Structural and magnetic phase transitions in Cs2[FeCl5(H2O)]

The compound is magnetoelectric but not multiferroic with an erythrosiderite-related structure. We present a comprehensive investigation of its structural and antiferromagnetic phase transitions by polarization microscopy, pyroelectric measurements, x-ray diffraction and neutron diffraction. At about K, the compound changes its symmetry from Cmcm to I2/c, with a doubling of the original c-axis. This transformation is associated with rotations of the octahedra and corresponds to an ordering of the molecules and of the related bonds. A significant ferroelectric polarization can be excluded for this transition by precise pyrocurrent measurements. The antiferromagnetic phase transition occurring at results in the magnetic space group , which perfectly agrees with previous measurements of the linear magnetoelectric effect and magnetization.