Dimorphism in iron(II) methylphosphonate: Low-temperature crystal structure and temperature-dependent Mössbauer studies of a new form of the layered weak ferromagnet Fe[(CH3PO3)(H2O)]

A second form of the literature-known layered weak ferromagnet Fe[(CH3PO3)(H2O)] has been isolated. The crystal structure determination of this new form (2) has been carried out at image, 200 and 130 K. It crystallizes in the orthorhombic space group Pmn21: image, image, image, while form (1) crystallizes in the space group Pna21: image, image, image. Mössbauer spectroscopy on form (2) has been performed in the temperature range 4–300 K; and, at image, a drastic change in the quadrupole splitting (ΔE) and a broadening of the doublet components is noticed. But surprisingly, on cooling the crystal, no structural change is observed, which could account for the increase in image. Below image, 57Fe spectra transform into hyperfine splitting patterns which reveal a magnetically ordered state in agreement with the results of earlier magnetic susceptibility studies.