We report the structural changes of three $\mathrm{YF}{\mathrm{e}}_2{\mathrm{O}}_{4-\delta }$ ($\delta <0.1$) specimens using high resolution synchrotron x-ray powder diffraction between 80 and 300 K. All samples adopt a rhombohedral cell at room temperature (space group $R\overline{3}m$). This cell becomes unstable for the three samples on cooling, and the oxygen-poor specimen ($\delta \sim 0.1$) shows a single transition at 240 K. The nearly stoichiometric ($\delta \le 0.03$) compounds exhibit two structural transitions with decreasing temperature at about 240 and 200 K. Each transition is revealed by an anomaly in the heat capacity measurements and a jump in the electric resistivity. Below 240 K, a strong splitting of some diffraction peaks is accompanied by the occurrence of superstructure peaks that follow the propagation vector $\mathbf{k}=(1/7,-2/7,9/7)$. The cell symmetry is then triclinic, and the structural transition is characterized by an expansion of the $c$ axis coupled to a contraction of the other two lattice parameters. There are 49 nonequivalent sites for Fe atoms with a maximum charge disproportionation of $\sim 0.5{\mathrm{e}}^{-}$. Upon cooling at 200 K, the previous superstructure peaks begin to vanish, and finally they are replaced by a new set of superstructure peaks following the propagation vector $\mathbf{k}=(1/4,1/2,1/4)$ with respect to the rhombohedral cell. The transition is also reflected in sudden changes in the lattice parameters that seem to smooth the changes observed in the previous transition. The new cell is also triclinic, and there are 48 nonequivalent Fe sites with a maximum charge disproportionation of $\sim 0.7{\mathrm{e}}^{-}$. Both phases coexist in a wide temperature range because this second transition is not completed at 80 K. A symmetry mode analysis indicates a complicated pattern for the charge distribution in the Fe sublattice of both distorted structures but clearly discard any bimodal distribution of only two types of Fe cations. Therefore, the sharp jumps in the electric resistivity at the phase transitions are clearly correlated with two different structural changes. Finally, the oxygen stoichiometry seems to be a key factor in the stabilization of the different distorted structures.

• Received 29 December 2015
• Revised 19 February 2016

DOI:https://doi.org/10.1103/PhysRevB.93.184110