Hydrothermal Synthesis and Characterization of Two Cobalt Phosphates Based on Double-Four-Ring Units with Fluorine-Occlusion and Phosphite-Substitution Modifications

Two new fluorinated microporous cobalt phosphates, (H2en)0.5[F0.25Co(H0.5PO4)0.5(PO4)0.5]·0.25H2O (1) and
(H2en)0.5[Fx(H2O)0.25-xCo(HPO3)x(PO4)1-x]·0.25H2O (x = 0.17; 2), have been synthesized from different P sources,
H3PO4 and H3PO3, in the presence of ethylenediamine and F ions. Both structures of 1 and 2 are based on the
similar secondary building unit of a double four ring (D4R). 1 crystallizes in the orthorhombic space group Cmcm
with a = 14.895(1) Å, b = 9.7133(9) Å, c = 13.688(1) Å, V = 1980.4(3) Å3, and Z = 16 and is built up from fully
F-occluded double-four-ring units via corner-sharing Co and P polyhedra and edge-sharing Co polyhedra. 2 crystallizes
in the tetragonal space group I4̄2m with a = 10.2033(4) Å, c = 9.5545(9) Å, V = 994.7(1) Å3, and Z = 8 and
adopts an ACO-type framework with strict alternation of corner-sharing Co and P polyhedra. On the basis of the
evidence from ion chromatography studies and elemental analyses, 2 contains both phosphate and phosphite
ions, and the amount of F ions in 2 is the same as the amount of phosphite ions. This implies that some PO43-
groups in 2 are substituted by HPO32- groups with the same amount of F- occlusion in the center of D4R as
charge compensation. Magnetic susceptibility measurements of both 1 and 2 appear that they contain mainly
antiferromagnetic interaction (θ = −27.8 K for 1 and −35.0 K for 2). However, the magnetic coupling is much more
complicated in compound 1 because of the Co−O−Co linkages in it. Low-field temperature-dependent magnetization
curves of 1 showed spontaneous magnetization in the form of two continuous bumps, one sharp and one broad.
This may be related to the temperature dependence of two related but different types of spin canting. At 2 K, 1
also showed a large coercive field of about 5000 Oe.