Crystal structure and magnetism of Fe₂(OH)[B₂O₄(OH)]

The structure and magnetism of Fe₂(OH)[B₂O₄(OH)] are reported. Powder x-ray diffraction reveals a characteristic structure containing two crystallographically independent zigzag-ladder chains of magnetic Fe²+ ions. Magnetization measurements reveal a phase transition at 85 K, below which a weak spontaneous magnetization (approximate to 0.15 μ_B/Fe) appears. Below 85 K, magnetization increases with decreasing temperature down to 70 K, below which it decreases and approaches a constant value at low temperature. The Mossbauer spectrum at room temperature is composed of two paramagnetic doublets corresponding to the two crystallographic Fe²+ sites. Below 85 K, each doublet undergoes further splitting because of the magnetic hyperfine fields. The temperature dependence of the hyperfine field is qualitatively different for the two distinguishable Fe²+ sites. This is responsible for the anomalous temperature dependence of the magnetization.