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Mass spectrometric and weight-loss effusion measurements of the sublimation of thulium triiodide
Journal of the Less-Common
Metals, 139 (1988) 331 - 343
331
MASS SPECTROMETRIC AND WEIGHT-LOSS EFFUSION MEASUREMENTS OF THE SUBLIMATION OF THULIUM TRIIODIDE J. H. DETTINGMEIJER Philips Lighting Division,
and H. R. DIELIS 5600 MD Eindhoven
[The Netherlands)
(Received November 1,1987)
Summary The equilibrium vapour pressure over solid thulium triiodide between 874 K and 1032 K was measured by determining the effusion of vapour out of a Knudsen cell by means of mass spectrometry (MS) and weight loss (WL). A significant dimer concentration was found in this temperature range. From the experiments the following second law thermodynamic data were derived (expressed in kcal mol-’ and cal mol-’ K-’ respectively): AH (subl, 956, Tm13) = 54.1, AH (subl, 956, Tm&) = 68.0 (MS); AH (subl, 946, TmI,) = 54.2, AH (subl, 946, Tm,I,) = 68.5 (WL); AS (subl, 946, TmIs) = 33.9, AS (subl, 946, TmJ,) = 47.6 (WL). These data were extrapolated to 298 K using estimated literature data for the specific heats.
1. Introduction Since the first successful incandescent halide lamp [l], interest in chemical transport reactions [2] has increased steadily. The early attempts to understand or even predict lamp transport processes by equilibrium calculations [3, 4, 51 were hindered by a lack of reliable thermochemical data. Since the development of metal-halide discharge lamps containing additives to improve efficacy and colour rendition, the chemical complexity of these lamps has increased, increasing the need for improved calculation facilities as well as thermochemical data. Sophisticated computer programs have been developed by Schnedler [6] and were recently used for transport calculations on a thulium iodide lamp by van Erk and Rietveld [7]. Mass spectra of thulium triiodide have been determined by Hirayama et al. [8]. Ion intensity ratios were measured and TmI,+ was found to be the most abundant ion. From the absence in the spectra of ions of the 0022-5088/88/$3.50
@ Elsevier Sequoia/Printed in The Netherlands
332
polymeric species, it was concluded that TmI, evaporates congruently. Weight-loss Knudsen effusion measurements on several rare earth iodides were made and sublimation thermodynamic data were derived assuming congruent evaporation [ 91. After the detection of dimeric scandium triiodide [lo, 111, it was decided to investigate the evaporation of thulium triiodide paying especial attention to the possibility of dimer formation.
2. Experimental
details
Thulium iodide of purity better than 99.5% was obtained from Anderson. It was handled in a glove-box in a dry nitrogen atmosphere. After filling the Knudsen cells, their orifices were sealed with naphthalene [ 121 prior to their introduction into the vacuum systems. The effusion rate was measured using a type RH microbalance from Cahn. The Knudsen cell was made of molybdenum; the orifice area was 8.516 X 10v4 cm*, its length was 7.2 X lo-* cm. A Clausing factor of 0.818 was derived from these dimensions [ 131. Temperatures were measured with two calibrated chromel-alumel thermocouples situated near the cell but not touching it; temperature gradients were minimized by enclosing the cell and the thermocouples in a copper cylinder. We estimate the temperature accuracy to be within 2 K. The apparatus was checked with a trial run on zinc; the second law sublimation enthalpy and entropy were found to be 31.3 kcal mol-’ and 28.5 cal mol-’ K-’ respectively, in excellent agreement with literature data [14]. Mass spectrometric measurements were performed with an Extranuclear Laboratories quadrupole mass spectrometer equipped with a Varian Knudsen cell system [ 151. This Knudsen cell was also made of molybdenum; the orifice dimensions were 1.96 X 10e3 cm* and 0.1 cm. Temperatures were measured by means of two calibrated chromel-alumel thermocouples inserted in small holes in the bottom of the Knudsen cell. The temperature accuracy was estimated to be better than 5 K. The quadrupole housing and the Knudsen-cell compartment were differentially pumped. The background particles were distinguished from species originating from the Knudsen cell by means of a shutter. To prevent excessive fragmentation, experiments were carried out at a relatively low electron energy of 28 eV.
3. Results 3.1. Mass spectrometry Mass spectrometric measurements were made in the temperature region between 886 and 1025 K. The expected ionic species TmI,+ (n = 0, 1,2, 3) were detected, and lg(l+T) was plotted us. T-l, where I+ is the ion intensity
333
and T the absolute temperature; I+ T is proportional to the partial pressure of the particular species. Since the lines were not parallel, as would have been the case if all ionic species had one molecular precursor, the presence of dimeric thulium triiodide was suspected. On scanning the appropriate mass range with increased sensitivity, we indeed found peaks of TmJ,+ (m = 3, 4, 5). The ionic abundances of all the species found are listed in Table 1. The results of a typical experiment (MS run number 11) are listed in Table 2 and plotted in Fig. 1. The ionic intensity relative to the sum of all ionic intensities is plotted logarithmically against the absolute temperature in Fig. 2; the relative intensity of TmI,’ decreases with temperature, that of Tm21,+ increases, while the relative intensities of Tm+, TmI+ and TmI,+ exhibit an intermediate temperature dependence. This strongly suggests the existence of two parent molecules, TmI, and its dimer Tm&, from which the ions TmI,+ and TmJ,+ originate respectively, while the ions Tm+, TmI+ and TmI*+ are fragmentation products of both parent molecules and thus show TABLE Ionic
1
abundance
in TmI3
spectrum
at 970 K
Ion
Relative
intensity
TABLE
2
Ionic intensities
I,,+
Tm+
TmI+
Tm12+
TmIs+
Tm21;
Tm&+
Tm&’
35
28
100
72
2
8
16
T
ITrqI 3+
zTm,I 4 +
ITm;I 5 +
5.469 7.192 3.793 8.554 2.516 10.022 1.468 -
27.30 34.24 17.54 41.08 12.15 49.85 -
50.93 63.33 33.33 73.93 23.81 87.97 -~
7.465 59.87 10.72 52.48 14.40 36.70 20.22 28.18
14.64 108.9 20.38 92.73 28.20 66.29 38.80 52.63
us. temperature
ITmI +
ITXllI *+
ha,+
(K)
977.5 961.0 984.5 967.5 987.0 940.5 994.0 950.0 1001.0 933.5 1008.5 945.0 990.5 954.5 973.0
133.5 78.5 146.0 32.8 176.0 45.5 213.0 25.6 257.0 40.3 161.0 53.5 92.5
89.3 50.0 104.5 63.0 119.0 25.5 147.0 35.5 183.0 20.4 222.0 30.5 135.0 42.0 77.0
298.3 183.0 366.5 227.5 415.5 97.3 501.0 134.3 617.0 79.0 742.0 115.0 461.0 155.0 272.0
213.5 131.8 258.0 162.0 286.0 75.3 343.5 102.0 413.5 59.4 489.5 85.5 321.5 115.3 194.0
977.0 984.0 965.0 989.5 956.5 995.0 941.5 943.5 1001.5 952.0 996.0 961.0 987.0 969.5 979.0
12.961 2.360 10.632 3.169 7.728 4.277 6.172
5
-
4.5 -
4
-
3.5 --
\ 1.00
Fig. 1. Mass spectrometry:
1.02
Tm,lf
1.04 -_lf)'/t (1(-p
plot of lg(l+T)
us. 103/T.
the observed intermediate temperature has been observed in scandium chloride To improve accuracy, generally performed, one on peaks of relatively intensity peaks, different instrument set. To obtain information about the extra measurements were carried out at
dependence. Analogous behaviour [16] and scandium iodide [lo, 111. two sets of measurements were high intensity, and one on the lowadjustments being chosen for each mbnomer:dimer ratio, however, two one constant instrument sensitivity.
335 4
3
I +-i---t:
Tm+
XW +=
.
a*_*
-
----
;
l
J_
TIllIf
f
2
i
0
Y . -1
t
950
mo
7-M) Fig. 2. Logarithmic plot of relative ionic abundance VS.temperature.
The data obtained are shown in Table 3. Assuming that TmI,+ and Tm&+ originate exclusively from TmI, and Tm$, parent molecules respectively, the sublimation entbalpies of monomer and dimer follow from the slopes shown in Table 3 for these ions. At the mean experimental temperature of 956 K, the sublimation enthalpy of the monomer was found to be 54.1 kcal mol-’ (226 kJ mol-I) that of the dimer 68.0 kcal mol-* (285 kJ mol-‘); the estimated error (95%) is +2 kcal mol-‘. For the reduction of the data from each experimental mean temperature to 956 K, literature C, data were used (see Section 3.3). As stated above, the ions Tm+, TmI+ and TmI,+ originate from both molecular precursors TmI, and Tm&, whereas TmIs’ and the ions of the Tm&,, ’ group (m = 3, 4, 5) are assumed to originate exclusively from each
969 938 946 954
927 970
10 11
!f (K)
1 2 3 4
Experiment number
61.0 59.6
59.0 60.6 58.8 60.4
Tm+
65.1 62.2
61.9 62.5 61.8 62.2
Tmi’
Slopes of Ig(l+T) us. T-’ plots (kcal mol-‘)
TABLE 3
59.2 58.3
55.6 58.3 57.1 58.9
TmIz+
53.9 54.7
51.6 55.0 53.0 56.4
TrnI3+
10 11
5 6 7 8 9
Experiment number
927 970
963 960 96% 966 961
F (K)
68.3 68.5
-. 69.2 69.9 68.9
Tmz13+
72.9 69.8
68.1 69.7 69.1 68.1 70.1
Tm&+
70.9 66.3
67.7 68.1 67.4 67.4 68.1
Tmz15+
g 0)
337 TABLE
4
Dimer mole fraction
Experiment
US. temperature
Experiment
10
T(K)
Xdi
T WI
880 890 900
0.21 0.23 0.25 0.27 0.29 0.31 0.33 0.35 0.37 0.39 0.41
930 940 950
910 920 930 940 950 960 970 980
960 970 980 990 1000 1010
11 xdi
0.31 0.33 0.34 0.36 0.37 0.38 0.40 0.41 0.43
of these parent molecules respectively, Monomer-to-dimer ratios have been calculated from data of the two experiments in which complete spectra were evaluated (numbers 10 and 11 in Table 3), using a procedure that has been described elsewhere [ 11,161; the results are shown in Table 4. At least-squares-fit of the data yielded - 3.012
3.2. Weigh t-loss measurements The measurements were performed at temperatures between 874 and 1032 K. The vapour pressures were calculated from the observed weight losses by means of the equation P=
6.2639 X lo-’ AC
iI2
(2)
where P is the vapour pressure in atmospheres, Am the mass loss by effusion in milligrams, At the time lapse in hours, A the orifice area in cm2, C the Clausing factor, T the temperature in kelvin and M the molecular weight of the effusing vapour. For the particular cell used we find A = 8.516 X low4 and C = 0.818. In this temperature region where the dimer concentration is appreciable, the mean molecular weight M at each experimental temperature was calculated from eqn. (1) and substituted in eqn. (2), which yielded the total pressure. Equation (1) was used again to calculate the monomer and dimer pressures. Typical results are shown in Table 5 and in Fig. 3.
338
TABLE5 Weight-loss experiment 1
897.5
917.0 943.5 976.5 985.0 1002.0 993.0 966.5 959.0 874.0 906.0 919.5 937.0 954.0 971.5 986.0 1001.0 1012.0 996.5 982.0 882.0 903.0 922.0 939.0 953.5
dm/dt (mgh-')
dimer mole fraction
l&t x 106 btm)
p(Tml3)x lo6
p(Tm$6)x IO6
(atm)
(atm)
0.2835 0.5955 1.589 4.078 5.283 9.434 6.667 3.127 2.375 0.1312 0.3501 0.6035 1.009 1.832 3.341 5.741 9.857 14.20 8.658 5.216 0.1738 0.3560 0.6859 1.217 2.058
0.252 0.285 0.332 0.392 0.407 0.437 0.421 0.374 0.360 0.213 0.266 0.290 0.321 0.351 0.383 0.409 0.435 0.455 0.427 0.402 0.226 0.261 0.294 0.324 0.350
2.911 6.100 16.22 41.42 53.60 95.52 67.58 31.81 24.18 1.350 3.591 6.180 10.31 18.67 33.96 58.24 99.82 143.6 87.72 52.94 1.788 3.653 7.021 12.43 20.97
2.177 4.357 10.82 25.18 31.76 53.71 39.08 19.91 15.46 1.062 2.634 4.387 6.998 12.10 20.95 34.41 56.30 78.22 50.18 31.66 1.383 2.698 4.954 8.393 13.62
0.7341 1.742 5.395 16.25 21.84 41.81 28.50 11.90 8.721 0.2886 0.9575 1.792 3.309 6.564 13.01 23.84 43.51 65.37 37.54 21.29 0.4049 0.9552 2.067 4.034 7.355
Least-squares fits of the data obtained in two weight-loss measurements yielded second-law enthalpy and entropy data for the sublimation of monomeric and dimeric thulium triiodide. These results are presented in Table 6. The estimated error (at 95% confidence limits) in the weight-loss data is r1.5 kcal mol-’ for enthalpies and k1.9 cal mol-’ for entropies. 3.3. Thermochemistry of sublimation of Tm13 Sublimation data at mean experimental temperatures are summarized in Table 7. For comparison, the data of Hirayama et al. [9] have also been listed, as well as their data treated with our monomer-to-dimer ratios (eqn. To extrapolate these data to 298 K, specific heat data for TmI,(s), TmI,(g) and Tm,I,(g) are needed. To our knowledge, no experimental C, data for these species exist. For related rare earth halides, however, these data do exist in work by Dworkin and Bredig [ 171. It appears that (Hr - HZ& values for GdX(s)
339
1.0
1.10
1.05
-
Fig. 3. Weight loss: logarithmic temperature.
I.15
f03/:(K-l)
plot of total vapour pressure against the reciprocal
(XSl, Br, I) vary little, and the values for DyCI, and HoCl, are also very similar. Thus, below phase transition temperatures, it was assumed that -~ H298) for TmI,(s) would be identical to the GdIs(s) value. (NT Adopting the constants presented by Dworkin and Bredig, we obtained for the specific heat of solid TmI,: C,(T) = 24.288 + 0.00187’ - 10.002 X 3.0M4TV2 cal mol-’ This formula yields only slightly lower C!, values than that proposed by Myers and Graves [ 181. For gaseous TmI, we adopted C, data from Lynch [19] which we fitted to a polynomial. These data are almost identical to those of Myers and Graves [20] ; for instance the difference in (I-r, - H2ss) between the two approximations at 1000 K is no larger than 0.02 kcal mol-‘.
For the dimer, we adopted a proposition by Work [21], assuming C,,(Tm&(g)) = 2.2 C,,(TmI,(g)). The extrapolated 298 K sublimation data thus obtained are presented in Table 8, The third law sublimation enthalpies in Table 8 were calculated from weight-loss data. For the entropy of gaseous TmI,, the value of 102.73 eal mall’ K-r, calculated by Myers and Graves, was adopted. For solid TmI, we used the entropy value of 57.1 cal mol-” K-’ which was also used by Hirayama [9] ; this estimation originates from Latimer [ 221 and Westrum v31.
938 954
1 2
946 929
924
This work (WL) Hirayama et of. (WL)
Hiraysma et al. (WL) (modified)
67.3
59.1 68.48
-
aCongruent sublimation of TmI3 was assumed.
956
This work (MS)
~~A&,~~ (kcal mol-l)
63.0
54.2 -
54.1
(kcal mol-*)
AH mono
77.3
68.5 -
68.0
(kcal mol-‘)
&Ii
68.9 68.0
59.4 58.8
54.6 53.7
fw2 (kcal mol-‘)
“AHkt” (kcal molvl)
Sublimation data for TmIs at mean experimental temperatures
TABLE 7
!F (K)
Experiment number
Second law sublimation data by weight loss
TABLE 6
48.2
39.9 49.7a
-
-Astot )’ (cal mol-’
40.8 39.0
K-l)
42.8
33.9 -
-
(cal molll
As mono
35.0 32.7
K-‘)
55.6
47.6 -
-
(cal mol-’
A&
48.8 46.5
K-l)
nuxm
-
63.2 63.5
This work (WL) Hirayama et al. (WL) (modified)
(kcal mol-‘)
&p
data for Tm13 at 298 K
This work (MS)
Sublimation
TABLE 8
%:;-I 39.7 48.5
AH& (kcal mol-‘) 72.4 72.7 81.4
57.5 57.6 66.2
K-‘ f
54.8 63.6
-
A& (Cal mol-’
K-l)
E
342
Discussion Sublimation studies on TmI, by weight loss have been reported in the literature [9]. Since congruent sublimation of the solid was assumed in ref. 9, only the total pressure data of this work can be compared with the reported data. In both studies the plots of lg P,, t us. T-’ fit well onto a straight line, in the relatively small experimental temperature interval (in Fig. 4). In fact, however, the line, being the sum of two straight lines with different slopes, is not really straight. At low temperatures its slope approaches that of the monomer, at high temperatures that of the dimer. The monomer and dimer thermodynamic data obtained from weight-loss measurements, using the temperature-dependent monomer-to-dimer ratio derived from mass spectrometry, are themselves not a function of the experimental temperature, apart from corrections to C,. In comparing total pressure data, however, the influence of the temperature on slope and on the point of intersection with the ordinate must be taken into account. Unfortunately, no temperature influence can explain the difference in the mentioned parameters between this work and ref. 9. ,
1
I
I
1
10-’ T-h
5 f 10-4
1O-s
1O-6
Fig. 4. Total vapour pressure plotted logarithmically against 103/T: 3, Hirayama et al. [9].
1 and 2, this work;
343
The total pressures, however, do agree reasonably in the experimental temperature range. Thus, the third law sublimation enthalpies in ref. 9 and this work agree well, independent of the values of the entropies used. The agreement between the second and third law sublimation enthalpies is less good, dependent on the entropy values used. Here the lack of experimental entropy data is a problem, especially for the condensed phase. Excellent agreement, however, exists between the thermodynamic data for both monomer and dimer, obtained either by mass spectrometric investigations or weight loss experiments.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
E. G. Zubler and F. A. Mosby, Illum. Eng., 54 (1959) 734. H. Schafer, Chemische Transportreaktionen, VerIag Chemie, Weinheim, 1962. B. Kopelman and K. van Wormer, Zllum. Eng., 63 (1968) 176. G. M. Neumann and W. Knatz, 2. Naturforsch., Teil A, 26 (1971) 863. J. H. Dettingmeijer, B. Meinders and L. M. Nijland, J. Less-Common Met., 35 (1974) 159. E. Schnedler, Proc. Symp. High Temp. Light Sources, 85-2 (1985) 95. W. van Erk and T. Rietveld, Proc. Symp. High Temp. Light Sources, 85-2 (1985) 57. C. Hirayama, P. M. Castle, R. W. Liebermann, R. J. Zollweg and F. E. Camp, Inorg. Chem., 13 (1974) 2804. C. Hirayama, J. F. Rome and F. E. Camp, J. Chem. Eng. Data, 20 (1975) 1. C. Hirayama, P. M. Castle, W. E. Snider and R. L. Kleinosky, J. Less-Common Met., 57 (1978) 69. J. H. Dettingmeijer, H. R. Dielis, B. J. de Maagt and P. A. M. Vermeulen, J. LessCommon Met., 107 (1985) 11. J. R. McCreary and R. J. Thorn, J. Chem. Phys., 48 (1968) 3290. B. A. Gottwald, Vuk.-Tech., 22 (1973) 106. I. Barin, 0. Knacke and 0. Kubaschewsky, Thermochemical Properties of Inorganic Substances, Springer, Berlin, 1977. J. H. Dettingmeijer and B. Meinders, 2. onorg. olig. Chem., 400 (1973) 10. H. Schafer and K. Wagner, 2. anorg. allg. Chem., 450 (1979) 88. A. S. Dworkin and M. A. Bredig, High Temp. Sci., 3 (1971) 81. C. E. Myers and D. T. Graves, J. Chem. Eng. Data, 22 (1977) 440. D. A. Lynch, private communication (1973). C. E. Myers and D. T. Graves, J. Chem. Eng. Data, 22 (1977) 436. D. E. Work, J. Less-Common Met., 69 (1980) 383. W. M. Latimer, The Oxidation States of the Elements and their Potentials in Aqueous Solutions, Prentice-Hall, New York, 1952. E. F. Westrum, Adv. Chem. Ser., 71 (1967) 25.
Metals, 139 (1988) 331 - 343
331
MASS SPECTROMETRIC AND WEIGHT-LOSS EFFUSION MEASUREMENTS OF THE SUBLIMATION OF THULIUM TRIIODIDE J. H. DETTINGMEIJER Philips Lighting Division,
and H. R. DIELIS 5600 MD Eindhoven
[The Netherlands)
(Received November 1,1987)
Summary The equilibrium vapour pressure over solid thulium triiodide between 874 K and 1032 K was measured by determining the effusion of vapour out of a Knudsen cell by means of mass spectrometry (MS) and weight loss (WL). A significant dimer concentration was found in this temperature range. From the experiments the following second law thermodynamic data were derived (expressed in kcal mol-’ and cal mol-’ K-’ respectively): AH (subl, 956, Tm13) = 54.1, AH (subl, 956, Tm&) = 68.0 (MS); AH (subl, 946, TmI,) = 54.2, AH (subl, 946, Tm,I,) = 68.5 (WL); AS (subl, 946, TmIs) = 33.9, AS (subl, 946, TmJ,) = 47.6 (WL). These data were extrapolated to 298 K using estimated literature data for the specific heats.
1. Introduction Since the first successful incandescent halide lamp [l], interest in chemical transport reactions [2] has increased steadily. The early attempts to understand or even predict lamp transport processes by equilibrium calculations [3, 4, 51 were hindered by a lack of reliable thermochemical data. Since the development of metal-halide discharge lamps containing additives to improve efficacy and colour rendition, the chemical complexity of these lamps has increased, increasing the need for improved calculation facilities as well as thermochemical data. Sophisticated computer programs have been developed by Schnedler [6] and were recently used for transport calculations on a thulium iodide lamp by van Erk and Rietveld [7]. Mass spectra of thulium triiodide have been determined by Hirayama et al. [8]. Ion intensity ratios were measured and TmI,+ was found to be the most abundant ion. From the absence in the spectra of ions of the 0022-5088/88/$3.50
@ Elsevier Sequoia/Printed in The Netherlands
332
polymeric species, it was concluded that TmI, evaporates congruently. Weight-loss Knudsen effusion measurements on several rare earth iodides were made and sublimation thermodynamic data were derived assuming congruent evaporation [ 91. After the detection of dimeric scandium triiodide [lo, 111, it was decided to investigate the evaporation of thulium triiodide paying especial attention to the possibility of dimer formation.
2. Experimental
details
Thulium iodide of purity better than 99.5% was obtained from Anderson. It was handled in a glove-box in a dry nitrogen atmosphere. After filling the Knudsen cells, their orifices were sealed with naphthalene [ 121 prior to their introduction into the vacuum systems. The effusion rate was measured using a type RH microbalance from Cahn. The Knudsen cell was made of molybdenum; the orifice area was 8.516 X 10v4 cm*, its length was 7.2 X lo-* cm. A Clausing factor of 0.818 was derived from these dimensions [ 131. Temperatures were measured with two calibrated chromel-alumel thermocouples situated near the cell but not touching it; temperature gradients were minimized by enclosing the cell and the thermocouples in a copper cylinder. We estimate the temperature accuracy to be within 2 K. The apparatus was checked with a trial run on zinc; the second law sublimation enthalpy and entropy were found to be 31.3 kcal mol-’ and 28.5 cal mol-’ K-’ respectively, in excellent agreement with literature data [14]. Mass spectrometric measurements were performed with an Extranuclear Laboratories quadrupole mass spectrometer equipped with a Varian Knudsen cell system [ 151. This Knudsen cell was also made of molybdenum; the orifice dimensions were 1.96 X 10e3 cm* and 0.1 cm. Temperatures were measured by means of two calibrated chromel-alumel thermocouples inserted in small holes in the bottom of the Knudsen cell. The temperature accuracy was estimated to be better than 5 K. The quadrupole housing and the Knudsen-cell compartment were differentially pumped. The background particles were distinguished from species originating from the Knudsen cell by means of a shutter. To prevent excessive fragmentation, experiments were carried out at a relatively low electron energy of 28 eV.
3. Results 3.1. Mass spectrometry Mass spectrometric measurements were made in the temperature region between 886 and 1025 K. The expected ionic species TmI,+ (n = 0, 1,2, 3) were detected, and lg(l+T) was plotted us. T-l, where I+ is the ion intensity
333
and T the absolute temperature; I+ T is proportional to the partial pressure of the particular species. Since the lines were not parallel, as would have been the case if all ionic species had one molecular precursor, the presence of dimeric thulium triiodide was suspected. On scanning the appropriate mass range with increased sensitivity, we indeed found peaks of TmJ,+ (m = 3, 4, 5). The ionic abundances of all the species found are listed in Table 1. The results of a typical experiment (MS run number 11) are listed in Table 2 and plotted in Fig. 1. The ionic intensity relative to the sum of all ionic intensities is plotted logarithmically against the absolute temperature in Fig. 2; the relative intensity of TmI,’ decreases with temperature, that of Tm21,+ increases, while the relative intensities of Tm+, TmI+ and TmI,+ exhibit an intermediate temperature dependence. This strongly suggests the existence of two parent molecules, TmI, and its dimer Tm&, from which the ions TmI,+ and TmJ,+ originate respectively, while the ions Tm+, TmI+ and TmI*+ are fragmentation products of both parent molecules and thus show TABLE Ionic
1
abundance
in TmI3
spectrum
at 970 K
Ion
Relative
intensity
TABLE
2
Ionic intensities
I,,+
Tm+
TmI+
Tm12+
TmIs+
Tm21;
Tm&+
Tm&’
35
28
100
72
2
8
16
T
ITrqI 3+
zTm,I 4 +
ITm;I 5 +
5.469 7.192 3.793 8.554 2.516 10.022 1.468 -
27.30 34.24 17.54 41.08 12.15 49.85 -
50.93 63.33 33.33 73.93 23.81 87.97 -~
7.465 59.87 10.72 52.48 14.40 36.70 20.22 28.18
14.64 108.9 20.38 92.73 28.20 66.29 38.80 52.63
us. temperature
ITmI +
ITXllI *+
ha,+
(K)
977.5 961.0 984.5 967.5 987.0 940.5 994.0 950.0 1001.0 933.5 1008.5 945.0 990.5 954.5 973.0
133.5 78.5 146.0 32.8 176.0 45.5 213.0 25.6 257.0 40.3 161.0 53.5 92.5
89.3 50.0 104.5 63.0 119.0 25.5 147.0 35.5 183.0 20.4 222.0 30.5 135.0 42.0 77.0
298.3 183.0 366.5 227.5 415.5 97.3 501.0 134.3 617.0 79.0 742.0 115.0 461.0 155.0 272.0
213.5 131.8 258.0 162.0 286.0 75.3 343.5 102.0 413.5 59.4 489.5 85.5 321.5 115.3 194.0
977.0 984.0 965.0 989.5 956.5 995.0 941.5 943.5 1001.5 952.0 996.0 961.0 987.0 969.5 979.0
12.961 2.360 10.632 3.169 7.728 4.277 6.172
5
-
4.5 -
4
-
3.5 --
\ 1.00
Fig. 1. Mass spectrometry:
1.02
Tm,lf
1.04 -_lf)'/t (1(-p
plot of lg(l+T)
us. 103/T.
the observed intermediate temperature has been observed in scandium chloride To improve accuracy, generally performed, one on peaks of relatively intensity peaks, different instrument set. To obtain information about the extra measurements were carried out at
dependence. Analogous behaviour [16] and scandium iodide [lo, 111. two sets of measurements were high intensity, and one on the lowadjustments being chosen for each mbnomer:dimer ratio, however, two one constant instrument sensitivity.
335 4
3
I +-i---t:
Tm+
XW +=
.
a*_*
-
----
;
l
J_
TIllIf
f
2
i
0
Y . -1
t
950
mo
7-M) Fig. 2. Logarithmic plot of relative ionic abundance VS.temperature.
The data obtained are shown in Table 3. Assuming that TmI,+ and Tm&+ originate exclusively from TmI, and Tm$, parent molecules respectively, the sublimation entbalpies of monomer and dimer follow from the slopes shown in Table 3 for these ions. At the mean experimental temperature of 956 K, the sublimation enthalpy of the monomer was found to be 54.1 kcal mol-’ (226 kJ mol-I) that of the dimer 68.0 kcal mol-* (285 kJ mol-‘); the estimated error (95%) is +2 kcal mol-‘. For the reduction of the data from each experimental mean temperature to 956 K, literature C, data were used (see Section 3.3). As stated above, the ions Tm+, TmI+ and TmI,+ originate from both molecular precursors TmI, and Tm&, whereas TmIs’ and the ions of the Tm&,, ’ group (m = 3, 4, 5) are assumed to originate exclusively from each
969 938 946 954
927 970
10 11
!f (K)
1 2 3 4
Experiment number
61.0 59.6
59.0 60.6 58.8 60.4
Tm+
65.1 62.2
61.9 62.5 61.8 62.2
Tmi’
Slopes of Ig(l+T) us. T-’ plots (kcal mol-‘)
TABLE 3
59.2 58.3
55.6 58.3 57.1 58.9
TmIz+
53.9 54.7
51.6 55.0 53.0 56.4
TrnI3+
10 11
5 6 7 8 9
Experiment number
927 970
963 960 96% 966 961
F (K)
68.3 68.5
-. 69.2 69.9 68.9
Tmz13+
72.9 69.8
68.1 69.7 69.1 68.1 70.1
Tm&+
70.9 66.3
67.7 68.1 67.4 67.4 68.1
Tmz15+
g 0)
337 TABLE
4
Dimer mole fraction
Experiment
US. temperature
Experiment
10
T(K)
Xdi
T WI
880 890 900
0.21 0.23 0.25 0.27 0.29 0.31 0.33 0.35 0.37 0.39 0.41
930 940 950
910 920 930 940 950 960 970 980
960 970 980 990 1000 1010
11 xdi
0.31 0.33 0.34 0.36 0.37 0.38 0.40 0.41 0.43
of these parent molecules respectively, Monomer-to-dimer ratios have been calculated from data of the two experiments in which complete spectra were evaluated (numbers 10 and 11 in Table 3), using a procedure that has been described elsewhere [ 11,161; the results are shown in Table 4. At least-squares-fit of the data yielded - 3.012
3.2. Weigh t-loss measurements The measurements were performed at temperatures between 874 and 1032 K. The vapour pressures were calculated from the observed weight losses by means of the equation P=
6.2639 X lo-’ AC
iI2
(2)
where P is the vapour pressure in atmospheres, Am the mass loss by effusion in milligrams, At the time lapse in hours, A the orifice area in cm2, C the Clausing factor, T the temperature in kelvin and M the molecular weight of the effusing vapour. For the particular cell used we find A = 8.516 X low4 and C = 0.818. In this temperature region where the dimer concentration is appreciable, the mean molecular weight M at each experimental temperature was calculated from eqn. (1) and substituted in eqn. (2), which yielded the total pressure. Equation (1) was used again to calculate the monomer and dimer pressures. Typical results are shown in Table 5 and in Fig. 3.
338
TABLE5 Weight-loss experiment 1
897.5
917.0 943.5 976.5 985.0 1002.0 993.0 966.5 959.0 874.0 906.0 919.5 937.0 954.0 971.5 986.0 1001.0 1012.0 996.5 982.0 882.0 903.0 922.0 939.0 953.5
dm/dt (mgh-')
dimer mole fraction
l&t x 106 btm)
p(Tml3)x lo6
p(Tm$6)x IO6
(atm)
(atm)
0.2835 0.5955 1.589 4.078 5.283 9.434 6.667 3.127 2.375 0.1312 0.3501 0.6035 1.009 1.832 3.341 5.741 9.857 14.20 8.658 5.216 0.1738 0.3560 0.6859 1.217 2.058
0.252 0.285 0.332 0.392 0.407 0.437 0.421 0.374 0.360 0.213 0.266 0.290 0.321 0.351 0.383 0.409 0.435 0.455 0.427 0.402 0.226 0.261 0.294 0.324 0.350
2.911 6.100 16.22 41.42 53.60 95.52 67.58 31.81 24.18 1.350 3.591 6.180 10.31 18.67 33.96 58.24 99.82 143.6 87.72 52.94 1.788 3.653 7.021 12.43 20.97
2.177 4.357 10.82 25.18 31.76 53.71 39.08 19.91 15.46 1.062 2.634 4.387 6.998 12.10 20.95 34.41 56.30 78.22 50.18 31.66 1.383 2.698 4.954 8.393 13.62
0.7341 1.742 5.395 16.25 21.84 41.81 28.50 11.90 8.721 0.2886 0.9575 1.792 3.309 6.564 13.01 23.84 43.51 65.37 37.54 21.29 0.4049 0.9552 2.067 4.034 7.355
Least-squares fits of the data obtained in two weight-loss measurements yielded second-law enthalpy and entropy data for the sublimation of monomeric and dimeric thulium triiodide. These results are presented in Table 6. The estimated error (at 95% confidence limits) in the weight-loss data is r1.5 kcal mol-’ for enthalpies and k1.9 cal mol-’ for entropies. 3.3. Thermochemistry of sublimation of Tm13 Sublimation data at mean experimental temperatures are summarized in Table 7. For comparison, the data of Hirayama et al. [9] have also been listed, as well as their data treated with our monomer-to-dimer ratios (eqn. To extrapolate these data to 298 K, specific heat data for TmI,(s), TmI,(g) and Tm,I,(g) are needed. To our knowledge, no experimental C, data for these species exist. For related rare earth halides, however, these data do exist in work by Dworkin and Bredig [ 171. It appears that (Hr - HZ& values for GdX(s)
339
1.0
1.10
1.05
-
Fig. 3. Weight loss: logarithmic temperature.
I.15
f03/:(K-l)
plot of total vapour pressure against the reciprocal
(XSl, Br, I) vary little, and the values for DyCI, and HoCl, are also very similar. Thus, below phase transition temperatures, it was assumed that -~ H298) for TmI,(s) would be identical to the GdIs(s) value. (NT Adopting the constants presented by Dworkin and Bredig, we obtained for the specific heat of solid TmI,: C,(T) = 24.288 + 0.00187’ - 10.002 X 3.0M4TV2 cal mol-’ This formula yields only slightly lower C!, values than that proposed by Myers and Graves [ 181. For gaseous TmI, we adopted C, data from Lynch [19] which we fitted to a polynomial. These data are almost identical to those of Myers and Graves [20] ; for instance the difference in (I-r, - H2ss) between the two approximations at 1000 K is no larger than 0.02 kcal mol-‘.
For the dimer, we adopted a proposition by Work [21], assuming C,,(Tm&(g)) = 2.2 C,,(TmI,(g)). The extrapolated 298 K sublimation data thus obtained are presented in Table 8, The third law sublimation enthalpies in Table 8 were calculated from weight-loss data. For the entropy of gaseous TmI,, the value of 102.73 eal mall’ K-r, calculated by Myers and Graves, was adopted. For solid TmI, we used the entropy value of 57.1 cal mol-” K-’ which was also used by Hirayama [9] ; this estimation originates from Latimer [ 221 and Westrum v31.
938 954
1 2
946 929
924
This work (WL) Hirayama et of. (WL)
Hiraysma et al. (WL) (modified)
67.3
59.1 68.48
-
aCongruent sublimation of TmI3 was assumed.
956
This work (MS)
~~A&,~~ (kcal mol-l)
63.0
54.2 -
54.1
(kcal mol-*)
AH mono
77.3
68.5 -
68.0
(kcal mol-‘)
&Ii
68.9 68.0
59.4 58.8
54.6 53.7
fw2 (kcal mol-‘)
“AHkt” (kcal molvl)
Sublimation data for TmIs at mean experimental temperatures
TABLE 7
!F (K)
Experiment number
Second law sublimation data by weight loss
TABLE 6
48.2
39.9 49.7a
-
-Astot )’ (cal mol-’
40.8 39.0
K-l)
42.8
33.9 -
-
(cal molll
As mono
35.0 32.7
K-‘)
55.6
47.6 -
-
(cal mol-’
A&
48.8 46.5
K-l)
nuxm
-
63.2 63.5
This work (WL) Hirayama et al. (WL) (modified)
(kcal mol-‘)
&p
data for Tm13 at 298 K
This work (MS)
Sublimation
TABLE 8
%:;-I 39.7 48.5
AH& (kcal mol-‘) 72.4 72.7 81.4
57.5 57.6 66.2
K-‘ f
54.8 63.6
-
A& (Cal mol-’
K-l)
E
342
Discussion Sublimation studies on TmI, by weight loss have been reported in the literature [9]. Since congruent sublimation of the solid was assumed in ref. 9, only the total pressure data of this work can be compared with the reported data. In both studies the plots of lg P,, t us. T-’ fit well onto a straight line, in the relatively small experimental temperature interval (in Fig. 4). In fact, however, the line, being the sum of two straight lines with different slopes, is not really straight. At low temperatures its slope approaches that of the monomer, at high temperatures that of the dimer. The monomer and dimer thermodynamic data obtained from weight-loss measurements, using the temperature-dependent monomer-to-dimer ratio derived from mass spectrometry, are themselves not a function of the experimental temperature, apart from corrections to C,. In comparing total pressure data, however, the influence of the temperature on slope and on the point of intersection with the ordinate must be taken into account. Unfortunately, no temperature influence can explain the difference in the mentioned parameters between this work and ref. 9. ,
1
I
I
1
10-’ T-h
5 f 10-4
1O-s
1O-6
Fig. 4. Total vapour pressure plotted logarithmically against 103/T: 3, Hirayama et al. [9].
1 and 2, this work;
343
The total pressures, however, do agree reasonably in the experimental temperature range. Thus, the third law sublimation enthalpies in ref. 9 and this work agree well, independent of the values of the entropies used. The agreement between the second and third law sublimation enthalpies is less good, dependent on the entropy values used. Here the lack of experimental entropy data is a problem, especially for the condensed phase. Excellent agreement, however, exists between the thermodynamic data for both monomer and dimer, obtained either by mass spectrometric investigations or weight loss experiments.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
E. G. Zubler and F. A. Mosby, Illum. Eng., 54 (1959) 734. H. Schafer, Chemische Transportreaktionen, VerIag Chemie, Weinheim, 1962. B. Kopelman and K. van Wormer, Zllum. Eng., 63 (1968) 176. G. M. Neumann and W. Knatz, 2. Naturforsch., Teil A, 26 (1971) 863. J. H. Dettingmeijer, B. Meinders and L. M. Nijland, J. Less-Common Met., 35 (1974) 159. E. Schnedler, Proc. Symp. High Temp. Light Sources, 85-2 (1985) 95. W. van Erk and T. Rietveld, Proc. Symp. High Temp. Light Sources, 85-2 (1985) 57. C. Hirayama, P. M. Castle, R. W. Liebermann, R. J. Zollweg and F. E. Camp, Inorg. Chem., 13 (1974) 2804. C. Hirayama, J. F. Rome and F. E. Camp, J. Chem. Eng. Data, 20 (1975) 1. C. Hirayama, P. M. Castle, W. E. Snider and R. L. Kleinosky, J. Less-Common Met., 57 (1978) 69. J. H. Dettingmeijer, H. R. Dielis, B. J. de Maagt and P. A. M. Vermeulen, J. LessCommon Met., 107 (1985) 11. J. R. McCreary and R. J. Thorn, J. Chem. Phys., 48 (1968) 3290. B. A. Gottwald, Vuk.-Tech., 22 (1973) 106. I. Barin, 0. Knacke and 0. Kubaschewsky, Thermochemical Properties of Inorganic Substances, Springer, Berlin, 1977. J. H. Dettingmeijer and B. Meinders, 2. onorg. olig. Chem., 400 (1973) 10. H. Schafer and K. Wagner, 2. anorg. allg. Chem., 450 (1979) 88. A. S. Dworkin and M. A. Bredig, High Temp. Sci., 3 (1971) 81. C. E. Myers and D. T. Graves, J. Chem. Eng. Data, 22 (1977) 440. D. A. Lynch, private communication (1973). C. E. Myers and D. T. Graves, J. Chem. Eng. Data, 22 (1977) 436. D. E. Work, J. Less-Common Met., 69 (1980) 383. W. M. Latimer, The Oxidation States of the Elements and their Potentials in Aqueous Solutions, Prentice-Hall, New York, 1952. E. F. Westrum, Adv. Chem. Ser., 71 (1967) 25.