Transition-Metal Monofluorophosphate Ba2M2(PO3F)F6 (M = Mn, Co, and Ni): Varied One-Dimensional Transition-Metal Chains and Antiferromagnetism

Abstract

Four transition-metal monofluorophosphates, with a chemical formula of Ba₂M₂(PO₃F)F₆ (M = Mn, Co, Ni, and Cu), have been synthesized hydrothermally using phosphoric and fluorophosphoric acids. The structures of Ba₂M₂(PO₃F)F₆ were intensively investigated by single-crystal and powder X-ray diffraction. Their networks exhibited three-dimensional frameworks formed by cis-MO₂F₄ octahedra and PO₃F tetrahedra. Ba₂M₂(PO₃F)F₆ (M = Mn, Co, and Cu) are isostructural, crystallizing in the monoclinic space group P2₁/c, while Ba₂Ni₂(PO₃F)F₆ adopted an unpredetermined structure, crystallizing in the monoclinic space group P2₁/n. The different arrangements of the same cis-MO₂F₄ moieties and PO₃F groups within one-dimensional chains lead to different frameworks in this late-first-row transition-metal series. By a comparison of normalized bond lengths with their normal counterparts, the more elongated octahedral cis-MO₂F₄ units in Ba₂Cu₂(PO₃F)F₆ reveal the presence of static Jahn-Teller distortion, uncovering the puzzling reason for the exceptional structure of Ba₂Ni₂(PO₃F)F₆ in the series. Ba₂M₂(PO₃F)F₆ (M = Mn, Co, and Ni) forms spin chains of M²⁺ made of coupled dimers or tetramers, showing dominant antiferromagnetic behavior.