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

Abstract

Two new fluorinated microporous cobalt phosphates, (H2en)0.5[F(0.25)Co(H(0.5)PO4)0.5(PO4)0.5].0.25H2O (1) and (H2en)0.5[Fx(H2O)0.25(-x)Co(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) A, b = 9.7133(9) A, c = 13.688(1) A, V = 1980.4(3) A(3), 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 /42m with a = 10.2033(4) A, c = 9.5545(9) A, V = 994.7(1) A(3), 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 PO4(3-) groups in 2 are substituted by HPO3(2-) 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 (theta = -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.