Crystal structure of CaSiF6·2H2O(mP2) and reevaluation of the SiIV–F bond-valence parameter R0

Motaln, K.; Lozinšek, M.

doi:10.1107/S2056989023009349

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Volume 79| Part 12| December 2023| Pages 1121-1126

https://doi.org/10.1107/S2056989023009349

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Crystal structure of CaSiF₆·2H₂O(mP2) and reevaluation of the Si^IV–F bond-valence parameter R₀

Klemen Motaln ^a,^b and Matic Lozinšek ^a,^b ^*

^aJožef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia, and ^bJožef Stefan International Postgraduate School, Jamova cesta 39, 1000 Ljubljana, Slovenia
^*Correspondence e-mail: matic.lozinsek@ijs.si

Edited by W. T. A. Harrison, University of Aberdeen, United Kingdom (Received 17 October 2023; accepted 25 October 2023; online 2 November 2023)

The structure of a second polymorph of CaSiF₆·2H₂O [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 _∞²[Ca(H₂O)_2/1(SiF₆)_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 bond-valence parameter R₀. Based on a data set of 42 high-quality crystal structures containing 49 independent Si^IV coordination environments, a revised value of 1.534 Å was derived for R₀.

Keywords: calcium hexafluoridosilicate; bond-valence parameter; crystal structure; disorder; hydrogen bonding.

CCDC reference: 2303630

1. Chemical context

Calcium hexafluoridosilicate (CaSiF₆) and its hydrated form, calcium hexafluoridosilicate dihydrate (CaSiF₆·2H₂O), 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 CaSiF₆·2H₂O and its dehydration to CaSiF₆ 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 CaSiF₆·2H₂O crystallizes in the monoclinic crystal system (space group P2₁/n, Pearson symbol mP4) and exhibits a three-dimensional framework structure. In this work, the crystal structure of a second polymorph of CaSiF₆·2H₂O (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 R₀ and an improved value of R₀ was determined.

2. Structural commentary

The crystal structure of CaSiF₆·2H₂O(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 inversion 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 1.6859 Å (Table 1), with the bond lengths ranging from 1.6808 (9) to 1.6942 (9) Å, which is in good agreement with the Si—F distances observed in the crystal structures of CaSiF₆·2H₂O(mP4) (Frisoni et al., 2011) and SrSiF₆·2H₂O (Golovastikov & Belov, 1982 ), which span from 1.648 (4) to 1.701 (3) Å and 1.675 (5) to 1.700 (5) Å, respectively. The Ca atom is coordinated by six fluorine atoms at distances of 2.2965 (9)–2.4105 (9) Å originating from four neighbouring [SiF₆]^2– octahedra, two of which are bound to the metal centre in a bidentate and two in a monodentate manner. In turn, each [SiF₆]^2– octahedron is coordinated to four Ca²⁺ cations. The primary coordination sphere of the Ca²⁺ cation is completed by two water molecules, with a Ca—O distance of 2.4331 (13) Å, resulting in a distorted square antiprismatic coordination (Fig. 1). Such connectivity leads to the formation of _∞²[Ca(H₂O)_2/1(SiF₆)_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, R₀ = 1.842 Å (Ca–F), R₀ = 1.967 Å (Ca–O), and R₀ = 1.58 Å (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 R₀ (Section 5).

Table 1
Selected bond lengths (Å)

Ca1—F1	2.2965 (9)	Si1—F1	1.6809 (9)
Ca1—F2ⁱ	2.3783 (9)	Si1—F2	1.6827 (9)
Ca1—F3ⁱ	2.4105 (9)	Si1—F3	1.6942 (9)
Ca1—O1	2.4331 (13)
Symmetry code: (i) $[x, -y+1, z-{\script{1\over 2}}]$ .

Figure 1
The distorted square antiprismatic coordination environment of the Ca²⁺ cation in the crystal structure of CaSiF₆·2H₂O(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: (i) −x, y, −z + $[{1\over 2}]$ ; (ii) x, −y + 1, z − $[{1\over 2}]$ ; (iii) −x, −y + 1, −z + 1.]

Figure 2
A single _∞²[Ca(H₂O)_2/1(SiF₆)_4/4] layer viewed along [100], with the intralayer O—H⋯F hydrogen bonds depicted as dashed lines.

3. Supramolecular features

The crystal structure of CaSiF₆·2H₂O(mP2) exhibits both intralayer O—H⋯F and interlayer O—H⋯O hydrogen bonds (Table 2, Fig. 3). The intralayer hydrogen bonds are formed between the F3 atom and the non-disordered hydrogen atom H1, with an O1⋯F3 distance of 2.9042 (14) Å and a graph-set motif of S(6) (Etter et al., 1990 ). The oxygen atom O1 is involved in two further hydrogen bonds with the disordered hydrogen atoms H2A and H2B, forming O1—H2A⋯O1 and O1—H2B⋯O1 hydrogen bonds, with O⋯O distances of 2.902 (3) and 2.856 (3) Å, respectively, that link the adjacent _∞²[Ca(H₂O)_2/1(SiF₆)_4/4] layers.

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O1—H1⋯F3ⁱⁱ	0.78 (3)	2.19 (3)	2.9042 (14)	153 (3)
O1—H2B⋯O1ⁱⁱⁱ	0.90 (5)	1.98 (5)	2.856 (3)	167 (4)
O1—H2A⋯O1^iv	0.77 (5)	2.17 (5)	2.902 (3)	159 (4)
Symmetry codes: (ii) ; (iii) ; (iv) $[-x+1, y, -z+{\script{1\over 2}}]$ .

Figure 3
Selected fragment of the crystal structure of CaSiF₆·2H₂O(mP2) displaying intra- and interlayer hydrogen bonds, which connect the adjacent layers. Some of the disordered hydrogen atoms have been omitted for clarity.

4. Database survey

A search of the Inorganic Crystal Structure Database (ICSD, version January 2023; Bergerhoff et al., 1983 ; Zagorac et al., 2019 ) revealed that in addition to the aforementioned mP4 polymorph of CaSiF₆·2H₂O, twelve other hydrated hexafluoridosilicates of divalent cations have been crystallographically characterized to date. Most of them form hexahydrates with the general formula MSiF₆·6H₂O, where M = Mg (Syoyama & Osaki, 1972 ; Cherkasova et al., 2004 ), Cr (Cotton et al., 1992 ), Mn (Torii et al., 1997 ), Fe (Hamilton, 1962 ; Chevrier et al., 1981 ), Co (Lynton & Siew; 1973 ; Ray et al., 1973a ; Ray & Mostafa, 1996 ), Ni (Ray et al., 1973a), Cu (Ray et al., 1973b ), and Zn (Ray et al., 1973a). The aforementioned compounds all exhibit a similar structural motif composed of alternating discrete [M(H₂O)₆]²⁺ and [SiF₆]^2– octahedra, connected via O—H⋯F hydrogen bonds into a three-dimensional network. The only examples of tetrahydrated metal(II) hexafluoridosilicates are the isostructural CrSiF₆·4H₂O (Cotton et al., 1993 ) and CuSiF₆·4H₂O (Clark et al., 1969 ; Schnering & Vu, 1983 ; Troyanov et al., 1992 ; Cotton et al., 1993). In their crystal structures, infinite zigzag chains are formed by the coordination of two [SiF₆]^2– octahedra to the apical positions of the square-planar [M(H₂O)₄]²⁺ units. The resulting highly distorted octahedral coordination surrounding the metal centre is characteristic of the Jahn–Teller active cations. The individual chains in the structures are connected by O—H⋯F hydrogen bonds that link the terminal fluorine atoms of the [SiF₆]^2– units to the water molecules coordinating the metal centres of the adjacent chains. Lastly there are three examples of metal(II) hexafluoridosilicate dihydrates, the isostructural pair CaSiF₆·2H₂O(mP4) (Frisoni et al., 2011) and SrSiF₆·2H₂O (Golovastikov & Belov, 1982), and PbSiF₆·2H₂O (Golubev et al., 1991 ). All three compounds feature an extended three-dimensional framework structure and display water molecules bridging the metal centres, giving rise to dimeric [(H₂O)M(μ-H₂O)₂M(OH₂)]⁴⁺ units for M = Ca, Sr and the more complex [Pb₄(H₂O)₆]⁸⁺ units in the structure of PbSiF₆·2H₂O, which contain both μ- and μ₃-water molecules. The Ca²⁺ cation in CaSiF₆·2H₂O(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 Ca²⁺ ion belongs to a separate [SiF₆]^2– octahedron, which contrasts with the structure of the newly discovered mP2 polymorph, where both monodentate and bidentate coordination of the [SiF₆]^2– anions to the Ca²⁺ cations is observed (Fig. 4). Conversely, each [SiF₆]^2– anion in the structure of CaSiF₆·2H₂O(mP4) coordinates five neighbouring Ca²⁺ cations, leaving one terminal fluorine atom, which in turn accepts O—H⋯F hydrogen bonds from two water ligands.

Figure 4
Comparison of the crystal structures of CaSiF₆·2H₂O(mP4) (top) and CaSiF₆·2H₂O(mP2) (bottom), viewed along [010].

5. Redetermination of Si^IV–F bond-valence parameter R₀

In order to determine a more accurate value of the Si^IV–F bond-valence parameter R₀, 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 R₀ 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 R₁-value above 0.05. A data set of 42 crystal structures was obtained, containing a total of 49 independent Si^IV coordination environments, 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 Å). An improved value for the R₀ parameter, 1.534 Å, was obtained by averaging the R_0i values, which ranged from 1.508 to 1.562 Å. 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).

Table 3
Crystal structures used for the calculation of the new empirical R₀ bond-valence parameter for Si^IV–F

Compound	ICSD number	Reference	Si—F bond-length range (Å)	BVS for Si (R₀ from Brese & O'Keeffe, 1991)	BVS for Si (new R₀)
BaSiF₆	60882	(Svensson et al., 1986 )	1.688 (2)	4.481	3.968
(CH₃NH₃)₂SiF₆	110673	(Conley et al., 2002 )	1.6810 (12)–1.6828 (17)	4.559	4.037
(CH₇N₄)₂SiF₆·2H₂O	280103	(Ross et al., 1999 )	1.6797 (9)–1.6808 (9)	4.578	4.054
(CH₈N₄)SiF₆	280102	(Ross et al., 1999)	1.6684 (9)–1.7043 (9)	4.529	4.010
(C(NH₂)₂OH)₂SiF₆	63069	(Gubin et al., 1988 )	1.677 (2)–1.6971 (18)	4.513	3.996
(C(NH₂)₃)₂SiF₆	59237	(Waskowska, 1997 )	1.6805 (12)–1.6833 (8)	4.550	4.029
(C₄H₁₃N₅)SiF₆	166449	(Gel'mbol'dt et al., 2009 )	1.657 (3)–1.698 (3)	4.643	4.111
CaSiF₆·2H₂O(mP2)	Present work		1.6808 (9)–1.6942 (9)	4.507	3.991
[Co(NH₃)₅(NO₂)]SiF₆	280030	(Naumov et al., 1999 )	1.6769 (18)–1.6899 (13)	4.495	3.981
CrSiF₆·4H₂O	165384	(Cotton et al., 1993)	1.6640 (8)–1.6968 (8)	4.546	4.026
CsLiSiF₆	142874	(Stoll et al., 2021 )	1.667 (2)–1.699 (2)	4.479	3.966
[Cu(bpy)₂(H₂O)]SiF₆·4H₂O	133607	(Nisbet et al., 2021 )	1.6677 (10)–1.6947 (9)	4.574	4.050
[Cu{SC(NH₂)₂}₄]₂SiF₆	249750	(Bowmaker et al., 2008 )	1.663 (2)–1.696 (2)	4.585	4.060
CuSiF₆·4H₂O	165385	(Cotton et al., 1993)	1.6686 (8)–1.6973 (9)	4.510	3.993
CuSiF₆·6H₂O	34760	(Ray et al., 1973b)	1.679 (5)	4.591	4.066
			1.659 (6)–1.674 (6)	4.765	4.219
H₂SiF₆·4H₂O	40388	(Mootz & Oellers, 1988 )	1.666 (1)–1.696 (1)	4.553	4.031
H₂SiF₆·6H₂O	40389	(Mootz & Oellers, 1988)	1.677 (1)–1.704 (1)	4.447	3.938
H₂SiF₆·9.5H₂O	40390	(Mootz & Oellers, 1988)	1.680 (1)–1.697 (1)	4.454	3.944
			1.684 (1)–1.706 (1)	4.448	3.939
K₂SiF₆(cF4)	420429	(Kutoglu et al., 2009 )	1.6873 (16)	4.490	3.975
K₂SiF₆(hP2)	158483	(Gramaccioli & Campostrini, 2007 )	1.681 (2)–1.689 (2)	4.518	4.000
K₂SiF₆·KNO₃	417735	(Rissom et al., 2008 )	1.6782 (6)	4.601	4.074
KLiSiF₆	142875	(Stoll et al., 2021)	1.676 (1)–1.701 (1)	4.495	3.980
KNaSiF₆	71334	(Fischer & Krämer, 1991 )	1.641 (5)–1.678 (5)	4.860	4.304
K₃Na(SiF₆)(TaF₇)	122403	(Tang et al., 2021 )	1.665 (3)–1.702 (3)	4.558	4.036
K₃Na₄(BF₄)(SiF₆)₃	121301	(Bandemehr et al., 2020 )	1.650 (2)–1.699 (2)	4.535	4.015
			1.666 (2)–1.700 (1)	4.560	4.038
Li₂SiF₆	425923	(Hinteregger et al., 2014 )	1.685 (2)	4.518	4.000
			1.690 (2)–1.690 (8)	4.457	3.947
MgSiF₆·6H₂O	250196	(Cherkasova et al., 2004)	1.6888 (9)–1.7465 (10)	4.194	3.714
MnSiF₆·6H₂O	59274	(Torii et al., 1997)	1.690 (7)	4.457	3.947
			1.668 (7)–1.693 (7)	4.575	4.051
(NH₃OH)₂SiF₆·2H₂O	94567	(Kristl et al., 2002 )	1.6793 (10)–1.6837 (10)	4.570	4.046
(NH₄)₂SiF₆	54724	(Fábry et al., 2001 )	1.695 (1)–1.700 (1)	4.368	3.867
(N₂H₅)₂SiF₆	776	(Ouasri et al., 2019 )	1.6777 (4)–1.7101 (4)	4.476	3.963
(N₂H₆)SiF₆	35702	(Cameron et al., 1983 )	1.671 (1)–1.683 (1)	4.596	4.070
Na₂SiF₆	433134	(Zhang et al., 2017 )	1.6755 (14)–1.6756 (14)	4.635	4.104
			1.6907 (16)–1.6916 (11)	4.443	3.934
PbSiF₆·2H₂O	39358	(Golubev et al., 1991)	1.645 (10)–1.707 (10)	4.558	4.036
			1.664 (10)–1.716 (10)	4.411	3.906
Rb₂SiF₆	136303	(Rienmüller et al., 2021 )	1.693 (3)	4.421	3.915
[RuF(NH₃)₄(NO)]SiF₆	703	(Mikhailov et al., 2019 )	1.661 (1)–1.713 (2)	4.556	4.035
[Ru₂(H₂O)₂(NH₄)₈S₂](SiF₆)₂	111446	(Woods & Wilson, 2021 )	1.666 (2)–1.7065 (19)	4.552	4.031
SiF₄	48147	(Mootz & Korte, 1984 )	1.5401 (6)	4.455	3.945
SrSiF₆·2H₂O	20552	(Golovastikov & Belov, 1982)	1.675 (5)–1.700 (5)	4.502	3.987
[Tl₂(NH₃)₆]SiF₆·2NH₃	144214	(Rudel et al., 2021 )	1.687 (2)–1.6877 (15)	4.488	3.974
Tl₂SiF₆	136300	(Rienmüller et al., 2021)	1.686 (6)	4.505	3.989
Tl₃F[SiF₆]	136302	(Rienmüller et al., 2021)	1.688 (6)–1.695 (6)	4.439	3.931

Table 4
Comparison of the BVS calculation results for Si^IV of crystal structures collected in Table 3 employing the new R₀ parameter and the previously reported parameter

	R₀	Maximum BVS	Minimum BVS	Mean BVS	Standard deviation	% of data within ± 0.2 v.u.	% of data within ± 0.1 v.u.
This study	1.534	4.304	3.714	4.005	0.086	93.9	87.8
Brese & O'Keeffe (1991)	1.58	4.860	4.194	4.522	0.098	2.0	0

6. 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][SbF₆] (Gillespie & Landa, 1973 ) frozen under a protective layer of perfluorodecalin at 255 K. It is presumed that CaSiF₆·2H₂O(mP2) formed when the soda-lime glass was attacked by the HF forming during hydrolysis of the highly oxidizing Xe^II compound.

7. 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⁻¹ range with a resolution of 4 cm⁻¹ using the 532 nm laser line operating at 12.5 mW.

In the Raman spectrum of CaSiF₆·2H₂O(mP2) (Fig. 5) the bands observed at 677 and 500 cm⁻¹ correspond to the ν₁ and ν₂ modes of the [SiF₆]^2– anion, respectively. The bands at 425 and 392 cm⁻¹ can be assigned to the ν₅ 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⁻¹ region belong to the symmetric ν₁ and antisymmetric ν₃ O—H stretching of the coordinated water molecules, whereas the bands at 1649 and 3225 cm⁻¹ could likely be assigned to δ(HOH) (ν₂) and 2δ(HOH), respectively (Lacroix et al., 2018 ).

Figure 5
Raman spectrum of CaSiF₆·2H₂O(mP2).

8. 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.

Table 5
Experimental details

Crystal data
Chemical formula	CaSiF₆·2H₂O
M_r	218.20
Crystal system, space group	Monoclinic, P2/c
Temperature (K)	100
a, b, c (Å)	5.96605 (17), 5.13977 (12), 9.9308 (3)
β (°)	107.275 (3)
V (Å³)	290.78 (1)
Z	2
Radiation type	Cu Kα
μ (mm⁻¹)	12.29
Crystal size (mm)	0.15 × 0.08 × 0.02

Data collection
Diffractometer	XtaLAB Synergy-S, Dualflex, Eiger2 R CdTe 1M
Absorption correction	Gaussian (CrysAlis PRO; Rigaku OD, 2022 )
T_min, T_max	0.365, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	8322, 608, 598
R_int	0.051
(sin θ/λ)_max (Å⁻¹)	0.628

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.025, 0.070, 1.14
No. of reflections	608
No. of parameters	61
H-atom treatment	All H-atom parameters refined
Δρ_max, Δρ_min (e Å⁻³)	0.32, −0.37
Computer programs: CrysAlis PRO (Rigaku OD, 2022), SUPERFLIP (Palatinus & Chapuis, 2007 ), SHELXL2019/2 (Sheldrick, 2015 ), OLEX2 (Dolomanov et al., 2009 ), DIAMOND (Brandenburg, 2005 ) and publCIF (Westrip, 2010 ).

Supporting information

CCDC reference: 2303630

Crystal structure: contains datablock I. DOI: https://doi.org/10.1107/S2056989023009349/hb8080sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989023009349/hb8080Isup2.hkl

checkCIF report

Computing details top

Calcium hexafluoridosilicate dihydrate top

Crystal data top

CaSiF₆·2H₂O	F(000) = 216
M_r = 218.20	D_x = 2.492 Mg m⁻³
Monoclinic, P2/c	Cu Kα radiation, λ = 1.54184 Å
a = 5.96605 (17) Å	Cell parameters from 5902 reflections
b = 5.13977 (12) Å	θ = 7.8–75.3°
c = 9.9308 (3) Å	µ = 12.29 mm⁻¹
β = 107.275 (3)°	T = 100 K
V = 290.78 (1) Å³	Plate, colourless
Z = 2	0.15 × 0.08 × 0.02 mm

Data collection top

XtaLAB Synergy-S, Dualflex, Eiger2 R CdTe 1M diffractometer	608 independent reflections
Radiation source: micro-focus sealed X-ray tube, PhotonJet (Cu) X-ray Source	598 reflections with I > 2σ(I)
Mirror monochromator	R_int = 0.051
Detector resolution: 13.3333 pixels mm^-1	θ_max = 75.4°, θ_min = 7.8°
ω scans	h = −7→7
Absorption correction: gaussian (CrysalisPro; Rigaku OD, 2022)	k = −6→6
T_min = 0.365, T_max = 1.000	l = −12→12
8322 measured reflections

Refinement top

Refinement on F²	Primary atom site location: iterative
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F² > 2σ(F²)] = 0.025	All H-atom parameters refined
wR(F²) = 0.070	w = 1/[σ²(F_o²) + (0.0516P)² + 0.0488P] where P = (F_o² + 2F_c²)/3
S = 1.14	(Δ/σ)_max < 0.001
608 reflections	Δρ_max = 0.32 e Å⁻³
61 parameters	Δρ_min = −0.37 e Å⁻³
0 restraints

Special details top

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 atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq	Occ. (<1)
Ca1	0.000000	0.58252 (7)	0.250000	0.01360 (19)
Si1	0.000000	0.000000	0.500000	0.0135 (2)
F1	−0.06447 (16)	0.26688 (17)	0.39793 (9)	0.0187 (3)
F2	0.20424 (15)	0.17052 (18)	0.62125 (9)	0.0178 (2)
F3	−0.19861 (16)	0.09149 (15)	0.58239 (10)	0.0164 (3)
O1	0.3884 (2)	0.4033 (2)	0.36145 (15)	0.0190 (3)
H1	0.378 (5)	0.255 (6)	0.373 (3)	0.035 (7)*
H2B	0.469 (8)	0.483 (9)	0.441 (5)	0.016 (13)*	0.49 (5)
H2A	0.475 (8)	0.419 (7)	0.318 (5)	0.017 (13)*	0.51 (5)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Ca1	0.0176 (3)	0.0092 (3)	0.0144 (3)	0.000	0.00529 (18)	0.000
Si1	0.0181 (3)	0.0090 (3)	0.0140 (3)	0.0000 (2)	0.0059 (2)	0.0000 (2)
F1	0.0251 (5)	0.0124 (4)	0.0201 (5)	0.0025 (4)	0.0091 (4)	0.0041 (3)
F2	0.0189 (5)	0.0155 (4)	0.0190 (5)	−0.0010 (3)	0.0056 (4)	−0.0037 (3)
F3	0.0201 (5)	0.0120 (5)	0.0185 (5)	−0.0003 (3)	0.0080 (4)	−0.0020 (3)
O1	0.0199 (6)	0.0145 (6)	0.0224 (6)	−0.0008 (4)	0.0058 (5)	0.0006 (4)

Geometric parameters (Å, º) top

Ca1—Si1ⁱ	3.2815 (3)	Si1—F1^vi	1.6808 (9)
Ca1—Si1ⁱⁱ	3.2815 (3)	Si1—F1	1.6809 (9)
Ca1—F1	2.2965 (9)	Si1—F2^vi	1.6826 (9)
Ca1—F1ⁱⁱⁱ	2.2965 (9)	Si1—F2	1.6827 (9)
Ca1—F2^iv	2.3783 (9)	Si1—F3	1.6942 (9)
Ca1—F2^v	2.3783 (9)	Si1—F3^vi	1.6942 (9)
Ca1—F3^iv	2.4105 (9)	O1—H1	0.78 (3)
Ca1—F3^v	2.4105 (9)	O1—H2B	0.90 (5)
Ca1—O1	2.4331 (13)	O1—H2A	0.77 (5)
Ca1—O1ⁱⁱⁱ	2.4331 (13)

Si1ⁱⁱ—Ca1—Si1ⁱ	98.328 (10)	O1—Ca1—Si1ⁱ	112.20 (3)
F1ⁱⁱⁱ—Ca1—Si1ⁱ	86.58 (2)	O1ⁱⁱⁱ—Ca1—Si1ⁱⁱ	112.20 (3)
F1—Ca1—Si1ⁱ	169.60 (2)	O1ⁱⁱⁱ—Ca1—Si1ⁱ	96.74 (3)
F1ⁱⁱⁱ—Ca1—Si1ⁱⁱ	169.60 (2)	O1ⁱⁱⁱ—Ca1—O1	135.49 (6)
F1—Ca1—Si1ⁱⁱ	86.58 (2)	Ca1^vii—Si1—Ca1^v	180.0
F1—Ca1—F1ⁱⁱⁱ	90.11 (4)	F1—Si1—Ca1^v	82.60 (3)
F1—Ca1—F2^iv	158.57 (3)	F1—Si1—Ca1^vii	97.40 (3)
F1—Ca1—F2^v	79.82 (3)	F1^vi—Si1—Ca1^v	97.40 (3)
F1ⁱⁱⁱ—Ca1—F2^iv	79.82 (3)	F1^vi—Si1—Ca1^vii	82.60 (3)
F1ⁱⁱⁱ—Ca1—F2^v	158.57 (3)	F1^vi—Si1—F1	180.0
F1—Ca1—F3^iv	142.32 (3)	F1^vi—Si1—F2^vi	89.65 (5)
F1—Ca1—F3^v	100.99 (3)	F1—Si1—F2^vi	90.35 (5)
F1ⁱⁱⁱ—Ca1—F3^v	142.32 (3)	F1—Si1—F2	89.65 (5)
F1ⁱⁱⁱ—Ca1—F3^iv	100.99 (3)	F1^vi—Si1—F2	90.35 (5)
F1—Ca1—O1	76.07 (4)	F1—Si1—F3	89.83 (4)
F1ⁱⁱⁱ—Ca1—O1ⁱⁱⁱ	76.07 (4)	F1—Si1—F3^vi	90.17 (4)
F1ⁱⁱⁱ—Ca1—O1	72.88 (4)	F1^vi—Si1—F3	90.17 (4)
F1—Ca1—O1ⁱⁱⁱ	72.88 (4)	F1^vi—Si1—F3^vi	89.83 (4)
F2^v—Ca1—Si1ⁱⁱ	29.44 (2)	F2—Si1—Ca1^v	44.00 (3)
F2^v—Ca1—Si1ⁱ	99.95 (3)	F2—Si1—Ca1^vii	136.00 (3)
F2^iv—Ca1—Si1ⁱ	29.44 (2)	F2^vi—Si1—Ca1^v	136.00 (3)
F2^iv—Ca1—Si1ⁱⁱ	99.95 (3)	F2^vi—Si1—Ca1^vii	44.00 (3)
F2^iv—Ca1—F2^v	115.49 (5)	F2^vi—Si1—F2	180.0
F2^iv—Ca1—F3^v	77.00 (3)	F2—Si1—F3^vi	91.63 (4)
F2^v—Ca1—F3^v	58.87 (3)	F2^vi—Si1—F3	91.63 (4)
F2^v—Ca1—F3^iv	77.00 (3)	F2—Si1—F3	88.37 (4)
F2^iv—Ca1—F3^iv	58.87 (3)	F2^vi—Si1—F3^vi	88.37 (4)
F2^v—Ca1—O1ⁱⁱⁱ	82.87 (4)	F3—Si1—Ca1^v	45.25 (3)
F2^iv—Ca1—O1ⁱⁱⁱ	121.89 (4)	F3^vi—Si1—Ca1^v	134.75 (3)
F2^iv—Ca1—O1	82.87 (4)	F3—Si1—Ca1^vii	134.75 (3)
F2^v—Ca1—O1	121.89 (4)	F3^vi—Si1—Ca1^vii	45.25 (3)
F3^v—Ca1—Si1ⁱⁱ	29.94 (2)	F3—Si1—F3^vi	180.0
F3^v—Ca1—Si1ⁱ	87.56 (2)	Si1—F1—Ca1	155.57 (5)
F3^iv—Ca1—Si1ⁱ	29.94 (2)	Si1—F2—Ca1^v	106.56 (4)
F3^iv—Ca1—Si1ⁱⁱ	87.56 (2)	Si1—F3—Ca1^v	104.81 (4)
F3^v—Ca1—F3^iv	91.93 (4)	Ca1—O1—H1	110 (2)
F3^v—Ca1—O1	75.09 (4)	Ca1—O1—H2B	115 (3)
F3^v—Ca1—O1ⁱⁱⁱ	141.61 (4)	Ca1—O1—H2A	114 (3)
F3^iv—Ca1—O1	141.61 (4)	H1—O1—H2B	111 (3)
F3^iv—Ca1—O1ⁱⁱⁱ	75.09 (4)	H1—O1—H2A	107 (3)
O1—Ca1—Si1ⁱⁱ	96.74 (3)

Ca1^v—Si1—F1—Ca1	115.56 (12)	F2^vi—Si1—F1—Ca1	−108.01 (13)
Ca1^vii—Si1—F1—Ca1	−64.44 (12)	F2—Si1—F1—Ca1	71.99 (13)
Ca1^vii—Si1—F2—Ca1^v	180.000 (1)	F2^vi—Si1—F3—Ca1^v	−170.07 (5)
Ca1^vii—Si1—F3—Ca1^v	180.000 (1)	F2—Si1—F3—Ca1^v	9.93 (5)
F1^vi—Si1—F2—Ca1^v	−100.32 (4)	F3^vi—Si1—F1—Ca1	−19.64 (13)
F1—Si1—F2—Ca1^v	79.69 (4)	F3—Si1—F1—Ca1	160.36 (13)
F1—Si1—F3—Ca1^v	−79.72 (4)	F3^vi—Si1—F2—Ca1^v	169.84 (5)
F1^vi—Si1—F3—Ca1^v	100.28 (4)	F3—Si1—F2—Ca1^v	−10.16 (5)

Symmetry codes: (i) −x, y+1, −z+1/2; (ii) x, y+1, z; (iii) −x, y, −z+1/2; (iv) x, −y+1, z−1/2; (v) −x, −y+1, −z+1; (vi) −x, −y, −z+1; (vii) x, y−1, z.

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1···F3^vi	0.78 (3)	2.19 (3)	2.9042 (14)	153 (3)
O1—H2B···O1^viii	0.90 (5)	1.98 (5)	2.856 (3)	167 (4)
O1—H2A···O1^ix	0.77 (5)	2.17 (5)	2.902 (3)	159 (4)

Symmetry codes: (vi) −x, −y, −z+1; (viii) −x+1, −y+1, −z+1; (ix) −x+1, y, −z+1/2.

Comparison of the BVS calculation results for Si^IV of crystal structures collected in Table 2 employing the new R₀ parameter and the previously reported parameter top

	R₀	Maximum BVS	Minimum BVS	Mean BVS	Standard deviation	% of data within ± 0.2 v.u.	% of data within ± 0.1 v.u.
This study	1.534	4.304	3.714	4.005	0.086	93.9	87.8
Brese & O'Keeffe (1991)	1.58	4.860	4.194	4.522	0.098	2.0	0

Crystal structures used for the calculation of the new empirical R₀ bond-valence parameter for SiSi^IV–F top

Compound	ICSD number	Reference	Si—F bond-length range (Å)	BVS for Si (R₀ from Brese & O'Keeffe, 1991)	BVS for Si (new R₀)
BaSiF₆	60882	(Svensson et al., 1986)	1.688 (2)	4.481	3.968
(CH₃NH₃)₂SiF₆	110673	(Conley et al., 2002)	1.6810 (12)–1.6828 (17)	4.559	4.037
(CH₇N₄)₂SiF₆·2H₂O	280103	(Ross et al., 1999)	1.6797 (9)–1.6808 (9)	4.578	4.054
(CH₈N₄)SiF₆	280102	(Ross et al., 1999)	1.6684 (9)–1.7043 (9)	4.529	4.010
(C(NH₂)₂OH)₂SiF₆	63069	(Gubin et al., 1988)	1.677 (2)–1.6971 (18)	4.513	3.996
(C(NH₂)₃)₂SiF₆	59237	(Waskowska, 1997)	1.6805 (12)–1.6833 (8)	4.550	4.029
(C₄H₁₃N₅)SiF₆	166449	(Gel'mbol'dt et al., 2009)	1.657 (3)–1.698 (3)	4.643	4.111
CaSiF₆·2H₂O(mP2)	Present work		1.6808 (9)–1.6942 (9)	4.507	3.991
[Co(NH₃)₅(NO₂)]SiF₆	280030	(Naumov et al., 1999)	1.6769 (18)–1.6899 (13)	4.495	3.981
CrSiF₆·4H₂O	165384	(Cotton et al., 1993)	1.6640 (8)–1.6968 (8)	4.546	4.026
CsLiSiF₆	142874	(Stoll et al., 2021)	1.667 (2)–1.699 (2)	4.479	3.966
[Cu(bpy)₂(H₂O)]SiF₆·4H₂O	133607	(Nisbet et al., 2021)	1.6677 (10)–1.6947 (9)	4.574	4.050
[Cu{SC(NH₂)₂}₄]₂SiF₆	249750	(Bowmaker et al., 2008)	1.663 (2)–1.696 (2)	4.585	4.060
CuSiF₆·4H₂O	165385	(Cotton et al., 1993)	1.6686 (8)–1.6973 (9)	4.510	3.993
CuSiF₆·6H₂O	34760	(Ray et al., 1973b)	1.679 (5)	4.591	4.066
			1.659 (6)–1.674 (6)	4.765	4.219
H₂SiF₆·4H₂O	40388	(Mootz & Oellers, 1988)	1.666 (1)–1.696 (1)	4.553	4.031
H₂SiF₆·6H₂O	40389	(Mootz & Oellers, 1988)	1.677 (1)–1.704 (1)	4.447	3.938
H₂SiF₆·9.5H₂O	40390	(Mootz & Oellers, 1988)	1.680 (1)–1.697 (1)	4.454	3.944
			1.684 (1)–1.706 (1)	4.448	3.939
K₂SiF₆(cF4)	420429	(Kutoglu et al., 2009)	1.6873 (16)	4.490	3.975
K₂SiF₆(hP2)	158483	(Gramaccioli & Campostrini, 2007)	1.681 (2)–1.689 (2)	4.518	4.000
K₂SiF₆·KNO₃	417735	(Rissom et al., 2008)	1.6782 (6)	4.601	4.074
KLiSiF₆	142875	(Stoll et al., 2021)	1.676 (1)–1.701 (1)	4.495	3.980
KNaSiF₆	71334	(Fischer & Krämer, 1991)	1.641 (5)–1.678 (5)	4.860	4.304
K₃Na(SiF₆)(TaF₇)	122403	(Tang et al., 2021)	1.665 (3)–1.702 (3)	4.558	4.036
K₃Na₄(BF₄)(SiF₆)₃	121301	(Bandemehr et al., 2020)	1.650 (2)–1.699 (2)	4.535	4.015
			1.666 (2)–1.700 (1)	4.560	4.038
Li₂SiF₆	425923	(Hinteregger et al., 2014)	1.685 (2)	4.518	4.000
			1.690 (2)–1.690 (8)	4.457	3.947
MgSiF₆·6H₂O	250196	(Cherkasova et al., 2004)	1.6888 (9)–1.7465 (10)	4.194	3.714
MnSiF₆·6H₂O	59274	(Torii et al., 1997)	1.690 (7)	4.457	3.947
			1.668 (7)–1.693 (7)	4.575	4.051
(NH₃OH)₂SiF₆·2H₂O	94567	(Kristl et al., 2002)	1.6793 (10)–1.6837 (10)	4.570	4.046
(NH₄)₂SiF₆	54724	(Fábry et al., 2001)	1.695 (1)–1.700 (1)	4.368	3.867
(N₂H₅)₂SiF₆	776	(Ouasri et al., 2019)	1.6777 (4)–1.7101 (4)	4.476	3.963
(N₂H₆)SiF₆	35702	(Cameron et al., 1983)	1.671 (1)–1.683 (1)	4.596	4.070
Na₂SiF₆	433134	(Zhang et al., 2017)	1.6755 (14)–1.6756 (14)	4.635	4.104
			1.6907 (16)–1.6916 (11)	4.443	3.934
PbSiF₆·2H₂O	39358	(Golubev et al., 1991)	1.645 (10)–1.707 (10)	4.558	4.036
			1.664 (10)–1.716 (10)	4.411	3.906
Rb₂SiF₆	136303	(Rienmüller et al., 2021)	1.693 (3)	4.421	3.915
[RuF(NH₃)₄(NO)]SiF₆	703	(Mikhailov et al., 2019)	1.661 (1)–1.713 (2)	4.556	4.035
[Ru₂(H₂O)₂(NH₄)₈S₂](SiF₆)₂	111446	(Woods & Wilson, 2021)	1.666 (2)–1.7065 (19)	4.552	4.031
SiF₄	48147	(Mootz & Korte, 1984)	1.5401 (6)	4.455	3.945
SrSiF₆·2H₂O	20552	(Golovastikov & Belov, 1982)	1.675 (5)–1.700 (5)	4.502	3.987
[Tl₂(NH₃)₆]SiF₆·2NH₃	144214	(Rudel et al., 2021)	1.687 (2)–1.6877 (15)	4.488	3.974
Tl₂SiF₆	136300	(Rienmüller et al., 2021)	1.686 (6)	4.505	3.989
Tl₃F[SiF₆]	136302	(Rienmüller et al., 2021)	1.688 (6)–1.695 (6)	4.439	3.931

Funding information

Funding for this research was provided by: European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (Grant agreement No. 950625); Jožef Stefan Institute Director's Fund; Slovenian Research and Innovation Agency (N1-0189).

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