Searched For and Found: Analogies between Reduced Oxomolybdates and Cluster Compounds of Rare Earth Metals

Abstract

The concept of cluster condensation [1] has proved valuable as an ordering scheme for structures of metal-rich compounds, and it has frequently opened new areas.[2] Reduced oxomolybdates illustrate the value of the concept particularly well. In their structures discrete cluster anions [Mo 4n+2 O 6n+4 ] (n+3)À occur which are formed from n {Mo 6 O 12 } clusters linked through opposite edges of the Mo 6 octahedra. So far, monomeric and oligomeric entities with n = 1-6[3] are known as well as the polymeric Mo 4 O 6 chain with n = 1. [4] Oligomeric clusters occur in crystal structures which are composed of a single type of cluster only, as well as of different types of clusters in ordered [5] or disordered arrays.[6]With rare earth metals (Ln), compounds containing discrete [Ln 6 ZX 12 ] [7] and [Ln 10 (Z) 2 X 18 ] [8] clusters (X = halide) have been known for a long time. These compounds correspond to the initial members (n = 1 and 2, respectively) of the molybdenum cluster series. Owing to the electron deficiency of the rare earth metals, additional endohedral atoms or groups of atoms (Z) are incorporated in the cluster centers which (over)compensate for the lack of metal-metal bonding by strong LnÀZ bonding. [2a, 9] The polymeric chain Ln 4 ZX 6 was also found repeatedly.[10] Recently, we characterized Ln 14 (C 2 ) 3 I 20 (Ln = La, Ce) as the first representatives for n = 3.[11] The details of preparation of the ordered phases are quite critical. Minute deviations from the optimized reaction temperatures [12] lead to heterogeneous products which exhibit characteristic disorder phenomena in the crystals. The disorder is seen in single-crystal X-ray analyses, and its nature becomes clear from investigations of the real structure in the electron microscope.The heterogeneous character of a sample of La 14 (C 2 ) 3 I 20 prepared at 900 8C shows up in the X-ray as well as the electron diffraction pattern of a selected single crystal. The observed reflections in the latter are shown and interpreted (Figure 1) in terms of coherently intergrown domains of La 14 (C 2 ) 3 I 20 and La 10 (C 2 ) 2 I 18 clusters that are arranged layerwise. Real structures of this type [13] have not been detected before in cluster phases of rare earth metals; however, they are reminiscent of a similar intergrowth with oxomolybdates, for which larger clusters (n > 3) also frequently occur. [6] A systematic search for such larger clusters with rare earth metals finally was successful through concentration on crystals which attracted attention owing to diffuse streaks in their electron diffraction patterns. In Figure 2 simulated contrasts of a model structure are shown which is composed of cluster layers with n = 2, 3, and 4. There is a clear correlation between the respective length of the cluster and the corresponding contrasts at varying focus values. The different unit cells for each layer are evident. In the Scherzer focus [14] the distances between the elongated light spots (parallelogram corners) are a measure of the cluster length. ...

γ‐Brasses with Spontaneous Magnetization: Atom Site Preferences and Magnetism in the Fe‐Zn and Fe‐Pd‐Zn Phase Spaces

“…[7] 3 (T CO ) 6 over the reported phase space of γ-brass "Fe 4-x Zn 9+x ", i.e., e/a = 10.72-11.04, shows that first the IT and then the OH sites are next in line for substitution by Fe for Zn atoms (see Figure S1, Supporting Information), a result which agrees well with our experimental findings. Tightbinding electronic structure calculations on "Fe 4 Zn 9 " with Fe atoms in the OT and IT sites were, therefore, carried out to provide some additional insights into this site preference problem.…”

“…Prof. A. Simon and his group successfully accomplished these goals for the diverse compound classes of alkali metal suboxides, [1] reduced rare-earth halides, [2] and oxomolybdates. [3] Intermetallic compounds also offer an excellent medium to investigate such relationships because their structures are sufficiently robust to allow significant variations in chemical composition, and, thus, valence electron concentration, i.e., valence electron/atom (e/a) ratios.…”

From an Impurity to the Pure Compound – an Electron Microscopy Study

“…High resolution transmission electron microscopy; Rare earth metals; Halogenides; Clusters compounds enables full characterization [3]. Impurities of the same Ln 6 X 12 -type can not be detected by powder XRD, due to the relatively low intensity indicating the distinct ordering of atoms and voids.…”

Meldungen: Biologie in unserer Zeit 5/2016

“…The elucidation, understanding, and development of structure‐bonding‐property relationships in solids are fundamental goals of chemists, physicists, and materials scientists. Prof. A. Simon and his group successfully accomplished these goals for the diverse compound classes of alkali metal suboxides,1 reduced rare‐earth halides,2 and oxomolybdates 3. Intermetallic compounds also offer an excellent medium to investigate such relationships because their structures are sufficiently robust to allow significant variations in chemical composition, and, thus, valence electron concentration, i.e., valence electron/atom ( e / a ) ratios 4.…”