Reinvestigation of the crystal chemistry of phosgenite, Pb₂(CO₃)Cl₂

The chemical composition of phosgenite (ideal formula Pb 2 (CO 3 )Cl 2 , sp. gr.P4/mbm with a8.15 and c8.87 ) from Monteponi Mine (Iglesias, Sardinia, Italy), its crystal structure and its high-temperature behaviour up to the onset of decomposition were investigated by a series of chemical analytical and diffraction techniques, including singlecrystal X-ray (data collected at 293 K) and neutron diffraction (at 293 and 20 K), and in-situ high-temperature powder X-ray diffraction and thermogravimetric analysis.The concentrations of more than 65 elements were measured.The empirical mineralogical formula of phosgenite, obtained by the multi-analytical approach used in this study, is almost identical to the ideal one, with only a few elements measured above the detection limit: (Na 2 O+K 2 O+CaO+SiO 2 ) = 0.11 wt%.The concentration of other industrially relevant elements is insignificant.X-ray and neutron refinements, based on data collected at room temperature, confirm the previously reported general structural model of phosgenite, while providing a full description of the displacement parameters of all the atomic sites.The building unit of the crystal structure of phosgenite is represented by a Pb-polyhedron, in which Pb is coordinated by 5Cl + 4O (coordination number CN= 9), forming a monocapped square antiprism.The combination of face-sharing Pb-polyhedra generates dense layers parallel to (001), which are connected by (edge-sharing) CO 3 -groups to form the crystalline edifice.Low-temperature neutron diffraction data show evidence of a T-mediated phase transition toward a lower symmetry (space group P ), with a modest distortion of the building units of the structure.A tentative description of the low-T mechanisms, at the atomic scale, that can lead to the phase transition is provided.