X-ray Powder Structure Determination and Thermal Behavior of a New Modification of Pb(II) Selenite

The crystal structure of a new polymorphic form of lead(II) selenite (PbSeO 3 ) was solved directly from conventional X-ray powder diffraction data. Direct methods and Rietveld refinement techniques were used for the structure determination. The compound crystallizes in the monoclinic space group P 2 1 / c (no. 14) with cell dimensions a = 9.1587(1) Å, b = 8.0902(1) Å, c = 8.7932(1) Å, β = 103.032(1)°, and V = 634.76(2) Å 3 . The final refinement gave background excluded R p = 6.22%, R wp = 8.81%, and R B = 2.16% by using 45 structural and 15 profile parameters with 10 atoms in an asymmetric unit. The three-dimensional structure consists of 9-coordinated lead atoms, which are linked by three bidentate and three monodentate selenite groups. By combining thermal studies and X-ray powder diffraction, the starting of irreversible phase transition between both polymorphic forms was observed, as now reported structure transformed slowly from space group P 2 1 / c to the known structural form P 2 1 / m between 315 and 355 °C.