Fluorinated Phosphates, BaMPO4F (M = Mn, Fe), with One-Dimensional Channels: Structure and Magnetism

Abstract

Two transition metal fluoride phosphates, BaMPO₄F (M = Mn, Fe), are prepared through hydrothermal redox reactions and analyzed using XRD, XPS, Mössbauer spectroscopy, and density functional theory (DFT). The modified fluoride-rich hydrothermal routes combining two reducing sources, i.e., hydrazine and hypophosphorous acid, to grow Fe(II)-based fluorinated phosphates are described. Both crystal structures consist of three-dimensional (3D) frameworks formed by connected [M²⁺O₄F]^7– and PO₄^3– polyhedra, containing 8-membered rings (8MR) and 6MR cross-stitched 1D channels filled by Ba²⁺ ions. BaMnPO₄F is isostructural to BaZnPO₄F with an orthorhombic space group Pna2₁, while BaFePO₄F (S.G. P2₁2₁2₁) is isotypic to BaMPO₄F (M = Cu and Co) analogues. Magnetic susceptibilities χ(T) and specific heat C_p(T) for BaMnPO₄F (I) and BaFePO₄F (II) reveal antiferromagnetic ordering below 5.8 K (I) and 11.3 K (II), with all magnetic exchanges resulting from supersuper-exchange interactions. Above T_N, χ(T) is analyzed on the basis of the most plausible low-dimensional subunits, decoupled above the long-range AFM ordering. The analysis of the density-of-states (DOS) by DFT+U calculations validates the crystal-field splitting of the local MO₄F trigonal bipyramids with significant overlap due to their strong structural distortions, and validates the lower-energy levels of F-2p compared to O-2p levels due to their enhanced ionic character.