Mild hydrothermal synthesis, crystal structure, spectroscopic and magnetic properties of the [MxIIM2.5-xIII(H2O)2(HPIIIO3)y(PVO4)2-yF] [M = Fe, x=2.08, y=1.58; M = Co, Ni, x=2.5, y=2] compounds

The [(MxM2.5-xIII)-M-II(H2O)(2)((HPO3)-O-III)(y)((PO4)-O-V)(2-y)F] [M = Fe (1), x = 2.08, y = 1.58; M = Co (2), x = 2.5, y = 2; Ni x = 2.5. y = 21 compounds have been synthesized using mild hydrothermal conditions at 170 degrees C during five days. Single-crystals of (I) and (2), and polycrystalline sample of (3) were obtained. These isostructural compounds crystallize in the orthorhombic system, space group Aba2, with a = 9.9598(2), b = 18.8149(4) and c = 8.5751(2)angstrom for (1), a = 9.9142(7), b = 18.570(1) and c = 8.4920(5)angstrom for (2) and a = 9.8038(2), b = 18.2453(2) and c = 8.4106(1) angstrom for (3), with Z = 8 in the three phases. An X-ray diffraction study reveals that the crystal structure is composed of a three-dimensional skeleton formed by [MO5F] and [MO4F2] (M = Fe, Co and Ni) octahedra and [HPO3] tetrahedra, partially substituted by [PO4] tetrahedra in phase (1). The IR spectra show the vibrational modes of the water molecules and those of the (HPO3)(2-) tetrahedral oxoanions. The thermal study indicates that the limit of thermal stability of these phases is 195 degrees C for (1) and 315 degrees C for (2) and (3). The electronic absorption spectroscopy shows the characteristic bands of the Fe(II), Co(II) and Ni(II) high-spin cations in slightly distorted octahedral geometry. Magnetic measurements indicate the existence of global antiferromagnetic interactions between the metallic centers with a ferromagnetic transition in the three compounds at 28, 14 and 21 K for (1), (2) and (3), respectively. Compound (1) exhibits a hysteresis loop with remnant magnetization and coercive field values of 0.72 emu/mol and 880Oe, respectively. (C) 2009 Elsevier Inc. All rights reserved.