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_{2}PO

_{3}F was determined at 290 and 130 K, while that of Cs

_{2}PO

_{3}F was determined at 240 and 100 K. Both compounds belong to the -K

_{2}SO

_{4}family. The structure analysis did not reveal signs of a phase transition in either compound. Crystal chemical considerations do not favour the presence of a phase transition in either Rb

_{2}PO

_{3}F or Cs

_{2}PO

_{3}F. However, glass-like phase transitions were observed by differential scanning calorimetry in slightly humid samples at 175 and 177 K for Rb

_{2}PO

_{3}F and Cs

_{2}PO

_{3}F, respectively, but were not observed in well dried samples. The bond distances are normal and Cs

_{2}PO

_{3}F is twinned.

### Supporting information

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270106016350/sq3009sup1.cif | |

Structure factor file (CIF format) https://doi.org/10.1107/S0108270106016350/sq3009I_290sup2.hkl | |

Structure factor file (CIF format) https://doi.org/10.1107/S0108270106016350/sq3009I_130sup3.hkl | |

Structure factor file (CIF format) https://doi.org/10.1107/S0108270106016350/sq3009II_240sup4.hkl | |

Structure factor file (CIF format) https://doi.org/10.1107/S0108270106016350/sq3009II_100sup5.hkl |

The structures were prepared by neutralization of stoichiometric amounts of Rb_{2}CO_{3} (3.0375 g, 0.013153 mol) or Cs_{2}CO_{3} (4.2855 g, 0.013153 mol) and H_{2}PO_{3}F. H_{2}PO_{3}F was obtained from a solution of (NH_{4})_{2}PO_{3}F·H_{2}O (2 g, 0.013153 mol) that was passed through a catex column. (NH_{4})_{2}PO_{3}F·H_{2}O was prepared by the method described by Schülke & Kayser (1991), and the raw material of (NH_{4})_{2}PO_{3}F·H_{2}O prepared by this method was recrystallized in order to remove contamination of NH_{4}H_{2}PO_{4}. The volume of the eluted solution of H_{2}PO_{3}F was about 120 ml in both cases. The solutions were put into an evacuated desiccator over P_{4}O_{10}. Crystals appeared within one week. Technical details of the calorimetric experiments are provided in the CIF. For K_{2}PO_{3}F, no anomaly was found in the temperature region 96–483 K. 20 measurements of Rb_{2}PO_{3}F and 24 measurements of Cs_{2}PO_{3}F were carried out in the temperature region 96–433 K for six different samples of each compound. A reproducible and a reversible glassy-like phase transition took place at 175 K for Rb_{2}PO_{3}F and 177 K for Cs_{2}PO_{3}F, with varying value of Δ*C*_{p}, unless the sample was carefully dried and placed in an aluminium pan in a dry-box.

Data collection: *COLLECT* (Hooft, 1998) for I_290, I_130; *CrysAlis CCD* (Oxford Diffraction, 2005) for II_240, II_100. Cell refinement: *HKL* *SCALEPACK* (Otwinowski & Minor, 1997) for I_290, I_130; *CrysAlis RED* (Oxford Diffraction, 2005) for II_240, II_100. Data reduction: *HKL* *DENZO* (Otwinowski & Minor, 1997) and *SCALEPACK* for I_290, I_130; *CrysAlis RED* for II_240, II_100. For all compounds, program(s) used to solve structure: *SIR97* (Altomare *et al.*, 1999); program(s) used to refine structure: JANA2000 (Petříček *et al.*, 2000). Molecular graphics: *ORTEP-3 for Windows* (Farrugia, 1997) for I_290. For all compounds, software used to prepare material for publication: JANA2000.

Rb_{2}PO_{3}F | 10 images collected for cell determiantion. Detector distance: 40 mm for cell determination. Rotation per frame: 1 deg for cell determination. Generator setting: 50 kV/20mA. |

M = 268.9_{r} | D_{x} = 3.514 (1) Mg m^{−}^{3} |

Orthorhombic, Pnma | Mo Kα radiation, λ = 0.71069 Å |

Hall symbol: -P 2ac 2n | Cell parameters from 3814 reflections |

a = 7.8714 (2) Å | θ = 1.0–27.5° |

b = 6.1236 (2) Å | µ = 19.49 mm^{−}^{1} |

c = 10.5424 (3) Å | T = 290 K |

V = 508.16 (3) Å^{3} | Prism, colourless |

Z = 4 | 0.18 × 0.15 × 0.12 mm |

F(000) = 488 |

Nonius KappaCCD diffractometer | 630 independent reflections |

Radiation source: fine-focus sealed tube | 570 reflections with I > 3σ(I) |

Graphite monochromator | R_{int} = 0.079 |

Detector resolution: 9.091 pixels mm^{-1} | θ_{max} = 27.5°, θ_{min} = 3.2° |

Rotation method data acquisition using ω scans | h = −10→10 |

Absorption correction: gaussian (Coppens, 1970) | k = −7→7 |

T_{min} = 0.073, T_{max} = 0.173 | l = −13→13 |

7793 measured reflections |

Refinement on F | 0 constraints |

R[F^{2} > 2σ(F^{2})] = 0.018 | Weighting scheme based on measured s.u.'s w = 1/[σ^{2}(F) + 0.0004F^{2}] |

wR(F^{2}) = 0.030 | (Δ/σ)_{max} = 0.008 |

S = 1.21 | Δρ_{max} = 0.65 e Å^{−}^{3} |

630 reflections | Δρ_{min} = −0.46 e Å^{−}^{3} |

41 parameters | Extinction correction: B-C type 1 Lorentzian isotropic (Becker & Coppens, 1974) |

0 restraints | Extinction coefficient: 0.132 (12) |

Rb_{2}PO_{3}F | V = 508.16 (3) Å^{3} |

M = 268.9_{r} | Z = 4 |

Orthorhombic, Pnma | Mo Kα radiation |

a = 7.8714 (2) Å | µ = 19.49 mm^{−}^{1} |

b = 6.1236 (2) Å | T = 290 K |

c = 10.5424 (3) Å | 0.18 × 0.15 × 0.12 mm |

Nonius KappaCCD diffractometer | 630 independent reflections |

Absorption correction: gaussian (Coppens, 1970) | 570 reflections with I > 3σ(I) |

T_{min} = 0.073, T_{max} = 0.173 | R_{int} = 0.079 |

7793 measured reflections |

R[F^{2} > 2σ(F^{2})] = 0.018 | 41 parameters |

wR(F^{2}) = 0.030 | 0 restraints |

S = 1.21 | Δρ_{max} = 0.65 e Å^{−}^{3} |

630 reflections | Δρ_{min} = −0.46 e Å^{−}^{3} |

The sample masses: 30±10 mg; scanning rate 10 K/min, aluminium 40 µl or 30 µ pans. ? |

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

Rb1 | 0.17362 (5) | 0.25 | 0.08089 (3) | 0.02586 (15) | |

Rb2 | 0.00298 (5) | 0.25 | 0.71819 (3) | 0.02500 (15) | |

P | 0.24073 (12) | 0.25 | 0.41616 (7) | 0.0174 (3) | |

O1 | 0.3059 (4) | 0.25 | 0.5501 (3) | 0.0303 (9) | |

O2 | 0.2759 (3) | 0.0447 (3) | 0.34441 (17) | 0.0357 (7) | |

F | 0.0372 (3) | 0.25 | 0.4316 (3) | 0.0473 (10) |

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U^{11} | U^{22} | U^{33} | U^{12} | U^{13} | U^{23} | |

Rb1 | 0.0254 (3) | 0.0261 (3) | 0.0260 (3) | 0 | 0.00001 (14) | 0 |

Rb2 | 0.0246 (3) | 0.0281 (3) | 0.0223 (3) | 0 | 0.00084 (13) | 0 |

P | 0.0175 (5) | 0.0179 (5) | 0.0167 (4) | 0 | −0.0001 (3) | 0 |

O1 | 0.0335 (16) | 0.0342 (16) | 0.0231 (13) | 0 | −0.0081 (12) | 0 |

O2 | 0.0495 (14) | 0.0255 (10) | 0.0321 (10) | 0.0046 (10) | 0.0011 (10) | −0.0092 (8) |

F | 0.0187 (13) | 0.079 (2) | 0.0444 (17) | 0 | 0.0015 (11) | 0 |

P—O1 | 1.502 (3) | Rb1—O2^{i} | 3.1539 (19) |

P—O2 | 1.493 (2) | Rb1—F | 3.850 (3) |

P—O2^{i} | 1.493 (2) | Rb1—F^{vii} | 2.865 (3) |

P—F | 1.610 (3) | Rb2—O1 | 2.971 (3) |

Rb1—O1^{ii} | 3.207 (3) | Rb2—O1^{viii} | 2.894 (3) |

Rb1—O1^{iii} | 3.0832 (3) | Rb2—O2^{ix} | 2.918 (2) |

Rb1—O1^{iv} | 3.0832 (3) | Rb2—O2^{x} | 2.838 (2) |

Rb1—O2 | 3.1539 (19) | Rb2—O2^{xi} | 2.838 (2) |

Rb1—O2^{ii} | 3.464 (3) | Rb2—O2^{xii} | 2.918 (2) |

Rb1—O2^{iii} | 3.1031 (19) | Rb2—F | 3.034 (3) |

Rb1—O2^{v} | 3.1031 (19) | Rb2—F^{ix} | 3.4593 (14) |

Rb1—O2^{vi} | 3.464 (3) | Rb2—F^{xiii} | 3.4593 (14) |

O1—P—O2 | 114.37 (10) | O2—P—F | 103.63 (12) |

O1—P—O2^{i} | 114.37 (10) | O2^{i}—P—O2 | 114.70 (12) |

O1—P—F | 104.19 (16) | O2^{i}—P—F | 103.63 (12) |

O2—P—O2^{i} | 114.70 (12) |

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

Rb_{2}PO_{3}F | F(000) = 488 |

M = 268.9_{r} | D_{x} = 3.560 (1) Mg m^{−}^{3} |

Orthorhombic, Pnma | Mo Kα radiation, λ = 0.71069 Å |

Hall symbol: -P 2ac 2n | Cell parameters from 4214 reflections |

a = 7.8403 (2) Å | θ = 1–27.5° |

b = 6.1034 (2) Å | µ = 19.75 mm^{−}^{1} |

c = 10.4813 (3) Å | T = 130 K |

V = 501.56 (3) Å^{3} | Prism, colourless |

Z = 4 | 0.18 × 0.15 × 0.12 mm |

Nonius KappaCCD diffractometer | 623 independent reflections |

Radiation source: fine-focus sealed tube | 581 reflections with I > 3σ(I) |

Graphite monochromator | R_{int} = 0.084 |

Detector resolution: 9.091 pixels mm^{-1} | θ_{max} = 27.5°, θ_{min} = 3.2° |

Rotation method data acquisition using ω scans | h = −10→10 |

Absorption correction: gaussian (Coppens, 1970) | k = −7→7 |

T_{min} = 0.067, T_{max} = 0.179 | l = −13→13 |

7775 measured reflections |

Refinement on F | 0 constraints |

R[F^{2} > 2σ(F^{2})] = 0.021 | Weighting scheme based on measured s.u.'s w = 1/[σ^{2}(F) + 0.0004F^{2}] |

wR(F^{2}) = 0.033 | (Δ/σ)_{max} = 0.010 |

S = 1.39 | Δρ_{max} = 0.83 e Å^{−}^{3} |

623 reflections | Δρ_{min} = −0.71 e Å^{−}^{3} |

41 parameters | Extinction correction: B-C type 1 Lorentzian isotropic (Becker & Coppens, 1974) |

0 restraints | Extinction coefficient: 0.111 (12) |

Rb_{2}PO_{3}F | V = 501.56 (3) Å^{3} |

M = 268.9_{r} | Z = 4 |

Orthorhombic, Pnma | Mo Kα radiation |

a = 7.8403 (2) Å | µ = 19.75 mm^{−}^{1} |

b = 6.1034 (2) Å | T = 130 K |

c = 10.4813 (3) Å | 0.18 × 0.15 × 0.12 mm |

Nonius KappaCCD diffractometer | 623 independent reflections |

Absorption correction: gaussian (Coppens, 1970) | 581 reflections with I > 3σ(I) |

T_{min} = 0.067, T_{max} = 0.179 | R_{int} = 0.084 |

7775 measured reflections |

R[F^{2} > 2σ(F^{2})] = 0.021 | 41 parameters |

wR(F^{2}) = 0.033 | 0 restraints |

S = 1.39 | Δρ_{max} = 0.83 e Å^{−}^{3} |

623 reflections | Δρ_{min} = −0.71 e Å^{−}^{3} |

The sample masses: 30±10 mg; scanning rate 10 K/min, aluminium 40 µl or 30 µ pans. ? |

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

Rb2 | 0.00450 (5) | 0.25 | 0.71800 (3) | 0.01349 (16) | |

Rb1 | 0.17204 (5) | 0.25 | 0.07970 (3) | 0.01320 (16) | |

P | 0.24068 (14) | 0.25 | 0.41541 (8) | 0.0100 (3) | |

O1 | 0.3079 (4) | 0.25 | 0.5505 (3) | 0.0159 (8) | |

O2 | 0.2729 (3) | 0.0427 (3) | 0.34255 (18) | 0.0191 (6) | |

F | 0.0349 (3) | 0.25 | 0.4337 (3) | 0.0243 (9) |

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U^{11} | U^{22} | U^{33} | U^{12} | U^{13} | U^{23} | |

Rb2 | 0.0140 (3) | 0.0142 (3) | 0.0122 (3) | 0 | 0.00057 (13) | 0 |

Rb1 | 0.0131 (3) | 0.0129 (3) | 0.0136 (3) | 0 | −0.00027 (13) | 0 |

P | 0.0101 (5) | 0.0097 (5) | 0.0101 (5) | 0 | 0.0002 (3) | 0 |

O1 | 0.0181 (15) | 0.0162 (16) | 0.0136 (13) | 0 | −0.0036 (11) | 0 |

O2 | 0.0260 (11) | 0.0138 (10) | 0.0175 (9) | 0.0020 (9) | 0.0009 (8) | −0.0038 (8) |

F | 0.0104 (13) | 0.0422 (18) | 0.0203 (14) | 0 | 0.0000 (10) | 0 |

P—O1 | 1.511 (3) | Rb1—O2^{i} | 3.133 (2) |

P—O2 | 1.499 (2) | Rb1—F | 3.863 (3) |

P—O2^{i} | 1.499 (2) | Rb1—F^{vii} | 2.848 (3) |

P—F | 1.625 (3) | Rb2—O1 | 2.956 (3) |

Rb1—O1^{ii} | 3.164 (3) | Rb2—O1^{viii} | 2.875 (3) |

Rb1—O1^{iii} | 3.0710 (3) | Rb2—O2^{ix} | 2.885 (2) |

Rb1—O1^{iv} | 3.0710 (3) | Rb2—O2^{x} | 2.818 (2) |

Rb1—O2 | 3.133 (2) | Rb2—O2^{xi} | 2.818 (2) |

Rb1—O2^{ii} | 3.472 (2) | Rb2—O2^{xii} | 2.885 (2) |

Rb1—O2^{iii} | 3.091 (2) | Rb2—F | 2.989 (3) |

Rb1—O2^{v} | 3.091 (2) | Rb2—F^{ix} | 3.4550 (13) |

Rb1—O2^{vi} | 3.472 (2) | Rb2—F^{xiii} | 3.4550 (13) |

O1—P—O2 | 114.75 (10) | O2—P—F | 103.15 (11) |

O1—P—O2^{i} | 114.75 (10) | O2^{i}—P—O2 | 115.10 (12) |

O1—P—F | 103.63 (15) | O2^{i}—P—F | 103.15 (11) |

O2—P—O2^{i} | 115.10 (12) |

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

Cs_{2}PO_{3}F | The sample is hygroscopic and therefore the crystal was put into the silicon grease. The worse standard uncertainties of the lattice parameters with regard to the measurement at 100 K is difficult to explain. |

M = 363.8_{r} | D_{x} = 4.129 Mg m^{−}^{3} |

Orthorhombic, Pnma | Mo Kα radiation, λ = 0.71069 Å |

Hall symbol: -P 2ac 2n | Cell parameters from 9848 reflections |

a = 8.308 (2) Å | θ = 3.1–26.9° |

b = 6.3812 (9) Å | µ = 12.66 mm^{−}^{1} |

c = 11.036 (2) Å | T = 240 K |

V = 585.07 (19) Å^{3} | Prism elongated along a, colourless |

Z = 4 | 0.25 × 0.10 × 0.03 mm |

F(000) = 632 |

Oxford Diffraction Model?? CCD diffractometer | 974 independent reflections |

Radiation source: Enhance (Mo) X-ray Source | 832 reflections with I > 3σ(I) |

Graphite monochromator | R_{int} = 0.041 |

Detector resolution: 8.3438 pixels mm^{-1} | θ_{max} = 26.9°, θ_{min} = 3.1° |

ω scans | h = −10→10 |

Absorption correction: analytical CrysAlis RED (Oxford Diffraction, 2005).
Analytical numeric absorption correction using a multifaceted crystal
model based on expressions derived by Clark & Reid (1995). | k = −8→8 |

T_{min} = 0.168, T_{max} = 0.460 | l = −13→13 |

6666 measured reflections |

Refinement on F | 0 constraints |

R[F^{2} > 2σ(F^{2})] = 0.022 | Weighting scheme based on measured s.u.'s w = 1/[σ^{2}(F) + 0.0004F^{2}] |

wR(F^{2}) = 0.037 | (Δ/σ)_{max} = 0.007 |

S = 1.48 | Δρ_{max} = 0.58 e Å^{−}^{3} |

974 reflections | Δρ_{min} = −0.71 e Å^{−}^{3} |

42 parameters | Extinction correction: B-C type 1 Lorentzian isotropic (Becker & Coppens, 1974) |

0 restraints | Extinction coefficient: 0.061 (6) |

Cs_{2}PO_{3}F | V = 585.07 (19) Å^{3} |

M = 363.8_{r} | Z = 4 |

Orthorhombic, Pnma | Mo Kα radiation |

a = 8.308 (2) Å | µ = 12.66 mm^{−}^{1} |

b = 6.3812 (9) Å | T = 240 K |

c = 11.036 (2) Å | 0.25 × 0.10 × 0.03 mm |

Oxford Diffraction Model?? CCD diffractometer | 974 independent reflections |

Absorption correction: analytical CrysAlis RED (Oxford Diffraction, 2005).
Analytical numeric absorption correction using a multifaceted crystal
model based on expressions derived by Clark & Reid (1995). | 832 reflections with I > 3σ(I) |

T_{min} = 0.168, T_{max} = 0.460 | R_{int} = 0.041 |

6666 measured reflections |

R[F^{2} > 2σ(F^{2})] = 0.022 | 42 parameters |

wR(F^{2}) = 0.037 | 0 restraints |

S = 1.48 | Δρ_{max} = 0.58 e Å^{−}^{3} |

974 reflections | Δρ_{min} = −0.71 e Å^{−}^{3} |

The sample masses: 30±10 mg; scanning rate 10 K/min, aluminium 40 µl or 30 µ pans. ? |

The twinning matrix relating the minor and dominating states (denoted with indices "m" and "d", respectively) is: (1 0 0) (a where ϕ=arctg(c/b). This means that for the transformation equation is: (1 0 0) (a The values of the matrix differ only minutely (\sim1/1000) from the pertinent values regarding rotation by 120º: (1 0 0) (a The idealized transformation is thus: (1 0 0) (a The refinement with idealized twinning matrix resulted in the same indicators of the refinement as with the non-idealized matrix given above. (This contrasts to the refinement at 100 K.) The overlapped diffractions The refined volume proportion of the minor domain equalled to: 0.135 (1). The difference electron density given in _refine_diff_density_max and _refine_diff_density_min was calculated with the help of the formula The alternative calculation with |

^{2}) top

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

Cs1 | 0.17416 (5) | 0.25 | 0.08366 (4) | 0.02138 (15) | |

Cs2 | −0.00153 (5) | 0.25 | 0.71578 (4) | 0.02044 (16) | |

P | 0.2453 (2) | 0.25 | 0.41694 (15) | 0.0148 (4) | |

O1 | 0.2985 (6) | 0.25 | 0.5465 (4) | 0.0251 (15) | |

O2 | 0.2828 (5) | 0.0509 (6) | 0.3494 (3) | 0.0289 (11) | |

F | 0.0521 (5) | 0.25 | 0.4245 (5) | 0.0386 (17) |

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U^{11} | U^{22} | U^{33} | U^{12} | U^{13} | U^{23} | |

Cs1 | 0.0235 (3) | 0.0196 (3) | 0.0210 (3) | 0 | 0.00055 (18) | 0 |

Cs2 | 0.0204 (3) | 0.0223 (3) | 0.0186 (3) | 0 | 0.00077 (16) | 0 |

P | 0.0154 (7) | 0.0136 (8) | 0.0153 (8) | 0 | −0.0007 (7) | 0 |

O1 | 0.031 (3) | 0.027 (3) | 0.017 (3) | 0 | −0.004 (2) | 0 |

O2 | 0.040 (2) | 0.0216 (18) | 0.0253 (19) | 0.0038 (17) | −0.0004 (18) | −0.0067 (15) |

F | 0.018 (2) | 0.062 (4) | 0.036 (3) | 0 | 0.001 (2) | 0 |

P—O1 | 1.496 (5) | Cs1—O2^{i} | 3.321 (4) |

P—O2 | 1.506 (4) | Cs1—F | 3.896 (5) |

P—O2^{i} | 1.506 (4) | Cs1—F^{vii} | 3.141 (4) |

P—F | 1.608 (5) | Cs2—O1 | 3.115 (5) |

Cs1—O1^{ii} | 3.435 (5) | Cs2—O1^{viii} | 3.105 (5) |

Cs1—O1^{iii} | 3.2249 (7) | Cs2—O2^{ix} | 3.109 (4) |

Cs1—O1^{iv} | 3.2249 (7) | Cs2—O2^{x} | 3.027 (4) |

Cs1—O2 | 3.321 (4) | Cs2—O2^{xi} | 3.027 (4) |

Cs1—O2^{ii} | 3.568 (4) | Cs2—O2^{xii} | 3.109 (4) |

Cs1—O2^{iii} | 3.240 (4) | Cs2—F | 3.245 (5) |

Cs1—O2^{v} | 3.240 (4) | Cs2—F^{ix} | 3.571 (2) |

Cs1—O2^{vi} | 3.568 (4) | Cs2—F^{xiii} | 3.571 (2) |

O1—P—O2 | 114.31 (17) | O2—P—F | 103.43 (19) |

O1—P—O2^{i} | 114.31 (17) | O2^{i}—P—O2 | 115.1 (2) |

O1—P—F | 104.2 (3) | O2^{i}—P—F | 103.43 (19) |

O2—P—O2^{i} | 115.1 (2) |

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

Cs_{2}PO_{3}F | F(000) = 632 |

M = 363.8_{r} | D_{x} = 4.177 Mg m^{−}^{3} |

Orthorhombic, Pnma | Mo Kα radiation, λ = 0.71069 Å |

Hall symbol: -P 2ac 2n | Cell parameters from 9972 reflections |

a = 8.2821 (8) Å | θ = 3.1–26.6° |

b = 6.3577 (6) Å | µ = 12.81 mm^{−}^{1} |

c = 10.9827 (8) Å | T = 100 K |

V = 578.30 (9) Å^{3} | Prism elongated along a, colourless |

Z = 4 | 0.25 × 0.09 × 0.03 mm |

Oxford Diffraction Mode ?? CCD diffractometer | 939 independent reflections |

Radiation source: Enhance (Mo) X-ray Source | 849 reflections with I > 3σ(I) |

Graphite monochromator | R_{int} = 0.038 |

Detector resolution: 8.3438 pixels mm^{-1} | θ_{max} = 26.6°, θ_{min} = 3.7° |

ω scans | h = −10→10 |

Absorption correction: analytical CrysAlis RED (Oxford Diffraction, 2005).
Analytical numeric absorption correction using a multifaceted crystal
model based on expressions derived by Clark & Reid (1995). | k = −7→7 |

T_{min} = 0.161, T_{max} = 0.434 | l = −13→13 |

6856 measured reflections |

Refinement on F | 0 constraints |

R[F^{2} > 2σ(F^{2})] = 0.017 | Weighting scheme based on measured s.u.'s w = 1/[σ^{2}(F) + 0.0004F^{2}] |

wR(F^{2}) = 0.036 | (Δ/σ)_{max} = 0.014 |

S = 1.52 | Δρ_{max} = 0.37 e Å^{−}^{3} |

939 reflections | Δρ_{min} = −0.41 e Å^{−}^{3} |

42 parameters | Extinction correction: B-C type 1 Lorentzian isotropic (Becker & Coppens, 1974) |

0 restraints | Extinction coefficient: 0.040 (5) |

Cs_{2}PO_{3}F | V = 578.30 (9) Å^{3} |

M = 363.8_{r} | Z = 4 |

Orthorhombic, Pnma | Mo Kα radiation |

a = 8.2821 (8) Å | µ = 12.81 mm^{−}^{1} |

b = 6.3577 (6) Å | T = 100 K |

c = 10.9827 (8) Å | 0.25 × 0.09 × 0.03 mm |

Oxford Diffraction Mode ?? CCD diffractometer | 939 independent reflections |

Absorption correction: analytical CrysAlis RED (Oxford Diffraction, 2005).
Analytical numeric absorption correction using a multifaceted crystal
model based on expressions derived by Clark & Reid (1995). | 849 reflections with I > 3σ(I) |

T_{min} = 0.161, T_{max} = 0.434 | R_{int} = 0.038 |

6856 measured reflections |

R[F^{2} > 2σ(F^{2})] = 0.017 | 42 parameters |

wR(F^{2}) = 0.036 | 0 restraints |

S = 1.52 | Δρ_{max} = 0.37 e Å^{−}^{3} |

939 reflections | Δρ_{min} = −0.41 e Å^{−}^{3} |

The sample masses: 30±10 mg; scanning rate 10 K/min, aluminium 40 µl or 30 µ pans. ? |

The twinning matrix relating the minor and dominating states (denoted with indices "m" and "d", respectively) is: (1 0 0) (a where ϕ=arctg(c/b). The absolute values of the numerical values of the elements with goniometric functions differ only minutely (2/1000) from the pertinent values of the elements of the rotation matrix with rotation by 120º. Nevertheless, in difference to the refinement at 240 K the twinning matrix that was applied was (1 0 0) (a and not (1 0 0) (a because the indicators of refinement in the latter case were worse. The overlapped diffractions The refined volume proportion of the minor domain equalled to: 0.135 (1). The difference electron density given in _refine_diff_density_max and _refine_diff_density_min was calculated with the help of the formula The alternative calculation with |

^{2}) top

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

Cs1 | 0.17283 (4) | 0.25 | 0.08342 (3) | 0.00966 (15) | |

Cs2 | −0.00169 (4) | 0.25 | 0.71520 (4) | 0.00956 (15) | |

P | 0.2464 (2) | 0.25 | 0.41686 (13) | 0.0072 (4) | |

O1 | 0.2984 (6) | 0.25 | 0.5490 (4) | 0.0134 (13) | |

O2 | 0.2825 (4) | 0.0483 (5) | 0.3489 (3) | 0.0143 (9) | |

F | 0.0491 (5) | 0.25 | 0.4253 (4) | 0.0184 (13) |

^{2}) top

U^{11} | U^{22} | U^{33} | U^{12} | U^{13} | U^{23} | |

Cs1 | 0.0092 (2) | 0.0093 (3) | 0.0106 (3) | 0 | 0.00006 (14) | 0 |

Cs2 | 0.0084 (3) | 0.0107 (3) | 0.0096 (2) | 0 | 0.00066 (13) | 0 |

P | 0.0071 (7) | 0.0055 (8) | 0.0091 (8) | 0 | −0.0003 (6) | 0 |

O1 | 0.013 (2) | 0.015 (2) | 0.013 (2) | 0 | 0.0023 (19) | 0 |

O2 | 0.0180 (15) | 0.0105 (15) | 0.0145 (16) | 0.0033 (14) | 0.0021 (14) | −0.0048 (12) |

F | 0.0076 (18) | 0.032 (2) | 0.016 (2) | 0 | 0.0014 (16) | 0 |

P—O1 | 1.514 (5) | Cs1—O2^{i} | 3.312 (3) |

P—O2 | 1.513 (3) | Cs1—F | 3.892 (4) |

P—O2^{i} | 1.513 (3) | Cs1—F^{vii} | 3.118 (4) |

P—F | 1.637 (4) | Cs2—O1 | 3.083 (5) |

Cs1—O1^{ii} | 3.425 (5) | Cs2—O1^{viii} | 3.073 (5) |

Cs1—O1^{iii} | 3.2101 (6) | Cs2—O2^{ix} | 3.082 (3) |

Cs1—O1^{iv} | 3.2101 (6) | Cs2—O2^{x} | 3.008 (3) |

Cs1—O2 | 3.312 (3) | Cs2—O2^{xi} | 3.008 (3) |

Cs1—O2^{ii} | 3.556 (3) | Cs2—O2^{xii} | 3.082 (3) |

Cs1—O2^{iii} | 3.220 (3) | Cs2—F | 3.212 (4) |

Cs1—O2^{v} | 3.220 (3) | Cs2—F^{ix} | 3.5551 (17) |

Cs1—O2^{vi} | 3.556 (3) | Cs2—F^{xiii} | 3.5551 (17) |

O1—P—O2 | 114.64 (15) | O2—P—F | 102.99 (16) |

O1—P—O2^{i} | 114.64 (15) | O2^{i}—P—O2 | 115.82 (19) |

O1—P—F | 103.3 (2) | O2^{i}—P—F | 102.99 (16) |

O2—P—O2^{i} | 115.82 (19) |

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

Experimental details

(I_290) | (I_130) | (II_240) | (II_100) | |

Crystal data | ||||

Chemical formula | Rb_{2}PO_{3}F | Rb_{2}PO_{3}F | Cs_{2}PO_{3}F | Cs_{2}PO_{3}F |

M_{r} | 268.9 | 268.9 | 363.8 | 363.8 |

Crystal system, space group | Orthorhombic, Pnma | Orthorhombic, Pnma | Orthorhombic, Pnma | Orthorhombic, Pnma |

Temperature (K) | 290 | 130 | 240 | 100 |

a, b, c (Å) | 7.8714 (2), 6.1236 (2), 10.5424 (3) | 7.8403 (2), 6.1034 (2), 10.4813 (3) | 8.308 (2), 6.3812 (9), 11.036 (2) | 8.2821 (8), 6.3577 (6), 10.9827 (8) |

V (Å^{3}) | 508.16 (3) | 501.56 (3) | 585.07 (19) | 578.30 (9) |

Z | 4 | 4 | 4 | 4 |

Radiation type | Mo Kα | Mo Kα | Mo Kα | Mo Kα |

µ (mm^{−}^{1}) | 19.49 | 19.75 | 12.66 | 12.81 |

Crystal size (mm) | 0.18 × 0.15 × 0.12 | 0.18 × 0.15 × 0.12 | 0.25 × 0.10 × 0.03 | 0.25 × 0.09 × 0.03 |

Data collection | ||||

Diffractometer | Nonius KappaCCD diffractometer | Nonius KappaCCD diffractometer | Oxford Diffraction Model?? CCD diffractometer | Oxford Diffraction Mode ?? CCD diffractometer |

Absorption correction | Gaussian (Coppens, 1970) | Gaussian (Coppens, 1970) | Analytical CrysAlis RED (Oxford Diffraction, 2005).
Analytical numeric absorption correction using a multifaceted crystal
model based on expressions derived by Clark & Reid (1995). | Analytical CrysAlis RED (Oxford Diffraction, 2005).
Analytical numeric absorption correction using a multifaceted crystal
model based on expressions derived by Clark & Reid (1995). |

T_{min}, T_{max} | 0.073, 0.173 | 0.067, 0.179 | 0.168, 0.460 | 0.161, 0.434 |

No. of measured, independent and observed [ I > 3σ(I)] reflections | 7793, 630, 570 | 7775, 623, 581 | 6666, 974, 832 | 6856, 939, 849 |

R_{int} | 0.079 | 0.084 | 0.041 | 0.038 |

(sin θ/λ)_{max} (Å^{−}^{1}) | 0.649 | 0.649 | 0.636 | 0.629 |

Refinement | ||||

R[F^{2} > 2σ(F^{2})], wR(F^{2}), S | 0.018, 0.030, 1.21 | 0.021, 0.033, 1.39 | 0.022, 0.037, 1.48 | 0.017, 0.036, 1.52 |

No. of reflections | 630 | 623 | 974 | 939 |

No. of parameters | 41 | 41 | 42 | 42 |

Δρ_{max}, Δρ_{min} (e Å^{−}^{3}) | 0.65, −0.46 | 0.83, −0.71 | 0.58, −0.71 | 0.37, −0.41 |

Computer programs: *COLLECT* (Hooft, 1998), *CrysAlis CCD* (Oxford Diffraction, 2005), *HKL* *SCALEPACK* (Otwinowski & Minor, 1997), *CrysAlis RED* (Oxford Diffraction, 2005), *HKL* *DENZO* (Otwinowski & Minor, 1997) and *SCALEPACK*, *CrysAlis RED*, *SIR97* (Altomare *et al.*, 1999), JANA2000 (Petříček *et al.*, 2000), *ORTEP-3 for Windows* (Farrugia, 1997), JANA2000.

_{2}SO

_{4}family: C1: Length (Å) of the interaction C1; C1/CR: the ratio of C1 to the sum of crystal radii (Shannon, 1976); M11(bv): bond valence of M11 (Brese & O'Keeffe (1991); Sum: bond valence sum of the cation M11; M11(bv)/Sum ???? top

Compound | C1 | C1/CR | M11(bv) | Sum | M11(bv)/Sum |

K_{2}SO_{4}^{a} | 2.714 (2) | 0.902 | 0.2065 (13) | 1.0795 (18) | 0.191 (2) |

K_{2}CrO_{4}^{b} | 2.681 (1) | 0.891 | 0.2256 (9) | 0.9358 (12) | 0.241 (1) |

K_{2}SeO_{4}^{c} | 2.610 (6) | 0.867 | 0.273 (4) | 0.943 (5) | 0.290 (6) |

K_{2}PO_{3}F^{d} | 2.671 (6) | 0.908 | 0.159 (3) | 0.893 (5) | 0.178 (4) |

Rb_{2}SO_{4}^{e} | 2.921 (2) | 0.945 | 0.1676 (11) | 1.0759 (17) | 0.156 (1) |

Rb_{2}CrO_{4}^{f} | 2.876 (15) | 0.931 | 0.189 (8) | 0.984 (10) | 0.192 (8) |

Rb_{2}SeO_{4}^{g} | 2.810 (6) | 0.909 | 0.228 (3) | 0.943 (4) | 0.242 (4) |

Rb_{2}PO_{3}F^{h} | 2.865 (3) | 0.949 | 0.149 (1) | 0.913 (2) | 0.163 (1) |

Cs_{2}SO_{4}^{e} | 3.177 (2) | 0.978 | 0.1293 (14) | 1.061 (3) | 0.122 (2) |

Cs_{2}CrO_{4}^{i} | 3.136 (2) | 0.965 | 0.1445 (9) | 0.968 (2) | 0.149 (10) |

Cs_{2}SeO_{4}^{j} | 3.038 (4) | 0.935 | 0.187 (2) | 0.916 (3) | 0.204 (3) |

Cs_{2}PO_{3}F^{h} | 3.141 (4) | 0.988 | 0.1118 (13) | 0.901 (3) | 0.124 (2) |

Tl_{2}SO_{4}^{k} | 2.844 (11) | 0.926 | 0.163 (5) | 0.838 (6) | 0.195 (7) |

Tl_{2}CrO_{4}^{l} | 2.70 (9) | 0.879 | 0.24 (6) | 0.90 (6) | 0.27 (8) |

Tl_{2}SeO_{4}^{m} | 2.660 (18) | 0.866 | 0.268 (13) | 0.801 (15) | 0.334 (2) |

Notes:(a) Ojima et al. (1995); (b) Toriumi & Saito (1978); (c) González-Silgo et al. (1996); (d) Payen et al. (1979); (e) Weber et al. (1989); (f) Aleksovska et al. (1998); (g) Takahashi et al. (1987); (h) this study; (i) Morris et al. (1981); (j) Zúñiga et al. (1991); (k) Wallez et al. (2004); (l) Carter & Margulis (1972); (m) Fábry & Breczewski (1993); |

_{2}SO

_{4}family: a/b, a/c, b/c: the unit-cell ratios. M11-O/F-

*X*: the angle (°) contatined by the atoms involved in the interaction C1 top

Compound | a/b | a/c | b/c | M11-O/F-X |

K_{2}SO_{4}^{a} | 1.2959 (3) | 0.74247 (6) | 0.5730 (1) | 179.7 (2) |

K_{2}CrO_{4}^{b} | 1.2945 (6) | 0.7374 (2) | 0.5696 (1) | 178.96 (15) |

K_{2}SeO_{4}^{c} | 1.276 (3) | 0.732 (2) | 0.573 (1) | 176.4 (3) |

K_{2}PO_{3}F^{d} | 1.269 (2) | 0.7427 (8) | 0.5854 (8) | 166.0 (5) |

Rb_{2}SO_{4}^{e} | 1.3079 (6) | 0.7490 (2) | 0.5726 (2) | 179.2 (2) |

Rb_{2}CrO_{4}^{f} | 1.317 (1) | 0.7460 (4) | 0.5665 (2) | 178.3 (8) |

Rb_{2}SeO_{4}^{g} | 1.292 (1) | 0.7375 (2) | 0.5708 (3) | 179.5 (4) |

Rb_{2}PO_{3}F^{h} | 1.28542 (8) | 0.74664 (4) | 0.58085 (4) | 171.6 (2) |

Cs_{2}SO_{4}^{e} | 1.3170 (7) | 0.7533 (3) | 0.5720 (2) | 178.7 (3) |

Cs_{2}CrO_{4}^{i} | 1.338 (1) | 0.7524 (8) | 0.5625 (5) | 177.70 (11) |

Cs_{2}SeO_{4}^{j} | 1.3021 (5) | 0.7430 (2) | 0.5706 (3) | 178.7 (3) |

Cs_{2}PO_{3}F^{h} | 1.3020 (5) | 0.7528 (3) | 0.5782 (2) | 175.4 (3) |

Tl_{2}SO_{4}^{k} | 1.3182 (3) | 0.7352 (2) | 0.5577 (2) | 179 (7) |

Tl_{2}CrO_{4}^{l} | 1.338 (2) | 0.7374 (8) | 0.5509 (7) | 178 (4) |

Tl_{2}SeO_{4}^{m} | 1.3025 (8) | 0.7250 (4) | 0.5566 (3) | 178.1 (14) |

Note: The references are the same as in Table 1. |

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Interest in these compounds was generated by a previous analysis of the members of the β-K

_{2}SO_{4}family with simple (non-complex) cations (Fábry & Pérez-Mato, 1993) [see also Beznosikov (1993) and da Silvaet al.(2005)]. This analysis resulted in the setting of criteria that enable qualitative distinction of the simple-cation members of the β-K_{2}SO_{4}family that undergo low-temperature phase transitions from the stablephases.There are two symmetry-independent cations in the structures belonging to the β-K

_{2}SO_{4}family. The cation that is surrounded by 11 (or ten) anionic ligands (i.e.O atoms in the prototypic compound β-K_{2}SO_{4}) is less firmly bound than that with nine closest neighbours. The former cation will be hereafter calledM11. Low-temperature phase transitions occur in those structures where the cationM11 is significantly underbonded (Brown, 1992). Typically, the underbonding of this cation is accompanied by the presence of a very short cation–anion ligand interaction. Hereafter, this short interaction will be denominated asC1.C1 is often the shortest cation–anion interaction in β-K_{2}SO_{4}structures. Its contribution to the overall bond-valence sum tends to be high in the compounds where the low-temperaturephase transitions occur. The shortening ofC1 is significant with regard to the sum of the ionic radii (Shannon, 1976).C1 is approximately parallel to the unit-cell parameterain thePnmasetting (Fig. 1). Therefore the unit-cell ratiosa/banda/calso correlate with occurrences of the low-temperature phase transitions in this compound.This is shown in Fig. 2, which depicts the plot of

a/banda/cunit-cell ratios for the title and related compounds. K_{2}SeO_{4}and Tl_{2}SeO_{4}are representatives of the β-K_{2}SO_{4}structures with confirmed low-temperature phase transitions (Yamadaet al., 1984; Frieseet al., 2004, respectively). One of these compounds is situated towards the left corner of the plot and the other lies well below the dashed line. This line depicts the loci that conform to the orthohexagonal metric of the unit cell,i.e.the loci for which the conditionc=b^{1/2}is fulfilled; the shorter the ratio, the more probable the low-temperature phase transitions. It can also be seen that the majority of the compounds are situated below the dashed line. The plot shows a certain regularity in the positions of the compounds either with corresponding cations or corresponding anions. For example, it can be seen that Tl compounds tend to be situated below the respective Cs compounds with the same anion, and that there is a similar tendency for the positions of the K, Rb and Cs compounds with the same anion. (A hypothetical structure of Tl_{2}PO_{3}F would be in the vicinity ofa/b≈ 1.31 anda/c≈ 0.736,i.e.in the region where a low-temperature phase transition can be expected.) Tables 1 and 2 list the numerical values of the criteria given above.It follows from Fig. 2 that the flouorotrioxophosphates are unusual in their positions, especially in K

_{2}PO_{3}F. Two opposite criteria are present; it is situated on the left side, but it is also situated above the dashed line. By analogy to the selenates, it can be assumed that a low-temperature phase transition would be more likely in K_{2}PO_{3}F than in Rb_{2}PO_{3}F and even in Cs_{2}PO_{3}F. [In K_{2}PO_{3}F, no phase transition was detected by a differential scanning calorimetry experiment (Vaněk, 2004) in the temperature region 96–483 K.]The reason for the unusual positions of the fluorotrioxophosphates is plausibly related to the size and symmetry of the [PO

_{3}F]^{2−}anion. The P—F bond distance is longer than the P—O bond lengths. It is of interest that in K_{2}PO_{3}F (Payenet al., 1979), as well as in the title structures, theC1 interaction involves the F atom. On the other hand, the longer bond P—Fis accommodated inthe structure by a significantly larger deviation ofC1 from parallelism with theaaxis (cf.Fig. 1 and Table 2.)We were interested in confirming the hypothesis that Rb

_{2}PO_{3}F and Cs_{2}PO_{3}F belong to the β-K_{2}SO_{4}family and that fluorine is involved in theC1 interaction. It has also been of interest to find the deviation ofC1 from being parallel to theaaxis. Although low-temperature phase transitions were not expected, we still carried out differential-scanning calorimetry experiments.In carefully dried samples of either compound, no phase transitions were observed by differential scanning calorimetry. However, even in slightly humid samples (the title compounds are hygroscopic), phase transitions were observed at 175 and 177 K for Rb

_{2}PO_{3}F and Cs_{2}PO_{3}F, respectively. The measured enthalpies were not reproducible. The structure determinations did not reveal signs of phase transitions in either compound.The influence of humidity can perhaps be related to contradictory reports on the phase transition in K

_{2}SO_{4}. Gesiet al.(1982) observed a phase transition in K_{2}SO_{4}at 56 K by dielectric measurements; Ahmed (1996) confirmed the presence of the phase transition at this temperature by powder diffraction. The low-temperature phase is monoclinic (P2_{1}/n11). It should be noted that the latter author had recrystallized the sample and had dried it before the preparation of the powder for the diffraction experiment. On the other hand, Ojimaet al.(1995) did not observe any phase transition down to 15 K by single-crystal diffractometry.The interatomic distances are normal in the title structures. As expected, the

C1 interaction involves fluorine in both compounds.The twinning in Cs

_{2}PO_{3}F is remarkable. It is related to the lattice parametersbandc. The ratioc/bis ~ 3^{1/2}and mimics orthohexagonal lattice parameters. The approximation of the lattice parametersbandcto the orthohexagonal metric in Cs_{2}PO_{3}F could be predicted taking into account the positions of the related compounds in Fig. 1. The lattice parameters as well as their ratios in the β-K_{2}SO_{4}family are dependent on the constituent ions, thus confirming the ionic character of these compounds (Aleksovskaet al., 1998). It is also of interest that only two and not three domain states were observed. Details regarding the twinning are given in the CIF for both determinations of Cs_{2}PO_{3}F.We have also synthesized Tl

_{2}PO_{3}F and determined its structure. It was found that it belongs to an unprecedented new structural type with a space-group typeR3 orR3. The structure determination will be published in the near future.