A magnetic and Mössbauer study of the layered compound (NH4)Fe(PO4) · H2O

(NH4)Fe(PO4) · H2O was prepared using hydrothermal methods and studied using bulk magnetic and Mössbauer spectroscopic techniques. The compound appears to be isostructural with (NH4)Co(PO4) · H2O and (NH4)Cd(PO4) · H2O. Orthorhombic cell constants (Å) a = 5.684(1), b = 8.803(1), and c = 4.818(5) were determined from powder data on highly oriented samples. The structure consists of FeO6 layers stacked normal to the b-axis separated by NH+4 ions. Nominally polycrystalline samples were found to have a high degree of crystallographic orientation relative to the b-axis and the samples showed anisotropy in the magnetic and Mössbauer studies. Variable temperature susceptibility and magnetization studies from 300 to 4.2 K showed a Curie-Weiss region (nearly isotropic) to 80 K and a short-range ordered region to 26 K, below which long-range magnetic order occurs. The long-range order is characteristic of a uniaxial antiferromagnetic (b-axis preferred) with a remnant spontaneous moment due, presumably, to sublattice canting. The short-range regime gives evidence for antiferromagnetic interlayer and ferromagnetic intralayer exchange. Effective moments and Weiss constants derived from the paramagnetic data for the field parallel and perpendicular to the b-axis, respectively, were 5.33(5) μB, 5.41(5) μB and −69(1) K, −64(1) K. The temperature dependence of the Mössbauer hyperfine field (Heff = 22.3(1) at 6 K) showed the transition at 26 K to be of first order. The quadrupole splitting was strongly temperature dependent between 300 K (1.34(1) mm sec−1) and 6 K (2.96(1) mm sec−) and η remained small (0.06(1) mm sec−) and constant over this range. The angle between Vzz and the hyperfine field direction was found to be 62(2)°. From an oriented sample the angle between Vzz and the b-axis was found to be 50(5)°. Thus, the hyperfine field makes an angle 12(5)° with the b-axis which is not inconsistent with the bulk magnetic data.