High-pressure polymorph of ${\mathrm{LuFe}}_2{\mathrm{O}}_4$ with room-temperature antiferromagnetic order

Branded for its potential electronic ferroelectricity, charge-ordered $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4$ has a layered Fe triangular framework, whose topology is a source of degeneracy, both at the charge and spin levels. Here we present an in-depth characterization of $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4\text{−}hp$, the high-pressure $\left(hp\right)$ polymorph of $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4$, using electron, x-ray, and neutron diffraction, combined with transport and magnetization measurements. We show that $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4\text{−}hp$ is characterized by a misfit-related monoclinic structure, accommodating a buckled triangular ${\left[\mathrm{Lu}\right]}_{\infty }$ layer and two shifted adjacent rectangular ${\left[\mathrm{Fe}\right]}_{\infty }$ planes belonging to a distorted rock salt–type layer. The release of the geometric frustration of the Fe magnetic lattice in the $hp$ form leads to collinear antiferromagnetic ordering at $T_N=380\mathrm{K}$. Possible coexistence of charge and magnetic orders in this material opens research pathways for the design of tunable multifunctional devices using high-pressure techniques.

Figure 1

(a) Rietveld refinement profile of the x-ray synchrotron diffraction pattern of the $Pm$ cell of $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4\text{−}hp$ at 300 K (experimental data: open circles; calculated profile: continuous line; allowed Bragg reflections: vertical marks. The difference between the experimental and calculated profiles is displayed at the bottom of the graph). Stars indicate Bragg intensity resulting from the displacement along $c$ of the Lu positions. The second row of vertical marks corresponds to untransformed $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4$ [14]. (b) [010], [001], and [100] electron diffraction patterns of $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4\text{−}hp$. The circled reflections on the [001] pattern highlight the link with the hexagonal-like $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4$ phase. The 0021 satellite (using four indices hklm with $m=1/4b^{*}+1/2c^{*}$) is shown on the [100] pattern.

Figure 2

(Top) [010] HREM images; the brighter contrast is associated with the positions of the Lu atoms, zones of higher electron density: the calculated Lu positions in the $C2/m[\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4$, (a)] and $Pm [\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4\text{−}hp$, (b)] forms are superimposed. In the [010] projections of the crystal structures, Lu atoms are in blue, and Fe atoms in orange and green to differentiate the top and bottom planes of the bilayer. This color code is kept throughout. (Bottom) High-pressure transformation of the initial triangular array of Fe ions (a) to a rectangular one (b), leading to a NaCl-type structure and to an increase $\left(3\to 4\right)$ of the in-plane oxygen coordination of Fe. The orange and green arrows schematize the displacements of the iron atoms during the transformation. (c) Structural relationships between the cells of $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4\text{−}hp$ and $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4-\mathrm{O}hp$ [11], following twinning and nanotwining mechanisms (see text), illustrated through the [010] ED patterns. The light blue spots in the upper panel are associated with the reflections of the calculated electron diffraction pattern of the monoclinic $Pm$ cell (blue dashed lines) of $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4\text{−}hp$. The system of reflections and the cell resulting from the formation of twins (mirror parallel to the layer plane) and slips are in red (middle panel).

Figure 3

(a) Neutron diffraction patterns of $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4\text{−}hp$ in the 310–380 K range. Inset: evolution of the normalized magnetic moment (with respect to the 10 K value) with temperature. (b) Observed (red circles), calculated (solid line), and difference (bottom line) magnetic intensity profile of $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4\text{−}hp$ (gray areas correspond to artifacts at crystal Bragg peaks positions). (c) Perspective view of the $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4\text{−}hp$ antiferromagnetic structure and projection of a single RS layer in the $ab$ plane, showing the comparison with the FeO magnetic order (red dotted line), corresponding to ferromagnetic ${\left(111\right)}_{\mathrm{cubic}}$ planes (outlined in red) coupled antiferromagnetically.

Figure 4

(From left to right) Evolution with pressure (at RT), increasing temperature (from RT to 730 K, in vacuum), and decreasing temperature (from 730 K back to RT, in vacuum) of the neutron diffraction patterns, showing the structural phase transition $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4\to \mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4\text{−}hp$ around 8 GPa, and its reversibility ($\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4\text{−}hp\to \mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4$, starting around 520 K in vacuum). Arrows in orange (purple) indicate the pressure or temperature range in which $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4\left(\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4\text{−}hp\right)$ is identified on the diffraction patterns. The dotted red line marks the antiferromagnetic transition temperature of $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4\text{−}hp$, and the red letter $m$ the position in $Q$ of the corresponding main antiferromagnetic Bragg peak.

Figure 5

Temperature evolution of the resistivity ρ of the $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4\text{−}hp$ sample heated in vacuum $\left(P=2.{10}^{-7}\mathrm{bars}\right)$ up to 565 K (1, full purple circles), then cooled down to RT after transformation into $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4$ (2, full orange circles), and heated up again to demonstrate reproducibility (3, hollow orange triangles). $\rho \left(T\right)$ of the as-synthesized $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4$ is also shown (hollow gray circles) for comparison purposes. Inset: [001] bright-field image of $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4\text{−}hp$.