Bis(ammonium) fluorophosphate at room temperature

The title room-temperature phase of (NH₄)₂(PO₃F) is orthorhombic (Pna2₁) and is related to the β-K₂SO₄ structure family. The title structure consists of ammonium cations, NH₄⁺, and fluoro­phosphate anions, (PO₃F)²⁻. These ions are connected by N—H...O hydrogen bonds. Two-centre N—­H...F hydrogen bonds are not present in the structure. Phase transitions were detected at 251±2 and 274±2 K during cooling and heating, respectively.

S1. Comment

The present structure determination had several objectives. Firstly, although the preparation of the title structure has been described previously (e.g. Schülke & Kayser, 1991), and its powder patterns have even been recorded in the Powder Diffraction File (ICDD, 2001) as unindexed entries, Nos. 18–0097 and 21–0022, its structure has not been reported, athough the hydrate, (NH₄)₂(FPO₃)·H₂O, was determined by Berndt & Sylvester (1972), Perloff (1972) and Durand et al. (1978).

Secondly, since the fluorophosphate anion is closely related to the sulfate anion, it is of interest to compare the structure of the title compound with that of (NH₄)₂SO₄. In particular, it is of great interest to find out whether or how the fluorine is involved in the hydrogen bonding. There are examples, such as NH₄SO₃F (O'Sullivan et al., 1970), the α and β modifications of NH₄HPO₃F (Prescott et al., 2002), NH₄PO₂F₂ (Harrison & Trotter, 1969), and (NH₄)₂(FPO₃)·H₂O (Perloff, 1972), where fluorine does not participate in the hydrogen bonding at all.

Thirdly, a phase transition can be expected in the title compound since, in the related compound (NH₄)₂SO₄, a ferroelectric phase transition (Pnam → Pna2₁) was observed at about 223 K (e.g. Mattias & Remeika, 1956).

(NH₄)₂SO₄ belongs to the structural type of β-K₂SO₄. Okaya et al. (1958) mentioned formation of a superstructure at room temperature in some cases. Hoshino et al. (1958) sometimes observed the doubling of the b and c axes with respect to the basic unit cell.

The structure of (NH₄)₂SO₄ has been determined several times by single-crystal diffraction methods, both below and above the phase transition. Schlemper & Hamilton (1966) carried out a neutron diffraction study of the ferrolectric and paraelectric phases, Hasebe (1981) studied the structure by X-ray diffraction at several temperatures both below and above the phase transition, and González-Silgo et al. (1997) redetermined the structure of the paraelectric phase. All these determinations coincided in finding that the sulfate is fairly regular. Jain & Bist (1974), however, inferred from Raman spectra that the sulfate anion should be distorted during the phase transition.

In contrast with the sulfate molecules, all these studies showed that the ammonium cations are quite irregular. For example, Schlemper & Hamilton (1966) stated that the H—N—H angles ranged from 100.2 (8)–116 (1)° and 104.7 (9)–118.5 (2)° for two independent ammonium cations in the paraelectric phase. Moreover, Hasebe (1981) found that both ammonium ions are disordered, both below and above the phase transition temperature, in contrast with the results published by Schlemper & Hamilton (1966) and González-Silgo et al. (1997).

The present structure determination of the title compound shows that it is closely related to the β-K₂SO₄ prototypic phase. Furthermore, no constituent groups of the title compound are disordered at room temperature. The ammonium groups are distorted in a similar way to those in (NH₄)₂SO₄, and the F atom, indeed, is not involved in the two-centred hydrogen bonds (Table 2), in contrast with the O atoms. The terminology used herein for the characterization of the hydrogen bonds is based on Jeffrey (1995.)

Table 2 shows that each of the atoms O1 and O3 accepts three H atoms, while atom O2 accepts two H atoms via two-centred hydrogen bonds. However, the small value of the N2—H8···O2ⁱⁱ angle and the presence of a short N2···O2^vi contact in N2—H8···O2^vi enable us to consider these latter two contacts as bifurcated N—H.. \atop. O \atop O hydrogen bonds. The values of the H···O—P angles (see Supplementary data) indicate participation of O-atom orbitals in the hydrogen bonding.

The hydrogen bonding in the title compound differs from that of (NH₄)₂SO₄, where each anionic ligand (i.e. O in the sulfate anion), in contrast with F in the title structure, is an acceptor of a two-centered hydrogen bond (Schlemper & Hamilton, 1966). On the other hand, most of the hydrogen bonds in (NH₄)₂SO₄ are weaker than those in (NH₄)₂(FPO₃).

During cooling of three samples, we observed cracking of the single crystals. A more careful cooling in 2 K steps showed that this cracking takes place between 252–250 K, most probably due to a phase transition. A phase transition was confirmed by a differential scanning calorimetry (DSC) experiment (see Experimental). The DSC curves show a broad (20 K) peak with an extrapolated onset, i.e. the phase transition temperature is at 251±2 K on cooling (exothermic effect) and 274±2 K on heating (endothermic effect). The absolute value of the enthalpy change was 3±0.5 J g^-1. The temperature hysteresis and cracking of the crystals suggest a first-order character for the transition.

The observed DSC peaks were sometimes corrugated. This, together with the width of the peaks, indicates that the transition does not start in various parts of the sample simultaneously. An additional peak was observed at ~145 K. The enthalpy change of this peak was dependent on the thermal history of the sample: previous heating above 310 K increased the enthalpy change. This peak was ascribed to the 5–10 mass % of (NH₄)H₂PO₄ present in the sample; cf. the phase transition temperature of 148 K quoted in the literature (e.g. Pérès et al., 1997). This impurity obviously originated from the hydrolysis of (NH₄)₂(FPO₃).

The stability of β-K₂SO₄ phases is discussed in Fábry & Pérez-Mato (1994). It is shown that the structural instability of members of this family can be related to the ratios of the pseudohexagonal unit-cell axis (for the setting of the title compound, it is the a axis) to the other two. In (NH₄)₂(FPO₃), the ratio a:b = 0.700 is extremely small, while a:c = 1.315 is relatively high. These values indicate, indeed, the possibility of a phase transition that has been confirmed. For comparison, the respective values for a:b and a:c in K₂SO₄ (McGinnety, 1972) are 0.742 and 1.297, while for the paraelectric phase of (NH₄)₂SO₄ (Schlemper & Hamilton, 1966), they are 0.732 and 1.299.

The file No. 18–0097 of the Powder Diffraction File (ICDD, 2001) can be well indexed using the present structure determination, while the file No. 21–0022 cannot.

Work on the investigation of the ferroic properties, especially the ferroelectricity, of the title compound, and the structure determination of a low-temperature phase, are being undertaken. Preparation of samples of the systems K₂PO₃F and (NH₄)_{2 - x}K_xPO₃F, as well as of K₂PO₃F—K₂SO₄ and (NH₄)₂SO₄-(NH₄)₂(FPO₃), is in progress.

Table 2. Geometric details of the N—H···O hydrogen bonds and the closest N—H···F contacts; N—O and N—F distances up to 3.3 Å are included (Å, °)

S2. Experimental

The title compound was prepared using a slight modification of the method of Schülke & Kayser (1991). Instead of NH₄HF₂, we used the product formed immediately after the neutralization of (NH₄)₂CO₃ and HF in molar ratio 1:2, and the mixture was then heated to 438 (5) K with stoichiometric amounts of H₃PO₄ and urea. The product was dissolved in water and acetone and left to crystallize at room temperature over P₄O₁₀. The precipitated product was a mixture of crystals of (NH₄)₂(FPO₃) and (NH₄)H₂PO₄, which differed in the habit of their crystals; (NH₄)₂(FPO₃) crystallized as tiny tetrahedra, while (NH₄)H₂PO₄ crystallized as tiny plates. (NH₄)₂(FPO₃) is slightly hygroscopic; at higher relative humidity, it is deliquescent, changing to the monohydrate. Further preparations with (NH₄)HF₂ showed that a longer heating resulted in the subsequent precipitation of (NH₄)₂(FPO₃)·H₂O or (NH₄)₂(FPO₃). Samples from other batches, which were prepared by the unmodified method of Schülke & Kayser (1991), were used for the differential scanning calorimetry (DSC) experiment [PerkinElmer Pyris Diamond DSC, using Pyris software (PerkinElmer Instruments, 2001), m = 7–20 mg, scanning rate 10 K min^-1, temperature calibration on extrapolated onsets of phase transition peaks in cyclopentane and cyclohexane, aluminium pans]. The (NH₄)₂(FPO₃) crystals from these further batches grew as needles.

S3. Refinement

The H atoms were clearly observable in the difference Fourier map and could be refined isotropically. The resulting N—H distances varied from 0.78 to 0.91 Å, while the angles varied from 101 to 117°. Therefore, the bond lengths to the H atoms were restrained to 0.90 (5) Å. The restraints to the N—H distances had a negligible influence on the final R-factor values. Applying restraints to the H—N—H angles resulted in higher R-factors of 0.023 and 0.033 for observed and all reflections, respectively. Therefore, having regard to the known shape of ammonium ions in related compounds determined by neutron diffraction, no restraints were applied to the H—N—H angles.

Computing details

Data collection: COLLECT (Nonius, 1997-2000); cell refinement: HKL SCALEPACK (Otwinowski & Minor, 1997); data reduction: HKL DENZO (Otwinowski & Minor, 1997) and SCALEPACK; program(s) used to solve structure: SHELXS86 (Sheldrick, 1986); program(s) used to refine structure: JANA2000 (Petříček & Dušek, 2000); molecular graphics: ORTEPIII (Burnett & Johnson, 1996); software used to prepare material for publication: JANA2000.

Figures

Fig. 1. A view of the unit cell of (NH4)2(FPO3) along the c axis. Symmetry-independent atoms are labelled and N—H···O hydrogen bonds are shown as dashed bonds. Displacement ellipsoids are drawn at the 30% probability level and arabic numerals refer to H atoms.

bis(ammonium) fluorophosphate

Crystal data

(NH4)2(FO3P)	F(000) = 280
Mr = 134	Dx = 1.633 Mg m^−3
Orthorhombic, Pna21	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2c -2n	Cell parameters from 4119 reflections
a = 7.9481 (2) Å	θ = 1.0–27.5°
b = 11.3472 (3) Å	µ = 0.44 mm^−1
c = 6.0425 (3) Å	T = 292 K
V = 544.97 (3) Å^3	Pyramid, colourless
Z = 4	0.43 × 0.34 × 0.30 mm

Data collection

Nonius KappaCCD area-detector diffractometer	1211 independent reflections
Radiation source: fine-focus sealed X-ray tube	1173 reflections with I > 3σ(I)
Graphite monochromator	Rint = 0.026
ω scans	θmax = 27.5°, θmin = 3.1°
Absorption correction: multi-scan (SORTAV; Blessing, 1997)	h = −10→10
Tmin = 0.834, Tmax = 0.879	k = −14→14
6096 measured reflections	l = −7→7

Refinement

Refinement on F	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.022	All H-atom parameters refined
wR(F^2) = 0.031	w = 1/(σ^2F + 0.0001F^2)
S = 1.76	(Δ/σ)max = 0.001
1211 reflections	Δρmax = 0.51 e Å^−3
96 parameters	Δρmin = −0.34 e Å^−3
8 restraints	Extinction correction: Becker & Coppens (1974) type 1 Lorentzian isotropic
0 constraints	Extinction coefficient: 0.000138 (19)

Special details

Experimental. Lattice parameters were determined on several samples, twice on a diffractometer with a CCD detector, three times on a diffractometer with a point detector.

Refinement. The Flack parameter (matrix /1 0 0/0 1 0/0 0 1/) converged to the value -0.01 (9), thus indicating unequivocally the choice of the absolute structure. The indicators of the refinement of the inverted structure resulted in the values _refine_ls_R_factor_gt 0.023 _refine_ls_wR_factor_gt 0.033 _refine_ls_R_factor_all 0.024 _refine_ls_wR_factor_ref 0.033 _refine_ls_goodness_of_fit_ref 1.86 _refine_ls_goodness_of_fit_obs 1.89 Because of a clear indication of the absence of inversion twinning, the Flack parameter was not included in the final refinement.

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

	x	y	z	Uiso*/Ueq
P	0.30924 (3)	0.37324 (2)	0.25	0.02187 (11)
N1	0.85814 (14)	0.46079 (9)	0.2665 (3)	0.0292 (3)
N2	0.91018 (14)	0.80919 (10)	0.2611 (3)	0.0343 (3)
F	0.49926 (8)	0.41610 (7)	0.2586 (2)	0.0437 (2)
O1	0.21318 (12)	0.48190 (9)	0.3206 (2)	0.0319 (3)
O2	0.30211 (15)	0.27216 (11)	0.4070 (2)	0.0422 (4)
O3	0.28164 (13)	0.34436 (10)	0.0099 (2)	0.0331 (3)
H1	0.9676 (17)	0.4632 (12)	0.274 (4)	0.033 (4)*
H2	0.826 (2)	0.4780 (17)	0.130 (2)	0.036 (5)*
H3	0.809 (2)	0.5117 (17)	0.363 (3)	0.030 (5)*
H4	0.838 (2)	0.3860 (12)	0.304 (3)	0.036 (5)*
H5	0.846 (2)	0.7622 (16)	0.352 (3)	0.032 (4)*
H6	1.008 (2)	0.8253 (17)	0.327 (3)	0.052 (6)*
H7	0.859 (2)	0.8760 (13)	0.271 (4)	0.041 (4)*
H8	0.898 (3)	0.777 (2)	0.130 (3)	0.082 (8)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
P	0.0219 (2)	0.0219 (2)	0.0218 (2)	−0.00026 (8)	0.00069 (14)	−0.00011 (16)
N1	0.0259 (5)	0.0326 (5)	0.0290 (7)	0.0019 (4)	−0.0026 (6)	−0.0025 (6)
N2	0.0359 (6)	0.0290 (5)	0.0379 (7)	−0.0017 (4)	−0.0026 (7)	0.0008 (7)
F	0.0232 (3)	0.0496 (4)	0.0583 (5)	−0.0021 (3)	−0.0030 (5)	−0.0034 (7)
O1	0.0301 (4)	0.0285 (5)	0.0370 (6)	0.0025 (3)	0.0017 (4)	−0.0061 (4)
O2	0.0602 (8)	0.0316 (6)	0.0349 (7)	0.0021 (4)	0.0049 (5)	0.0088 (5)
O3	0.0396 (4)	0.0362 (5)	0.0236 (6)	−0.0009 (4)	0.0001 (4)	−0.0044 (4)

Geometric parameters (Å, º)

P—F	1.5875 (8)	N1—O3^iii	2.8785 (18)
P—O1	1.512 (1)	N2—F^v	3.348 (2)
P—O2	1.489 (1)	N2—F^vi	3.321 (2)
P—O3	1.503 (1)	N2—F^vii	3.1966 (14)
N1—H1	0.87 (1)	N2—O1^vii	2.8636 (16)
N1—H2	0.88 (2)	N2—O2^ii	2.877 (2)
N1—H3	0.91 (2)	N2—O2^v	3.160 (2)
N1—H4	0.89 (2)	N2—O3^iii	2.761 (2)
N2—H5	0.92 (2)	N2—O3^vi	2.9017 (19)
N2—H6	0.89 (2)	O1—H1^viii	1.983 (14)
N2—H7	0.86 (2)	O1—H2^iii	1.951 (17)
N2—H8	0.88 (2)	O1—H7^ix	2.007 (17)
N1—F	2.8975 (13)	O2—H4^x	1.920 (15)
N1—O1^i	2.8508 (15)	O2—H8^iii	2.16 (2)
N1—O1^ii	2.829 (2)	O3—H3^ii	1.99 (2)
N1—O1^iii	3.457 (2)	O3—H5^ii	1.848 (18)
N1—O2^iv	2.8117 (18)	O3—H6^xi	2.016 (19)
F—P—O1	102.79 (5)	N2—H5—F^vi	86.0 (12)
F—P—O2	104.55 (6)	N2—H5—O1^vii	66.9 (10)
F—P—O3	103.73 (7)	N2—H5—O2^ii	75.8 (12)
O1—P—O2	115.41 (7)	N2—H5—O3^iii	173.8 (18)
O1—P—O3	112.12 (7)	N2—H5—O3^vi	60.4 (10)
O2—P—O3	116.17 (7)	N2—H6—F^vi	118.0 (15)
H1—N1—H2	109 (2)	N2—H6—F^vii	96.9 (13)
H1—N1—H3	112.1 (18)	N2—H6—O1^vii	59.8 (10)
H1—N1—H4	101.1 (15)	N2—H6—O2^v	91.3 (15)
H2—N1—H3	109.4 (17)	N2—H6—O3^iii	53.3 (11)
H2—N1—H4	113.4 (19)	N2—H6—O3^vi	171 (2)
H3—N1—H4	111.2 (18)	N2—H7—F^vi	91.4 (18)
H5—N2—H6	109.7 (18)	N2—H7—F^vii	126.1 (15)
H5—N2—H7	101.9 (19)	N2—H7—O1^vii	171 (2)
H5—N2—H8	104 (2)	N2—H7—O2^ii	70.3 (15)
H6—N2—H7	101.6 (19)	N2—H7—O3^iii	59.7 (12)
H6—N2—H8	126 (2)	N2—H7—O3^vi	61.0 (13)
H7—N2—H8	112 (2)	N2—H8—F^v	116 (2)
N1—H1—O1^i	173 (2)	N2—H8—O2^ii	139 (2)
N1—H1—O1^ii	60.3 (16)	N2—H8—O2^v	111 (2)
N1—H1—O2^iv	64.0 (9)	N2—H8—O3^iii	63.8 (15)
N1—H1—O3^i	136.5 (19)	H1^viii—O1—H2^iii	90.2 (9)
N1—H1—O3^iii	56.0 (11)	H1^viii—O1—H7^ix	129.3 (7)
N1—H2—F	87.4 (11)	H2^iii—O1—H7^ix	92.7 (9)
N1—H2—O1^i	53.6 (9)	H4^x—O2—H8^iii	94.1 (9)
N1—H2—O1^ii	172.4 (16)	H3^ii—O3—H5^ii	96.2 (8)
N1—H2—O3^iii	62.9 (11)	H3^ii—O3—H6^xi	98.1 (7)
N1—H3—F	89.4 (12)	H5^ii—O3—H6^xi	95.9 (8)
N1—H3—O1^i	57.0 (8)	P—O1—H1^viii	111.7 (5)
N1—H3—O1^ii	52.9 (11)	P—O1—H2^iii	122.8 (5)
N1—H3—O1^iii	133.1 (15)	P—O1—H7^ix	108.8 (5)
N1—H3—O2^iv	55.1 (10)	P—O2—H4^x	120.6 (6)
N1—H3—O3^iii	163.9 (18)	P—O2—H8^iii	128.6 (7)
N1—H4—F	91.7 (12)	P—O3—H3^ii	107.7 (6)
N1—H4—O1^i	61.0 (10)	P—O3—H5^ii	136.3 (6)
N1—H4—O2^iv	176 (2)	P—O3—H6^xi	115.6 (6)
N1—H4—O3^iv	131.3 (17)

Symmetry codes: (i) x+1, y, z; (ii) −x+1, −y+1, z−1/2; (iii) −x+1, −y+1, z+1/2; (iv) x+1/2, −y+1/2, z; (v) −x+3/2, y+1/2, z−1/2; (vi) −x+3/2, y+1/2, z+1/2; (vii) x+1/2, −y+3/2, z; (viii) x−1, y, z; (ix) x−1/2, −y+3/2, z; (x) x−1/2, −y+1/2, z; (xi) −x+3/2, y−1/2, z−1/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H2···F	0.88 (2)	2.80 (2)	2.898 (1)	87 (1)
N1—H3···F	0.91 (2)	2.76 (2)	2.898 (1)	89 (1)
N1—H4···F	0.89 (2)	2.73 (2)	2.898 (1)	92 (1)
N1—H1···O1^i	0.87 (1)	1.98 (1)	2.851 (2)	173 (2)
N1—H2···O1^ii	0.88 (2)	1.95 (2)	2.829 (2)	172 (4)
N1—H3···O3^iii	0.91 (2)	1.99 (2)	2.879 (2)	164 (2)
N1—H4···O2^iv	0.89 (2)	1.92 (2)	2.812 (2)	176 (2)
N2—H6···F^vii	0.89 (2)	2.96 (2)	3.197 (1)	97 (1)
N2—H7···F^vii	0.86 (2)	2.61 (2)	3.197 (1)	126 (2)
N2—H7···O1^vii	0.86 (2)	2.01 (2)	2.864 (2)	171 (2)
N2—H8···O2^ii	0.88 (2)	2.16 (2)	2.877 (2)	139 (2)
N2—H8···O2^v	0.88 (2)	2.74 (2)	3.160 (2)	111 (2)
N2—H5···O3^iii	0.92 (2)	1.85 (2)	2.761 (2)	174 (2)
N2—H6···O3^vi	0.89 (2)	2.02 (2)	2.902 (2)	171 (2)

Symmetry codes: (i) x+1, y, z; (ii) −x+1, −y+1, z−1/2; (iii) −x+1, −y+1, z+1/2; (iv) x+1/2, −y+1/2, z; (v) −x+3/2, y+1/2, z−1/2; (vi) −x+3/2, y+1/2, z+1/2; (vii) x+1/2, −y+3/2, z.