V. J. LYONS
11$2
Vol. 63
TABLE IV (Table V) and tetraethyllead (ref. 1). The heat THEMOLALENTROPY OF TETRAMETHYLLEAD IN CAL.D E G . - ~ of vaporization of tetraethyllead ~ z J 0 g 8 . 1 6 was estiAT 298.16"K. S.sta(liq) Vaporization, AHv/T S(idea1) - S( real) , estimated Compression, R In P
76.48 f 0.20 30.44 0.03 -6.47
So(ideal gas a t 1 atm.)
100.48 f 0.20
TABLE V THERMODYNAMIC PROPERTIES AT 298.16"K. 4C(c, graphite)
+ 6H2(g) + Pb(c)Liquid = (CHa)4Pb(liq or g) Gas
AHfo2sa.is, kcal. mole-' ASfo29&16,
AFfo29&16,
f23.5
cal. deg.-l mole-' kcal. mole-'
Log Kf298.16
- 131.7 +62.8 -46.0
$32.6 -107.7 $64.7 -47.4
differed from the reliable one found subsequently by the rotating-bomb method by 39 kcal. mole-l, but in the opposite direction from the difference found for tetramethyllead. The Pb-C Thermochemical Bond Energy and Bond Dissociation Energy.-The Pb-C thermochemical bond energy was calculated from the heat-of-formation values of both tetramethyllead
mated to be 13 kcal. mole-', the values of heat of atomization of elements were obtained from Circular 50011 and the other bond energies were assigned the values E(C-H) = 98.85 kcal. and E(C-C) = 83.1 kcal., given by Cass, et aZ.12 The values obtained for E(Pb-C) are 34.9 kcal. for tetramethyllead and 31.7 kcal. for tetraethyllead. The difference of 3.2 kcal. corresponds to 12.8 kcal. in the heat of atomization. Thus, the assumption of constant bond energies is a poor approximation for these compounds. The average Pb-C bond dissociation energy, one fourth of AH298.16 for the gas-phase reaction PbR4 = Pb 4R, was calculated by use of Field and Franklin'~'~ values of 31 and 24 kcal. mole-' for AHj of methyl and ethyl radicals, respectively. The values thus obtained were 34 kcal. for tetramethyllead and 32 kcal. for tetraethyllead, or approximately the same as the thermochemical bond energies calculated above.
+
.
(12) R. C. Cass, 8. E . Fletcher, C. T. Mortimer, H. D. Springalland T. R. White, J . Chem. Boc., 1406 (1958). (13) F. H. Field and J. L. Franklin, "Electron Impact Phenomena and the Properties of Gaseous Ions," Academic Press, Inc., New York, N. Y., 1957, p . 129.
THE DISSOCIATION PRESSURE OF ZnAsa BY V. J. LYONS International Business Machines Corporation, Research Laboratory, Poughkeepsie, NEWYork Received December 11 1068 ~
Two standard methods have been employed to measure the dissociation pressure of ZnAsz in the range 612 to 770". A dew-point method was used from 612 to 759" and triple-point measurements covered the range 756 to 770". The melting point of the compound was found to be 768 & 1" a t an arsenic pressure of 3.3 atmospheres. I n the temperature range 612 to 740" the data may be represented by the equation log P = - 12300/T 15.0. The heat of dissociation calculated from the slope of the equation is 18.8 kcal./mole.
+
Introduction The compound ZnAsz is a semi-conductor having an energy gap of approximately 0.92 e.v. at 300°K. During the course of investigation of various properties of the compound a degree of thermal instability, resulting from the dissociation of the compound, was observed. Since thermal dissociation introduces special problems relevant to the growth of single crystals and to the precision of subsequent physical measurements, a study was made of the dissociation reaction for the compound. Thus dissociation pressures were measured. The data presented herein cover the temperature range 600" to the melting point. Experimental The dissociation pressure of a solid compound may be defined as the sum of the vapor pressures of the dissociation products in equilibrium with the solid compound at a specified temperature.2 In the case where only one of the dissociation products is volatile, the pressure may be expressed in terms of the volatile product in equilibrium with the solid (1) W. J. Turner, A. S. Fischler. V. J. Lyons and W. E. Reese, "The Electrical and Optical Properties of ZnAsr." presented at The American Physical Society Meeting, Chicago, Nov. 29, 1958. (2) See for example, E. P. Egan, J. E. Potts and G . D. Potts, I n d . Eno. Chem., 58, 454 (1946).
compound. Dissociation of ZnAsz proceeds according to the reaction l/sAs4 ZnAsz = '/sZn3As2 Since the vapor pressure of arsenic is considerably higher than the vapor pressures of ZnaAs2and ZnAs2, the dissociation pressure is essentially equal to the arsenic pressure.4 Two different methods were used to measure the dissociation pressure of ZnAs2. In the temperature range 600 to 759" a "dew-point'' method was employed, while in the range 750" to the melting point, ZnAs, composition melting points were observed under various arsenic pressures thereby providing triple-point measurements. Overlapping of the temperature ranges covered by each method provided a means of checking the reliability of the methods. In the "dew-point" method, messurement of the arsenic pressure in a closed system can be accomplished by observa: , "dew-point."* tion of its condensation temperature, i . ~ its Dissociation of the compound in order to provide an arsenic atmosphere for the measurement will result in the formation of a layer of ZnaAsz on the sample surface. In order t o minimize the thickness of the ZnsAs2 layer, a large sample surface area to reaction tube volume ratio is necessary. Therefore, approximately 10 g. of ZnAsz was broken into small pieces (1-2 mm. in diameter) prior to the runs. The
+
(3) The compoiind ZnsAsz does not dissociate in the temperature ranee studied. (4) Arsenio pressures were obtaincd from two sources: (a) "International Critical Tables," 3b, 207; (b) R. E. Honiq, R . C. A . Reu., 18, No. 2, 195 (1957). (5) K. Weiser, THISJOURNAL, 61, 513 (1957).
b
DISSOCIATION PRESSURE OF ZINC ARSENIDE
July, 1.959
ZnAsz was synthesized at 775" by direct combination of Zn and As in a sealed uartz tube under an arsenic pressure of two atmospheres. single crystal of the compound was grown by directional freezing in a temperature gradient. The material was stoichiometric ZnAsp as shown by wet chemical analysis. The spectrographically pure monocrystalline ZnAsI, contained in a carbon-coated quartz boat, was placed a t one end of a quartz tube. The tube was then filled with HI to a pressure of 10 mm. rior to sealing off. Thermocouple wells were located at eacg end of the reaction tube for measuring decomposition and condensation temperatures. The reaction tube was placed in a furnace consisting of two contiguous windings of nichrome ribbon on a quartz tube providing separate heaters for decomposition and condensation. The furnace was enclosed in a second quartz tube such that the entire reaction tube was visible. After the two heaters were raised to the same temperature, the temperature of the condensation furnace ( Tz)was lowered in steps of 5' while the decomposition temperature ( T I )was maintained constant.6 A 10 minute equilibration time was allowed after each change in Tz. A short blast of air then was directed into the thermocouple well ( Tz)thus momentarily lowering the temperature 10-20" in the immediate area of the well. The air blast was used to ensure against errors which might occur due to supersaturation of the arsenic vapor. The temperature was lowered in this manner until the air blast caused arsenic crystals to appear on the well. Ta was then varied over a narrow range and the process was repeated until the condensation temperature was established through observation of the condensation and sublimation of the arsenic crystals. The decomposition temperature was then increased to a new value and equilibrated for 20-30 minutes prior to repeating the process. At a decomposition temperature of 612", the arsenic Condensation temperature could be determined with an accuracy of f10". At higher decomposition temperatures, however, the condensation temperatures were determined with 4-6' since the arsenic sublimed more rapidly at these temperatures. In the temperature range 730 to 760" there was a noticeable increase in the quantity of ZnsAs2formed a8 a product of the dissociation. Two problems were thus encountered. The first problem involved the gradual sublimation of ZnaAsz away from the sample surface and subsequent condensation of the compound in the area of the temperature gradient between the two furnaces. Therefore, the decomposition pressures could be recorded only in the order of increasing sample temperature since reduction of the sample temperature, after decomposition at an elevated temperature, would result in recombination of As with ZnaAsI at two different temperatures: (1) the sample temperature, and (2) a lower temperature in the gradient area. The second problem was encountered when the sample reached the ( Zn3AsI.ZnAsz) eutectic temperature (750') and B liquidus phase appeared on the sample surface. As the sample temperature was increased the liquidus phase increased in volume and eventually covered the entire surface. Since this condition could not be distinguished visually from a completely molten hase, samples of larger surface area were used for the higE temperature dissociation. This resulted in the observed presence of both solidoand liquid Further phases at the decomposition temperature of 759 dissociation at higher temperatures increased the possibility of the entire sample becoming a liquid solution of (ZnAsz. ZnsAs2)in which case the system becomes bivariant. In order to increase the accuracy of the measurements made near the melting point of the compound, a different method was employed wherein arsenic vapor was maintained in equilibrium with a ZnAs, composition at the melting point of the sample. The primary objectives in employing this method were to establish the melting point of the stoichiometric compound and to determine the equilibrium arsenic pressure at the melting point. The method employed an apparatus similar to that used by van den Boomgaard and Schol' in their study of the compounds GaAs, InAs and InP. The furnace was the same as that used in the dew-point measurements. A quantity of ZnAsz
1143
104
1
.
(6) Temperatures were measured with Pt-Pt, 10% Rh thermocouples which were subsequently calibrated with an N.B.S. standardized PtrPt, 10% Rh thermocouple. (7) Boomgaard and Sohol, Philips Res. Rpts., 12, 127 (1957).
>
103
i4 Ei 102 a;
I
10
1 60.97
1.0
1.05 1000/T, OK. Fig. 1.
1.10
1.13
was contained in a graphite boat and a large excess of arsenic was kept in a cylindrical reservoir at the opposite end of the reaction tube. The reaction tube was evacuated to 1 X 10-b mm. prior to sealing. After melting the ZnAsz under an arsenic pressure determined by its temperature, t,he melt was mainhined a t a temperature of -780" for periods of from two to six hours. This permitted the melt composition to adjust to the equilibrium composition re resented by the arsenic vapor pressure. The melt was &en frozen under the constant arsenic pressure. Accurate freezing points of the melt could not be recorded due to considerable supercooling of the melt. (Supercooling occurred over the entire temperature range covered by the experiment regardless of the melt composition.) The sample then was heated a t the constant rate of 1°/min. The latent heat of fusion was thus observed as a plrtteau in the curve of sample temperature va. heating time. Sample melting could be observed visually. Melting points of various compositions were measured under arsenic pressures of from 1.5 to 3.6 atmospheres. The melting points were recorded first in the order of increasing arsenic pressures and then with decreasing arsenic pressures. This provided a check on the established equilibrium of the system. There was no variation between melting points taken with increasing or decreasing arsenic pressure. A maximum in the melting point curve occurred at an arsenic pressure of 3.3 atmospheres. Since at this point arsenic was in equilibrium with both solid and liquid ZnAs2, the pressure represents the dissociation pressure of ZnAsz at its melting point. The melting point of the compound was found to be 768 f 1'. The previously reported melting point was 771 O .s
Discussion A plot of the experimental data is shown in Fig. 1. In the temperature range 600 t o 740°, the data may be represented as a straight line and fitted to the equation log P =
-l2? + 15.0
(1)
(8) M. Hansen, "Constitution of Binary Alloys." second edition, McGraw-Hill Book Go., Inc., New York, N. Y . , 1958, pp. 185-186.
G. PARRAVANO, H. G. FRIEDRICK AND M. BOUDART
1144
where P is pressure in millimeters of mercury and T is absolute temperature. The equilibrium constant for the dissociation reaction ZnAsz
51 ZnaAaz + 31 Asa
(2)
may be expressed as the arsenic pressure only since the vapor pressures of ZnaAsz and ZnAsz are negligible in the temperature range studied. The assumption of only As4molecules in the vapor state is based on a calculation of the fraction of As4 molecules dissociated using data tabulated by Stull and Sinke.e I n the temperature range 730 to 830” the fraction of As4 molecules dissociated to form As2 molecules is approximately 1 X hence the contribution due to As2 molecules may be considered negligible. The equilibrium constant is then K,’ = P‘/aAB4
(3)
Vol. 63
TABLE I TABULATION OF EXPERIMENTAL DATA Dew Point Measurements ZnAsa, temp., OC.
As condensation temp., OC.
pressure, mm.
612 655 695 720 759
420-440 498-507 535-541 569-573 619-623
6.03-11.5 62.2-79.0 160.7-199.8 357.1-390.7 1035.2-1122.0
AS
Triple-point Measurements ZnAsz
%(e:,
A8 temp., OC.
pressure, mm.
756.9 754.9 758.7 757.4 764.9 763.7 763.4 764.4 765.5 766.9 766.9 766.9 768.5 767.8 766.1 765.7
623.0 624.0 632.9 633.8 646.3 647.5 647.9 655.0 655.0 660.7 661.0 663.4 665.4 665.6 666.4 666.5
1122 1143 1361 1387 1762 1803 1815 2075 2075 2307 2317 242 1 2512 2512 2559 2559
AS
in the temperature range described for equation 1. I n the temperature range 750” to the compound melting point, the experimental data show an increase in the slope of the dissociation pressure curve. Since the Zn-As phase diagram indicates a degree of solubility of arsenic in liquid ZnAsz, a positive slope would be expected for arsenicrich compositions near the ZnAsz stoichiometry and, therefore, an infinite slope would occur at the ZnAsz melting point. Hence, an increase in the slope of the curve below the compound melting point, as suggested by the experimental data, is not a1together unexpected in this system. No attempt P us. 1/T using the straight line approximationlo was made to determine the equilibrium constant AH near the melting point because of insufficient data I n P = - RT --- + C on the reactions involved. The over-all heat for the reaction corresponding is 18.8 kcal./mole of solid ZnAsz. to equation 2 as calculated from the slope of log Acknowledgments.-The author wishes to ex(9) I). R. Stull and G. C. Sinke, “Thermodynamic Properties of press his thanks to Dr. G. A. Silvey and Dr. K. the Elements,” Advanoes in Chemistry Series, No. 18, 1956. Weiser for their valuable suggestions and discus(IO) K . Denbigh, “Principles of Chemical Equilibrium,” Cambridge sions. University Press, 1965, Ch. 6.
THE SLOW STEP IN CHEMISORPTION. THE POSSIBLE ROLE OF THE SOLID ADSORBENT. I1 BY G. PARRAVANO,’ H. G. FRIEDRICK AND M. BOUDART Department of Chemical Engineering, University of Notre Dame, Notre Dame, Indiana Received December fd,1968
The rates of adsorption of hydrogen and deuterium on a variety of zinc oxide samples are identical over a wide temperature range. It is concluded that the activated complex does not involve the adaorbed species. The slow step in this chemisorption process is associated with the thermal generation of active centers.
Introduction In a previous paper under the same title,Z the phenomenon of slow chemisorption was critically analyzed in the light of data existing at that time. It was recalled that the commolily accepted mechanism of “activated” adsorption was not acceptable in certain cases. This mechanism assumes that (1) Department of Chemical and Metallurgical Engineering, University of Michigan, Ann Arbor, Michigan. (2) M. Boudart and H. TayloF, L. Farkas Memorial Volume, Re@?archCouncil of Igrael, Jerq$alem, 1952, p. 228,
only a fraction of the molecules hitting vacant surface sites will be adsorhed, namely, the fraction Possessing an excess elleWY E in translational motion. Such a simple picture of the activation energy E for adsorption leads to serious difficulties. In particular, it provides no explanation for the fact that rates of slow chemisorption of hydrogen and deuterium should sometimes be identical, a striking experimental fact discovered at princeton by by ’ace and Tay10r3 and ‘Onfirmed (3) J. Pace and H, S. Taylor, J . Chew. Phys., 2, 578 (1934).
.
C
