Magnetoelectric coupling in the mixed erythrosiderite [(NH4)1-xKx]2[FeCl5(H2O)]

We present a study of the dielectric, structural, and magnetic properties of the multiferroic or linear magnetoelectric substitution series [(NH4)(1-x)K-x](2) [FeCl5(H2O)]. Pyroelectric currents, magnetic susceptibilities, and thermodynamic properties were examined on large single crystals of the erythrosiderite compounds and detailed magnetic-field versus temperature phase diagrams are derived for three different substitution levels. With increasing potassium concentration the material is tuned from a multiferroic (x <= 0.06) to a linear magnetoelectric (x >= 0.15) ground state. In contrast to the respective pure parent compounds with x = 0 or 1, however, the ferroelectric or linear magnetoelectric polarization in none of the substituted samples is switchable by external electric fields because these samples exhibit a significant electric polarization already above the magnetic ordering transition. The polarization arises at a higher-lying structural phase transition that is examined by THz spectroscopy, and, on a deuterated pure single crystal, by comprehensive neutron-diffraction experiments. The structural phase transition is attributed to an ordering of NH4+ tetrahedra but does not break inversion symmetry in the pure material, while a finite K content causes pyroelectricity.