A powder X-ray diffraction study of lead chloride oxalate Pb 2 Cl 2 (C 2 O 4 ): ab initio structure determination and thermal behavior

Mixed lead chloride oxalate, Pb 2 Cl 2 (C 2 O 4 ), has been obtained in a polycrystalline form in the course of a study on precursors of nanocrystalline PZT-type oxides. Its crystal structure has been solved ab initio from powder diffraction data collected using a monochromatic radiation from a conventional X-ray source. The symmetry is monoclinic, space group C 2/ m , the cell dimensions are a =5.9411(3) A, b =5.8714(4) A, c =9.4212(4) A, β=95.232(4)° and Z=2. The structure consists of a stacking of complex double sheets, built from lead polyhedra, parallel to (001) and connected together through oxalate groups. The lead atom is nine-fold coordinated by four O atoms from one bidentate and two monodentate oxalate groups and five Cl atoms. The polyhedron can be described as a highly distorted square antiprism mono-capped by a Cl atom. The thermal behavior of lead chloride oxalate, in vacuum and in air, is carefully described from temperature-dependent powder diffraction and thermogravimetric measurements. It is shown that reaction pathways are complicated by the identification of various oxide chloride phases.