Observation of Vacancies, Faults, and Superstructures in <i>Ln</i>₅Mo₂O₁₂ (<i>Ln</i> = La, Y, and Lu) Compounds with Direct Mo–Mo Bonding

Among oxide compounds with direct metal–metal bonding, the Y₅Mo₂O₁₂ (<i>A</i>₅<i>B</i>₂O₁₂) structural family of compounds has a particularly intriguing low-dimensional structure due to the presence of bioctahedral <i>B</i>₂O₁₀ dimers arranged in one-dimensional edge-sharing chains along the direction of the metal–metal bonds. Furthermore, these compounds can have a local magnetic moment due to the noninteger oxidation state (+4.5) of the transition metal, in contrast to the conspicuous lack of a local moment that is commonly observed when oxide compounds with direct metal–metal bonding have integer oxidation states resulting from the lifting of orbital degeneracy typically induced by the metal–metal bonding. Although a monoclinic <i>C</i>2/<i>m</i> structure has been previously proposed for <i>Ln</i>₅Mo₂O₁₂ (<i>Ln</i> = La–Lu and Y) members of this family based on prior single crystal diffraction data, it is found that this structural model misses many important structural features. On the basis of synchrotron powder diffraction data, it is shown that the <i>C</i>2/<i>m</i> monoclinic unit cell represents a superstructure relative to a previously unrecognized orthorhombic <i>Immm</i> subcell and that the superstructure derives from the ordering of interchangeable Mo₂O₁₀ and LaO₆ building blocks. The superstructure for this reason is typically highly faulted, as evidenced by the increased breadth of superstructure diffraction peaks associated with a coherence length of 1–2 nm in the <i>c</i>* direction. Finally, it is shown that oxygen vacancies can occur when <i>Ln</i> = La, producing an oxygen deficient stoichiometry of La₅Mo₂O_11.55 and an approximately 10-fold reduction in the number of unpaired electrons due to the reduction of the average Mo valence from +4.5 to +4.05, a result confirmed by magnetic susceptibility measurements. This represents the first observation of oxygen vacancies in this family of compounds and provides an important means of continuously tuning the magnetic interactions within the one-dimensional octahedral chains of this system