Synthesis of Vanadium Oxide Gels from Peroxovanadic Acid Solutions: A 51V NMR Study

MIL-21 or [FeIII5-xVIIIx(H2PO4)4(HPO4)4F4(H2O)2, 4(H2+yN–(CH2)2–NH2+y)] was prepared hydrothermally (453 K, autogenous pressure, 3 days) in the presence of ethylenediamine. Its structure was determined by single-crystal X-ray diffraction. MIL-21 is tetragonal (space group P-421c, No. 114, with cell parameters a=17.2023(2) ?, c=13.3086(2) ?, V= 3938.27(9) ?3, Z=4. Its three-dimensional network can be described from the connection via phosphate tetrahedra of infinite corner-sharing cis–trans octahedral chains of metallic atoms in a random distribution and isolated FeO4(H2O)2 octahedra. The (Fe3+, V3+) infinite chains belong to the KTP type. M?ssbauer and structure refinements demonstrate this segregation. This structure exhibits small tunnels and cages in which the ethylenediamine molecules are located. Moreover, MIL-21 shows a ferrimagnetic behavior below 13 K.