lonothermal synthesis of open-framework metal phosphates with a Kagome lattice network exhibiting canted anti-ferromagnetism

摘要

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 Kagome 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 (I) 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(l). 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,-... 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 Kagome 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 Co~(2+) and Fe~(2+).