*P*6

_{3}

*mc*) of (NH

_{4})

_{2}SiF

_{6}was grown accidentally. This new modification has the unit-cell

*c*parameter doubled with respect to a previously reported trigonal form (

*P*

*m*1) 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 (NH

_{4})

_{2}MnF

_{6}, K

_{2}GeF

_{6}and Rb

_{2}GeF

_{6}.

### Supporting information

Crystallographic Information File (CIF) https://doi.org/10.1107/S160053680101501X/wn6052sup1.cif | |

Structure factor file (CIF format) https://doi.org/10.1107/S160053680101501X/wn6052Isup2.hkl |

The present compound grew from a solution which was prepared by neutralization
of stoichiometric amounts of (NH_{4})_{2}CO_{3} and H_{2}PO_{3}F 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).

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

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).

2(NH_{4})·(SiF_{6}) | D_{x} = 2.047 Mg m^{−}^{3} |

M = 178.15_{r} | Mo Kα radiation, λ = 0.71069 Å |

Hexagonal, P6_{3}mc | 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 |

Kuma-4 diffractometer | θ_{max} = 30.0°, θ_{min} = 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 |

R_{int} = 0.020 | intensity decay: 1.5% |

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 | Δρ_{max} = 0.39 e Å^{−}^{3} |

361 reflections | Δρ_{min} = −0.39 e Å^{−}^{3} |

31 parameters |

2(NH_{4})·(SiF_{6}) | Z = 2 |

M = 178.15_{r} | Mo Kα radiation |

Hexagonal, P6_{3}mc | µ = 0.46 mm^{−}^{1} |

a = 5.8955 (10) Å | T = 290 K |

c = 9.599 (1) Å | 0.40 × 0.35 × 0.30 mm |

V = 288.93 (8) Å^{3} |

Kuma-4 diffractometer | R_{int} = 0.020 |

1682 measured reflections | 3 standard reflections every 200 reflections |

361 independent reflections | intensity decay: 1.5% |

348 reflections with I > 3σ(I) |

R[F^{2} > 2σ(F^{2})] = 0.018 | 31 parameters |

wR(F^{2}) = 0.022 | All H-atom parameters refined |

S = 1.70 | Δρ_{max} = 0.39 e Å^{−}^{3} |

361 reflections | Δρ_{min} = −0.39 e Å^{−}^{3} |

^{2}) top

x | y | z | U_{iso}*/U_{eq} | ||

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)* |

^{2}) top

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) |

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. |

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. |

Experimental details

Crystal data | |

Chemical formula | 2(NH_{4})·(SiF_{6}) |

M_{r} | 178.15 |

Crystal system, space group | Hexagonal, P6_{3}mc |

Temperature (K) | 290 |

a, c (Å) | 5.8955 (10), 9.599 (1) |

V (Å^{3}) | 288.93 (8) |

Z | 2 |

Radiation type | Mo Kα |

µ (mm^{−}^{1}) | 0.46 |

Crystal size (mm) | 0.40 × 0.35 × 0.30 |

Data collection | |

Diffractometer | Kuma-4 diffractometer |

Absorption correction | – |

No. of measured, independent and observed [ I > 3σ(I)] reflections | 1682, 361, 348 |

R_{int} | 0.020 |

(sin θ/λ)_{max} (Å^{−}^{1}) | 0.703 |

Refinement | |

R[F^{2} > 2σ(F^{2})], wR(F^{2}), S | 0.018, 0.022, 1.70 |

No. of reflections | 361 |

No. of parameters | 31 |

No. of restraints | ? |

H-atom treatment | All H-atom parameters refined |

Δρ_{max}, Δρ_{min} (e Å^{−}^{3}) | 0.39, −0.39 |

Computer programs: KM4B8, JANA2000 (Petříček & Dušek, 2000), JANA2000, *ORTEPIII* (Burnett & Johnson, 1996).

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) |

Symmetry codes: (i) −y+1, x−y+1, z; (ii) −x+y, −x+1, z. |

D—H···A | D—H | H···A | D···A | D—H···A |

N1—H1n1···F1^{iii} | 0.85 (3) | 2.49 (4) | 2.975 (2) | 117 (3) |

N1—H1n1···F1^{iv} | 0.85 (3) | 2.65 (4) | 2.962 (2) | 103 (3) |

N1—H1n1···F2^{v} | 0.85 (3) | 2.28 (2) | 2.972 (1) | 139 (3) |

N1—H1n1···F2^{iv} | 0.85 (3) | 2.28 (2) | 2.972 (1) | 139 (3) |

N2—H1n2···F1^{vi} | 0.82 (3) | 2.58 (3) | 3.023 (1) | 115 (2) |

N2—H1n2···F1^{vii} | 0.82 (3) | 2.58 (3) | 3.023 (1) | 115 (2) |

N2—H1n2···F2^{viii} | 0.82 (3) | 2.29 (3) | 3.022 (2) | 149 (3) |

N1—H2n1···F1^{ix} | 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^{iv} | 0.8 (1) | 2.45 (7) | 2.962 (2) | 124 (3) |

N2—H2n2···F2^{vi} | 0.86 (8) | 2.62 (5) | 3.241 (2) | 130 (1) |

N2—H2n2···F2^{x} | 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) |

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

Until now, two modifications of (NH

_{4})_{2}SiF_{6}have been known (ICSD; Bergerhoffet al., 1983; PDF-2, 2001): trigonalP3m1 (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 (NH

_{4})_{2}SiF_{6}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 K

_{2}GeF_{6}(Bode & Brockmann, 1952) and Rb_{2}GeF_{6}(ibid.). The structure is also isostructural with (NH_{4})_{2}MnF_{6}(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.