Ionothermal synthesis of open-framework metal phosphates with a Kagomé lattice network exhibiting canted anti-ferromagnetism

Abstract

Four open-framework transition-metal phosphates; (NH₄)₂Co₃(HPO₄)₂F₄ (1), (NH₄)Co₃(HPO₄)₂(H₂PO₄)F₂ (2), KCo₃(HPO₄)₂(H₂PO₄)F₂ (3), and KFe₃(HPO₄)₂(H₂PO₄)F₂ (4); are prepared by ionothermal synthesis using pyridinium hexafluorophosphate as the ionic liquid. Single-crystal X-ray diffraction analyses reveal that the four compounds contain cobalt/iron–oxygen/fluoride layers with Kagomé topology composed of interlinked face-sharing MO₃F₃/MO₄F₂ octahedra. PO₃OH pseudo-tetrahedral groups augment the [M₃O₆F₄] (1)/[M₃O₈F₂] layers on both sides to give M₃(HPO₄)₂F₄ (1) and M₃(HPO₄)₂F₂ (2–4) layers. These layers are stacked along the a axis in a sequence AA…, resulting in the formation of a layer structure for (NH₄)₂Co₃(HPO₄)₂F₄(1). In NH₄Co₃(HPO₄)₂(H₂PO₄)F₂ and KM₃(HPO₄)₂(H₂PO₄)F₂, the M₃(HPO₄)₂F₂ layers are stacked along the a axis in a sequence AA_i… and are connected by [PO₃(OH)] tetrahedra, giving rise to a 3-D open framework structure with 10-ring channels along the [001] direction. The negative charges of the inorganic framework are balanced by K⁺/NH₄⁺ ions located within the channels. The magnetic transition metal cations themselves form layers with stair-case Kagomé topology. Magnetic susceptibility and magnetization measurements reveal that all four compounds exhibit a canted anti-ferromagnetic ground state (T_c = 10 or 13 K for Co and T_c = 27 K for Fe) with different canting angles. The full orbital moment is observed for both Co²⁺ and Fe²⁺.