P. J. FICALORA, J. W. HASTIE,AND J. L . MARGRAVE
1660
Mass Spectrometric Studies at High Temperatures. XXVII.
The Reactions
of Aluminum Vapor with S,(g), Se,(g), and Te,(g) by P. J. Ficalora, J. W. Hastie, and J. L. Margrave Departmen,t of Chemistry, Rice University, Houston, Texas 77001 (Received October 3, 1067)
High-temperature studies of the various species present when Al(g) is equilibrated with Sp(g), Se2(g),or Tez(g) have yielded thermodynamic data for the molecules .41X(g), &X(g), and Al2Xz(g) when X = S, Se, or Te. The species 41Te2(g)has also been observed. Bond energies have been evaluated and the trends correlated on the basis of the periodic table.
Introduction Mass spectrometric have shown that AlzO&) vaporizes to form AlO(g), AlzO(g),and AlzOz(g) molecules, and the heats of reaction involving the dissociation of the various species to the gaseous elements or lower molecular weight species were measured. The present study of the reactions of Al(g) with Sz(g) and Sez(g) and Tez(g) provides sufficient data for correlating the energetics of the various reactions for the different chalcogens and obtaining a more fundamental understanding of the nature of the bonds involved.
given in Figure 1, and breaks in the curves indicate the formation of an ion from several precursors. Appearance potentials (Table I) were measured by the vanishing-current method with water and argon6 as calibrants. Since the ionization potentials of A1 and 82 are known5 and could be reproduced to within *0.2 eV, they served as a set of secondary standards. Table I : Appearance Potentials (AP)” and Relative.Intensities X Ion
AP
YE~---AP
AlX+
9.5 7.7 9.9
9.5 9.0 9.5
0
Experimental Section A Bendix time-of-flight mass spectrometer and Knudsen assembly similar to those previously described3j4were used. Aluminum metal contained in a small tantalum crucible was placed in a larger tantalum Knudsen cell, having a knife-edge effusion hole of 0.5-mm diameter, which was loaded with either CrzS3, Cr2Sea, or Cr2Te3. The vaporization of aluminum produced Al(g), and the decomposition of Cr2X3produced Xz(g) where X = S, Se, or Te. Heating was accomplished by radiation and electron bombardment from tungsten filaments, and temperatures were measured with a Pt-Pt-lOyo Rh thermocouple located in a hole in the bottom of the cell. I t was found necessary to protect the thermocouple from the X2(g)with a 0.5-mm tantalum foil cap.
Composition of the Vapor
+
Al(y) &(g) Equilibria. The molecular ions observed in this system and their relative intensities at 15 eV are given in Table I. The ions were identified as species effusing from the Knudsen cell by their mass, their isotopic distribution, shutterability, and their appearance potentials. Ionization-efficiency curves showed that several molecular precursors existed for some of the ions; e.g., the AlS+ ion intensity was due to AlS(g) as well as fragments from -41~sand AlZSz(g). The ionization-efficiency curves for this system are The Journal of Physical Chemistry
A&X+
AliXz+ AlX2+ a
.,.
. ,.
I _
----Ter--AP
ySe---
I
AP
I
0.95 1.0 0.04
8.3 6.0 9.0
0.60 1.0 0.90
...
,
..
AP is given in electron volts (10.5).
. ..
9.0 10.0 10.0 6.5
I
0.50 0.55 1.0 0.85
Arbitrary units.
The shapes of the ionization-efficiency curves as well as the appearance potentials indicate that Al+, Sz+, AlS+, and AlzS2+are parent ions up to several electron volts above their appearance potentials. S+ and Ah+ were observed as fragments, KO higher polymers of sulfur molecules were observed. The ion-intensitytemperature data, shown in Figure 2 , were collected a t 10.5 eV, i.e., below the lowest break in any of the ionization efficiency curves, so that there were no significant contributions due to fragmentation. AZ(g) Xez(g) Equilibria. The appearance poten-
+
(1) J. Drowart, G. DeMaria, R. P. Burns, and M. G. Inghram, J. Chem. Phya., 3 2 , 1366 (1960). (2) R. F. Porter, P. Shissel, and M. G. Inghram, ibid., 2 3 , 339 (1955). (3) D. B. Harrington in “Encyclopedia of Spectroscopy,” C. F. Clark, Ed., Reinhold Publishing Corp., New York, N. Y., 1960, p 828. (4) A . Kant, J . Chern. Phys., 41, 1872 (1964). (6) R . W. Kiser, “Introduction to Mass Spectrometry and Its Applications,” Prentice-Hall, Inc., Englewood Cliffs, N. J., 1965.
1661
MASSSPECTROMETRIC STUDIES AT HIGHTEMPERATURES
ENERGY (EV)
Figure 1. Ionization-efficiency curves for species in the Al(g)-S&) system. IOYT
(OK)
Figure 3. Ion-intensity data for the Al(g)-Sez(g) system as a function of temperature.
+
Al(g) Tez(g) Equilibria. Appearance potentials and relative intensities of the species observed in this system are given in Table I. The appearance potentials of the secondary standards Al+ and Ten+ were reproduced7to within h 0 . 2 eV. The ions Al+, Ten+, AlTe+, AlzTe+, and AITez+ were found to be parents while Te+ and Alz+ were fragments. The ion AITez+ has no analog in the preceding systems and AITez(g) is very stable. The absence of AlX2 where X = 0, S, or Se is consistent with the trends in the thermodynamic properties of these systems, as indicated in Table 11, and a similar behavior occurs in the Sn(g)-Sz(g),8 - S e ~ ( g ) , ~and -Ten(g) systems. No higher polymers of tellurium were observed. All of the ion-intensity data shown in Figure 4 were taken at electron energies where fragmentation was negligible.
Calculations Figure 2. Ion-intensity data for the AI(g)-Ss(g) system as a function of temperature.
tials and the relative intensities of the species observed in this system are given in Table I. Again the appearance of the secondary standards, AI6 and Sez,6were reproduced to within A0.2 eV. The ions Al+, Sez+, AlSe +, and AlzSe2+ were found to be parents, while Se+ and Alz+ were fragments. No higher polymers of selenium were observed. All ion-intensity data were collected a t 10.0 eV for the same reason mentioned previously and are shown in Figure 3.
The heat of vaporization of aluminum measured in this study (AHv0298= 75.0 rt 2.0 kcal/mol) is in excellent agreement with the accepted valuelOjll sug(6) J. Berkowitz and W. A. Chupka, J. Chem. Phys., 45, 4289 (1966). (7) R. Colin, I n d . Chim. Belge, 26, 51 (1961). (8) R. Colin and J. Drowart, J. Chem. Phys., 37, 1120 (1962). (9) R. Colin and J. Drowart, Trans. Faraday Soc., 60, 673 (1964). (10) L. Brewer and A. W. Searcy, J. Amsr. Chem Soc., 73, 6309 (1951). (11) “JANAF Thermochemical Tables,” D. R. Stull, Ed., The Dow Chemical Co., Midland, Mich., 1963; U.S. Government Document NO.PB-168-370.
Volume 78, Number 6
M a y 1968
P. J. FICALORA, J. W. HASTIE, AND J. L. MARGRAVE
1662 Table I1 : Second-Law Enthalpies and Entropies"
-
+ '/zXz = AlX + '/zXZ = AlzX + Xz = AlzX2 Al + A1X = AlzX 2A1X AlzXz A1 + Xz = AlXz AlX + '/zXz = AlXa
A1 2A1 2A1
A1 f AlXz = A12Xz
-
AHOT (tempc = 129Z°K,
koal/mol
-AS% (tempc = 1414OK, = S), eu
k ca 1/ m o1
- ASOT (tempc = 129Z°K, X = Se), eu
57 f 5 187 f 7 249 131 5 135 f 5
47.0 107.0 152.0 60.3 58.0
19.8 36.1 22.0 16.3 17.7
53.5 122.4 182.9 74.3 75.5
27.5 54.8 24.0 27.3 31.0
x
Reactionb
-
AHOF (tempc = 1414'K, x = S), koal/mol
AHOT (tempc = 2300°K, = 0).
+
... ...
...
" Uncertainties are =4-6%, unless indicated.
... ....
.
.
I
x
...
... ...
All species are gaseous.
X = Se),
-
AHOT (tempc = 1292'K,
X = Te), kcal/mol
56.5 129.7 305.5 72.3 90.0 125.5 68.0 80.0
... ... ...
... ... ...
-ASOT (tempc = 1292OK, X = Te), eu
31.3 59.5 21.6 28.2 41.0 71.3 34.2 34.6
Median temperature.
1 E L E CTR 0 N E GAT I V I T Y
( PAUL I N G SCALE 1
Figure 5. Dissociation energies of aluminum chalcogenides as a function of the chalcogen electronegativities.
5x10-6 I
1
recoverable after each experiment. An X-ray powder diffraction pattern, taken in the usual way, showed lines due to aluminum alone. Therefore, aluminum was used as an internal standard to convert ion intensities to pressures using the relation
p
10~1~ Figure 4. Ion-intensity data for the Al(g)-Tez(g) system as a function of temperature. (OK)
gesting that Al(1) was probably at unit activity. Further evidence for this fact is that elemental aluminum and not a solid sulfide, selenide, or telluride was The Journal of Physical Chemistrg
Fs
E("-) cry E - A
where P is the pressure,12I is the ion intensity, T is the temperature, u is the ionization cross section, y is the multiplier efficiency, A is the appearance potential, and E is the energy of the ionizing electrons. Cross sections were estimated by using the Otvos and Stevensonl3 rule and the data of R/Iann.l4 Efficiency terms relative to aluminum were then calculated from the relation16 (12) An. N. Nesmeyanov, "Vapour Pressure of the Elements," Academic Press Inc., New York, N. Y., 1963, p 234. (13) J. W. Otvos and D. P. Stevenson, J. Amer. Chern. Soc., 78,646 (1956). (14) J. B. Mann, J . Chem. Phys., 46, 1646 (1967). (15) R. C. Schoonmaker and R. F. Porter, ibid., 30, 283 (1969).
MASSSPECTROMETRIC STUDIES AT HIGHTEMPERATURES
2
(!5)"%
Yz where M is the molecular weight. With these partial pressures, free energies for the various reactions were obtained, and a combination of these energies with the second-law enthalpies provided corresponding entropies, as given in Table 11. Since free-energy functions are available for Al(g), Sz(g), and AlS(g)," a third-law calculation for the reaction Al(g)
+ '/zSz(g)
= AlS(d
yielded AH02gR = -35.4 f 2.0 kcal/mol, as compared to a second-law value of AH02g8 = -37.3 f 3.0 kcal/mol. The agreement between second- and third-law values allows an uncertainty of 5 f 3 eu for the calculated entropies. Finally, the dissociation energies of AlS(g), AlSe(g), and AlTe(g), and heats of formation of the various species were calculated as presented in Table 111, based on the following heats of reaction. l/&(g) = S(g)le l/k3ez(g) = Se(g)" l/zTez(g) = Te(g)l8
AH02g8
= 51.0 f 1.5 kcal/mol
AHozgs= 37.5
f
1.0 kcal/mol
26.3
f
1.0 kcal/mol
AH02g8
=
The selenides and tellurides were assumed to have similar heat capacities to those of the sulfides.'l
Discussion The dissociation energy of A1S is thus established as 86.0 + 3.0 kcal/mol. By analogy with the A10 molecule and by adding 17% to the dissociation energy obtained from a linear extrapolation of the vibrational levels, the JANAF tables predicted Doz9,(A1S) = 95 kcal/mol, in fair agreement. The A1X bond strengths can be used to elucidate the nature of the bonds. If one assumes that the bonds are covalent, then the dissociation energies should increase with the increasing polarizability from oxygen to tellurium. The values shown in Table I11 along with a value of 115.0 & 5.0 kcal/mol for the heat of dissociation of AlO(g)" show that the above is not the
1663
case; in fact, the dissociation energies decrease through the oxygen family. If the relation given by Pauling, l9 relating dissociation energy to electronegativity, is used to calculate the difference in electronegativities between the A1X species and this difference is used to deduce the per cent ionic characterjZ0one finds that the A10 bond is 98% ionic, the A1S bond is 95%) the AlSe bond is 93%, and the AlTe bond is 90% ionic. If the above calculations are meaningful, then the dissociation energies for the aluminum chalcogenides should follow the trend for the changes of electronegativities from oxygen to tellurium. Figure 5 shows the variation of dissociation energies with the electronegativities from oxygen to tellurium. The fact that the dissociation energies follow the changes in electronegativities indicates that the A1X bonds are largely ionic. Furthermore, this trend allows an estimation of D o (AlPo) equal to 70 f 3 kcal/mol. The trend in the heats of dimerization also proves valuable in understanding the bonding in these molecules. If one assumes that the structure of AlzXz(g), the dimer, is like that of the alkali halide dimers,20the relatively small increase in stability from AlZSz(g) to AlzTez(g) can be explained. This assumption is considered consistent with the high per cent of ionic character of the A1X bond. The repulsive terms in the coulombic expression for the potential energy of the AlZXz(g)molecules should decrease in value from X = S to X = Te owing to the increasing size of the chalcogen ions. If the repulsive terms decrease faster than the attractive terms, one should find, and does in this case, that the stability of the molecules increases from X = S t o X = Te. The unusually high stability of the Al2O2(g)molecule ( D o o= 365 & 7 kcal/mol)l is the reverse of what one might expect to find from the above discussion and indicates a very different type of bonding in this case. This behavior has been noted for the Al2X(g) molecules by I\lal'tsev,zl who suggested that the subsulfide should have a bond type and configuration different from that of the suboxide.
Acknowledgment. This work was supported by the National Aeronautics and Space Administration and by the Robert A. Welch Foundation. (16) J. Berkowitz and J. R. Marquart, J . Chem. Phys., 30, 283 (1963). (17) M. Jeunehomme, Ph.D. Thesis, University of Brussels, Brussels, 1962.
Table I11 : Dissociation Energies of AlX Molecules ( X = 0, S, Se, and Te) Donos, Species
kcal/mol
A10 A1S AlSe AlTe
115 i z 5 . 0 86.0-1: 3.0 81.0 i 3.0 7 4 . 0 & 3.0
(18) R. F. Porter, J . Chem. Phya., 34, 583 (1961). (19) L. Pauling, "The Nature of the Chemical Bond," 3rd ed., Cornel1 University Press, Ithaca, N. Y., 1960. (20) 6 . H. Bauer and R. F. Porter, "Molten Salt Chemistry," M. Blander, Ed., Interscience Publishing Corp., New York, N. Y., 1964, p 650. (21) A. A. Mal'tsev and V. F. Shevel'kov, Teplofiz. Vys. Temp., 2, 650 (1964).
Volume 72, Number 6
May 1968
