Synthesis and structure of ternary molybdenum oxides MMo₈O₁₀ (M = Li or Zn) having orthogonal nonintersecting octahedral cluster chains

The new compounds LiMo₈O₁₀ (I) and ZnMo₈O₁₀ (II) have been prepared via reactions in sealed Motubes at 1370-1450°C between Li₂MoO₄, MoO₃, and Mo (3:16:29 mole ratio), and ZnO, MoO₃, and Mo (1:3:5 mole ratio), respectively. The isomorphous compounds are tetragonal, space group I4₁md, Z = 4, with lattice constants a = 5.8515(6) Å, c = 24.783(3)Å for I, and a = 5.8961(4) Å, c = 24.840(3)Å for II, as determined from Guinier X-ray powder patterns. Single crystal structure determination shows that these compounds are constructed from infinite chains consisting of edge-shared octahedral cluster units. The structure can be viewed as a stacking along the c axis of layers consisting of parallel chains, with the chains in successive layers oriented alternately along the a and b axes. Intra- and interlayer crosslinking of the chains by MoOMo bonds is represented in the connective formula [(Mo₂Mo _{4 2}O _{6 2}O _{4 3})O _{2 3}] for the octahedral cluster subunits. MoMo bonding within the subunits is strong, but very irregular compared to analogous compounds containing such chains. Subunit MoMo bond order sums indicate that the bond distortions are effective in accommodating a higher electron count in metal-metal bonding states. It is shown that the structures are related to the spinel and rock salt structures through an ordered-defect arrangement of the Mo and O atoms.