The recently synthesized compound K0.19Ba3.81Mo22O34 is a representative of the ternary and quaternary reduced molybdenum oxides in which clusters built from molybdenum octahedra are arranged in layers of composition AxByMo4n+2O6n+4. The number of trans-edgesharing molybdenum octahedra in the cluster is represented by n. The general concept of cluster condensation is formulated and discussed elsewhere. For K0.19Ba3.81Mo22O34, n is equal to 5. A ternary representative, likewise with n = 5, is In6Mo22O34.By x-ray structure analysis, K0.19Ba3.81Mo22O34 proved to crystallize in space group P21/a with parameters a = 0.9908(2) nm, b = 0.9353(2) nm, c = 1.5951(3) nm, and β = 98.78(2)°. The potassium atoms were found to reside on the same sites as the barium atoms with an almost statistical distribution.We studied K0.19Ba3.81Mo22O34 by high-resolution electron microscopy (HREM). Small fragments of a crystal were investigated using a Philips CM30/ST microscope operating at 300 kV (point resolution 0.19 nm). At appropriate orientations of the crystal fragments, HREM images reveal domains of two ordered polytypes: a monoclinic and an orthorhombic one (Figs. 1 and 2).
Structure analysis of amorphous materials by high resolution electron microscopy.