Structure and magnetic properties of new trigonal iron-boracite, Fe ³B⁷O¹³(OH)

The magnetic properties of a newly synthesized compound, Fe ₃B₇O₁₃(OH), are reported. The space group and lattice constants at room temperature are R3cH (trigonal, #161), a - 8:590(3) Å and c = 21:107(7) Å. In this compound, three Fe²⁺ ions form an equilateral triangular trimer sharing a hydroxide ion, and the trimers further construct a trigonal lattice. The magnetic susceptibility exhibits an antiferromagnetic phase transition accompanied by a steep drop in susceptibility at T_N ≈ 4:8 K under small magnetic fields. Mossbauer spectroscopy indicates that Fe ions are divalent. It also suggests a simple spin structure in which all of the spins are equally canted from the principal axis of the electric-field gradient below T_N. This magnetic ground state is, however, easily destroyed by almost 1 T of magnetic field regardless of its direction. In addition to this transition, successive metamagnetic transitions are induced by larger magnetic fields. The additional transitions are strongly dependent on the direction of the applied magnetic field, resulting in a rich magnetic phase diagram.