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

Abstract The [ M x II M 2.5 - x III ( H 2 O ) 2 ( HP III O 3 ) y ( P V O 4 ) 2 - y F ] [ M =Fe ( 1 ), x =2.08, y =1.58; M =Co ( 2 ), x =2.5, y =2; Ni ( 3 ), x =2.5, y =2] compounds have been synthesized using mild hydrothermal conditions at 170 °C during five days. Single-crystals of ( 1 ) and ( 2 ), and polycrystalline sample of ( 3 ) were obtained. These isostructural compounds crystallize in the orthorhombic system, space group Aba 2, with a =9.9598(2), b =18.8149(4) and c =8.5751(2) A for (1), a =9.9142(7), b =18.570(1) and c =8.4920(5) A for (2) and a =9.8038(2), b =18.2453(2) and c =8.4106(1) A 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 [ M O 5 F] and [ M O 4 F 2 ] ( M =Fe, Co and Ni) octahedra and [HPO 3 ] tetrahedra, partially substituted by [PO 4 ] tetrahedra in phase ( 1 ). The IR spectra show the vibrational modes of the water molecules and those of the (HPO 3 ) 2− tetrahedral oxoanions. The thermal study indicates that the limit of thermal stability of these phases is 195 °C for ( 1 ) and 315 °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 880 Oe, respectively.