Prescott, Hillary A.: The Crystal Structures and Thermal Behavior of Hydrogen Monofluorophosphates and Basic Monofluorophosphates with Alkali Metal and N-containing Cations

69

Kapitel 5. Thermal Analysis

The crystallographic study of the hydrogen monofluorophosphates lead to investigations on the thermal behavior of a select number of these compounds. The existence of first-order phase transitions [82] similar to those found in the hydrogen sulfates was also examined. Compounds were selected for measurement depending on their composition and structure. The results provide an overview of the thermal decomposition for the different types of substance in this class of compounds. The investigations on the thermal behavior of the sodium compounds, NaHPO3F and NaHPO3F·2.5H2O, gave insight on the influence of the crystal water on the salt's thermal decomposition. The study of the CsHPO3F compound was of particular interest based on phase transitions found for the isoelectronic CsHSO4, a well-known proton conductor [2]. The hydrogen monofluorophosphate with an organic cation and a structure similar to CsHPO3F, [NHEt3]HPO3F, was also investigated thermally.

Thermal studies have been carried out on the monofluorophosphates, CaPO3F·2H2O [21, 22], SrPO3F·H2O [23, 24], and Mg(NH4)2(PO3F)2·2H2O [25] and the hydrogen monofluorophosphate, KHPO3F [26, 27]. The decomposition of the basic monofluorophosphates yielded the diphosphate, M2P2O7 (M = Sr, Ca), as the end product, whereas KHPO3F decomposed to the cyclo-triphosphate, K3[P3O9]. In the case of KHPO3F, condensation reactions began at 413 K with the release of HF rather than H2O


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[27]. At this temperature, unreacted KHPO3F was found along with the cyclo-triphosphate. A variation in decomposition was observed, when KHPO3F was heated directly to temperatures above 463 K. In this case, an array of polyphosphates with and without fluorine: Kn[PnO3n-1F2], Kn+1[PnO3nF], and Kn[PnO3n+1] were obtained by the escape of both HF and H2O [27]. The basic monofluorophosphate, CaPO3F·2H2O, was studied with MS-coupled thermogravimetry [21]. The MS gas analysis showed the release of H2O+, HF+, and POF2+, a fragment of POF3, at different stages throughout the decomposition of the Ca salt. Fluorination of the solid sample with gaseous HF resulted in the formation of small amounts of POF3, which was observed by the IC curve of m/z 85 for POF2+.

The KH2PO4 compound decomposes to the end product of metaphosphate, KPO3, above 573 K [83]. The sodium phosphate, NaH2PO4, decomposes to Na2H2P2O7 between 473 and 513 K [84] according to the following path of decomposition [85]:

NaH2PO4 Na2H2P2O7 (NaPO3)n Na3P3O9

Three stable, crystalline phases of NaPO3 exist, which give identical solutions [19]. Thus, the thermal study of the hydrogen monofluorophosphates should contribute to an understanding of their decomposition and could show similarities to the hydrogen phosphates or KHPO3F, or, in the case of the hydrate, the CaPO3F·2H2O. The thermal behavior of the compounds, NaHPO3F, NaHPO3F·2.5H2O, CsHPO3F, and [NHEt3]HPO3F, were studied in N2 or air with a TA-MS skimmer coupled system described in Sect. 2.1 Differential Thermal Analysis. The results of these thermal investigations are presented here.

5.1 The Sodium Salts: NaHPO3F and NaHPO3F·2.5H2O

The compounds, NaHPO3F and NaHPO3F·2.5H2O, had three and four step decompositions, respectively (Fig. 19a and b). In both cases, decomposition was complete by 673 K. The initial temperatures of decomposition varied between the compounds. Dehydration of NaHPO3F·2.5H2O started immediately after heating, whereas the anhydrous salt was stable up to about 373 K. The decompositions of the sodium compounds were simulated and the intermediate and end products were characterized by H and F elemental analysis, XRD, 31P and 19F NMR (when soluble), and IR to understand the distinct steps of decomposition.


71

Fig. 19 STA curves for (a) NaHPO3F and (b) NaHPO3F·2.5H2O both in N2 showing the respective three and four step decompositions.

5.1.1 The Thermal Behavior of NaHPO3F

The hydrogen monofluorophosphate, NaHPO3F, decomposes in a three step process with a total mass loss of 15.41% (1.04 mg, 8.5 mmol). Endothermic effects are observed between ca. 423-483, 523-573, and 598-648 K (Fig. 19a). The IC curves, m/z 17 (OH+), 18 (H2O+), 19 (F+), and 20 (HF+), show three corresponding maxima for these intervals (Fig. 20a). Therefore, the loss of mass in the samples can be explained by the simultaneous release of H2O and HF up to 673 K. The formation of POF3 was only confirmed for the first step of


72

decomposition by the detected maxima between 423 and 498 K in the IC curves of m/z 50 (PF+) and 47 (PO+), both fragments of POF3 [21] (Fig. 20b). However, the absolute amount of these fragments is significantly less than the others due to the smaller scale of 10-12 A (Fig. 20b). The quantitative interpretation of the data showed a total mass loss of 0.24 (0.013 mmol) and 1.01 mg (0.053 mmol) for H2O and HF, respectively (Tab. 45), which corresponds to yields of 23.6 and 96.1 %, respectively.

Fig. 20 Measured IC and TG curves of NaHPO3F. (a) Maxima are observed for the IC curves, m/z 17 (OH+), 18 (H2O+), 19 (F+), and 20 (HF+) for each of the endothermic processes. (b) The IC curves of m/z 50 (PF+) and 47 (PO+) showing a maximum for the first step of decomposition.


73

Tab. 45 Quantitative interpretation of the IC curves, m/z 18 and 19, for NaHPO3F postcalibration

 

T [K]a

Deltam (TG) [mg]

A [10-6 A·s]

mH2O (PTA) [mg]b

mHF(PTA) [mg]c

Deltam1

372...506

0.57

0.165

 

 

Deltam2

506...591

0.19

0.044

 

 

Deltam3

591...683

0.28

0.094

 

 

SigmaDeltami

 

1.04

0.303

0.24

1.01

cCalculated with A=5.172·106 mg/A·s from the calibration with NaHF2·0,12 H2O in N2; with the partial area of 372-683K calculated as one

Simulated experiments were carried out at 498, 573, and 673 K. The elemental analyses showed a reduction in the hydrogen contents with increasing temperature (Tab. 46). The amount of free fluoride remained constant except for a higher value of 1.1 % found in the sample heated to 498 K. On the other hand, the total fluoride contents (Seel) diminished steadily as the temperature increased. Thus, it seems P-F bonds are broken gradually resulting in the stepwise release of HF throughout the entire process, which is supported by the three maximums found for m/z 19 and 20 shown in Fig. 20a. The final Seel value of 1.1 % obtained after the sample was heated to 673 K indicated the almost complete release of fluoride in the form of HF or POF3. The "absence" of H (0.025 ap 0%) implies a hydrogen-free end product.

Tab. 46 Elemental analysis of NaHPO3F at RT and after being heated to the indicated temperature

T/K

RT [exp.(calcd)]

498

573

673

H /%

0.7 (0.82)

0.44

0.045

0.025

F (50 mL H2O / Seel)/ %

0.3 / 13.0 (15.57)

1.1 / 7.9

0.4 / 3.4

0.5 / 1.1

While the elemental analyses were quite effective in showing what escaped from the melt, the identification of the products or residue formed after heating was characterized by 31P and 19F NMR and XRD. The NMR spectra after heating to 498 K showed that several products of condensation were obtained in the first step of decomposition, which was also observed in [27]; in the second step (573 K), no new products were formed (Tab. 47). The 31P NMR spectra were identical for 498 and 573 K except for a change in the product ratios. The major phase at 498 K was the stable diphosphate anion, H2P2O72-, with 47%. The triphosphate anion, (P3O9)3-, indicated by the singulet at ca. -20 ppm was obtained in 81% in the mixture at 573 K and was the end product at 673 K. The low H contents (0.045%) found at 573 K agrees with the NMR data, which shows that the hydrogen-free anions, P2O5F22- and P3O93-, account for 90% of the phosphorus species obtained at this


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temperature.

Tab. 47 31P NMR data (delta) for the products obtained after heating NaHPO3F to the indicated temperature; J is given in parenthesis with the product ratios in %

T/K

RT

%

498

%

573

%

673

HPO3F-

-3.6 (d, 908 Hz)

81

 

 

 

 

 

H2PO4-

0.8 (s)

19

 

 

 

 

 

H2P2O72-

 

 

-10.0 (s)

47

-9.8 (s)

4

 

HP2O6F2-

 

 

-10.1 (s), -16.9 (d, 933 Hz)

18

10.0 (s), -16.8 (d, 931 Hz)

4

 

P2O5F22-

 

 

-17.8 (dt, JPF = 940, JPF´ = 8 Hz)

11

-17.7 (dt, 940 Hz)

10

 

P3O93-

 

 

-20.9 (s)

13

-20.7 (s)

81

-20.8 (s)

?

 

 

-13.2 (s)

3

-13.1 (s)

1

 

The double triplet found at -17.8/-17.7 ppm for the temperatures, 498 and 573 K, was assigned to the difluorodiphosphate anion, P2O5F22- [86], with the corresponding double triplet at -73.7 ppm with JFP = 942 and JFP´ = 10 Hz in the 19F spectra. A second doublet found in the 31P and 19F spectra at -16.9/-16.8 and -74.0 ppm, respectively, with JPF = 933 Hz implied a second, fluorinated, condensated phosphate in the melt. The chemical shift of the doublet similar to that of the P2O5F22- anion and the singulet at -10.1 ppm in the 31P spectra suggest the intermediate HP2O6F2- anion [87, 88], which was also formed in the decomposition of KHPO3F, when initially heated to temperatures above 463 K [27]. The singulet observed at -13.2/-13.1 ppm for 498/573 K, respectively, could not be interpreted.

XRD confirmed the cyclo-triphosphate, Na3P3O9 [89, 90], as the end product. The patterns measured for 573 and 673 K were identical except for weak peaks at 21.4, 25.8, 27.9, and 31.1 Å (573 K) due to the incomplete decomposition of the intermediates at 573 K. The pattern measured for the melt heated to 498 K could not be interpreted because of the numerous phases present (Tab. 47).

The following can be concluded about the path of decomposition:


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The following reactions can be understood as a rough schema for the stepwise decomposition of NaHPO3F, but are not mechanistic and other products and combinations can not be ruled out.

Above 498 K

2 NaHPO3F rarr Na2P2O5F2 + H2O

2 NaHPO3F rarr Na2HP2O6F + HF

Na2P2O5F2 + H2O rarr Na2HP2O6F + HF

Na2P2O5F2 + 2 H2O rarr Na2H2P2O7 + 2 HF

Na2HP2O6F + H2O rarr Na2H2P2O7 + HF

Reaction 6

Reaction 7a

Reaction 7b

Reaction 8a

Reaction 8b

3 Na2H2P2O7 rarr 2 Na3P3O9 + 3 H2O

Reaction 9a

3 Na2HP2O6F rarr 2 Na3P3O9 + 3 HF

Reaction 9b

3 Na2P2O5F2 + 3 H2O rarr 2 Na3P3O9 + 6 HF

Reaction 10

Overall without consideration of the minimal loss of H2O and formation of fluorinated diphosphates:

3 NaHPO3F rarr Na3P3O9 + 3 HF

Reaction 11

The decomposition of anhydrous NaHPO3F resembles that of KHPO3F reported in [27] with identical end products. The diphosphate, Na2H2P2O7, which is more stable than that of the potassium, is also found as an intermediate product.

5.1.2 The Thermal Behavior of NaHPO3F·2.5H2O

The decomposition of NaHPO3F·2.5H2O involved four steps with endothermic effects between RT-373, 448-473, 530-573, and 598-633 K (Fig. 19b) [91]. The total mass loss of 34.8% was higher than that found for NaHPO3F. The IC curves for m/z 17 (OH+), 18 (H2O+), 19 (F+), and 20 (HF+) are shown in Fig. 21a and b. The release of HF was not observed until the second step of decomposition at temperatures of about 473 K and continued on with two additional maxima up to 723 K (Fig. 21a), whereas dehydration started immediately after heating and was even observed in the dry gas flow at RT (Fig. 21b). Therefore, the removal and addition of crystal water from the structure was examined.


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Fig. 21 Measured IC curves with the TG and DTG graphs of NaHPO3F2.5H2O (a) for m/z 19 (F+) and 20 (HF+) and (b) for m/z 17 (OH+) and 18 (H2O+)

The 2.5 moles of crystal water could be completely removed after NaHPO3F·2.5H2O was left to stand in vacuum at RT for 12 h. The ease with which the crystal water can be removed without heating was confirmed by XRD; the pattern of the treated NaHPO3F·2.5H2O was identical to that of NaHPO3F [71]. Experiments in air showed that 1.7 moles of crystal water, which corresponded to a mass increase of 25.5%, could be recovered by simply leaving the sample to stand in air over a period of 6 d. The experimental and theoretical changes in mass for both experiments are given in (Tab. 48).


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Tab. 48 Behavior of NaHPO3F·2.5H2O at RT

 

12 h Vacuum

In air

Change in mass /% (calcd)

-28.8 (26.95)

+25.5 (36.88)

Inconsistencies due to immediate dehydration were observed in the total amount of mass lost during thermal decomposition. The loss of crystal water accounted for 97.1% of the total mass loss in the first stage of decomposition. After that, the fraction of H2O responsible for the total mass loss decreased significantly with the simultaneous release of HF and other species above 473 K (Tab. 49).

Tab. 49 Quantitative interpretation of the IC curves, m/z 18, postcalibration for NaHPO3F·2.5H2O

 

T [K]a

Deltam (TG) [mg]

A [10-6 A·s]

Deltam (PTA) [mg]b

Deltam1

29...150

2.76

5.415

2.68

97.1

Deltam2

160...240

0.94

0.272

0.13

13.8

Deltam3

250...306

0.31

0.081

0.04

12.9

Deltam4

306...392

0.43

0.227

0.11

25.6

cFraction of H2O in the TG step, Deltami

Information on the products formed during decomposition was acquired by simulated experiments carried out at 393, 493, and 673 K. Again, the H contents decreased with higher temperatures (Tab. 50) as in the case of NaHPO3F. However, inconsistencies were observed in the fluoride analyses. The total fluoride contents increased initially from 9.2 to 11.7% at 393 K before decreasing to a final value of 1.0% very similar to the 1.1% found for NaHPO3F. At 393 K, 92% of the entire fluoride in the sample was found as free fluoride in the melt. This and the two singulets in the 19F spectra (Tab. 52) imply that the fluoride does not immediately escape the melt in the form of HF, although the P-F bond has been broken; maxima in the IC curves for HF and F were first observed at about 473 K (Fig. 21a). The release of HF after heating to 493 K correlated with (a) a reduction in the amount of free fluoride to 39 %, (b) the disappearance of the singulet at -151 ppm in the 19F spectra, and (c) the observed maximum in Fig. 21a.

Tab. 50 Elemental analysis of NaHPO3F·2.5H2O at RT and after being heated to the indicated temperature

T/K

12 h Vacuum

[exp.(calcd)]

RT [exp.(calcd)]

393

493

673

H /%

0.9 (0.82)

3.3 (3.59)

2.3

0.9

0.02

F (50 mL H2O/ Seel)/ %

0.1 / 14.2 (15.57)

0.3 / 9.2 (11.38)

10.8 / 11.7

2.7 / 6.9

0.1 / 1.0

The presence of the HPO3F- and H2PO4- anions was detected in the melt at 393 and 493 K


78

by 31P NMR (Tab. 51). The 31P spectrum of the melt heated to 393K showed that the HPO3F-/H2PO4- signal ratio was reversed when compared with the spectrum of the RT product; equivalent amounts of the HPO3F- and H2PO4- anions are found in the melt at 493 K. A strong singulet at -10.0 ppm confirmed the condensation of phosphorus and the presence of the H2P2O72- anion as the major product in the second step of decomposition (493 K) identical to NaHPO3F melt at 498 K. On the other hand, a doublet in the 19F spectra was observed for the HPO3F- anion throughout the decomposition and two singulets were observed for the melt heated to 393 K (Tab. 52) not observed in the decomposition of NaHPO3F. The singulet at -151 ppm was assigned to HF not yet released from the melt [92]. The other singulet at -131 ppm ({F1}) could not be interpreted. An analysis of the product ratios in the 19F spectra shows that the majority of fluorine is in the form of HF at 393 K (73%), but at higher temperatures, HF leaves the melt (the singulet at -151 ppm disappears) and HPO3F- becomes the major product containing fluoride. Interestingly enough, the signal ratio of HPO3F-/{F1} remains approximately constant from 393 to 493 K with a value of ca. 2.4.

Tab. 51 31P NMR data (delta) for NaHPO3F·2.5H2O and the products obtained after heating to the indicated temperature; JPF is given in parenthesis with the product ratios in %

T/K

12 h Vacuum

%

RT

%

393

%

493

%

HPO3F-

-3.6 (d, 908 Hz)

90

-3.7 (d, 909 Hz)

89

-3.8 (d, 908 Hz)

9

-3.7(d, 909 Hz)

22

H2PO4-

0.8 (s)

10

0.7 (s)

11

0.8 (s)

91

0.7 (s)

22

H2P2O72-

 

 

 

 

 

 

-10.0(s)

56

Tab. 52 19F NMR data (delta) for NaHPO3F·2.5H2O and the products obtained after heating to the indicated temperature; JPF is given in parenthesis

T/K

12 h Vacuum

RT

393

%

493

%

HPO3F-

-74.9 (d, 908 Hz)

-74.8 (d, 909 Hz)

-74.8 (d, 904 Hz)

19

-74.8(d, 904 Hz)

71

{F1}

 

 

-131 (s)

8

-131 (s)

29

HF-

 

 

-151 (s)

73

 

 

The XRD patterns measured for the tempered melts were difficult to interpret except for the pattern of the melt heated to 673 K. This pattern could be assigned to a monoclinic (NaPO3) phase [93] and not the cyclo-triphosphate as in the case of NaHPO3F. Interestingly enough, the IR spectra were identical for NaHPO3F·2.5H2O and NaH2PO4, both tempered to 673 K; the spectrum of the end product of anhydrous NaHPO3F varied (Fig. 22).


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Fig. 22 IR spectra for the tempered (a) NaHPO3F, (b) NaHPO3F·2.5H2O, and (c) NaH2PO4 (673 K).


80

The following can be concluded about the decomposition of NaHPO3F·2.5H2O:

With this information, the following nonmechanistic reactions can be formulated for the decomposition of NaHPO3F·2.5H2O demonstrating the simultaneous release of HF and H2O; again, other products could not be ruled out.

Up to 393 K

NaHPO3F·2.5H2O rarr NaHPO3F + 2.5 H2O

Reaction 12a

NaHPO3F·2.5H2O rarr NaH2PO4 + 1.5 H2O + HF

Reaction 12b

Ongoing

NaHPO3F + NaHPO3OH rarr Na2H2P2O7 + HF

Reaction 13a

2 NaHPO3OH rarr Na2H2P2O7 + H2O

Reaction 13b [84]

At 673 K

n Na2H2P2O7 rarr 2 (NaPO3)n + n H2O

Reaction 14 [85]

5.1.3 Comparison

The NaHPO3F and NaHPO3F·2.5H2O both have stepwise decompositions, yet the intermediate and end products formed during these processes vary. The crystal water in NaHPO3F·2.5H2O was released between RT and 373 K during an additional stage of decomposition. Differences in the intermediate and final products formed were based on the hydrolysis of the P-F bond with crystal water in NaHPO3F·2.5H2O and the direct heating of NaHPO3F to over 463 K. The fluorinated diphosphates, Na2H2P2O5F2 and Na2HP2O6F, the diphosphate, Na2H2P2O7, and the cyclo-triphosphate, Na3P3O9, were formed in the first step of decomposition for NaHPO3F with traces of them still existing in the melt tempered to 573 K. In the case of the hydrate, only the anions of H2PO4- and


81

H2P2O72-, were found during the decomposition of the hydrate, because decomposition of NaHPO3F·2.5H2O involved the formation of phosphate. The complete reaction of the HPO3F- anion prior to 498 K was implied by the absence of the corresponding doublet in the 31P spectrum of the tempered NaHPO3F, whereas the HPO3F- anion was observed in the spectrum of NaHPO3F·2.5H2O tempered to 493 K. The release of HF and condensation reactions took place at temperatures above 393 K for both Na compounds. This agrees with the findings in [27] that condensation reactions began at 413 K. Thus, it seems that while the NaHPO3F condensates directly, the hydrate is first hydrolyzed; condensation then takes place with phosphate and monofluorophosphate explaining the absence of fluorinated diphosphates and the end product identical to that of the NaH2PO4. This discrepancy could also be based on the different paths of decomposition and consequent varying heating regimes, which has also been commented on in [27].

Not only do the courses of decomposition, but also the end products formed differ for NaHPO3F and NaHPO3F·2.5H2O. In the case of NaHPO3F, a cyclo-triphosphate was formed. The hydrate decomposed to the corresponding metaphosphate, (NaPO3)n, identical to the end product of the NaH2PO4 below 773 K [85].

5.2 The Thermal Behavior of CsHPO3F

The cesium hydrogen monofluorophosphate was measured in air between RT and 773 K. Endothermic maxima were found at 452 and 507 K (Fig. 23).

Fig. 23 STA graphs measured for CsHPO3F showing a total loss of mass at 6.97%.


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In comparison with the hydrogen sulfate [2], a first-order phase transition was not observed. CsHPO3F melts at 443.7 K and then decomposes directly to the end product at 748 K without forming stable intermediates. Thus, the decomposition of the cesium compound is quite different from that observed for the sodium salts. The release of HF was much more gradual than in the case of the sodium compounds and began around 398 K continuing up to 748 K (Fig. 24a). On the other hand, the temperature range, in which H2O escaped the melt, was narrower (between 448 and 673 K) (Fig. 24a). A short break was observed for both species at 507 K (endothermic effect) (Fig. 24a). The formation of the fluorination product, POF3, above 473 K was confirmed by the maximum for m/z 47 (PO+) and 88 (PF3+) [21] shown in Fig. 24b.

Fig. 24 IC graphs for (a) m/z 18 (H2O+), 19 (F+), and 20 (HF+) and (b) m/z 47 (PO+) and 88 (PF3+) shown with the DTA and TG data of CsHPO3F.


83

The hydrogen phosphate, CsH2PO4, is reported to undergo dehydration at ca. 508 K [85] and (CsPO3)n is synthesized by the dehydration of CsH2PO4 [85]. Based on the similar behavior of KHPO3F [27], and NaHPO3F, it can be assumed that CsHPO3F decomposes to form the cyclo-triphosphate, Cs3P3O9 (Reaction 15). A total mass loss of 8.62% (1.56 mg) corresponds to the condensation of the hydrogen monofluorophosphate to the cyclo-triphosphate with the release of HF shown in Reaction 15. The total mass loss (6.97 %, 1.27 mg) found amounts to 81% of the theoretical value.

3 CsHPO3F rarr Cs3P3O9 + 3 HF

Reaction 15

Again, the occurrence of H2O is not considered in the overall reaction for the thermal degradation of CsHPO3F. However, it can be assumed that small amounts of H2O are released as in the decompositon of NaHPO3F supported by the formation of fluorinated diphosphates.

5.3 The Thermal Behavior of [NH(CH2CH3)3]HPO3F

The thermal behavior of [NHEt3]HPO3F was investigated between RT-993 K and was quite different than that of the alkali metal hydrogen monofluorophosphates (Fig. 25).

Fig. 25 STA graphs of [NHEt3]HPO3F showing the progression (course) of decomposition.

The steps of decomposition are less distinct than in the case of the sodium compounds. The first endothermic effect, the melting point, is lower than that of the cesium salt and was


84

observed at 393.1 K. In succeeding steps of decomposition up to 684 K, exothermic and endothermic processes overlap each other. A pronounced endothermic process then occurs at 684 K, which corresponds to the decomposition of the organic cation (Fig. 26a). Above this temperature, indistinct exothermic processes take place.

Fig. 26 IC curves of [NHEt3]HPO3F for (a) m/z 18 (H2O+), 19 (F+), 20 (HF+), 26 (C2H2+) and (b) m/z 88 (PF3+), 104 (POF3+), and 85 (POF2+)

The emission of HF first began at 531 K, a higher temperature than for the alkali metal hydrogen monofluorophosphate (ca. 473 K), whereas dehydration was observed immediately after melting (393.1 K) (Fig. 26a). An integral quantitative analysis of the TG graph for the entire path of decomposition (331 to 931 K) found a total mass loss of


85

92.27% (10.71 mg, 0.053 mmol) much higher than those found for the alkali metal compounds (Tab. 53). In comparison with the alkali metal hydrogen monofluorophosphate, an end product, such as a triphosphate or metaphosphate, was not observed based on the absence of a stabilizing cation, instead a small amount of a black residue was left over. Both the organic cation and the HPO3F- anion seem to decompose and escape the melt as diverse volatile products. This is reflected by small H2O and HF fractions (7 and 17%, respectively) of the total mass lost. The secondary formation of POF3 due to the presence of HF in the melt was confirmed with the observation of various POF3 fragments [21]: PO, PF, PF2, PF3, POF3, and POF2. Only those of PF3, POF3, and POF2 are shown in Fig. 26b; the points of release vary. These fragments were not as abundant in the decompositions of the alkali metal hydrogen monofluorophosphates. The formation of POF3 was also supported by the constant release of fluorine (m/z 19) above 723 K (Fig. 26a).

Tab. 53 Quantitative interpretation of the TG graph and IC curves, m/z 18 and 19

 

T [K]

A [10-6 A·s]

m (PTA) [mg/mmol]

mTG [mg/mmol]

DeltamH2O

402...931

0.941

0.74 / 0.041

 

DeltamHF

531...931

0.356

1.82 / 0.096

 

Deltamtotal

330...683

 

 

10.71 / 0.053

5.4 Summary

The thermal behavior of NaHPO3F, NaHPO3F·2.5H2O, CsHPO3F, and [NHEt3]HPO3F are quite different depending on the cation and presence of crystal water in the structure. The sodium salts, NaHPO3F and NaHPO3F·2.5H2O, both have decompositions involving three and four steps, respectively. The anhydrous salt initially decomposed to the intermediate condensation products, Na2H2P2O5F2, Na2HP2O6F, Na2H2P2O7, and Na3P3O9, after heating to 498 K with the cyclo-triphosphate, Na3P3O9, as the end product at 673 K. On the other hand, the decomposition of the hydrate, NaHPO3F·2.5H2O, involves the initial release of the crystal water and the formation of the corresponding phosphate, NaH2PO4 (393 K). After that, HF escapes the melt and condensation occurs forming the diphosphate, Na2H2P2O7. A metaphosphate, (NaPO3)n, was obtained as the end product, which was identical to the decomposition product of NaH2PO4. The cesium compound, CsHPO3F, melts at 443.7 K and then decomposes directly to the end product without the formation of stable intermediates. The decomposition of [NHEt3]HPO3F is similar to the CsHPO3F. Melting occurs at 373.1 K and then the compound decomposes gradually with a break at 707.6 K. A black residue was left over as the final product based on an almost complete loss of mass (92.27 %).


86

Tab. 54 Observed temperatures for the escape of HF and H2O (K)

 

NaHPO3F

NaHPO3F·2.5H2O

CsHPO3F

[NHEt3]HPO3F

HF release

 

 

 

 

Initial temperature

448

448

473

531

First maximum

473

473

498

673

H2O release

 

 

 

 

Initial temperature

423

RT

473

393

First maximum

<473

<373

<498

>473 (broad)

The following two types of decomposition were observed:

yet the compounds can be grouped differently according to the temperature at which H2O and HF escaped the melt (Tab. 54). The anhydrous salts of Na and Cs have similar behavior with regard to the release of H2O and HF. A comparison of the hydrate with these anhydrous salts shows that, as expected, H2O is released from the hydrate at much lower temperatures. HF, on the other hand, escapes the melt of NaHPO3F·2.5H2O at temperatures identical to those of NaHPO3F. The thermal behavior of [NHEt3]HPO3F differed from that of the hydrate and anhydrous salts, NaHPO3F and CsHPO3F. In this case, H2O escaped the melt at temperatures lower than those observed for the anhydrous alkali metal salts, while the release of HF started later than that for the MHPO3F. The broadness of the maximum also suggests another type of mechanism compared with that of the alkali metal compounds.


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