CATER,LEE, JOHNSON, RAUH,AND EICK
2684
Vaporization, Thermodynamics, and Dissociation Energy of Lanthanum Monosulfide'
by E. David Cater, Thomas E. Lee, Ernest W. Johnson, Department of Chemistry, University of Iowa, Iowa City, Iowa
Everett G. Rauh, Argonne National Laboratory, Argonns, Illinois
and Harry A. Eick Department of Chemistry, Michigan State University, East Lansing, Michigan
(Received March 1, 1966)
The congruent vaporization of lanthanum monosulfide has been studied over the temperature range 2012 to 2490OK. by the effusion technique with the aid of vacuum balance and mass spectrometer. The principal vapor species is LaS, and calculation shows that 3 to 10% of the vapor is La S. The vapor pressure of solid LaS is given by log Patm= 7.365 - 2.873 X 104/T. The heat and entropy of sublimation a t 2240OK. are A H 0 2 2 d 0 = 131.5 f 1.0 kcal./mole and A S o n 4 0 = 33.5 h 0.6 e.u. Estimated thermal data yield A H O B ~= 141.4 f 2.0 and AHoo = 141.7 f 2.0 kcal./mole. The dissociation energy of gaseous LaS is 137 d= 6 kcal./mole or 5.9 0.2 e.v. The quoted uncertainty for is the statistical standard deviation; the other uncertainties are estimated limits of accuracy, and the values apply if the ground state of LaS(g) is 42. The lattice parameter of LaS a t room temperature is 5.854 0.002 A. The melting point of LaS is a t least as high as2327 f 20'.
+
*
*
Introduction Recent work on the dissociation energies of gaseous oxides, particularly of metals, has resulted in several summary paper^^-^ in which the periodic behavior and bonding have been discussed. It is of interest to obtain quantitative data on dissociation energies of sulfide molecules for comparison with the oxides to aid in the understanding of the bonding. Lanthanum monoxide, one of the most stable gaseous monoxides, has a dissociation energy of 8.3 e . ~ .Thus, ~ by analogy one expects a reasonable stability for a gaseous lanthanum monosulfide molecule. This paper combines the results of two independent investigations of the sublimation of lanthanum monosulfide carried out a t the University of Iowa (UI) and Argonne National Laboratory (ANL) to obtain the dissociation energy of gaseous LaS as well as to characterize the thermodynamics of sublimation of solid LaS. The JOUTTUZ~ of Physical chemistry
Materials Previous studies on and the means of preparation of the solid phases in the lanthanum-sulfur system are summarized by Flahaut6 and by Samsonov and Radzikovskaya.' The known solid phases are LaS, (1) Based in part on work performed under the auspices of the U. S. Atomic Energy Commission. Taken in part from the M.S. Thesis of Thomas E. Lee and the Ph.D. Thesis of Ernest W. Johnson, University of Iowa, 1964. (2) R. $ Ackermann and R. J. Thorn, in "High Temperature Technology, proceedings of a symposium, Stanford Research Institute, 1963. (3) M. S. Chandrasekharaiah, J . Phys. Chem., 68, 2020 (1964). (4) R. J. Ackermann and R. J. Thorn, in Progr. Ceram. Sei., 1, 39 (1961). (5) R.J. Ackermann, E. G. Rauh, and R. J. Thorn, J . Chem. Phys., 40,883 (1964). (6) J. Flahaut, Bull. SOC. chim. Prance, 1282 (1960). (7) G. V. Samsonov and S. V. Radzikovskaya, Russ. Chem. Rar., 30, 28 (1961).
VAPORIZATION, THERMODYNAMICS, AND DISSOCIATION ENERGY OF LAS
La3S4, La2S3, LaS2, and an oxysulfide, La20zS. The phases La3& and La2S3are probably limiting compositions in a single-phase region.* The monosulfide, LaS, is a refractory, gold-colored, metallic-looking solid with the NaC1-type crystal structure. We have prepared monosulfide samples by a variety of techniques: (1) direct union of stoichiometric amounts of the elements in an initially evacuated fused-silica tube; (2) reaction of excess H2S with finely divided metal (produced by decomposition of the hydride under vacuum) to give the yellow sesquisulfide, followed by reduction to LaS with La203 La a t high temperatures; (3) reaction of H2S with La203 to produce the oxysulfide or sesquisulfide, followed by reduction to the monosulfide with carbon or lanthanum metal. I n all cases the product was homogenized by annealing under high vacuum in tungsten or molybdenum crucibles inductively heated to temperatures from 1800 to 2100O. After the samples had been heated for several hours a t elevated temperatures, the background pressure dropped below torr, and the heating was discontinued. Debye-Scherrer patterns of the materials were taken with Norelco cameras of 11.4-em. diamet2r and copper radiation of wave length Kal 1.54050 A. Only from samples heated to high temperatures were lines of LaS obtained. Those samples chosen for vaporization studies had patterns containing only lines of LaS or LaS with a barely detectable second phase, La3S4. Lattice parameters of LaS samples annealed for long periods under high vacuum, and residues from actual vapor pressure studies were constant within the precision of omeasurement. For example, values of 5.852 to 5.857 A. and 5.854 to 5.855 8.were obtained, respectively, from samples coexisting with La& and from residues from vaporization measurements which contained only LaS. Presumably, there is essentially no solid solubility of sulfur in LaS. Our "best value" for the lattice parameter is 5.854 i 0.002 A., compared to previously reported values of 5.788, 5.860165.842,'"& and 5.84.1°b For Lassl our values of 8.722 It 0.005 and 8.717 f 0.002 A. may be compared with the previously reported value of 8.730 A.ll Diffraction lines of oxide phases were not obtained from samples used in the vaporization studies. The methods of preparation and the purity of the starting materials precluded contamination of the samples with measurable impurities other than oxygen. During both weight loss and mass spectrometric effusion studies, once a small percentage of the sample had been vaporized so that oxygen contamination was eliminated (see below), it was found that the vaporization behavior became univariant and reproducible with temperature until the sample was essen-
+
2685
tially completely vaporized. LaS remained the predominant vapor species. We conclude that the sublimation of lanthanum monosulfide is congruent and assume that any deviations from stoichiometry during the measurements a t high temperatures may be neglected in calculating the thermodynamic properties reported herein. To determine the melting point, samples of LaS were inductively heated under vacuum a t successively higher temperatures in a sintered tungsten effusion cell, and the residues were observed after each heating. After heatings a t 2220 and 2300' no indication of melting was found. On being heated to 2327 i 20' for 4 min., a 53-mg. sample lost 17 mg. by effusion, 10 mg. was sublimed to the underside of the lid, and 26 mg. disappeared by diffusion into the bottom of the cell, perhaps as a liquid since such diffusion had not occurred a t lower temperatures. The melting point of LaS is thus a t least 2327 i 20°, but may be higher. Flahaut reports6 that a sample of LaS did not melt at 2200'. Another reported melting pointe of 1970' may have been low because of eutectic formation.
Vacuum Balance Measurements Absolute rate of effusion measurements were performed a t UI. An Ainsworth recording vacuum balance Type RV-AU-2 was evacuated through a liquid nitrogen trap by an oil diffusion pump using silicone oil. The effusion cell was suspended from one balance arm by two tungsten wires, 0.1-em. diameter and approximately 84-em. length, into a water-jacketed, fused-silica condenser which had a sighting window and prism a t its bottom. A 1 X 2-em. glass plate, rotatable by means of a ball and socket joint, was located between the suspension wires about 23 em. above the effusion cell to prevent swinging of the cell during heating. The effusion cells were of sintered tungsten12of approximately 80% of theoretical density; their design was essentially that of Ackermann and Rauh.I3 The orifices were of 0.125-em. nominal diameter and of 0.125-em. nominal depth. A traveling microscope was used to measure actual orifice diameters. The values were corrected for thermal expansion for calculation of vapor pressures. A radiation shield of strips of tantalum, 1 em. wide by 10 em. long, (8) W. H. Zachariasen, Acta Cryst., 2, 57 (1949). (9) M. Picon and M. Patrie, Compt. rend., 242, 1321 (1956). (10) As cited in ref.2: (a) A. Iandelli, Guzz. chim. ital., 85,881 (1955); (b) N. P. Zvereva, Dok2. Akad. Nuuk SSSR, 113,333 (1957). (11) M.Picon and J. Flahaut, Compt. rend., 243, 2074 (1956). (12) Obtained from Philips Metalonics. (13) R. J. Aokermann and E. G . Rauh, J. Chem. Phys., 36, 448 (1962).
Volume 69,Number 8 August 1966
CATER,LEE, JOHNSON, RAuH,AND EICK
2686
surrounded the cell. Temperatures were measured by means of a Leeds and Northrup disappearingfilament optical pyrometer that had been calibrated by intercomparison with a pyrometer which had been calibrated a t ANL by the method of rotating sectors.14 During weight loss measurements the pyrometer was sighted into a blackbody hole (0.125 cm. in diameter and 0.70 cm. deep) in the bottom of the effusion cell. An intercomparison of temperatures measured in orifice and blackbody hole was made, and the reported experimental temperatures correspond to orifice temperatures. All temperatures were corrected for transmissivity of window and prism. The effusion cell was heated inductively by power generated in a coil of flattened copper tubing by a Ther-monic Model 2500 generator. Background pressure, measured by a cold cathode ionization gauge, was less than 5 X torr during all effusion measurements. Thirty weight loss measurements were made in the temperature range 2066 to 2348OK. Apparent weight losses ranged from 0.03 to 9.43 mg., and heating times were from 30 to 370 min. In all cases the suspension of the previously weighed effusion cell was clamped to preclude swinging, and then power was applied to heat the cell as rapidly as possible to the desired temperature. The temperature was maintained for the desired time (and measured at intervals during this time) after which the power was shut off. The effusion cell suspension was then undamped and the weight recorded for 3 to 15 min. until constant weight was obtained. Continuous weighing during heating was not possible because of the forces exerted by the high-frequency field on the effusion cell. Occasional manual power adjustment maintained the temperature constant within Ago. The time required for the cell to reach temperature (less than 1 min.) was always short relative to the time a t temperature. Data were taken in several series of increasing and decreasing temperatures. The effusion cell was maintained under vacuum a t all times, except that more sample was added after the third weight loss measurement. The 12 measurements with weight losses greater than 1.9 mg. were arbitrarily retained for calculation of vapor pressures, and these data are recorded in Table I and plotted in Figure 1. These points were all among the last 18 taken. Analysis of the data from all 30 weight loss measurements showed a very large scatter around a straight line in a plot of log p us. 1/T, particularly among points of least weight loss. However, no trend due to length of heating time, order of temperature, or total elapsed weight loss was apparent. It was felt that static charges on the cell were responsible for the scatter in the data, which for weight losses The Journal of Physical Chemwtry
Table I : Data from Vacuum Balance Experiments” Third-law
Wt. Expt. no.
Time, min.
T , OK.
19 21 23 25 26 27 29 32 33 34 35 36
30 45 60 240 30 60 195 370 120 120 90 90
2297 2299 2289 2243 2348 2321 2166 2143 2210 2261 2289 2347
a
loss, mg.
AHOo,
kcal./ mole
Log 107~. 10’/T, atm. ‘K.-1
2.00 1.929 4.354 2.85 1.907 4.350 2.90 1.789 4.369 5.64 1.469 4.458 3.02 2.113 4.259 2.70 4.308 1.761 1.95 1.093 4.617 5.60 1.271 4.666 2.47 1.411 4.525 4.52 1.678 4.423 4.20 1.774 4.369 9.43 2.130 4.261 Av. (no. 27 and 32 omitted)
141.0 141.3 142.0 142.6 141.9 144.1 141.8 138.7 141.3 141.1 142.1 141.6 141.7
Orifice area = 0.01413 cmS2at temperature.
between 0.3 and 1.0 mg. corresponded to up to *65% of the pressure. Thus, the arbitrary 1.9-mg. cutoff was made. For calculation of pressures it was assumed that the vaporization occurred entirely according to the process LaS(s) = LaS(g)
(1)
Pressures in Table I were calculated from the Knudsen equation l5
in which w is the weight loss in grams, a the orifice area in cm.2, t the time in seconds, T the temperature in degrees Kelvin, and M the molecular weight of LaS, 170.95; K , = 0.8013 is the so-called Clausing factor’6 for an orifice of length equal to its diameter; K , = 1.048 is a correction6Ja applied to account for vapor condensing on the suspension wires. An unweighted least-squares treatment of log P vs. 1/T yields, after rejection of points 27 and 32 which had excessive residuals log P a t m
(7.47
f
0.65) - (28960
* 1480)/T
(3)
The quoted uncertainties are statistical standard deviations. This equation is of the form ~~
(14) See, for example, R. J. Thorn and G . H. Winslow, “Recent Developments in Optical Pyrometry,” paper 63-WA-224 presented before the Annual Meeting of the American Society of Mechanical Engineers, Nov. 17-23, 1963. (15) See, for example, the discussion of S. Dushman and J. M. Lafferty in “Scientific Foundations of Vacuum Technique,” 2nd Ed., John Wiley and Sons, Inc., New York, N. Y., 1962.
VAPORIZATION, THERMODYNAMICS, AND DISSOCIATION ENERGY OF LAS
AH" 1 ~-
AS" --
10gp =
2.303R
2.303R
(4)
T
The slope and intercept yield for the sublimation of LaS a t the temperature 2240OK. in the middle of the = 132.5 d= 6.8 kcal./mole and ASOZNO= range 34.17 i 2.97 e.u.
OK
2500 3.0
2400 '
1
2300 '
2200
1
'
2100
2'
I
1
I
NORMALIZATION POINT 2240°K
2.5
89;
'2.0
Log Po+m=(-2.873f0.021) IO4/ T +(7.365 f 0.094)
a" E
*0
1.5
0
s
1 .a
0 VACUUM BALANCE (TOTAL
WEIGHT LOSS) OAX
0.5
NORMALIZED MASS SPECTROMETRIC DATA 70 e.v. 50 e.v. x 30 e.v. (4 SERIES)
A
4
I
1
1
I
I
I
I
I
104/T
Figure 1. Temperature dependence of vapor pressure of LaS from vacuum balance (total weight loss) and mass spectrometric measurements. Mass spectrometric data normalized at 224OOK.
Mass Spectrometric Studies The vaporization of LaS from tungsten cells was studied mass spectrometrically both at ANL and UI. I n each case the spectrometer employed was a Bendix Model 12-101 time-of-flight instrument operated in the pulsed mode. At UI a standard Bendix Knudsen cell assembly was used; a t ANL, the same cell as has been described previo~s1y.l~I n both studies it was found that the principal ion currents observed were due to Lao+, LaS+, La+, and S+. No polymers of these
2687
species were detected. The Lao+, due to vaporization of LaO(g) from samples containing slight oxygen contamination, eventually disappeared below the limit of detectability. Rough appearance potential curves obtained at UI are shown in Figure 2. The energy scale was determined by equating the appearance potential of the ion a t mass 28 to the ionization potential of Nz, 15.6 v.lS (The fragmentation behavior indicated the peak to be predominantly Nz rather than CO.) The appearance potentials of Laof and LaS+, approximately 5 v., indicate these to be primary ions. The appearance potential of La+, about 12 v., shows it to be predominantly a fragment since the fist ionization potential of La is 5.6 v.I6 At ANL, measurements were made of ion currents of La+, LaS+, and S+ as a function of temperature at electron energies from 20 to 70 v. Temperatures were measured by sighting directly into the orifice of the effusion cell with a Leeds and Northrup optical pyrometer which had been calibrated by the method of rotating sectors.14 The general apparatus and procedure were essentially the same as described by Ackermann and Rauh.'3 Four sets of data were obtained from two different samples of LaS at temperatures from 2012 to 2490°K. Slopes were calculated by leastsquares treatment of log I + T us. 1/T. Since the product of ion current and temperature (I+T)is proportional to the partial pressure of a species, the slopes are presented in Table I1 as heats of vaporization in units of kcal./ mole. It is seen that slopes for La+ and S+ are the same, within the experimental uncertainty, as those for LaSf at various energies. Furthermore, all are in agreement with the second-law heat of vaporization of LaS obtained from the vacuum balance. This indicates again that LaS+ is a primary ion and La+ and S+ are predominantly fragments. In the final section of the paper calculations are presented which show that the partial pressures of La and S in equilibrium with LaS at 2240°K. are less by a factor of 10 to 20 than that of LaS. Several other sets of data were obtained but are not reproduced here because the results were identical, within the experimental errors, with those in Table 11. For use in the final calculations of thermodynamic properties of LaS we have selected all data taken on LaS+ at 30-v. energy and for good measure have included the two sets of data on LaS+, respectively, at 70 and 50 g.
(16) F. H. Field and J. L. Franklin,"Electron Impact Phenomena," Academic Press Inc., New York, N. Y . , 1957.
Volume 69, Number 8
August 1966
CATER, LEE, JOHNSON, RAUH,AND EICK
2688
'wLKi 5
value of log I f T in a given set to convert from log I + T to log P L ~ s . In Figure 1 the vacuum balance and mass spectrometric pressures so calculated are plotted as log P us. 1 / T . The final second-law heat and entropy of sublimation are obtained from the slope and intercept of the least-squares line through all of these points
1
E
log Patrn= (7.365
2 4
z
f
0.094) - (28,730
* 210)/T
(5)
The heat of sublimation at temperature is thus AHozZ40 131.5 i 1.0 kcal./mole, where the uncertainty is the statistical standard deviation. Converted to room temperature and zero degrees by the enthalpy data from Table 111, estimated as described below, this becomes AHoz9s = 141.4 f 2.0 and AHoo = 141.7 f 2.0 kcal./mole, where the quoted uncertainties are estimated. =
i: 23 :
IO
OO-
20
30
ELECTRON ENERGY (VOLTS)
Table lI1: Estimated Thermodynamic Quantities
Figure 2. Appearance potential measurements on La0 +, LaS+, and La+ from oxygen-contaminated LaS. Nz+ measurements included to establish energy scale.
-LaS(g)-
Table 11: Mass Spectrometric Data on LaS+, La+, S+ Expt. no.
LaS-7 LaS-8 LaS-9
Species
LaS+ LaS+ LaS+ LaS+ LaS+ La+
S+ LaS-10
LaS+ La+
S+
Energy, Slope (as A H O F ) , v. kcal./mole
30 30 30 50 70 30 30 30 30 30
Av. for all LaS +
131.3 f 2 . 1 129.4 f 1 . 5 131.7 f 2 . 8 132.3 f 3 . 5 133.0 f 3 . 5 134.6 f 3 . 3 1 3 6 . 0 f 4.6 131.3 f 2 . 3 133.9 f 2 . 2 127.8 f 2 . 6 131.5
Temp. range, OK.
2155-2451 2147-2490 2 137-2450 2137-2450 2137-2450 2137-2450 2224-2450 2012-2321 2012-2321 2012-2321
No. of points
7 6 9 9 9 9 7 15 15 15
Thermodynamics of LaS Second-Law Treatment. It is seen that the secondlaw heats determined by vacuum balance and mass spectrometer are in agreement with each other. In order to get "best" values for the heat and entropy of sublimation, the mass spectrometric values of I+T for LaS+ were converted to pressures, and a leastsquares treatment of these values combined with the vacuum balance points was made. For this purpose, a constant was added to the intercept of the leastsquares line for log I+T vs. 1 / T for each set of data on LaS+ so that at 2240'K. log I f T = log PL&S obtained from eq. 3. This same constant was added to each The Journal of Physical Chemislry
T,OK.
oal./mole
deg. mole
-LaS(s)- H'o, cal./mole
2100 2200 2240 2300 2400 298.15
18,120 19,010 19,370 19,900 20,800 2,219
78.71 79.13 79.29 79.52 79.91 61.66
27,590 29,000 29,550 30,380 31,780 2,550
f290
f1.06 f3.05
it870
HOT
Estimated error a t 2240 OK.a
- H'o,
S O T , cal./
H O T
S O T , oal./
deg. mole
44.65 45.29 45.54 45.91 46.50 17.50 12.2
a Error estimated for &20% in re, 2 ~ 5 0 % in we, f l e.u. in Cp(LaS,s), f l e.u. in So2ss(LaS,s),f300 cal. in ( H ' z ~ B- H"o) (La&). This uncertainty includes the difference between 4 x and 2 2 or 4r ground states.
'
The entropy of sublimation is obtained from the intercept of eq. 5 after application of a small correction. The normalization of log P was made under the assumption that only molecules existed in the vapor. The value from the intercept, 33.70 f 0.43 e.u., must be decreased by 0.18 e.u. to correct for the presence (see below) of atoms in the vapor. To the standard deviation of the intercept we add an additional uncertainty of f 0 . 3 e.u. to account for uncertainties in the experimental pressure determination and the calculation of the proportion of atoms in the vapor. Thus, the entropy of sublimation of LaS is AS02240 = 33.5 rfr: 0.6 ex. Third-Law Treatment. A so-called third-law treatment of the vacuum balance data was performed for
VAPORIZATION, THERMODYNAMICS, AND DISSOCUTION ENERGY OF LAS
comparison with the second-law heat and entropy. Entropies and enthalpy functions calculated for LaS(s) and (g) are listed in Table I11 along with estimated limits of accuracy. The third-law value of Moo for the vaporization is shown in the last column of Table I for each experimental temperature. Molecular parameters chosen for LaS(g) were re = 2.38 8. (20% less than the La-S distance in the solid) and ue = 490 cm.-‘ (60% of the value for LaO).I7 The ground state of the LaS molecule was taken to be 41;, the same as deduced by Akerlindl’ for Lao. The entropy of solid LaS a t 298OK. was estimated to be 17.5 e.u. by the method of Grqinvold and Westrumls from the magnetic susceptibilityg of 281 X c.g.s. unit. The enthalpy (H0298 - Hoo) for solid LaS was estimated to be 2550 cal./mole by comparison with the values 2590 for CeSg and 2667 for US.2o These latter are metallic, refractory compounds, isomorphous with and having physical properties similar to those of LaS. For the extrapolation from 298OK. to the experimental temperatures an average heat capacity of 13.9 caI./ mole deg., 2.0 cal./mole deg. higher than the classical 6R, is reasonable by comparison with the heat capacities of all oxides and sulfides of type AB tabulated by Kelley.21 The third-law treatment gives for sublimation of LaS AHoo = 141.7 f 7.6 kcal./mole. There is no trend in Noo with temperature of measurement. From log PLaS a t 2240OK. as calculated from eq. 3 or 5 and the estimated entropy, 33.75 e.u. a t 2240°K., one obtains AHon4,, = 132.1 f 6 kcal./mole. These errors are estimated as indicated in Table 111. The third-law values are in excellent agreement with the second-law values M O O = 141.8 f 2.0, m 0 2 2 4 0 = 131.5 f 1.0 kcal./mole, and ASOZUO= 33.5 f 0.6 e.u. The accord between second- and third-law treatments may be somewhat fortuitous in view of the large uncertainties in the third-law values. The second-law values would seem inherently trustworthy, having been derived from two techniques, using several samples, a t two laboratories, by two sets of workers. Alternatively, the excellent agreement may be taken as corroborating the choice of electronic ground state, in which case the uncertainty in the third-law values is overestimated,
Dissociation Energy of LaS It was originally hoped that the heat of dissociation of LaS might be directly determined from the (hoped for) observable equilibrium
LaS(g) = L 4 g )
+ S(g)
(6)
One could then calculate the heat of formation of LaS(s)
2689
and (g) directly. Since the elemental ions appeared to be almost entirely fragments in these studies, we require a heat of formation of LaS(s) to obtain the dissociation energy. It is not available. A reasonable estimate of the heat of formation of LaS(s) from metal and rhombic sulfur is AHot,29s= -113 f 5 kcal./ mole if one compares the known heats of formation of Cess4,Ce2S3,and CeS with those of La2S3 and La3S4.22 From the thermal data for sulfur,23for La(g),24Ackermann and Rauh’s value of 100.0 kcal./mole for the heat of vaporization of La a t 2980,13the estimated heat of formation and enthalpy functions for LaS(s), and the presently determined heat of vaporization of LaS, one obtains the dissociation energy Do(LaS) = 137 f 5 kcal./mole or 6.0 f 0.2 e.v. The molecule LaS is thus less stable than the molecule La0 by the amount Do(LaS)/Do(LaO)= 0.72. From the free energy of vaporization a t 2240OK. and appropriate other values cited above, one obtains for reaction 6 at 2240°, log K = 1.60 X lov8. Since S 3.5 X atm. at 2240°, it from eq. 3 or 5 P L ~ = = 0.07. Befollows that in a closed system PLJPL~S cause of the differences in molecular weight of the species, in an effusioncell P L a / P L a S = 0.098. The observation that I + L was ~ due almost entirely to fragmentation is thus corroborated.
Acknowledgment. This work was supported by the United States Atomic Energy Commission a t the University of Iowa under Contract No. AT(l1-1)-1182. T. E. L. and E. W. J. wish to thank the Commission for research assistantships, and H. A. E. and E. D. C. express their appreciation for temporary appointments as Research Associates a t Argonne National Laboratory. The Lunex Co., Pleasant Valley, Iowa, kindly provided some of €he samples of lanthanum. (17) (a) L. Akerlind, Arkiv Fgsik, 22, 41, 65 (1962); (b) U. Uhler and L. Akerlind, %%id., 19, 1 (1961). (18) F. Grpvold and E. F. Westrum, Jr., Inorg. Chem., 1, 36 (1962). (19) E. G. King and W. W. Weller, U. S. Department of the Interior, Bureau of Mines, Report of Investigations, No. 5485, 1959. W e have performed a graphical integration of their data to get H O z s s HOO = 2590 cal./mole. (20) E. F. Westrum, Jr., and R. R. Walters, reported by E. F. Westrum, Jr., and F. Gr@nvold in “Thermodynamics of Nuclear Materials,” International Atomic Energy Agency, Vienna, 1962, pp. 24-32. (21) K. K. Kelley, U. S. Bureau of Mines Bulletin 584, U. S. Government Printing Office, Washington, D. C., 1960. (22) R. L. Montgomery, U. S.Department of the Interior, Bureau of Mines, Report of Investigations, NO. 5468, 1959, has recalculated the heats of fobnation of cerium and lanthanum sulfides on the basis of more recent heats of solution of cerium and lanthanum. (23) G. N. Lewis and M. Randall, “Thermodynamics,” 2nd Ed., revised by K. Piteer and L. Brewer, McGraw-Hill Book Co., Inc., New York, N. Y., 1961, pp. 671-673. (24) D. R. Stull and G. C. Sinke, “Thermodynamic Properties of the Elements,” American Chemical Society, Washington, D. C., 1956.
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Volume 69, Number 8 August 1966
