We report on the crystal structure and magnetism of the iron-based oxyfluoride Bi${}_4$Fe${}_5$O${}_{13}$F, a material prototype of the Cairo pentagonal spin lattice. The crystal structure of Bi${}_4$Fe${}_5$O${}_{13}$F is determined by a combination of neutron diffraction, synchrotron x-ray diffraction, and transmission electron microscopy. It comprises layers of FeO${}_6$ octahedra and FeO${}_4$ tetrahedra forming deformed pentagonal units. The topology of these layers resembles a pentagonal least-perimeter tiling, which is known as the Cairo lattice. This topology gives rise to frustrated exchange couplings and underlies a sequence of magnetic transitions at $T_1=62$ K, $T_2=71$ K, and $T_N=178$ K, as determined by thermodynamic measurements and neutron diffraction. Below $T_1$, Bi${}_4$Fe${}_5$O${}_{13}$F forms a fully ordered non-collinear antiferromagnetic structure, whereas the magnetic state between $T_1$ and $T_N$ may be partially disordered according to the sizable increase in the magnetic entropy at $T_1$ and $T_2$. Bi${}_4$Fe${}_5$O${}_{13}$F reveals unanticipated magnetic transitions on the pentagonal Cairo spin lattice and calls for a further work on finite-temperature properties of this strongly frustrated spin model.

• Received 9 October 2012

DOI:https://doi.org/10.1103/PhysRevB.87.024423