Preparation and Crystal Structures of Ni(NH3)2Cl2 and of Two Modifications of Ni(NH3)2Br2 and Ni(NH3)2I2

The iron oxycarbonate Sr4Fe2O6CO3 is reported. The crystal structure and magnetic order were determined by magnetic susceptibility, powder neutron diffraction, and neutron scattering studies. The structure is tetragonal between 10 and 400 K (I4/mmm, a=3.88907(4) ? and c=27.9818(4) ? at 295 K), and is related to a 4:3 type Ruddlesden–Popper phase with CO3 plaques in place of the middle layer of transition metal octahedra. The three-dimensional magnetic ordering transition temperature is 361 K. The magnetic order is antiferromagnetic within the FeO2/SrO/SrO/FeO2 blocks and ferromagnetic between blocks.