Charge disproportionation in tetragonal la₂MoO₅, a small band gap semiconductor influenced by direct Mo-Mo bonding

The structure of the novel compound La₂MoO₅ has been solved from powder X-ray and neutron diffraction data and belongs to the tetragonal space group P4/m (no. 83) with a = 12.6847(3) A˚ and c = 6.0568(2) A˚ and with Z = 8. It consists of equal proportions of bioctahedral (Mo₂O₁₀) and square prismatic (Mo₂O₈) dimers, both of which contain direct Mo-Mo bonds and are arranged in 1D chains. The Mo-Mo bond length in the Mo₂O₁₀ dimers is 2.684(8) A˚, while there are two types of Mo₂O₈ dimers with Mo-Mo bonds lengths of 2.22(2) and 2.28(2) A˚. Although the average Mo oxidation state in La₂MoO₅ is 4+, the very different Mo-Mo distances reflect the fact that the Mo₂O₁₀ dimers contain only Mo⁵⁺ (d¹), while the prismatic Mo₂O₈ dimers only contain Mo³⁺ (d³), a result directly confirmed by density function theory calculations. This is due to the complete disproportionation of Mo⁴⁺, a phenomenon which has not previously been observed in solid-state compounds. La₂MoO₅ is diamagnetic, behavior which is not expected for a nonmetallic transition-metal oxide whose cation sites have an odd number of d-electrons. The resistivity displays the Arrhenius-type activated behavior expected for a semiconductor with a band gap of 0.5 eV, exhibiting an unusually small transport gap relative to other diamagnetic oxides. Diffuse reflectance studies indicate that La₂MoO₅ is a rare example of a stable oxide semiconductor with strong infrared absorbance. It is shown that the d-orbital splitting associated with the Mo₂O₈ and Mo₂O₁₀ dimeric units can be rationalized using simple molecular orbital bonding concepts. (Graph Presented).