Fe(II)/Fe(III) Mixed-Valence Fluorophosphate Frameworks and Their Magnetic Characterization

We report the synthesis and characterization of Fe(II)/Fe(III) mixed-valence fluorophosphate frameworks. Both (N 2 H 5 )Fe 3 (PO 4 )F 6 ( I ) and (NH 4 ) 4 Fe 5 (PO 4 ) 2 (PO 3 F) 2 F 4 (OH) 2 ( II ) reveal three-dimensional (3D) frameworks possessing cross-stitched one-dimensional (1D) channels filled by N 2 H 5 + or NH 4 + ions. I is built from FeF 4 O 2 octahedra and PO 4 tetrahedra, forming 8-member rings (8MR) and 6MR in a 1D channel, while II is constructed from trans - FeO 4 F 2, cis- FeO 4 O 2, FeO 6 octahedra, and PO 4 and PO 3 F tetrahedra, forming bow-shaped 10MR channels running along the b -axis and hexagon-shaped 6MR channels running along the [011] axis and [1–10] axis, respectively. Structurally, I contains the ∞ [Fe 2.5+ 2 F 6 O 2 ] 5– double chains along the a -direction, which are interconnected by tran s-Fe 3+ F 4 O 2 octahedra through shared fluorine vertices to form a 3D architecture. In contrast, II features diamond-chain motifs along the b -direction composed of triangular [Fe 2+ 3 O 4 F 4 ] 4+ units. These diamond chains are further linked by both trans -Fe 3+ O 4 F 2 octahedra and cis -Fe 3+ O 4 F 2 via shared fluorine corners, resulting in a distinct 3D network. Magnetization and Mössbauer spectroscopy measurements revealed that I undergoes an antiferromagnetic-to-ferromagnetic phase transition at 30 K, whereas II exhibits strong competition between antiferromagnetic and ferromagnetic interactions below 40 K. Comprehensive analysis confirms that the magnetic behaviors of both compounds are dominated by their respective 1D structural motifs: double chains in I and diamond chains in II.