Grandreefite, Pb2F2SO4: Crystal structure and relationship to the lanthanide oxide sulfates, Ln2O2SO4

Grandreefite, Pb₂F₂SO₄, has been determined to be monoclinic rather than orthorhombic as reported in its origrnal description. Its cell parameters are a = 8.667(1), b = 4.4419(6), c = 14.242(2) Å, β = 107.418(2)°(Z = 4) and the space group is A2/a. The crystal strucrure, refined to R = 0.032 for 399 observed reflections, consists of layer fragments of the β-PbF₂ structure parallel to (100) alternating with layers of isolated sulfate tetrahedra. The layer sequence in the unit cell is SO₄-PbF₂Pb-SO₄-PbF₂Pb-SO₄. Pb is coordinated to four F and four O forming a distorted bicapped trigonal prism. The structure of grandreefite is closely related to that of the lanthanide oxide sulfates, Ln₂O₂SO₄. Electrostatic bond strength and empirical bond-valence considerations suggest that the topologies of the structures differ with respect to the ligation of the sulfate oxygen atoms to the large cations.