads.

The simulation of HREM images according to the multislice formalism (EMS programme-package, P. Stadelmann, Lausanne) was performed using the lattice parameters obtained from the X-ray data together with the generated sets of atomic positions for both modifications. The latter were generated under the assumption that the oligomeric clusters are built from ideal molybdenum octahedra and that the indium polycations are linearly arranged with constant In-In distances. The oxygen atoms have been ignored. Although the simulated images (specimen thickness: 100 nm; defocus value: -100 nm) - presented as inserts in the experimental micrographs - reflect the overall features, they are not exact in their details. When using the data from the full X-ray structure analyses, the image simulations, which are very sensitive to the position of the indium polycations, completely failed. This disrcepancy may be attributed either to the X-ray data not being completely correct with regard to the indium positions or to the influence of beam damage, the latter affecting mainly the indium polycations but not the lengths of oligomeric clusters nor their stacking sequences. Detailed studies will be carried out to enlighten this problem.

Figure 3: A large monoclinic domain in the [110] orientation with stacking sequence ABCDA. An intergrowth layer with 5-membered clusters is indicated by an arrowhead.

Figure 4: A large orthorhombic domain in the [110] orientation with stacking sequence ABABA. Intergrowth layers with 5-membered clusters are indicated by arrowheads. Here, they are repeating every other layer, thus forming an intergrowth structure with chemical composition In₁₁Mo₄₀O₆₂.

(R. Ramlau, A. Simon and E. Fais)

From the yearbook of the institute ("Wissenschaftlicher Tätigkeitsbericht") 1993

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