A new modification of di­ammonium hexa­fluoro­silicate, (NH₄)₂SiF₆

A hexagonal modification (P6₃mc) of (NH₄)₂SiF₆ was grown accidentally. This new modification has the unit-cell c parameter doubled with respect to a previously reported trigonal form (Pm1) of the title compound. The H atoms in the present structure are ordered. Three or even four F atoms are hydrogen-bond acceptors for each H atom. The structure is isostructural with modifications of (NH₄)₂MnF₆, K₂GeF₆ and Rb₂GeF₆.

Key indicators

• Single-crystal X-ray study
• T = 290 K
• Mean [sigma](Si-F) = 0.001 Å
• R factor = 0.018
• wR factor = 0.022
• Data-to-parameter ratio = 11.6

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry


 Alert Level C:



PLAT_420  Alert C D-H Without Acceptor       N(2)   -   H(22N)            ?

General Notes



REFLT_03
         From the CIF: _diffrn_reflns_theta_max           29.98
         From the CIF: _reflns_number_total                361
         Count of symmetry unique reflns          191
         Completeness (_total/calc)            189.01%
         TEST3: Check Friedels for noncentro structure
         Estimate of Friedel pairs measured       170
         Fraction of Friedel pairs measured     0.890
         Are heavy atom types Z>Si present          yes
          Please check that the estimate of the number of Friedel pairs is
          correct. If it is not, please give the correct count in the
          _publ_section_exptl_refinement section of the submitted CIF.


 0 Alert Level A = Potentially serious problem

 0 Alert Level B = Potential problem

 1 Alert Level C = Please check

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S1. Comment

Until now, two modifications of (NH₄)₂SiF₆ have been known (ICSD; Bergerhoff et al., 1983; PDF-2, 2001): trigonal P3m1 (Schlemper & Hamilton, 1966; PDF-2 44–1424 and 72–1548) and cubic Fm3 m (Hanic, 1966; PDF-2 07–0013, 72–1552 and 72–1759). The trigonal and cubic modifications are also known by their respective mineralogical names bararite and cryptohalite. The cubic modification is stable at room temperature (Schlemper & Hamilton, 1966). In the cubic modification, the positions of the H atoms were determined; they were not determined in the trigonal modification. According to both, the neutron-diffraction study (Schlemper & Hamilton, 1966) and the electron-diffraction study (Vainshtein & Stasova, 1956) the H atoms in the cubic modification are disordered.

In this study, two modifications were grown in the same beaker, viz. hexagonal and cubic. The crystals of each modification were quite easily distinguishable in cross-polarized light as well as by their habit. While the habit of the cubic modification was cubic, the hexagonal modification grew as trigonal or hexagonal pyramids. The typical size of both crystal modifications was several tenths of a millimetre.

Attempts at chemical analysis were hindered by the small amount of available crystals. An electron microprobe analysis [Jeol JXA733 (Jeol Ltd., Tokyo, Japan) with X-ray analyzer KEVEX (Delta Class Analyzer; Kevex Instruments, San Carlos, California, USA)] was not successful in the determination of the proportion of constituent elements P, Si, O and F because of the fluffy grains on the surface of the sample. These fluffy grains contained predominantly Si, F and K. It should be noted that the microprobe analysis was not performed on the sample that was used for the structure determination. Nevertheless, the satisfactory result of the structure analysis shows that (NH₄)₂SiF₆ can also form a superstructure, either as a transitory state or stabilized by traces of K.

The previously determined trigonal modification shows similarities with the present structure in the orientation of the Si—F bonds with regard to the unit-cell axes. It should be noted that the structure determined in this study is isostructural with one of the known modifications of K₂GeF₆ (Bode & Brockmann, 1952) and Rb₂GeF₆ (ibid.). The structure is also isostructural with (NH₄)₂MnF₆ (Kaskel & Straehle, 1997) where the positions of the H atoms were determined.

The H atoms were readily seen in the difference Fourier maps and could be easily refined with no applied constraints or restraints. All the H atoms are ordered. Each N atom in the structure is surrounded by six F atoms with fairly similar N···F distances. The distribution of four H atoms among six surrounding F atoms results in the distances between the H and F atoms being unequal despite the regularity of the N···F distances. Table 2 lists the closest N···F distances and angles.

S2. Experimental

The present compound grew from a solution which was prepared by neutralization of stoichiometric amounts of (NH₄)₂CO₃ and H₂PO₃F in the molar proportion 1:1. Most probably some of the fluorophosphate anions have decomposed with production of HF which reacted with the glass of the beaker. In addition to the structure which is reported here, the cubic modification was also grown, in fact, in larger amount. Its lattice parameters corresponded to those determined by e.g. Hanic (1966).

S3. Refinement

No restraints or constraints were applied. The inversion-twinning proportion determined by refinement turned out to be insignificant.

Computing details

Cell refinement: KM4B8; data reduction: JANA2000 (Petříček & Dušek, 2000); program(s) used to solve structure: JANA2000; program(s) used to refine structure: JANA2000; molecular graphics: ORTEPIII (Burnett & Johnson, 1996).

Figures

Fig. 1. The unit-cell contents of (NH4)2SiF6 viewed along the c axis.

(I)

Crystal data

2(NH4)·(SiF6)	Dx = 2.047 Mg m^−3
Mr = 178.15	Mo Kα radiation, λ = 0.71069 Å
Hexagonal, P63mc	Cell parameters from 68 reflections
a = 5.8955 (10) Å	θ = 8.5–19.1°
c = 9.599 (1) Å	µ = 0.46 mm^−1
V = 288.93 (8) Å^3	T = 290 K
Z = 2	Pyramid, colourless
F(000) = 180	0.40 × 0.35 × 0.30 mm

Data collection

Kuma-4 diffractometer	θmax = 30.0°, θmin = 4.0°
w/2θ scans	h = −7→7
1682 measured reflections	k = 0→7
361 independent reflections	l = −13→13
348 reflections with I > 3σ(I)	3 standard reflections every 200 reflections
Rint = 0.020	intensity decay: 1.5%

Refinement

Refinement on F	All H-atom parameters refined
R[F^2 > 2σ(F^2)] = 0.018	Weighting scheme based on measured s.u.'s w = 1/(σ^2(F) + 0.0001F^2)
wR(F^2) = 0.022	(Δ/σ)max = 0.0001
S = 1.70	Δρmax = 0.39 e Å^−3
361 reflections	Δρmin = −0.39 e Å^−3
31 parameters

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Si	0.333333	0.666667	0.25	0.01751 (12)
F1	0.06201 (16)	0.53101 (8)	0.14734 (10)	0.0276 (2)
F2	0.19679 (9)	0.39358 (19)	0.35039 (10)	0.0289 (3)
N1	0.666667	0.333333	0.3731 (2)	0.0235 (4)
N2	0	0	0.5855 (2)	0.0269 (5)
H1n1	0.586 (3)	0.172 (6)	0.394 (4)	0.089 (10)*
H1n2	−0.146 (4)	−0.073 (2)	0.549 (3)	0.071 (8)*
H2n1	0.666667	0.333333	0.291 (13)	0.19 (5)*
H2n2	0	0	0.675 (8)	0.10 (2)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.0239 (5)	0.0239 (5)	0.0227 (7)	0.0119 (2)	0	0
N2	0.0247 (5)	0.0247 (5)	0.0315 (12)	0.0123 (2)	0	0
Si	0.01716 (15)	0.01716 (15)	0.0182 (2)	0.00858 (7)	0	0
F1	0.0221 (4)	0.0301 (3)	0.0279 (4)	0.0110 (2)	−0.0072 (2)	−0.00360 (14)
F2	0.0297 (3)	0.0244 (4)	0.0308 (4)	0.0122 (2)	0.00440 (15)	0.0088 (3)

Geometric parameters (Å, º)

Si—F1	1.700 (1)	Si—F2	1.695 (1)
Si—F1^i	1.700 (1)	Si—F2^i	1.695 (1)
Si—F1^ii	1.700 (1)	Si—F2^ii	1.695 (1)
H1n1—N1—H1n1^iii	115 (3)	F1—Si—F2	89.68 (5)
H1n1—N1—H1n1^iv	115 (3)	F1—Si—F2^i	179.22 (5)
H1n1—N1—H2n1	104 (2)	F1—Si—F2^ii	89.68 (5)
H1n1^iii—N1—H1n1^iv	115 (3)	F1^i—Si—F1^ii	89.77 (5)
H1n1^iii—N1—H2n1	104 (2)	F1^i—Si—F2	89.68 (5)
H1n1^iv—N1—H2n1	104 (2)	F1^i—Si—F2^i	89.68 (5)
H1n2—N2—H1n2^v	103 (3)	F1^i—Si—F2^ii	179.22 (5)
H1n2—N2—H1n2^vi	103 (3)	F1^ii—Si—F2	179.22 (5)
H1n2—N2—H2n2	115 (2)	F1^ii—Si—F2^i	89.68 (5)
H1n2^v—N2—H1n2^vi	103 (3)	F1^ii—Si—F2^ii	89.68 (5)
H1n2^v—N2—H2n2	115 (2)	F2—Si—F2^i	90.87 (6)
H1n2^vi—N2—H2n2	115 (2)	F2—Si—F2^ii	90.87 (6)
F1—Si—F1^i	89.77 (5)	F2^ii—Si—F2^i	90.87 (6)
F1—Si—F1^ii	89.77 (5)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1n1···F1^vii	0.85 (3)	2.49 (4)	2.975 (2)	117 (3)
N1—H1n1···F1^vi	0.85 (3)	2.65 (4)	2.962 (2)	103 (3)
N1—H1n1···F2^iii	0.85 (3)	2.28 (2)	2.972 (1)	139 (3)
N1—H1n1···F2^vi	0.85 (3)	2.28 (2)	2.972 (1)	139 (3)
N2—H1n2···F1^viii	0.82 (3)	2.58 (3)	3.023 (1)	115 (2)
N2—H1n2···F1^ix	0.82 (3)	2.58 (3)	3.023 (1)	115 (2)
N2—H1n2···F2^v	0.82 (3)	2.29 (3)	3.022 (2)	149 (3)
N1—H2n1···F1^x	0.8 (1)	2.45 (7)	2.962 (2)	124 (3)
N1—H2n1···F1^i	0.8 (1)	2.45 (7)	2.962 (2)	124 (3)
N1—H2n1···F1^vi	0.8 (1)	2.45 (7)	2.962 (2)	124 (3)
N2—H2n2···F2^viii	0.86 (8)	2.62 (5)	3.241 (2)	130 (1)
N2—H2n2···F2^xi	0.86 (8)	2.62 (5)	3.241 (2)	130 (1)
N2—H2n2···F2^xii	0.86 (8)	2.62 (5)	3.241 (2)	130 (1)

Symmetry codes: (i) −y+1, x−y+1, z; (iii) −y+1, x−y, z; (v) −y, x−y, z; (vi) −x+y, −x, z; (vii) x−y+1, x, z+1/2; (viii) x−y, x, z+1/2; (ix) y−1, −x+y−1, z+1/2; (x) x+1, y, z; (xi) y, x, z+1/2; (xii) −x, −y, z+1/2.