Structural and magnetic properties of mixed-metal (Bi₂O₂)(Fe_1-xM_xF₄) Aurivillius oxyfluorides

A series of mixed-metal Aurivillius oxyfluorides of the ideal formula [Bi₂O₂][Fe_1-xM_xF₄] was synthesized by hydrothermal synthesis with M = Mn, Co, and Ni. We first re-examined the Fe-only compound and deduced that despite the observation of inhomogeneous lattice parameters between batches, the iron valence remains constant around Fe^∼2.5+ in all samples measured using Mössbauer spectroscopy. The mixed valency charge compensation is mainly assigned to the formation of Bi vacancies. For the mixed Fe/M phases, the most common observation, using various diffraction techniques, of long-range ordering between tilted [(Fe,M)F₆] octahedra in the perovskite layers is reminiscent of the Fe, Co, and Ni parent members. This validates the possibility of well-defined anion-ordering, despite the mixing of cations with different ionic radii. A qualitative matching between the lattice evolution along the Fe/M solid solutions and our DFT relaxed ideal models supports this idea. Differences in the magnetic structures are observed between the single-metal and the mixed Fe/M compositions, while retaining ordered magnetic structures and escaping spin-glass behavior despite disordered Fe/M ions. In contrast to the non-collinear antiferromagnetic spin arrangements obtained in most of the parent cases, the majority of the mixed Fe/M compounds show a collinear structure with spins aligned along the c-axis, similar to the single-metal M = Mn²⁺ (L = 0) case in which spin-orbit coupling is absent. This suggests the predominant role of the spin contribution to the ordering of the magnetic moments as soon as both Fe and M intervene.