Ferromagnetic and Crystal Structure of Ludlamite, Fe3(PO4)2·4H2O, at 4.2°K

A neutron-diffraction study of Fe3(PO4)2·4H2O below the Curie temperature has determined both the crystal and the magnetic structure. At 4.2°K, the crystal remains monoclinic, with lattice constants a=10.541±0.010, b=4.638±0.008, c=9.285±0.010 Å, β=100°43.7′±4.0′ and two formula weights in the unit cell. The nuclear and Shubnikov space groups are both P21/a. The iron—oxygen octahedral distances range from 2.019±0.010 through 2.317±0.010 Å, with average distance 2.154 Å. The phosphate group is very nearly a regular tetrahedron, with the phosphorus—oxygen distance ranging from 1.521±0.008 through 1.553±0.007 Å. The average O–H distance is 1.00 Å, and the shortest O–H···O bond is 2.546±0.005 Å. The magnetic unit cell coincides with the nuclear cell. The spins on each linear triad of iron atoms, separated by 3.267±0.004 Å, are ferromagnetically coupled and are essentially parallel. Triad spin components in the ac plane, related by the screw-axis or glide-plane operators, are antiferromagnetically coupled. Spin components parallel to the b axis are ferromagnetically coupled. The spins are very nearly normal to the a axis and are inclined 10° out of the ac plane, always in the same sense, forming a canted spin array. The magnetic moment per iron atom is 4.5±0.2 μB. The effective ferromagnetic moment, based on this spin array, is 0.81 μB in excellent agreement with Mays' NMR measurement.