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Abstract:
Four open-framework transition-metal phosphates; (NH4)(2)CO3(HPO4)(2)F-4
(1), (NH4)CO3(HPO4)(2)(H2PO4)F-2 (2), KCo3(HPO4)(2)(H2PO4)F-2 (3), and
KFe3(HPO4)(2)(H2PO4)F-2 (4); are prepared by ionothermat 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 Kagorne topology composed of
interlinked face-sharing MO3F3/MO4F2 octahedra. PO3OH pseudo-tetrahedral
groups augment the [M3O6F4] (1)/[M3O8F2] layers on both sides to give
M-3(HPO4)(2)F-4 (1) and M-3(HPO4)(2)F-2 (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)(2)CO3(HPO4)(2)F-4(1). In
NH4CO3(HPO4)(2)(H2PO4)F-2 and KM3(HPO4)2(H2PO4)F-2, the M-3(HPO4)(2)F-2
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 Kagome topology. Magnetic susceptibility and
magnetization measurements reveal that all four compounds exhibit a
canted antiferromagnetic 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+
