Synthesis and Characterization of the Crystal Structure and Magnetic Properties of the New Fluorophosphate LiNaCo[PO₄]F

The new compound LiNaCo­[PO₄]F was synthesized by a solid state reaction route, and its crystal structure was determined by single-crystal X-ray diffraction measurements. The magnetic properties of LiNaCo­[PO₄]F were characterized by magnetic susceptibility, specific heat, and neutron powder diffraction measurements and also by density functional calculations. LiNaCo­[PO₄]F crystallizes with orthorhombic symmetry, space group <i>Pnma</i>, with <i>a</i> = 10.9334(6), <i>b</i> = 6.2934(11), <i>c</i> = 11.3556(10) Å, and <i>Z</i> = 8. The structure consists of edge-sharing CoO₄F₂ octahedra forming CoFO₃ chains running along the <i>b</i> axis. These chains are interlinked by PO₄ tetrahedra forming a three-dimensional framework with the tunnels and the cavities filled by the well-ordered sodium and lithium atoms, respectively. The magnetic susceptibility follows the Curie–Weiss behavior above 60 K with θ = −21 K. The specific heat and magnetization measurements show that LiNaCo­[PO₄]F undergoes a three-dimensional magnetic ordering at <i>T</i>_<i>mag</i> = 10.2(5) K. The neutron powder diffraction measurements at 3 K show that the spins in each CoFO₃ chain along the <i>b</i>-direction are ferromagnetically coupled, while these FM chains are antiferromagnetically coupled along the <i>a</i>-direction but have a noncollinear arrangement along the <i>c</i>-direction. The noncollinear spin arrangement implies the presence of spin conflict along the <i>c</i>-direction. The observed magnetic structures are well explained by the spin exchange constants determined from density functional calculations