Chirality and ferromagnetism in NiBPO4(OH)2 containing helix edge-sharing NiO6 chains

Two isotypic borophosphates MBPO4(OH)2 (M=Mg, Ni) have been hydrothermally synthesized and structurally characterized by powder X-ray diffraction in the space group P3121. Nickel (or magnesium) atoms are octahedrally coordinated. The octahedra share edges to form helix chains around the three-fold screw-axis. Boron and phosphorus atoms are both tetrahedrally coordinated. The BO4 and PO4 tetrahedra are alternately connected, forming vierer-single chains. These two kinds of chains are intersected in the three-dimensional framework structure. NiBPO4(OH)2 can be considered as a quasi-one-dimensional magnet because the shortest Ni2+–Ni2+ distance within the helix chain is about 3.187(1) Å, while the shortest inter-chain connection of the nickel ions is through a BO4 group (5.650(1) Å). Both dc and alternating current (ac) susceptibilities and isothermal magnetization have been measured on powder sample. The intra- and inter-chain interactions are proved to be both ferromagnetic, and a long-range ordering is established below 2.2 K in NiBPO4(OH)2.