Charge Order in ${\mathrm{LuFe}}_2{\mathbf{O}}_4$: An Unlikely Route to Ferroelectricity

We present the refinement of the crystal structure of charge-ordered ${\mathrm{LuFe}}_2{\mathrm{O}}_4$, based on single-crystal x-ray diffraction data. The arrangement of the different Fe-valence states, determined with bond-valence-sum analysis, corresponds to a stacking of charged Fe bilayers, in contrast with the polar bilayers previously suggested. This arrangement is supported by an analysis of x-ray magnetic circular dichroism spectra, which also evidences a strong charge-spin coupling. The nonpolar bilayers are inconsistent with charge order based ferroelectricity.

Figure 1

(a) Monoclinic crystal structure $C2/m$ of ${\mathrm{LuFe}}_2{\mathrm{O}}_4$ ($a=5.95\AA$, $b=10.30\AA$, $c=16.96\AA$, $\beta =96.72°$). The refined data were measured at 210 K. The ferrimagnetic high-field spin order and ${\mathrm{Fe}}^{3+/2+}$ charge order is represented by arrows and different colors, respectively. (b) Lu and O atoms drawn as thermal ellipsoids in the projection along $a$. For comparison, the Lu positions at 350 K are displayed as spheres. (c), (d) O coordination at 210 K and 350 K for the ${\mathrm{Fe}}^{2+/3+}$ minority (small spheres indicate the O positions at 350 K).

Figure 2

(a) Composite precession image of crystal 1 in the $(0k\ell {)}_{\mathrm{Mon}}$ plane indexed in the new monoclinic cell, measured at 210 K. (b) Intensity distribution along $(02\ell {)}_{\mathrm{Mon}}$ for two crystals at 210 K.

Figure 3

(a) Fe and Lu valence states for different sites from the bond-valence-sum method. (b) XMCD spectra across the Fe $L_{2/3}$ edge at 120 K and 260 K (inset). The XAS spectra with $H$ parallel and antiparallel to the incoming beam by changed photon polarization were averaged by subtracting them from each other [37].