Synthesis, X-Ray Powder Structure and Magnetic Properties of layered Ni (II)Methylphosphonate and Ni(II) octadecylphosphonate

[Ni(CH3PO3)(H2O)] (1) and [Ni{CH3-(CH2)17-PO3}(H2O)](2)were synthesised by reaction of (NiCl2)6H2O and the relevant phosphonic acid in water in presence of urea. The compounds were characterised by elemental and thermogravimetric analyses, UV-visible and IR spectroscopy, and their magnetic properties were studied by using a SQUID magnetometer. The crystalstructu re of 1 was determined ab initio from X-ray powder diffraction data and refined by the Rietveld method. The crystals of (1) are orthorhombic,space group Pmn21, with a= 5.587(1), b=8.698(1), c=4.731(1) A. The compound has a hybrid, layered structure made up of alternating inorganic and organic layers along the b direction of the unit-cell. The inorganiclayers consist of NiII ions octahedrally coordinated by five phosphonate oxygen atoms and one oxygen atom from the water molecule. These layers are separated by bilayers of methyl groups and van der Waals contacts are established between them. A preliminary structure characterisation of compound 2 suggests the crystallisation in the orthorhombic system with the following unit-cell parameters: a=5.478(7), b=42.31(4), c= 4.725(3)A.The oxidation state of the Ni ion in both compounds is +2, and the electronic configuration is d8 (S=1), as determined from static magnetic susceptibility measurements above 50K. Compound 1 obeys the Curie-Weiss law at temperatures above 50K; the Curie (C) and Weiss (theta) constants were found to be 1.15 cm3Kmol-1 and -32K, respectively. The negative value of theta indicates an antiferro-magnetic exchange coupling between near-neighbouring Ni(II) ions. No sign of 3D antiferromagnetic long-range order is observed down to T=5K, the lowest measured temperature. Compound 2 is paramagnetic above T=50 K, and the values of C and theta were found to be 1.25 cm3Kmol-1 and -24 K, respectively. Below 50 K the magnetic behavior of 2 is different from that of 1. Zero-field cooled (zfc) and field-cooled (fc) magnetisation plots do not overlap below T=21 K. The irreversible magnetisation, DMfc-zfc, obtained as a difference from fc and zfc plots starts to increase at T=20K, on lowering the temperature, and it becomes steady at T=5 K. The presence of spontaneous magnetisation below T=20 K indicates a transition to a weak-ferromagnetic state for compound (2).