High-Resolution Electron Microscopy Investigations on the Real Structure of Reduced Barium Oxomolybdates: I. Polytypism and Chemical Intergrowth in Ba3Mo18O28

Ba3Mo18O28, a reduced ternary molybdenum oxide in the seriesMn±DELTAMo4n+2O6n+4withn=4, is a cluster compound containing Mo4n+2O8n+10clusters withn=4transedge-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 1Mmodification of Ba3Mo18O28an orthorhombic 2Opolytype was discovered, probably crystallizing in space groupPnam. Lattice parameters were estimated, and a rough structure model was generated. The real structure of Ba3Mo18O28is characterized by: (i) intergrowth of both ordered polytypes, 1Mand 2O; (ii) stacking disorder; (iii) polysynthetic microtwinning of the 1Mpolytype; (iv) disordered chemical intergrowth of Mo4n+2O8n+10-cluster layers withn=2, 3, and 5; and (v) chemical intergrowth of the phase BaMo6O10(formallyn=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.