Synthesis and characterization of reduced ternary alkali metal molybdenum oxides

Aleandri, Lorraine
Major Professor
Robert E. McCarley
Committee Member
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A series of novel layered reduced Na and Li Mo-oxides is reported. The two dimensional framework is quite unique among the structure types previously observed for reduced molybdenum oxides;Synthetic exploration of oxides related to Na[subscript]2Mo[subscript]3O[subscript]6 (Hubert, P. H. C. R. Acad. Sc. Paris 1966, 262, 1189) led to the isolation of a series of monoclinic layered Na-Mo-oxides, Na[subscript]0.41MoO[subscript]2, Na[subscript]0.51MoO[subscript]2, and Na[subscript]0.66MoO[subscript]2. The basic framework of these compounds consists of oxide layers alternating with sheets of Na and Mo atoms. Within the Mo layers, the metal atoms shift from ideal octahedral sites toward neighboring Mo atoms to form extended chains of fused rhomboidal clusters;Structurally, from Na[subscript]0.41MoO[subscript]2 to Na[subscript]0.66MoO[subscript]2, the Na ions shift from octahedral to trigonal prismatic sites; and oxygen packing changes from nonsequential to ABBCCA;The stacking of layers toward cubic close packing, ABCABC, was realized in the analogous Li-Mo-oxide system. A material with ccp oxygen layers, LiMoO[subscript]2, was isolated as the product from the high temperature reaction between Li[subscript]2MoO[subscript]4 and Mo metal. Neutron powder diffraction Rietveld analysis shows that LiMoO[subscript]2 is isomorphous with [alpha]-NaFeO[subscript]2 and reveals for Mo-Mo bonded compounds an unprecedented arrangement, an infinite hexagonal sheet. DOS and COOP curves from extended Huckel band calculations indicate that all Mo-Mo bonding states are filled within the O-Mo-O layers;The results of the extended Huckel calculations suggest that as the Mo bonding states are emptied, via oxidation by delithiation, the Mo sheet should rearrange to form Mo ribbons or some other type of clustering favored by the lower electron to Mo ratio. Indeed, such a structural transition of the hexagonal structure, in partially delithiated Li[subscript] 1-xMoO[subscript]2, to a monoclinic framework with infinite Mo bonded zig-zag chains has been observed;Synthetic efforts in the heavier alkali metal ternary system, K-Mo-oxide, have led to a novel compound containing Mo octahedral clusters. Partial characterization through single crystal diffraction data suggests the structure contains discrete segments of chains comprised of edged shared Mo[subscript]6 units. ftn*DOE Report IS-T-1330. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.