Monoclinic FeCl2·2H2O orders antiferromagnetically at TN = 23 K, and the magnetic structure consists of two sublattices of -FeCl2-chains lying along the c-axis. The coupling along the chains is ferromagnetic with weak antiferromagnetic coupling between chains. Application of an external magnetic field along the easy axis (a) induces phase transitions at H1 = 39 kOe and at H2 = 46 kOe.

We report the observation of the three phases using the Mössbauer effect in a single crystal of FeCl2·2H2O cut parallel to the c-axis and placed at 32° to the gamma ray beam and magnetic field Ho so that Ho was parallel to the easy axis alpha. The results may be summarized as follows: (1) For H0 < H1 we observe two superposed spectra with equal intensities due to the external field adding and subtracting to the hyperfine field in the spin down and spin up sublattices respectively; (2) For H1 < Ho < H2 the relative intensities of the spin up to spin down spectra are roughly 3:1. (3) For H2 < Ho, only one spectrum is observed. These observations are consistent with the antiferromagnetic==>ferrimagnetic==>paramagnetic model of Narath1 from susceptibility measurements.