posted on 2017-10-26, 16:07authored byDiane M. Colabello, Elizabeth M. Sobalvarro, John P. Sheckelton, Joerg C. Neuefeind, Tyrel M. McQueen, Peter G. Khalifah
Among oxide compounds
with direct metal–metal bonding, the Y5Mo2O12 (A5B2O12) structural family of compounds has a particularly
intriguing low-dimensional structure due to the presence of bioctahedral B2O10 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 C2/m structure has
been previously proposed for Ln5Mo2O12 (Ln = 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 C2/m monoclinic
unit cell represents a superstructure relative to a previously unrecognized
orthorhombic Immm 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 c* direction. Finally,
it is shown that oxygen vacancies can occur when Ln = La, producing an oxygen deficient stoichiometry of La5Mo2O11.55 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.