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

A structural phase transition occurs for [Bi2O2(OH)](NO3) between 173 K (space group Cmc21; previous single-crystal X-ray data) and 100 K (space group Pna21; current single-crystal X-ray data).

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 Cmc2 1 at 173 K) to a crystal structure with lower symmetry (space group Pna2 1 at 100 K).The Cmc2 1 !Pna2 1 group-subgroup relationship between the two crystal structures is klassengleiche with index 2.In contrast to the crystal structure in Cmc2 1 with orientational disorder of the nitrate anion, disorder does not occur in the Pna2 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) À anions above and (NO 3 ) À anions below the cationic layer.Whereas the (OH) À anion shows strong bonds to the Bi III cations, the (NO 3 ) À anion weakly binds to the Bi III cations of the cationic layer.A rather weak O-H� � �O hydrogenbonding interaction between the (OH) À anion and the (NO 3 ) À anion links adjacent sheets along [001].

Chemical context
During hydrothermal phase-formation studies of synthetic montanite, a bismuth(III) oxidotellurate(VI) mineral with composition Bi 2 TeO 6 •nH 2 O (0 � n � 2/3; Missen et al., 2022), small amounts of basic bismuth(III) nitrate [Bi 2 O 2 (OH)](NO 3 ) were also obtained when the starting materials Bi(NO 3 ) 3 •5H 2 O (Herpin & Sudarsanan, 1965;Lazarini, 1985), Te(OH) 6 and KOH were reacted under hydrothermal conditions.It is worth noting that no minerals containing both Bi and (NO 3 ) À have yet been found and described, with all examples of these compounds being synthetic.A routine unit-cell search at 100 K for selected crystals revealed unit-cell parameters very close to those of previously reported [Bi 2 O 2 (OH)](NO 3 ) (Henry et al., 2005; 173 K single-crystal X-ray data), however not with a C-centred but with a primitive orthorhombic unit cell.We therefore decided to determine the crystal structure based on the 100 K data and report here the results of this study.

Structural commentary
The previous crystal-structure determination and refinement of [Bi 2 O 2 (OH)](NO 3 ) in space group Cmc2 1 resulted in a model with the nitrate anion being disordered over two possible orientations.As noted in the original report, this disorder could not be resolved: 'Attempts to further lower the symmetry to order those anions was not successful and no supercell spots were detected on single crystal and powder diffraction data' (Henry et al., 2005).The current single-crystal X-ray diffraction data clearly revealed space group Pna2 1 , and the observed disorder of the nitrate anion does not prevail in the primitive unit cell, indicating that an apparent structural phase transition has taken place between 173 K and 100 K. Fig. 1 shows the Ba ¨rnighausen tree (Ba ¨rnighausen, 1980;Mu ¨ller, 2013) indicating the group-subgroup relationship between the two space groups and the associated crystal structures, denoted in the following as [Bi 2 O 2 (OH)](NO 3 )-I for the 173 K data in space group Cmc2 1 and as [Bi 2 O 2 (OH)](NO 3 )-II for the 100 K data in space group Pna2 1 .The latter is a klassengleiche subgroup of Cmc2 1 with index 2.All atoms in [Bi 2 O 2 (OH)](NO 3 )-I that are located on sites with mirror symmetry, viz.atoms Bi1, Bi2, O2, O3 and N1, lie on general positions in [Bi 2 O 2 (OH)](NO 3 )-II.The O1 site in the higher-symmetry structure splits into two sites (O1A, O1B) in the lower-symmetry structure, and the two disordered (half-occupied) sites O4 and O5 fully order.
Apart from the ordering of the (NO 3 ) À group, the general structural set-up is very similar in the two crystal structures.
Individual bond lengths of the structure units in the two polymorphs differ slightly (Table 1); numerical values are discussed in the following paragraph only for [Bi 2 O 2 (OH)](NO 3 )-II.Within the [Bi 2 O 2 ] 2+ layer, the two Bi III cations exhibit four bonds each [range 2.1964 (8)-2.657(12) A ˚] to the O1A and O1B atoms that, in turn, are tetrahedrally surrounded by the Bi III cations.Such anioncentered [OBi 4 ] tetrahedra are a common structural motif in inorganic bismuth(III) compounds (Krivovichev et al., 2013).Additional strong Bi III -O interactions of 2.335 (9) and 2.493 (9) A ˚include the O2 atom of the hydroxide anion in the adjacent layer.On the other hand, the nitrate anion is only weakly bonded to the Bi III cations of the cationic layer, with four Bi1-O3 interactions ranging from 2.868 (9) to 2.942 (9) A ˚, and another weak Bi2-O4 bond of 3.080 (10) A ˚. Overall, both Bi III cations have eight oxygen atoms as coordination partners.The [Bi1O 8 ] coordination polyhedron can be described as a distorted square antiprism, whereas the [Bi2O 5 (OH) 3 ] coordination polyhedron shows a significantly greater distortion (Table 1) and is difficult to derive from a simple geometric shape.In both cases, the 6s 2 free electron pair E of Bi III located at the top of the {BiO 4 } square-pyramid (as defined by the four short Bi-O bonds) is made responsible for the distortion of the polyhedra.The resulting stereochemical effect appears to be less pronounced for the  [Bi1O 8 ] coordination polyhedron, but is much clearer with the [Bi2O 5 (OH) 3 ] coordination polyhedron.This behaviour might possibly be explained by the stronger repulsive interaction between E and the surrounding (OH) À groups.The ordered (NO 3 ) À group in [Bi 2 O 2 (OH)](NO 3 )-II has an average N-O bond length of 1.264 A ˚, which is slightly longer but within the single standard deviation of the mean literature value of 1.247 (29) A ˚calculated for 468 N-O bonds in nitrates (Gagne ´& Hawthorne, 2018).The O-N-O bond angles range from 118.7 (10) to 121.3 (11) � , indicating a slight angular distortion.However, the (NO 3 ) À group does not deviate from planarity as observed for many nitrates, with deviations of up to 0.02 A ˚ (Jarosch & Zemann, 1983).In [Bi 2 O 2 (OH)](NO 3 )-II, the root-mean-square deviation of fitted atoms is 0.0014 A ˚, with a deviation for N1 of À 0.003 (10) A ˚from the plane defined by O3, O4(x À 1, y, z) and O5.
As shown in Fig. 2, the hydrogen-bonding schemes in the two [Bi 2 O 2 (OH)](NO 3 ) polymorphs are different.Based on the closest O2� � �O contacts between the hydroxide and the nitrate anion, the acceptor changes from O5 The closest contact of O2 to O5 in polymorph-II then is 3.017 (14) A ˚and that of O2 to O4 in polymorph-I is 3.16 A ˚.The differences in hydrogenbonding correlate with the ordering of the (NO 3 ) À anion, which might be the driving force for the Cmc2 1 !Pna2 1 phase transition.A similar situation is found for the double salt (NH 4 ) 2 SeO 4 •3NH 4 NO 3 for which the high-temperature polymorph shows disorder of one of the nitrate groups that is fully resolved for the low-temperature polymorph (Weil et al., 2023).
[Bi 2 O 2 (OH)](NO 3 ) remains the only basic bismuth(III) nitrate for which this structural motif is known so far in the solid state.As shown for numerous other basic bismuth(III) nitrate phases obtained under hydrolytic conditions of Bi(NO 3 ) 3 •5H 2 O, the hexanuclear cation [Bi 6 O 4+x (OH) 4-x ] (6-x)+ with x = 0 and x = 1 was reported to be the predominant species (Nørlund Christensen et al., 2000).Later, Henry et al. (2005) gave a general formula of [Bi 6 O x (OH) 8-x ] (10-x)+ for the compositorial range of this complex cation.
A search of the Inorganic Crystal Structure Database (ICSD, version 2023_1; Zagorac et al., 2019) revealed the following basic bismuth(III) nitrate phases where this complex Comparison of bond lengths (A ˚) in the Pna2 1 and Cmc2 1 structures of [Bi 2 O 2 (OH)](NO 3 ).
O atoms marked with an asterisk show half-occupancy.

Synthesis and crystallization
Crystals of [Bi 2 O 2 (OH)](NO 3 ) were obtained in a hydrothermal reaction as a byproduct from a mixture of Bi(NO 3 ) 3 •-5H 2 O (0.0786 g), Te(OH) 6 (0.0124 g) and KOH (0.0060 g) in a 3:1:2 molar ratio.The reactants were intermixed and 3.62 g of water was added to achieve a 2/3 inner volume of the Teflon container.The reaction vessel was enclosed in a steel autoclave, heated to 473 K and reacted for a period of 69 days under autogenous pressure.The mixture was then cooled to room temperature by removing the autoclave from the oven.The solid material obtained after the reaction time was filtered off through a glass frit, washed with mother liquor, water and ethanol and dried in air.Aside from few light-yellow crystals of [Bi 2 O 2 (OH)](NO 3 ) with a plate-like form, all other products were cryptocrystalline.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Inspection of the diffraction data revealed twinning by a 180 � rotation about the c axis and inversion, which means that the Flack parameter could not be determined.After crystal structure solution, the atomic coordinates and atom labelling were adapted to the Cmc2 1 structure (Henry et al., 2005) for better comparison.The Bi atoms were refined with anisotropic displacement parameters, all other atoms with isotropic displacement parameters each; the H atom of the hydroxide anion (O2) could not be localized.The remaining maximum (3.03 e À A ˚À 3 ) and minimum (À 3.58 e À A ˚À 3 ) electron-density peaks are located 1.63 and 1.47 A ˚away from Bi2 and Bi1, respectively.

Figure 2
Figure 2 Crystal structures of the [Bi 2 O 2 (OH)(NO 3 )] polymorphs.Strong Bi-O bonds (2.20-2.62A ˚) are shown as black lines, weaker Bi-O bonds (2.80-3.00A ˚) as grey lines; colour code for both structures: Bi1 blue, Bi2 light blue, N1 purple, O2 associated with the OH group green, all other O atoms white; the O2� � �O hydrogen bond is displayed with green lines.(a) The Pna2 1 structure of Bi 2 O 2 (OH)](NO 3 )-II with atoms at the 97% probability level; symmetry codes refer to Table 1 and (b) the Cmc2 1 structure of [Bi 2 O 2 (OH)](NO 3 )-I (Henry et al., 2005) with atoms shown as spheres of arbitrary radius.In the inset, the nitrate group is rotated by 60 � to show the disorder present in Bi 2 O 2 (OH)](NO 3 )-I.