Crystal structure of CaSiF6·2H2O(mP2) and reevaluation of the SiIV–F bond-valence parameter R 0

The crystal structure of a second polymorph of CaSiF6·2H2O featuring a layered structure connected by hydrogen bonds is presented.

The structure of a second polymorph of CaSiF 6 •2H 2 O [calcium hexafluoridosilicate dihydrate; space group P2/c (No. 13), Pearson symbol mP2] was elucidated by single-crystal X-ray diffraction.It arose as an unexpected product when soda-lime glass was attacked by HF.Its crystal structure consists of infinite 1 2 [Ca(H 2 O) 2/1 (SiF 6 ) 4/4 ] layers oriented parallel to the bc-crystallographic plane, a unique motif among structurally characterized hydrated hexafluoridosilicates.The crystal structure also exhibits inter-and intralayer hydrogen bonds, with the interlayer O-H� � �O hydrogen bonds involving a disordered hydrogen atom.The large deviation between the calculated bond-valence sum for Si and the expected value prompted a redetermination of the empirical Si IV -F bondvalence parameter R 0 .Based on a data set of 42 high-quality crystal structures containing 49 independent Si IV coordination environments, a revised value of 1.534 A ˚was derived for R 0 .

Chemical context
Calcium hexafluoridosilicate (CaSiF 6 ) and its hydrated form, calcium hexafluoridosilicate dihydrate (CaSiF 6 •2H 2 O), are both commercially available chemicals that have found numerous uses, including as additives for cement manufacture (Smart & Roy, 1979), improving dentine remediation treatments (Kawasaki et al., 1996), and as precursors for synthesis of luminescent materials (Kubus & Meyer, 2013).Although the synthesis of CaSiF 6 •2H 2 O and its dehydration to CaSiF 6 were investigated more than 90 years ago (Carter, 1932), their crystal structures were determined relatively recently by laboratory-based powder X-ray diffraction using simulated annealing and Rietveld refinement (Frisoni et al., 2011).The study revealed that CaSiF 6 •2H 2 O crystallizes in the monoclinic crystal system (space group P2 1 /n, Pearson symbol mP4) and exhibits a three-dimensional framework structure.In this work, the crystal structure of a second polymorph of CaSiF 6 •2H 2 O (space group P2/c, Pearson symbol mP2) was determined by low-temperature single-crystal X-ray diffraction.The observed discrepancies between the calculated and expected bond-valence sum (BVS) for Si also provided the impetus for a reevaluation of the Si IV -F bond-valence parameter R 0 and an improved value of R 0 was determined.

Structural commentary
The crystal structure of CaSiF 6 •2H 2 O(mP2) features eight atoms in the asymmetric unit, with one hydrogen atom disordered over two positions.The Ca atom is located on a twofold rotation axis and the Si atom is situated on an inver-sion centre, whereas the light atoms all lie on general positions.The hexafluoridosilicate anion displays a nearly ideal octahedral coordination, with the cis-F-Si-F angles ranging from 88.37 (4) to 91.63 (4) � .The average Si-F bond length is 1.6859A ˚(Table 1), with the bond lengths ranging from 1.6808 (9) to 1.6942 (9) A ˚, which is in good agreement with the Si-F distances observed in the crystal structures of CaSiF 6 •2H 2 O(mP4) (Frisoni et al., 2011) and SrSiF 6 •2H 2 O (Golovastikov & Belov, 1982), which span from 1.648 (4) to 1.701 (3) A ˚and 1.675 (5) to 1.700 (5) A ˚, respectively.The Ca atom is coordinated by six fluorine atoms at distances of 2.2965 (9)-2.4105(9) A ˚originating from four neighbouring [SiF 6 ] 2-octahedra, two of which are bound to the metal centre in a bidentate and two in a monodentate manner.In turn, each [SiF 6 ] 2-octahedron is coordinated to four Ca 2+ cations.The primary coordination sphere of the Ca 2+ cation is completed by two water molecules, with a Ca-O distance of 2.4331 (13) A ˚, resulting in a distorted square antiprismatic coordination (Fig. 1).Such connectivity leads to the formation of 1 2 [Ca(H 2 O) 2/1 (SiF 6 ) 4/4 ] (Jensen, 1989) infinite layers, which extend along the bc-crystallographic plane and are stacked along the a-axis (Fig. 2), a structural motif that differs from all other hydrated hexafluoridosilicates.Bond-valence sum calculations (Brown, 2009) for Ca and Si using the parameters b = 0.37, R 0 = 1.842A ˚(Ca-F), R 0 = 1.967A ˚(Ca-O), and R 0 = 1.58A ˚(Si-F) obtained from the literature (Brown 2020;Brown & Altermatt, 1985;Brese & O'Keeffe, 1991), yielded 2.05 valence units (v.u.) for Ca and 4.51 v.u. for Si (expected values: 2 for Ca, 4 for Si).Similarly inflated values for the bond-valence sum of Si were also observed when other crystal structures of hexafluoridosilicates were examined, indicating the need to reevaluate the current Si IV -F parameter R 0 (Section 5).

Figure 1
The distorted square antiprismatic coordination environment of the Ca 2+ cation in the crystal structure of CaSiF 6 •2H 2 O(mP2).Displacement ellipsoids are drawn at the 50% probability level and hydrogen atoms shown as spheres of arbitrary radius.Hydrogen atom H2 is disordered over two sites with occupancies 0.49 (5) and 0.51 (5) [Symmetry codes: with the intralayer O-H� � �F hydrogen bonds depicted as dashed lines.

Figure 3
Selected fragment of the crystal structure of CaSiF 6 •2H 2 O(mP2) displaying intra-and interlayer hydrogen bonds, which connect the adjacent layers.Some of the disordered hydrogen atoms have been omitted for clarity.

Redetermination of Si IV -F bond-valence parameter R 0
In order to determine a more accurate value of the Si IV -F bond-valence parameter R 0 , the ICSD was searched for all crystal structures containing Si IV in an exclusively fluorine environment.To ensure that only high-quality data were used for the calculation of the R 0 parameter, the data set was limited to crystal structures solved by single-crystal X-ray diffraction at ambient or low-temperature conditions, excluding disordered structures or those with an R 1 -value above 0.05.A data set of 42 crystal structures was obtained, containing a total of 49 independent Si IV coordination envir- onments, including the compound presented herein (Table 3).
The R 0i value for each individual Si coordination environment was calculated using formula (A1.3) from the literature (Brown, 2002), which assumes a fixed value for the b parameter (0.37 A ˚).An improved value for the R 0 parameter, 1.534A ˚, was obtained by averaging the R 0i values, which ranged from 1.508 to 1.562 A ˚. BVS calculations employing the new empirical parameter yield significantly improved results compared to the calculations performed with the previously reported parameter, as 46 out of 49 evaluated coordination environments give a bond-valence sum within �0.2 v.u. of the expected value (3.8-4.2 v.u.), in contrast to only a single one when using the old parameter (Table 4).

Synthesis and crystallization
Colourless single crystals of the title compound were discovered to have grown serendipitously on a soda-lime watch glass containing a sample of [XeF][SbF 6 ] ( Gillespie & Landa, 1973) frozen under a protective layer of perfluorodecalin at 255 K.It is presumed that CaSiF 6 •2H 2 O(mP2) formed when the sodalime glass was attacked by the HF forming during hydrolysis of the highly oxidizing Xe II compound.

Raman spectroscopy
A Bruker Senterra II confocal Raman microscope was used to record the Raman spectrum on a randomly oriented single crystal of the title compound.The spectrum was measured at room temperature (297 K) in the 50-4250 cm À 1 range with a resolution of 4 cm À 1 using the 532 nm laser line operating at 12.5 mW.
In the Raman spectrum of CaSiF 6 •2H 2 O(mP2) (Fig. 5) the bands observed at 677 and 500 cm À 1 correspond to the � 1 and � 2 modes of the [SiF 6 ] 2-anion, respectively.The bands at 425 and 392 cm À 1 can be assigned to the � 5 mode, split due to the distortion of the anion from the ideal O h symmetry (Ouasri et al., 2002).The Raman bands observed in the 3300-3600 cm À 1 region belong to the symmetric � 1 and antisymmetric � 3 O-H stretching of the coordinated water molecules, whereas the bands at 1649 and 3225 cm À 1 could likely be assigned to �(HOH) (� 2 ) and 2�(HOH), respectively (Lacroix et al., 2018).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5.The positions of the hydrogen atoms, including the disordered one, were located in difference maps and freely refined, including their isotropic thermal parameter U iso (Cooper et al., 2010).The refinement of the disordered hydrogen atoms' occupancies, resulted in values of 0.51 (5) and 0.49 (5) for H2A and H2B, respectively.(Dolomanov et al., 2009), DIAMOND (Brandenburg, 2005) and publCIF (Westrip, 2010).

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. Fractional

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. Fractional
2 O(mP4) is coordinated by five fluorine and three oxygen atoms arranged in a distorted square-antiprismatic coordination.Each of the five fluorine atoms coordinated to the Ca 2+ ion belongs to a separate [SiF 6 ] 2-octahedron, which contrasts with the structure of the newly discovered mP2 polymorph, where both monodentate and bidentate coordination of the [SiF 6 ] 2-anions to the Ca 2+ cations is observed (

Table 3
Crystal structures used for the calculation of the new empirical R 0 bond-valence parameter for Si IV -F.

Table 4
Comparison of the BVS calculation results for Si IV of crystal structures collected in Table3employing the new R 0 parameter and the previously reported parameter.

Table 5
Experimental details.