High-Resolution Electron Microscopy Investigations on the Real Structure of Reduced Barium Oxomolybdates: I. Polytypism and Chemical Intergrowth in Ba3Mo18O28
✍ Scribed by R. Ramlau; R.E. McCarley; A. Simon
- Publisher
- Elsevier Science
- Year
- 1999
- Tongue
- English
- Weight
- 1009 KB
- Volume
- 142
- Category
- Article
- ISSN
- 0022-4596
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✦ Synopsis
Ba 3 Mo 18 O 28 , a reduced ternary molybdenum oxide in the series M n$ Mo 4n؉2 O 6n؉4 with n ؍ 4, is a cluster compound containing Mo 4n؉2 O 8n؉10 clusters with n ؍ 4 trans edge-sharing molybdenum octahedra. Crystals of the compound were investigated by high-resolution electron microscopy (HREM), selected-area electron diffraction (SAD), and electron probe microanalysis. Besides the known monoclinic 1M modification of Ba 3 Mo 18 O 28 an orthorhombic 2O polytype was discovered, probably crystallizing in space group Pnam. Lattice parameters were estimated, and a rough structure model was generated. The real structure of Ba 3 Mo 18 O 28 is characterized by: (i) intergrowth of both ordered polytypes, 1M and 2O; (ii) stacking disorder; (iii) polysynthetic microtwinning of the 1M polytype; (iv) disordered chemical intergrowth of Mo 4n؉2 O 8n؉10 -cluster layers with n ؍ 2, 3, and 5; and (v) chemical intergrowth of the phase BaMo 6 O 10 (formally n ؍ 1). In connection with particular defects, an irregular condensation of clusters was observed. Discrepancies between experimental HREM and SAD results and related computer simulations are discussed, as well as the reliability of X-ray structure results from defective single crystals.