8944
J . Phys. Chem. 1991, 93, 8944-8947
temperature of 2100 K, and a C / H / O ratio of 1:1:0.96. We did a calculation in which the oxygen concentration was reduced until the predicted BF yield matched the experimental BF yield. This condition occurred at an elemental ratio of C/H/O of 1 :I :0.52. In other words, experimental flames produce fullerenes under conditions much leaner than is predicted by an equilibrium analysis. It is interesting to compare this result to oxygen levels for soot inception. Equilibrium predicts that soot would not form until the C/O ratio exceeds unity. However, for benzene, the fuel used in our experiments, the soot inception level is at C/O = 0.76. Thus, flames produce both soot and fullerenes at oxygen concentrations much higher than is predicted by equilibrium. We have by no means experimentally mapped the fullerene production parameter space, but all other soot samples we have tested, which were from flames at higher pressures and lower temperatures, did not produce fullerenes. These results are extremely sensitive to the Nffvalue used. As stated above, the accuracy limits on the MNDO calculation for these types of molecules is f l kcal/mol per carbon atom. Decreasing the AHI by this amount increases the maximum equilibrium weight fraction for the base case system (C/H of unity, 1 atm) from 8.18 at 2350 K to 26.6% at 2250 K. Increasing the AH, by 1 kcal/mol per carbon atom drops the maximum equilibrium weight fraction of the base case to 5.0 X lod. The results are also sensitive to the vibrational frequencies used to compute the entropy and heat capacity. We have done these calculations using the frequency set of Wu et alef3 This data set came from a parameter fit of the Newton and Stanton frequencies. The geometric mean frequency is 870 cm-' as opposed to 914 cm-' for the Newton and Stanton set. The result for the base case (13) Wu,Z. C.; Jelski, D. A.;George, T. F. Chem. Phys. Lerr. 1987, 137, 291-21 94.
system was an increase in yield from 8.1% to 23.6% at 2350 K. The only fullerene included in these calculations was BF. Newton and Stantod have computed that C70 has a heat of formation 1 kcal/mol per carbon atom lower than Cm so it would be expected that C70 would be produced in larger quantities than Cb0. All experimental results to date have shown the opposite, so there may be a kinetic limitation to the larger fullerene. The vibrational frequencies were not available for C70 so its entropy and heat capacity could not be calculated. Conclusions Equilibrium calculations using the available thermodynamic properties for Buckminsterfullerene indicate that, possibly, relatively large quantities of the carbon cluster can be produced by pyrolysis of a hydrocarbon or from the rich combustion of a hydrocarbon. The limited experimental data available are in very good agreement with the equilibrium calculations. That is, fullerenes are produced in flames at a very high temperature (2100 K), a low pressure (0.053 atm), and a low oxygen concentration (C/O = 0.96). The experiments show that flames produce fullerenes at leaner conditions than predicted by the equilibrium model, but this result is in very good agreement with the trends seen in C / O ratio of soot inception. Buckminsterfullerene falls into a temperature window of thermodynamic stability between PAHs on the low-temperature side and acetylenes on the high-temperature side. Computed Buckminsterfullerene yields are favored by high carbon-to-hydrogen ratios, low oxygen concentrations, and low pressures.
Acknowledgment. This project was partially supported by the Department of Energy, under the Small Business Innovative Research Program, contract no. DE-FG03-90ER80999.000. Registry No. Car 99685-96-8; C2H2,74-86-2; CsH,, 71-43-2; 0, 7782-44-7;C, 7440-44-0; H, 1333-74-0.
Thermochemical Properties of the Osmium Oxides Lyn R. Watson, Terry Thiem, Rainer A. Dressler, Richard H. Salter, and Edmond Mucad* Phillips Laboratory, WSSI, Hanscom Air Force Base, Massachusetts 01 731 -5000 (Received: March 27, 1991)
The enthalpy, AH', for the gaseous equilibrium OsO,(g) s Os03(g) + '/202(g) was measured by using high-temperature mass spectrometry. Over the temperature range 1139-1471 K, a second-law heat of reaction AHollO5(II)= 161 f 14 kJ mol-' was obtained, which yields the standard enthalpy AH0298(II) = 164 f 14 kJ mol-', and a third-law heat of reaction AH0298(III)= 189 f 7 kJ mol-' was calculated by using the known and estimated molecular constants for OsO,(g) and and OsO3(g), respectively. Because of uncertainties in molecular constants, an average standard enthalpy of AHo298(II) AH0298(IlI),176 f 29 kJ mol-', is reported for the equilibrium. From this heat of reaction the heat of formation for gaseous Os03, ArHo298(0s03) = -163 f 29 kJ mol-', is obtained, leading to a bond energy for O-Os03 of 423 f 29 kJ mol-'. The ionization potentials for Os03 and Os04were found to be 11.4 f 0.2 and 12.3 f 0.2 eV, respectively. A small signal of OsOl was observed, and an upper limit of 12.2 f 0.4 eV for its ionization potential was obtained. It is concluded that osmium films which disappear when exposed to the low earth orbit environment probably do so by forming OsO,(g).
I. Introduction Osmium thin films, used as optical coatings in spectrometers because of their high reflectivity in the vacuum ultraviolet, are found to disappear quickly on space-borne instruments.' It is surmised that the disappearance is related to chemistry initiated by the collisions of high-velocity 0 atoms (-7.8 km/s) with surfaces. Possible causes for the mass loss include the following: the formation of volatile oxides of osmium; the distribution of the excess reaction energy to the material lattice, resulting in subsequent vaporization of reaction products or unreacted material;
or chipping of surface materials, particularly thin films. To determine which process causes the mass loss and develop a predictive understanding of the interaction between the highvelocity atmospheric oxygen atoms and the surfaces of materials, it is necessary (among other things) to determine the thermochemical properties of gaseous oxides of the materials.* For osmium, Os04(g) is a well-known gaseous oxidizing agent which is used for many applications, including as a biological fixative. The high vapor pressure of Os04(g) at room temperature has facilitated its study, and infrared,*5 ultraviolet: and photoelectron (2) Murad, E. J. Spacecr. Rockers 1989, 26, 145.
This article not subject to U.S.Copyright. Published 1991 by the American Chemical Society
Thermochemical Properties of Osmium Oxides
The Journal of Physical Chemistry, Vol. 9.5, No. 22, 1991 8945
spectra' have been obtained. The commonly accepted heat of formation of Os04(g),A r f f = -337.2 kJ/mol? was determined from the heats of formation and vaporization of O S O ~ ( S )Far .~ fewer data are available for the other osmium oxides. The equilibrium (1) OsOdg) * OSOdg) 4- Y202(g) was studied by Grimley et a1.I0 in the temperature range 1 100-1750 K using high-temperature mass spectrometry. These authors determined a reaction enthalpy PHOT ( T = 1400 K) = 49.4 f 4 kJ/mol ( 1 1.8 f 1 kcal/mol). No conclusive evidence was found for the existence of gas-phase OsOl and Os0 in equilibrium. It has been suggested" on the basis of free energy correlations that the only osmium oxides that can be of importance in equilibrium above 1500 K are of the form Os,O and 0 ~ ~ 0 2 . We have attempted to determine reliable thermochemical properties of the gaseous oxides of osmium. We observe the Os03 and Os04equilibrium in the gas phase; the ion 0s02+was observed and believed to be from the parent osmium dioxide molecule, but no evidence of OsO(g) is found. The ionization potentials of the oxides as well as their fragmentation are discussed. Thermochemical data determined from the results obtained are presented and compared to existing data. 11. Experimental Section The high-temperature mass spectrometer (Nuclide 12-60) used in the experiments has been described in detail elsewhere.I2 Briefly, a Knudsen cell containing osmium and an oxidizer is heated radiatively. Two types of oxidizer are used: V20S(s)and 02(g). The temperature of the cell is determined both by optical pyrometry and with thermocouples. In the latter case the thermocouples are pressed against the side of the Knudsen cell by means of a tantalum band. The molecular beam effusing from the cell orifice passes through a moveable shutter and enters an electron impact ion source where it is ionized. The ion beam is then focused and accelerated into a 60°, 0.3 m radius magnetic mass analyzer, where it is dispersed according to m / e . The mass-analyzed ions are detected by a channel electron multiplier. Mass spectra and ionization efficiency curves are obtained by computer-controlled repetitive scanning of the magnetic field and the electron energy, respectively. As indicated above, two methods are used to generate Os03(g) and OsO,(g): (a) A mixture of osmium powder and V20S(s)is placed in a platinum liner, which in turn is placed inside a tungsten Knudsen cell. In the initial heating of the mixture, high ion count rates of Os03+and Os04+are observed at a Knudsen cell temperature of 970 K, which is about 30 K above the melting point of V20s.13 We do not report any data for reaction 1 at this temperature because we did not observe 02(g). The osmium oxide ion signals at this temperature decrease rapidly and then disappear. No osmium oxides are observed when only osmium powder is present in the heated cell. The osmium oxides appear again at (3) Makarov, A. A.; Makarov. G . N.; Puretzky, A. A.; Tyakht, V. V. Appl. Phys. 1980, 23, 391. (4) Bazarov, E. N.; Gerasimov, G . A,; Guryev, K. 1.; Derbov, V. L.; Kovner, M. A.; Posudin, Yu. 1.; Potapov, S. K.; Chenin, V. A. J . Quant. Specrrosc. Radiat. Transfer 1911, 17, 7. ( 5 ) McDowell. R. S.:Radziemski. L. J.: Flicker. H.: Galbraith. H. W.: Kennedy, R. C.; Nereson, N. G.; Krohn, B. J.; Aldridge, J. P.; King, J. D: J . Chem. Phys. 1978, 69, 1513. (6) Roebber, J. L.; Weiner, R. N.; Russcl, C. A. J . Chem. Phys. 1974,60, 3 166. (7) Foster, S.; Felps, S.; Cusachs, L. C.; McGlynn, S. P. J . Am. Chem. Soc. 1973, 95, 5521. (8) Wagman, D. D.; Evans, W. H.; Parker, V. B.; Schemm, R. H.; Halow, I.; Bailey, S. M.; Chutney, K. L.; Nuttal, R. J. Phys. Chem. Ref. Dora 1982, 11. 2-190. (9) Nikol'skii, A. B.; Ryabov, A. N . Rum. J . Inorg. Chem. 1965, 10, I . (IO) Grimley, R. 7.;Burns, R. P.; Inghram, M. G . J . Chem. Phys. 1960, 33.. 308. ~.. (11) Brewer. L. Chem. Rev. 1953, 52, I . (12) Thiem, T.; Watson, L. R.; Dressler, R. A.; Salter, R. H.; Murad, E. Report GL-TR-90-0224; Geophysics Laboratory: Hans" Air Force Base, M..A.,. 1990. . .- - -. (13) Chase, M. W., Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N . J . Phys. Chem. Re/. Dora 1985,14, 1707.
TABLE 1: Ionization Potentials of Osmium Oxides (eV) Dillard Grimley Foster this work and Kiser" et al.IO et ai.' Os02
Os03 OsOI
I 12.2 f 0.4 11.4 f 0.2 12.3 f 0.2
(1 I .2) 12.97 f 0.12
12.3 f 1 12.6 f 1
12.320
TABLE I 1 ADDearance Potentials for Ions Formed from Oso~ .. - (ev) - . this work Dillard and Kiser'' OSOA * OsOa+ 16.4 f 0.4 17.00 f 0.10 * oso;+ 16.3 f 0.4 17.1 f 0.2
-
-
oso+
21.7 f 0.6 26.6 f 1.0
* os+
21.2 f 0.2 26.8 f 0.5
TABLE 111: AoDearance Potentials for Ions Formed from Os01 (eV) OSOJ* oso2+ =)
oso+ os+
=!+
this work 15.6 f 0.5 15.7 f 0.6 20.8 f 0.5
1150 K, as does 02(g). We did not observe any signals due to the gaseous oxides of vanadium. (b) In the second method, molecular oxygen is passed through osmium powder contained in an alumina cell equipped with a gas inlet tube. The partial pressures of the neutral species effusing from the Knudsen cell are determined from their ion currents by calibration of the instrument with a substance of known vapor pressure,14 in this case silver (99.99% purity). The AH,,, for silver calculated from the calibration data provides a check on the accuracy of the temperature measurement. The partial pressure of a particular species is given by where the subscripts i and Ag are species i and silver, respectively, P is the partial pressure, I is the ion count rate, T is the temperature, and 8 denotes the ionization cross section at electron energy E. The ion counts of all species in a particular gaseous equilibrium were measured at electron energies 3 eV above their threshold ionization potentials to avoid fragmentation of the molecules. However, in some cases the ion count rate of O2was measured at 80 eV due to low signal levels; a correction based on the measured electron energy dependence of the O2ionization cross section was applied to convert the measured ion count rate to that of the electron energy E. N o correction is made for the mass dependence of the channel electron multiplier detection efficiency, which is most significant at low ion energies.'s*'6 Since the energy at the detector cathode is 7 keV, we assume that the multiplier response is essentially mass independent. 111. Results A. Ionization and Appearance Potentiah. The vanishing current
method is used to determine the ionization potentials (IPS) and appearance potentials (APs) of the gaseous species, where the term appearance potential applies when the ion formed is a fragment of the parent molecule. These measurements represent vertical IPS and APs and are therefore not necessarily equal to the adiabatic values. Oxygen and xenon are used for reference in all cases. The IP for Os04+was measured below 1000 K, where no other osmium oxide species is observed in the equilibrium vapor. The IP(Os03+) was measured above 1300 K, where Os03 has appreciable concentrations; fragmentation products from OsO, do not interfere with this measurement. IP(Os02+)was measured at an average temperature of 1800 K. Table I presents data for the IPS Os02+,OsOp+, and Os04+, as measured in this work. The errors for the measurments are
~
(14) Inghram, M. G.; Drowart. J. Mass Spectrometry Applied to High Temperature Chemistry. In Proceedings of an International Symposium on High Temperature Technology;McGraw-Hill: New York, 1960; pp 219-240. (15) Burrous, C. N.; Lieber, A. J.; Zaviantseff, V. T. Reo. Sci. Instrum. 1967, 38, 1477. (16) Potter, W. E.; Mauersberger, K. Rev. Sci. Insrrum. 1972,43, 1327.
Watson et al.
8946 The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 TABLE IV: Equilibrium Constants and Third-Law Resuits for the Reaction OsO,(g) s Os03(g) + 1/203(g) Using V20,(s) as the Oxidizer third law third law kJ/mol T,K K,, Pall2 AH298,kJ/mol T,K Kq, Pall2 AH298,
I I24 I I39 I I83
1215 1258 1309 1337
0.108 0.108 0.137 0.232 0.598 0.868 I .24
181.6 184.3 188.7 196.1 185.0 188.2 188.2
1346 1367 1369 1385 1407 1471
1.22 2.07 1.84 2.06 2.91 4.23
189.6 186.5 188.1 188.9 187.8 191.5 -2
TABLE V: Equilibrium Constants and Third-Law Results for the Reaction Os04 ,= Os03 '/*02Using Oz(g) as Oxidizer third law third law T,K KW, AH0298,kJ/mol T,K Kq, AH0298.kJ/mol
+
897 897 981 1015 1015
0.009 0.012 0.020 0.046 0.056
164.4 162.0 175.4 171.5 169.3
1111 1111
1151 1204 1241
0.208 0.161 0.170 0.325 0.500
173.5 175.9 181.5 183.7 184.3
the estimated accuracies for the determination of the vanishing current. For comparison, data from other measurements are also included. The value of IP(Os04+) = 12.3 f 0.2 eV is in good agreement with the more accurate photoelectron measurement of 12.32 eV reported by Foster et al.' IP(Os03+) = 11.4 f 0.2 agrees within experimental error with the value of 12.6 f 1 eV reported by Grimley et a1.I0 Because of the low signal levels for Os02+,the value given in Table I represents an upper limit for IP(Os02). In addition, the APs for fragment ions from Os04and Os03are shown in Tables I1 and 111, respectively. For comparison, the values reported by Dillard and Kiser" for the fragmentation of Os04are also given in Table 11. B. Heat of Reaction for the Equilibrium Os04(g) P Os03(g) '/,02(g). The partial pressures of Os03(g) and Os04(g) were calculated by using the relationship given in eq 2 and assuming that the relative electron impact ionization cross sections for the two gases were equal a t ionizing energies 3 eV above threshold. For 02(g) and Ag(g) the electron impact ionization cross sections are known: 40,) = (0.1 15 X cm2) at 15.1 eVI8 and a(Ag) = (0.70 X cm2) at 10.5 eV.I9 The partial pressures calatm) to 5 Pa ( 5 X atm). culated ranged from lo-, (2 X By use of the partial pressures thus obtained, the equilibrium constant K(T) for reaction 1 can be calculated for use in the determination of second- and third-law heats. Second-law heats are obtained from the van't Hoff relationship [ M o ( I I ) = -d(R In K)/d( l/7')]; thus a plot of -R In K vs 1 / T yields the heat of reaction, AH, at the midpoint of the investigated temperature range. If the molecular constants of the constituents of the equilibrium are known, the third-law entropies or the free energy functions, AFEF, can be calculated and used to determine the third-law heat of reaction AH298(III)= - T [ R In K - AFEF]. A van't Hoff plot of the data using V205(s) as an oxidizer is shown in Figure I , and the data are summarized in Table IV. The second-law heat of reaction, AH298(II),is found to be 164 f 14 kJ mol-l, while the third-law AH298(III)is found to be 189 f 7 kJ mol-' by using the molecular constants given in the Appendix. The reported error is the statistical 2a. In addition to the above method for generating the oxides of Os, we also attempted to study equilibrium 1 by passing O,(g) through an alumina cell equipped with an alumina gas inlet tube. The results from this experiment, which are summarized in Table V, suggest that thermodynamic equilibrium was not established. This conclusion is reached because of the wide discrepancy between AH298(II)and AH298(111) and because AH298(III)shows a trend with temperature. Thus,
+
(17)Dillard, J. G.;Kiser, R. W. J. Phys. Chem. 1965,69, 3893. (18) Schulz, G. J. Phys. Reu. 1962,I t & 178. (19)Crawford, C.K.;Wang, K. 1. J. Chem. Phys. 1967,47,4667.The uA at 75 eV is given in this reference, and the value was corrected to 10.5 e g b y using an ionization efficiency curve obtained in our laboratory.
\.
t
-31
1 '
'
'
'
'
'
'
'
'
'
'
'
'
'
1
1 o'/T(K-')
Figure 1. van? Hoff plot for the reaction OsO, V20, as the oxidizer.
z OsO, + 1/202 using
we conclude that only the data from Os(s)-V205(s) are valid. Because of uncertainties in the molecular constants of OsO,(g), we report an average of M298(II) and AH298(III),176 f 29 kJ mol-', for equilibrium 1. Both the V205(s) and O,(g) experiments yield a AH298 that is considerably larger than that reported in an earlier study.'O The earlier results were obtained by flowing O2over Os(s) contained in an alumina liner inside a molybdenum Knudsen cell with a molybdenum gas inlet tube. The measurements may have been affected by MOO^)^+, which interferes with the Os04+mass peaks. To check this possibility, some experiments were conducted in which Os(s) was placed in a molybdenum Knudsen cell (no alumina liner) and O2 was passed through the sample. Large signals that were attributable to the dimer ion of Moo2 were observed at temperatures near 1400 K. Inclusion of a large MOO^)^+ signal in the Os04+measurement would lower the determined equilibrium constant, hence reducing the heat of reaction. C. AHr' [OsO,(g)] and D ( O s 0 3 4 ) . When AH298= 176 i 29 kJ mol-I derived above for equilibrium 1 is combined with AfHo298(oSo4(g)) = -337.2 kJ mol-' (no error is given in the reference), a value of -163 f 29 kJ mol-' is obtained for AfH298(0s03(g)). From the ArHo2ss(Os03(g))deduced above, we calculate D(OsO,-O) to be 423 f 29 kJ mol-I. The abundance of OsO2(g) was too low to be measured accurately. Therefore, we report a lower limit for D(Os02-O) using the appearance potential for OsO2+from OsO,, given in Table 111. For the purpose of the calculation, it is assumed that the measured appearance potential is equal to the adiabatic appearance potential. This leads to the lower limit of D(OsO2-O) L 328 f 87 kJ mol-'. IV. Discussion The data presented above indicate that the two oxidizers, VzOs(s) and 02(g), do not yield equally reliable results for reaction 1. Because the mixture V,O,(s)-Os(s) yields second- and third-law heats for reaction 1, which agree within experimental error, and because the third-law heat is essentially independent of temperature, we conclude that these measurements are in thermodynamic equilibrium. The discrepancy between second- and third-law heats for reaction 1 obtained by using the flow of 02(g) through powdered osmium is a good indication the system is probably not in equilibrium. Similar observations have been made by Berkowitz-Mattuck et a1.,20 who reported that the partial pressures of Mo02(g) and MoO,(g) desorbing from a molybdenum metal surface being struck with a stream of 02(g) molecules are smaller than the equilibrium partial pressures measured by Burns et aL2' over heated Mo02(c). Thus, for the case of 02(g)-Os(s) the partial pressures of Os04(g), OsO,(g), and Oz(g) are kinetically (20) Berkowitz-Mattuck, J. 9.;Blichler, A,; Engelke, J. L.;Goldstein, S. N. J . Chem. Phys. 1963,39, 2772. (21) Burns, R. P.; DeMaria, G.;Drowart, J.; Grimley, R. T. J. Chem. Phys. 1960,32, 1363.
The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 8947
Thermochemical Properties of Osmium Oxides
TABLE VU: CskulPted Free Energy Functions, -(GT- H = ) / T (cal mor' K-% for OsOI and OsO, T, K OsO, Os03 T,K OsO, Os03 1000 1500 2000 5000
109
105 10-11
10-3
10-16
IO-"
10-14
10-26
10-19
1045
1 0-44
I 0-34
10-22
10-12 10-'0
1044
800 900 1000 1100 1200
71.4591 79.1292 80.7341 82.2701 83.7376
74.5921 75.9120 77.1772 78.3853 79.5371
1300 1400
IS00 1600
85.1389 86.9776 87.1575 88.9825
80.6352 81.6827 82.6830 83.6395
and not thermodynamically controlled. The rate-limiting step may be the transition from 02(g) to adsorbed 0 atoms, a step that is considered necessary, for example, for the oxidation of platinum metaLZ It has been showna that under normal conditions above 150 K 02(g) is adsorbed by platinum metal surfaces in the form of atoms. It is recommended that the value for A$fm(Os03(g)) reported in this paper, -163 f 29 kJ/mol, be adopted. This value is less negative than the value derived by using the AHlm (corrected to AHB8)measured by Grimley et a1.I0 for reaction 1 which yields &Hm(OsO,(g)) = -283 kJ/mol. Nikol'skii et al.,24from a study of the thermal decomposition of osmium dioxide (prepared in their laboratory), in which they detected both Os04(g) and OsO,(g), derived 4kfm(OsO3(g)) = -159 f 40 kJ/mol, in agreement with the value reported in this paper. The agreement of the two different solid-phase methods [decomposition of OsOz(s) and reaction of Os(s) with V20s(s)] in the derived standard heat of formation of OsO,(g) suggests that a gas-phase equilibrium is established in both methods. Our attempt to calculate a heat for the disproportionation reaction 2OsO2(s) a OsO,(g) OS(S)
AfWm(OsOj) = -163 f 29 kJ mol-' reported in this paper. The data in Table VI show that Os04(g) is the dominant species to temperatures well above lo00 K. The OsO,(g) pressure exceeds that of Os04(g) between 1000 and 1500 K. At 1500 K, the pressure of OsOz(g) is equal to that of Os03(g). OsO(g) is the dominant species at 2000 K, and it is interesting to note that its pressure increases by only 2 orders of magnitude over the temperature range 2000-5000 K, compared to 4 orders of magnitude change over the range 1000-2000 K. Thus, depending on the effective equilibrium temperature, the formation of any one of the four osmium oxide species could be responsible for the loss of osmium metal in low earth orbit according to the hypothetical densities shown in Table VI. Osmium is the only metal that has been observed to lose mass in the low earth orbita2 The metals aluminum, silver, copper, iridium, nickel, and tungsten have all been shown to gain mass in the low earth orbit despite the fact that they have gaseous monoxides which are stable a t high temperatures.2 Osmium is the only one of these metals, however, that has a stable gaseous oxide a t room temperature, namely OsO,(g). This is a strong indication that the effective temperature is low and that the formation of Os04(g) is the cause of the disappearance of osmium metal when it is exposed in the low earth orbit environment.
proved futile, since for both oxidation methods the second- and third-law heats did not agree and because the third-law heats showed a trend with temperature in both cases. This may be a result of the fact that the experiments were performed at 1100 K and higher, above the disproportionation temperature of OsOz(s), 923 K.25 We undertook this investigation to gain insight into the cause and mechanism for the rapid disappearance of osmium coatings when they are exposed to atomic oxygen in low earth orbit.' The principal difficulty encountered in making an assessment is relating the high impact velocity 0 atoms to a thermodynamic temperature. The kinetic energy of the 0 atoms with a relative velocity of 7.7 km/s corresponds to a temperature of 60000 K, whereas the temperature of the bulk surface is 250 K.2 Furthermore, we know that nonequilibrium conditions exist in the spacecraft environment, since the ambient atmosphere at an altitude of 250 km consists primarily of 0 atoms (-70%, with a pressure of approximately atm), while the 02(g) density is approximately 2 orders of magnitude contrary to equilibrium conditions. Nevertheless, we have determined a hypothetical abundance of the gaseous osmium oxides at several temperatures, assuming a thermodynamic equilibrium between Os(s) and O(g). We show in Table VI calculated partial pressures of the osmium oxides in equilibrium with atm O(g). Thermochemical data for the calculations were taken from Schick2' and corrected for the
Acknowledgment. We thank K. H. Lau and D. L. Hildenbrand for supplying a tantalum heater and for advice on accurate temperature measurement. J. A. Gardner provided helpful insights during the development of this work. We thank AFOSR for supporting this work under Task 230362.
+
(221 Jehn. H. J . Less-Common Me?. 1984. 100. 321. (23) See, for example: Luntz, A. C.; Williams, M. D.; Bethune, D. S. J . Chem. Phys. 1988,89,4381. (24) Nikd'skii, A. B.; Semenov, G. A.; Veshnyakova, V. N. Russ. J . Inorg. Chem. 1967, 12, 57 I . (25) Brewer, L. Chem. Reu. 1953, 52, 1. (26) Handbook of Geophysics and the Space Environment; Jursa, A. S., Ed.: ADA 167000: National Technical Information M a :Springfield, VA, 1985. (27) Schick, H. L. Thermodynamics of Certain Refracrory Compounds; Academic Press: New York, 1966; Vol. 2.
Appendix
Free energy functions, for use in third-law calculations, and
(HT- HZg8)values, for use in second-law calculations, were ob-
tained from the following sources: Oz(g) were taken from Selected Values of the Thermodynamic Properties of the Elements.28 OsO,(g) were calculated by using the symmetry number u = 12; the vibrational frequencies 965.2, 332.9,960.7, and 329.0 cm-' with degeneracies of 1, 2, 3, and 3, re~pectively;~ Z3 = 8.933 X (from B = 0.1349'); and an electronic ground state with energy 0 and degeneracy 1. Calculated values are shown in Table VII. Os03(g) were calculated using u = 6 (D3hsymmetry); the vibrational frequencies 564, 347, 1040, and 320 cm-I with degeneracies of l, l, 2, and 2, respectively [the vibrational frequencies are the JANAF Table (1985 Supplement l)I3 values for the molecule W03, which weighs only 2 amu less than OsO,]; Z3 = 5.587 X calculated by using an O s 4 bond length of 1.88 A, which is estimated to be 10%longer than the known length of the bond in OsO4'; and an electronic ground state with energy 0 and degeneracy 1 . The values calculated are very similar to previously calculated values.27 Calculated values are shown in Table VII. Registry No. Os03,12036-24-7; OsO,, 208 16- 12-0; oxygen, 778244-1. (28) Hultgren, R.; Desai, P. D.; Hawkins, D. T.; Gleiser, M.; Kelley, K.; Wagman, D. Selected Values of the Thermodynamic Properries of the Elements; American Society for Metals: Metals Park, OH, 1973.
