A polycrystalline sample of LuFe${}_2$O${}_4$ has been investigated by means of powder synchrotron x-ray and neutron diffraction and transmission electron microscopy (TEM), along with Mössbauer spectroscopy and transport and magnetic properties. A monoclinic distortion is unambiguously evidenced, and the crystal structure is refined in the monoclinic $C$2/$m$ space group [$a_M$ $=$ 5.9563(1) Å, $b_M$ $=$ 3.4372(1) Å, $c_M$ $=$ 8.6431(1) Å, β $=$ 103.24(1)°]. Along with the previously reported modulations distinctive of the charge-ordering (CO) of the iron species, a new type of incommensurate order is observed, characterized by a vector $\vec{q}$${}_1$ $=$ ${\alpha }_1$${\vec{\mathrm{a}}}_M^{*}$ + ${\gamma }_1$${\vec{c}}_M^{*}$ (with ${\alpha }_1$ ≅ 0.55, ${\gamma }_1$ ≅ 0.13). In situ heating TEM observations from 300 to 773 K confirm that the satellites associated with $\vec{q}$${}_1$ vanish completely, only at a temperature significantly higher than the CO temperature. This incommensurate modulation has a displacive character and corresponds primarily to a transverse displacive modulation wave of the Lu cations position, as revealed by the high resolution, high angle annular dark field scanning TEM images and in agreement with synchrotron data refinements. Analyses of vacuum-annealed samples converge toward the hypothesis of a new ordering mechanism, associated with a tiny oxygen deviation from the O${}_4$ stoichiometry.

• Received 27 October 2011

DOI:https://doi.org/10.1103/PhysRevB.85.064102