[Pb2F2](SeO4): a heavier analogue of grandreefite, the first layered fluoride selenate

Abstract

Co-precipitation of PbF 2 and PbSeO 4 in weakly acidic media results in the formation of [Pb 2 F 2 ](SeO 4 ), the selenate analogue of the naturally occurring mineral grandreefite, [Pb 2 F 2 ](SO 4 ). The new compound is monoclinic, C2/c, a = 14.0784(2) Å, b = 4.6267(1) Å, c = 8.8628(1) Å, β = 108.98(1)°, V = 545.93(1) Å 3 . Its structure has been refined from powder data to R B = 1.55%. From thermal studies, it is established that the compound is stable in air up to about 300 °C, after which it gradually converts into a single phase with composition [Pb 2 O](SeO 4 ), space group C2/m, and lattice parameters a = 14.0332(1) Å, b = 5.7532(1) Å, c = 7.2113(1) Å, β = 115.07(1)°, V = 527.37(1) Å 3 . It is the selenate analogue of lanarkite, [Pb 2 O](SO 4 ), and phoenicochroite, [Pb 2 O](CrO 4 ), and its crystal structure was refined to R B = 1.21%. The formation of a single decomposition product upon heating in air suggests that this happens by a thermal hydrolysis mechanism, i.e., Pb 2 F 2 SeO 4 + H 2 O (vapor) → Pb 2 OSeO 4 + 2HF↑.