Antiferromagnetism, Anion Disorder, and Lattice Defects in the S = 5/2 [Bi₂O₂][Mn²⁺F₄] Aurivillius Compound

Abstract

[Bi₂O₂][MnF₄] is an Aurivillius oxyfluoride phase with Mn²⁺cations filling the one-layer perovskite subunits. Its synthesis as a single-phase material by a high-temperature solid-state route is complicated by the high volatility of fluorine in the used precursors and by its limited thermal stability above 400 °C. Its crystal structure was determined using synchrotron and neutron diffraction data. It shows structural singularities, highlighted by the anion positional disorder inherent to the I4/mmm space group, despite the evidence of strong local axial and equatorial octahedral tilts along with clues for ideal F^–and O^2–total segregation in the two-layered subunits. Contrarily to the M = Ni²⁺, Co²⁺, and Fe^2+/3+analogs, where similar tilts order in various supercells, for M = Mn²⁺only short-range ordering (SRO) was detected by electron diffraction. This was rationalized in terms of the d⁵ion isotropic nature for which weak distortions of the crystal field do not have an energy penalty and favor local octahedral defects acting as antiphase boundaries between nanometric SRO domains. DFT calculations validate equiprobable tilted [MnF₆] arrangements in the perovskite layers and suggest the possibility for an O ↔ F exchange between the [Bi₂O₂] and [MnF₄] modules, supported by HAADF imaging. The presence of oxygen and vacancies in the perovskite slabs was also detected in samples prepared at higher temperatures. [Bi₂O₂][MnF₄] magnetic structure shows a colinear antiferromagnetic arrangement below T_N = 19.8 K, with Mn²⁺spins (S = 5/2, L = 0) aligned parallel to the c-axis (M_z = 3.8(2) μ_B/Mn). This is again in contrast with the systematic spin-canting responsible for weak ferromagnetism in the previous M²⁺Aurivillius compounds. Finally, the modest fluoride ionic mobility was measured in a small temperature range, restricted by the stability of the compound in air.