Low-temperature modification of Ba(BF4)2(H2O)3

Ba(BF4)2(H2O)3, orthorhombic, space group C2221 at 300 K, becomes monoclinic at 280 K, space group P21. The spread of Ba—F distances is significantly larger in the LT modification, and one of the water molecules in the LT form exhibits positional disorder.


Structure description
Recently, the orthorhombic crystal structure of the compound Ba(BF 4 ) 2 (H 2 O) 3 was reported on the basis of room-temperature (RT) single-crystal data in space group C222 1 (Charkin et al., 2023).The authors observed a phase transition at decreasing temperature but were unable to solve the crystal structure of the low-temperature (LT) modification.
We have now succeeded in solving the crystal structure of LT-Ba(BF 4 ) 2 (H 2 O) 3 .
The asymmetric unit of LT-Ba(BF 4 ) 2 (H 2 O) 3 contains one Ba 2+ cation, two tetrahedral BF 4 À anions and three water molecules, one of which (O3) is disordered over two sets of sites with approximately equal occupancy [ratio 0.56 (2):0.44 (2)].The Ba 2+ cation has a coordination number of 10 and is coordinated by seven F ligands from six BF 4 À anions and by three water ligands (Fig. 1).In anhydrous Ba(BF 4 ) 2 (Bunic ˇet al., 2007), the Ba 2+ cation is surrounded by ten BF 4 À anions.The B(1)F 4 unit in LT-Ba(BF 4 ) 2 (H 2 O) 3 is bound to four Ba 2+ cations, while the B(2)F 4 unit is connected in a chelate mode to one Ba  1).
The RT unit cell in space group C222 1 with a = 7.1763 (6) A ˚, b = 18.052 (2) A ˚, c = 7.1631 (6) A ˚ (Charkin et al., 2023) is related to the LT mP unit cell in P2 1 by the transformation -a, -c, 1/2a + 1/2b, suggesting a translationengleiche symmetry relationship of index 2 (Mu ¨ller, 2013).Considering the significant difference in crystal density for both modifications (2.59 g cm-3 for the RT modification at 300 K, 2.63 g cm À3 for the LT modification at 280 K, and 2.59 g cm À3 at 150 K), it can be assumed that the formation of a structure with more effective packing is the driving force of the observed phase transition.
We also tried to determine the temperature of the phase transition.It is noteworthy that at 280 K the LT modification remains unchanged, with significantly enlarged unit-cell parameters (Table 2).At 300 K, an orthorhombic cell was indexed with 100% of all observed reflections and with similar lattice parameters as given by Charkin et al. (2023).Thus, we can conclude that the ordered oC Ð mP phase transition of Ba(BF 4 ) 2 (H 2 O) 3 (accompanied by twinning of the monoclinic LT phase) occurs between 280 and 300 K.

Synthesis and crystallization
Single crystals of Ba(BF 4 ) 2 (H 2 O) 3 were grown from an aqueous solution of Ba(BF 4 ) 2 .Barium carbonate was added in

Figure 1
The environment of the Ba 2+ cation in the crystal structure of LT-Ba(BF 4 ) 2 (H 2 O) 3 .Displacement ellipsoids are drawn at the 50% probability level.Hydrogen bonds are shown as dashed lines.[Symmetry codes: small portions to 40% wt HBF 4 .After completion of the gaseous CO 2 release, the solution was decanted from residual BaCO 3 .Evaporation of water at room temperature yielded small crystals of Ba(BF 4 ) 2 (H 2 O) 3 .Note that an excess of HBF 4 led to the formation of crystals of anhydrous Ba(BF 4 ) 2 in our experiments.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3.The obtained crystals suffer from racemic twinning and additionally show twinning by pseudo-merohedry at decreasing temperature.To avoid complicated refinement, many crystals were tested until a crystal with a Flack parameter (Flack, 1983) close to zero was found.The twin law corresponding to a 180 rotation around the [100] direction was determined, and the reflection array was indexed as a two-component twin with a negligible amount of non-indexed reflections.Because of the relatively small amount (BASF = 0.26) of the second domain, the final refinement was performed with a HKLF5-type file containing reflections from the first domain and overlapping reflections.
Because of unstable refinement, EADP commands in SHELXL (Sheldrick, 2015) were applied to the pair of disordered O3 atoms and also to the pair of B atoms.Hydrogen atoms were placed on calculated positions and refined with AFIX 7 restrictions.One reflection with an error/ e.s.d.ratio of 5.5 was omitted.

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes.

Figure 2
Figure 2 Crystal structure of LT-Ba(BF 4 ) 2 (H 2 O) 3 in a view approximately along [100].Displacement ellipsoids are the same as in Fig. 1.Display of hydrogen-bonding interactions was omitted for clarity.

Table 3
Experimental details.