HIGH-RESOLUTION ELECTRON MICROSCOPY INVESTIGATIONS ON THE REAL STRUCTURE OF REDUCED BARIUM OXOMOLYBDATES: I. POLYTYPISM AND CHEMICAL INTERGROWTH IN Ba3Mo18O28

摘要

Ba3Mo18O28, a reduced ternary molybdenum oxide in the series Mn+-Mo4n+2O6 n+4 with n=4, is a cluster compound containing Mo4n+2O8n+10 clusters with n=4 trans edge-sharing Mo octahedra. Crystals of the compound were investigated by HREM, selected-area electron diffraction (SAD), and electron probe microanalysis. Besides the known monoclinic 1M modification of Ba3Mo18O28 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 Ba3Mo18O28 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 Mo4n+2O8n+10-cluster layers with n=2, 3, and 5; and (v) chemical intergrowth of the phase BaMo6O10 (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. 21 refs.