One disorder out of two orders: Synthesis and crystal structures of cation-ordered PbNaF2NO3, anion-ordered Pb2OFNO3, and continuous disordered (Pb,Na)2(O,F)2-δNO3 solid solution with Sillén-derived structures

Abstract

Two new Sillen-like layered lead fluoride nitrates, PbNaF2NO3 and Pb2OFNO3, have been prepared at 300 C. PbNaF2NO3 is a structural analog of alkaline earth – bismuth oxyhalides, BaBiO2X and SrBiO2X (X = Cl – I), but not the corresponding nitrates, SrBiO2NO3 or BaBiO2NO3. Pb2OFNO3 is analogous to the corresponding halides, Pb2OFX (X = Cl, Br, I). Both structures belong to orthorhombic symmetry and demonstrate Na/Pb and O/F ordering, respectively. A continuous solid solution is formed between PbNaF2NO3 and Pb2OFNO3 which demonstrates neither cation nor anion ordering; the structure of intermediate composition Pb1.5Na0.5F1.5O0.5NO3 was refined in tetragonal symmetry and is indeed very close to that of PbBiO2NO3 and CaBiO2NO3. In PbNaF2NO3, the O:F ratio may be varied to a slight extent, PbNaF2-2yOyyNO3, which also breaks the Na – Pb cation ordering. Analogous fluoride halides could not be prepared. Structural analogies to lead, bismuth, and antimony oxyhalides are discussed