Dimorphism of [Bi₂O₂(OH)](NO₃) - the ordered Pna2₁ structure at 100 K

The re-investigation of [Bi₂O₂(OH)](NO₃), dioxidodibismuth(III) hydroxide nitrate, on the basis of single-crystal X-ray diffraction data revealed an apparent structural phase transition of a crystal structure determined previously (space group Cmc2₁ at 173 K) to a crystal structure with lower symmetry (space group Pna2₁ at 100 K). The Cmc2₁ → Pna2₁ group-subgroup relationship between the two crystal structures is klassengleiche with index 2. In contrast to the crystal structure in Cmc2₁ with orientational disorder of the nitrate anion, disorder does not occur in the Pna2₁ structure. Apart from the disorder of the nitrate anion, the general structural set-up in the two crystal structures is very similar: [Bi₂O₂]²⁺ layers extend parallel to (001) and alternate with layers of (OH)^- anions above and (NO₃)^- anions below the cationic layer. Whereas the (OH)^- anion shows strong bonds to the Bi^III cations, the (NO₃)^- anion weakly binds to the Bi^III cations of the cationic layer. A rather weak O-H⋯O hydrogen-bonding inter-action between the (OH)^- anion and the (NO₃)^- anion links adjacent sheets along [001].