Dimorphism of [Bi 2 O 2 (OH)](NO 3 ) - the ordered Pna 2 1 structure at 100 K.

The re-investigation of [Bi 2 O 2 (OH)](NO 3 ), 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 Cmc 2 1 at 173 K) to a crystal structure with lower symmetry (space group Pna 2 1 at 100 K). The Cmc 2 1 → Pna 2 1 group-subgroup relationship between the two crystal structures is klassengleiche with index 2. In contrast to the crystal structure in Cmc 2 1 with orientational disorder of the nitrate anion, disorder does not occur in the Pna 2 1 structure. Apart from the disorder of the nitrate anion, the general structural set-up in the two crystal structures is very similar: [Bi 2 O 2 ] 2+ layers extend parallel to (001) and alternate with layers of (OH) - anions above and (NO 3 ) - anions below the cationic layer. Whereas the (OH) - anion shows strong bonds to the Bi III cations, the (NO 3 ) - 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 (NO 3 ) - anion links adjacent sheets along [001].