Crystal Structure of Paramagnetic Ludlamite, Fe3(PO4)2·4H2O, at 298°K

Abstract

Paramagnetic Fe3(PO4)2·4H2O is monoclinic, with lattice constants a=10.541±0.005, b=4.646±0.004, c=9.324±0.005 Å and β=100°25.8′±1.0′ at 298°K, with two formula weights in the unit cell. The space group is P21/a. The positions of the iron, phosphorus, and oxygen atoms, determined by Ito and Mori in 1951, together with those of the hydrogen atoms, have been refined by the method of least squares. The integrated intensities of 6212 reflections in a reciprocal lattice hemisphere of radius (sinθ)/λ=1.02 Å−1 were measured with PEXRAD; 1919 symmetry-independent F(hkl) were significantly above background. The final agreement factor between measured and calculated structure factors was 0.0336. There are two crystallographically unrelated iron atoms per unit cell, each occupying a distorted oxygen octahedron; the iron—oxygen distances range from 2.016±0.002 through 2.362±0.002 Å. The average distance is 2.156 Å. The phosphate group is very nearly a regular tetrahedron, the range in P–O distance being from 1.536±0.002 through 1.549±0.002 Å. The average O–H distance is 0.87 Å. The shortest O–H···O bond is 2.541±0.003 Å. One phosphate—oxygen atom forms no hydrogen bond, one forms two, and the remaining two oxygen atoms form one hydrogen bond each. The thermal vibrations are significantly anisotropic.