Mass spectrometric studies at high temperatures—XXIII

j. inorg,nucl.Chem..1968,VoL30.pp. 729to 736. PergamonPressLtd. PrintedinGreatBritain

MASS

SPECTROMETRIC STUDIES TEMPERATURES - XXlII

AT

HIGH

V A P O R E Q U I L I B R I A O V E R M O L T E N NaSnF3 A N D KSnF3 J. W. HASTIE, K. F. ZMBOV* and J. L. MARGRAVE Rice University, Department of Chemistry, Houston, Texas 77001

(First received 1June 1967; in revised form 28August 1967)

appearance potential measurements on vapors from NaSnF.~ and KSnFa over the temperature range 77.5-895°K indicate the presence of SnF2, NaSnFs, Na~SnFo NaF, NaSnsFs and possibly (NaSnFs)t as well as SnFz, KSnF3 and KF, in their approximate relative order of abundance. From ion current vs. temperature data, second law heats for the reactions of SnFs and NaF (or KF) to form NaSnFs, NatSnF, and NaSntFs (or KSnF3) were derived. By using values of the activities of NaF and KF, obtained from an analogy with condensed phase data for similar mixtures, free energies were calculated for these reactions. Independent enthalpies of reaction were then obtained by use of estimated entropy data. Abstract-Mass spectral and

INTRODUCTION

THE PmBSENCE of complex m o l e c u l e s - o f the type AMXs or AMX4 where M is a di- or tri-valent metal, A an alkali metal and X a halogen-in the vapors over mixed halide systems has been well established using mass spectrometric [ 1-6], matrix spectroscopic[7] and vapor pressure methods[3, 8, 9]. A brief summary of the evidence prior to 1965 for the existence of such molecules has been given[2]. In several instances evidence was found for the presence, in small amount, of even more complex species such as A2AIFs, A~(AIF4)2, Li2BeF4[1], Na~BeF4[9], and CsCd2Cls[3]. Also the heats of formation of AMXs o r AMX4 from their component salts were of a similar magnitude while the free energies varied considerably. For example, where M is Pb, relatively large free energy values were found and only AMXs was observed to any appreciable extent[2, 3]. In order to study the more complex species mentioned above, systems containing ions of higher charge density e.g., NaSnFa and KSnFs were chosen for mass spectrometric and thermodynamic studies. *On leave from the Boris Kidrich Institute of Nuclear Sciences, Belgrade, Yugoslavia. 1. A. Biichler and J. L. Stauffer, Thermodynamics, Vol. 1, p. 271. International Atomic Energy Agency, Vienna, (1966). 2. H. Bloom and J. W. Hastie, Aust.J. Chem. 19, 1003 (1966). 3. J.W. Hastie, Ph, D. Thesis, University of Tasmania (1966). 4. G. I. Novikov and A. L. Kuz'menko, Vest. leningr, gos. Univ. 19 (16); Ser. Fiz. i Khim. 3, 165 (1964); Chem.Abstr. 62, 1317b (1965); Chem.Abstr. 62, 343 If (1965). 5. J. Berkowitz and W. A. Chupka, Ann. New YorkAcad. Sci. 79, 1073 (1959). 6. R.F. Porter and E. E. Zeller, J. chem. Phys. 33, 858 (1960). 7. A. Snelson, Optical Spectra of Some Low-Molecular Weight Compounds Using the Matrix Isolation Technique, Report No. IITRI-U6001-13, U.S. Army Research Office (1966). 8. G.A. Semenov and F. G. Gavryuchenko, Russ. d. inorg. Chem. 9, 123 (1964). 9. K. A. Sense and R. W. Stone, J. phys. Chem. 61, 337 (1957). 729

730

J . W . HASTIE, K. F. ZMBOV and J. L. M A R G R A V E EXPERIMENTAL

The mass spectrometer used was a 12-in. radius, 60 ° sector magnetic-focusing instrument, which together with the experimental procedure have been described previously[10]. The salts used were the commercially available samples of NaSnF3 and KSnF3 and they were contained in nickel Knudsen cells during the beatings. RESULTS

Table 1 presents a typical mass spectrum for the vapors in equilibrium with molten NaSnFs. The distinctive isotopic composition of Sn facilitated interpretation of the mass spectra. The resuRs for KSnF3 are analogous except that no Table 1. Mass spectrum of vapors over NaSnF3 at 490°C.

Ion

Relative intensity **

F+ HF + Na +* Sn + SnF + SnF2 + NaSnF ÷ NaSnF2 + NaSnF3 + Na~SnFa+

4"0 45"0 43.0 30.6 184.0 136.0 -7-0 64-2 ' 20.3 68.7 12"0 100-0 68.0 16"0 12"0

Na~SnF4 +

NaSn~F~ + NaSn2F~ + NaSn~F4 + Na~SnzFs+?

Appearance potential *'§ (eV)

¶ 16"3 -+ 0"3 9.9+0.2 15.5±0-3, (18.5) aj 9.0 ± 0.5, (12-5) 10.5 ± 0.3 13.0 ± 1.0 9.0_+ 0.3 8-8_ 0.3 9.5 _ 0.3 9"0 ± 0"4 ¶ ¶ ¶ ¶

Molecular precursor

-HF NaF SnF~, SnF SnF2, SnF SnFz NaSnF3 NaSnF3 NaSnF3 NasSnF4 Na~SnF4 NaSn2Fs NaSn~Fa NaSn2Fs NasSn2Fe?

*Molecular precursor is NaF up to at least 20 eV. ?This may also be written as the complex dimer [NaSnFs]~. ~The energy scale was calibrated from the IP o f H : O ( 12.67 eV). §Bracketted values indicate the apparent onset energy of a different source or process. rfThe theoretical energy for the process: e + SnF2 --- Sn + + F~ + 2e, is 15-7_ 0.5 eV. SAppearance potentials not recorded. **Intensities represent a summation over the isotopic species; electron energy = 50 eV.

species of mass higher than KSnF3 + were recorded, and heavy K - - S n - - F species were present in much smaller amounts than for the NaSnFa system. To establish the molecular precursors for these ions, ionization efficiency curves and hence appearance potentials (AP) were recorded. The values and assigned molecular precursors are given in Table 1. The corresponding values for the KSnF8 system did not differ significantly from those of NaSnFa except for K + t h e / I P of which was 9-5 ---0.3 eV, in agreement with K F being the precursor. 10. G. D. Blue, J. W. Green, R. G. Bautista and J. L. Margrave,J.phys. Chem.67, 877 (1963).

Mass spectrometric studies at high temperatures-XXIII

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Btichler et a/.[1] and Porter et a/.[6] found that a major part of the Li + ion intensity for LiBeFs and LiAIF4 systems was the result of electron impact fragmentation of the molecules LiBeFs and LiAIF4. However, the molecules NaPbCI8 and KPbCI3 have been found to fragment, at 20 eV ionizing energy, mainly as APbCI= + and APbCI + with no measurable N a + or K + ions[3]. Therefore the ionization effeciency curves for N a + and K + were recorded more accurately than is usual for this type of work, in order to facilitate correct assignment of the molecular precursors of these ions. A semi-log plot for Na +, shown in Fig. 1,

I00

N~

I.-L >re,

et,

<10 t.'Z b.I n.. n,. 0

8

9

I0 ELECTRON

I 12 ENERGY (EV)

14

Fig. 1. Ionization efficiency curve for Na ÷ from NaF, with H~O as a reference.

indicates an A P of 9.9+--0"2 eV, using the known ionization potential of H 2 0 (12.67 eV) [11] as the energy scale calibration. From the known bond energy of NaF[12] and the ionization potential of Na[11] and assuming that N a + is not formed with excess energy, the appearance potential would be 9.80+-0.2eV. Hence the ionization process is: N a F + e --* Na + + F + 2e and for energies up to at least 20 eV no other process was noted. Hence the molecular precursors of N a + and K + are taken to be N a F and KF. Also the data support a value of 112±5 kcal/mole for the dissociation energy of NaF. Subsequent ion current data for these and other ions were measured only a few eV above the appearance potentials to prevent the possible complication of there being several molecular precursors for a particular ion. During the course of an experiment the composition of the condensed phase altered considerably, due to the much higher volatility of SnF= as compared with N a F or K F and hence, no heat of vaporization data are reported. Vapor 11. R. W. Kiser, Introduction to Mass Spectrometry and its Applications, Prentice-Hall, New York (1965). 12. T. L. Cottrell, The Strengths of Chemical Bonds. Butterworths, London, (1958).

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J . W . HASTIE, K. F. ZMBOV and J. L . MARGRAVE

equilibria should be independent of the condensed phase composition and therefore equilibrium constants were recorded over a range of temperature. Data were obtained for both heating and cooling of the Knudsen cell. No equilibria involving alkali-halide dimers were recorded as the low temperature used favored the presence of mainly monomers. From the curves of log K~ vs. 1/T given in Fig. 2, second law enthalpies were obtained for the reactions listed in Table 2. As has been found in similar studies [6], it is difficult to eliminate

I ...........

'

'1'" ,0 (°~-') Fig. 2. Ion-current analogs of equilibrium constants at various temperatures.

scatter in the K~ values and therefore the second-law heats have fairly high uncertainty limits. An attempt to obtain better values was made by combining calculated free energy data with estimated entropy data.

Calculation offree energy data Thermodynamic activities are known for a sufficient number of analogous mixed salts to allow one to estimate reasonably accurate activities for the present mixtures. For example in the LiBeF3 molten mixture[5], LiF has an activity, a(LiF), of 0.025 and for the KPbCI3 mixture[13], a(KCl) = 0.05. It is generally the rule that the activities of,IX fall in the range 0.01-0.2, depending on the charge density ofA + or M 2+[3, 8]. From the known trends, reasonable values for a(NaF) and a(KF) in the present mixtures are 0.1 and 0.01, respectively. An uncertainty in the activities of a factor of 10 produces only several kcal/mole uncertainty in the free energy values; even a large composition change in the melt (e.g. from 50 to 30 mole per cent SnF~) alters the activity only by several orders of magnitude. The absolute equilibrium constants are then calculated as follows. By combining the a(NaF) and a(KF) values with the known standard state vapor pressure data for the monomers [ 14], one obtains the partial pressures, p(NaF) and p(KF). 13. J. L. Barton and H. Bloom, Trans, Faraday Soc. 55, 1792 (1959). 14. JANAF Thermochemical Tables, (Edited by D. R. Stull) No. PB-168-370, The Dow Chemical Company, Midland, Michigan, (Dec. 1964).

855 746 746 754

lb: KSnF8 ~=~K F + S n F 2

2 : NazSnF4 ~=~2 N a F + S n F ~

3 : Na~SnF4 ~ N a F + N a S n F s

4 : NaSnzF5 ~:~ SnFz+ NaSnF3

36-+ 12

69--+4

130-----8

49-+8

61+4

AH~* (kcal/mole)

--

--

--

54-+4

58--+4

AH~t (kcal/mole)

43-+ 13 §

32-+5

53-----9

19-+3

19-+3

AS~ (e.u.)

l ( N a +) × l(SnFz +) × T ! (NaSnF, +) I ( K +) x I(SnF2 +) x T l(KSnFz +) l(Na+)~× l(SnF2 +) x T 2 l(NasSnFs +) i ( N a +) × I ( N a S n F , +) × T I(Na2SnF~ + ) l(SnFz +) × I(NaSnF~ +) × T l(NaSnzF2 +)

K~

*Temperature ranges of 725-770°K and 815~895°K for NaSnF3 and KSnF3 studies, respectively, yielded second-law slopes. tCalculated from a combination of AS~.and AFt. ~ F ~ . values may be calculated from AH~, and AS~, the uncertainties being only +_2 kcal/mole. AS~ for reactions la and I b estimated as described in text. §Calculated firom K=p(SnFO×I(NaSnF2+)/I(NaSn2F2+), assuming a(SnF2)=0-2--0.15 and using standard state pressure data for SnF2 [15 !.

746

Average T* (°K ( + _ 2 ) )

la: NaSnFs ~ N a F + SnF2

Reaction (gas state)

Table 2. Vapor equih~ria over molten NaSnFa and KSnFs

t~

x

X

I

t~

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J . W . HASTIE, K. F. Z M B O V and J. L. M A R G R A V E

Now, for example, g 1 ~--

P ( N a F ) • P(SnF2) = k l ( N a +) • kl(SnF2 +) • T P (NaSnFa) kl (NaSnF2 + )

where the k is the machine sensitivity constant (not necessarily the same for each ion) and contains multiplier efficiency and ionization cross-section terms as well as allowing for differing intervals of electron energy above threshold for the ions. It is assumed that the k's for SnFz + and NaSnF~ + cancel, allowing for an error of a factor of 2. Therefore Kx =

P ( N a F ) I (SnF2 ÷) I(NaSnF2 +)

and similarily for the reactions, 1-3, K may be expressed in terms of p(NaF) and relative ion currents only. Hence, free energies are easily calculated and enthalpies are obtainable if entropy estimates are made.

Estimation of entropy and calculation of enthalpy Practically all of the molecules of the type AMX3 studied so far are formed from the binary halides with entropy decreases between 19 and 32 e.u. For example Novikov et a/.[4] found for Be-containing molecules the following (T = 900-1100°K): KBeC13 = KCI + BeCI~, AH~ = 48 ___5 kcal/mole, AS~ = 24 __ 2 e.u. NaBeCI3 = NaC1 + BeCI2, AH~.= 54 ___5 kcal/mole, AS~.= 23 ___2 e.u. NaBeF3 = N a F + BeF2, AH~.= 52_+ 2 kcal/mole, AS~.= 21 __+3 e.u. whereas Biicher and Stauffer[ 1] found: LiBeF3 = LiF + BeF~, AH~.= 60 kcal/mole, AS~ = 32 e.u. The larger entropy change for the reaction involving the Li-complex may be due to either an error in the data for this or the Na and K complexes, or alternatively to a marked changed in the structure ofABeX3 whenA is Li. Concerning this latter point Snelson[7] has found some evidence from i.r. spectra that LiA1F4 has a different structure from NaAIF4. The fact that the LiBeF3 molecule fragments more than similar molecules do on electron impact could also indicate a different structure. For the molecules APbX3 where A is Na, K, Rb or Cs and Xis C1, the entropy change is larger than that for X = F by several e.u. [3], consistent with the above observations on A BeXa molecules. Thus, if NaPbCla is formed with an entropy decrease of 20___ 1 e.u. then, for NaPbF3 the entropy decrease is 19--- 1 e.u. To compare NaSnFs with NaPbFa one can use the analogy of KCaCIs and KSrCIa[4], where the entropies are similar to each other to within +_.2e.u. Hence, the entropy 15. W. Fischer and T. Petzel, Z. anorg, allg. Chem. 333, 226 (1964).

Mass spectrometric studies at high temperatures-XXIII

735

associated with the dissociation of NaSnF3 and similarly KSnF3 should fall in the range 1 9 - 3 e.u. A test of this estimation is the consistency between the enthalpy data obtained from the combination of A ~ and AS~. with that obtained from d In KId(lIT). The results, as given in Table 2, indicate good agreement of both sets of enthalpy data for the reactions la and lb. As an independent check on this method the same procedure was carded out for a number of similar molecules using the data of other workers. For example, using the mass spectrometric data of Semenov [8] for KErCh an entropy of 31 e.u. was calculated and this agrees well with the value of 32 e.u. obtained by Novkov et a/.[4] using a vapor pressure method. Similarily the mass spectrometric and vapor pressure data for CsPbCla also yield good agreement for the entropy values [3]. As the second-law enthalpy data for reactions la and lb appear to be reliable one can use the second law values for reactions 2 and 3, combined with the related free energies, to calculate the entropies associated with these reactions. The values are given in Table 2. It is noted that the data are self-consistent in that,

AH~(rn 2) = AH~(rna) + AH~(rn 3) and AS~(rn 2) = AS~(rna) -~ AS~(rn 3). CONCLUSIONS

The results indicate that the complex species AnSnFn+2 (n = 1, 2) are extremely volatile as compared with the alkali halides. In fact if one wished to distill alkali-halides from a particular melt at fairly low temperatures, then the addition of a small amount of a salt such as SnF2 could serve the purpose. It is also seen that the complex molecule NaSnF3, and possibly KSnF3 to a much lesser extent, is fairly reactive, in that it can combine either with an N a F or SnF~ molecule to form new species. The appearance of Na~Sn2F5 + in the mass spectrum is evidence that NaSnF3 is capable of forming dimers with itself. As more data become available, it appears that, as a rule, the heat of formation of an AMX3 molecule is similar to the heat of dimerization o f / I X , as noted by Biichler et a/.[1] for LiBeF3. Currently the structures of these molecules are unknown and several models have been postulated[I, 3, 6]. The tetrahedral model proposed by Porter and Zeller[6] for LiAIF4 is favored at present. For NaSnF3 or KSnF3 it is likely that the lone 4s 2 electron pair on Sn assumes the fourth tetrahedral position. If these molecules are assumed to consist of ions, then a simple ionic model leads to enthalpies of formation that are in reasonable agreement with exporiment. The similarity in the values of the ionization potentials (Table 1) of NaSnF3 and Na2SnF4 suggests that no significant alteration to the energy levels in NaSnFa is caused by the addition of another N a F to it. In conclusion, it seems that group I V A metal di-halides are very susceptable to complex molecule formation with the alkali halides and, in view of the relatively low temperatures needed to vaporize the molecules, these species could prove very useful in further studies on complex salt molecules. The considerable range

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J. W. HASTIE, K. F. ZMBOV and J. L. MARGRAVE

of stable isotopes for Sn could be useful in spectroscopic studies to establish detailed molecular structures. There is no obvious reason to expect that GeX~ or SiX~ would not lend themselves to similar complex formation. Acknowledgement-This work has been supported by the National Aeronautics and Space Administration and by the Robert A. Welch Foundation.