X-ray Single-Crystal Structure and Magnetic Properties of Fe[CH<sub>3</sub>PO<sub>3</sub>)]·H<sub>2</sub>O:  A Layered Weak Ferromagnet

The crystal and molecular structure of the layered weak-ferromagnet Fe[CH<sub>3</sub>PO<sub>3</sub>]·H<sub>2</sub>O has been solved by X-ray single-crystal diffraction techniques. Crystal data for Fe[CH<sub>3</sub>PO<sub>3</sub>]·H<sub>2</sub>O are the following:  orthorhombic space group <i>Pna</i>2<sub>1</sub>; <i>a =</i>17.538(2), <i>b</i> = 4.814(1), <i>c</i> = 5.719(1) Å. The structure is lamellar, and it consists of alternating organic and inorganic layers along the <i>a</i> direction of the unit cell. The inorganic layers are made of Fe(II) ions octahedrally coordinated by five phosphonate oxygen atoms and one from oxygen of the water molecule. Each phosphonate group coordinates four metal ions, through chelation and bridging, making in this way a cross-linked Fe−O network. The resultant layers are then separated by bilayers of the methyl groups, with van der Waals contacts between them. The compound is air stable, and it dehydrates under inert atmosphere at temperatures above 120 °C. The oxidation state of the metal ion is +2, and the electronic configuration is d<sup>6</sup><i><sup> </sup></i><sup></sup>high spin (<i>S</i> = 2), as determined from dc magnetic susceptibility measurements from 150 K to ambient temperature. Below 100 K, the magnetic moment of Fe[CH<sub>3</sub>PO<sub>3</sub>]·H<sub>2</sub>O rises rapidly to a maximum at <i>T</i><sub>max</sub> ≅ 24 K, and then it decreases again. The onset of peak at <i>T</i> = 25 K is associated with the 3D antiferromagnetic long-range ordering, <i>T</i><sub>N</sub>. The observed critical temperature, <i>T</i><sub>N</sub>, is like all the other previously reported Fe(II) phosphonates, and it appears to be nearly independent of the interlayer spacing in this family of hybrid organic−inorganic layered compounds. Below <i>T</i><sub>N</sub>, the compound behaves as a “<i>weak ferromagnet</i>”, and represents the third kind of magnetic materials with a spontaneous magnetization below a finite critical temperature, ferromagnets and ferrimagnets being the other two types.