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

Two new Sillén–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 Pb_1.5Na_0.5F_1.5O_0.5NO₃ 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, PbNaF_2-2yO_yyNO₃, 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.