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[(CH3 PO3)(H2 O)] has been isolated. The crystal structure determination of this new form (2) has been carried out at , 200 and 130 K. It crystallizes in the orthorhombic space group Pmn 21 : a:5.7177(11), b=8.8093(18), C=4.8154(10) A , while form (1) crystallizes in the space group Pna 21 : a:17.58(2), b=4.814(1), 5.719(1) A. Mössbauer spectroscopy on form (2) has been performed in the temperature range 4–300 K; and, at , a drastic change in the quadrupole splitting (deltaE) 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 .Below , 57 Fe spectra transform into hyperfine splitting patterns which reveal a magnetically ordered state in agreement with the results of earlier magnetic susceptibility studies.