Charge Disproportionation in Tetragonal La₂MoO₅ , a Small Band Gap Semiconductor Influenced by Direct Mo–Mo Bonding

Journal Article · Wed Dec 31 00:00:00 EST 2014 · Journal of the American Chemical Society

DOI:https://doi.org/10.1021/ja511218g· OSTI ID:1261409

Colabello, Diane M. ^[1]; Camino, Fernando E. ^[2]; Huq, Ashfia ^[3]; Hybertsen, Mark ^[2]; Khalifah, Peter G. ^[4]

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1. Stony Brook Univ., NY (United States). Dept. of Chemistry
2. Brookhaven National Lab. (BNL), Upton, NY (United States). Center for Functional Nanomaterials (CFN)
3. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Spallation Neutron Source (SNS)
4. Stony Brook Univ., NY (United States). Dept. of Chemistry; Brookhaven National Lab. (BNL), Upton, NY (United States). Dept. of Chemistry

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) Å and c = 6.0568(2) Å 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 Mo₂O₁₀dimers is 2.684(8) Å, while there are two types of Mo₂O₈ dimers with Mo-Mo bonds lengths of 2.22(2) and 2.28(2) Å. 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+ (d(1)), 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. Lastly, we show that the d-orbital splitting associated with the Mo₂O₈ and Mo₂O₁₀ dimeric units can be rationalized using simple molecular orbital bonding concepts.

• Research Organization: Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Spallation Neutron Source (SNS)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: AC05-00OR22725; DMR-095564; AC02-06CH11357; AC02-98CH10886.
• OSTI ID: 1261409
• Journal Information: Journal of the American Chemical Society, Vol. 137, Issue 3; ISSN 0002-7863
• Publisher: American Chemical Society (ACS)
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
Metal-metal bonding
small band gap oxide semiconductor
charge disproportionation
infrared absorption
structure solution from powder diffraction data
1D chains
edge-sharing bioctahedra
square prisms