THEVAPORIZATION OF YTTERBIUM DICARBIDE
1697
The Vaporization of Ytterbium Dicarbide John M. Haschke and Harry A. Eick Department of Chemistry, Michigan State University, East Lansing, Michigan 48828
(Received November 3, 1967)
Samples of ytterbium dicarbide and ytterbium sesquicarbide have been prepared by direct reaction of the elements in sealed tantalum bombs. The vaporization of the dicarbide according to the reaction YbCZ(s) + Yb(g) 2C(s) has been studied by the target collection Knudsen effusion technique over the temperature range 1100-1550°K. An X-ray fluorescence method for analysis of the condensed effusate has been developed. Second- and third-law results are reported, and the A H f o z s sof YbCz(s)has been determined to be -18.0 1.0 kcal/gfw.
+
*
Introduction
Experimental Section
Current interest in the vaporization behavior and thermodynamics of lanthanide and alkaline earth dicarbides is evidenced by the number of investigations recently reported. Aside from recent descriptions of the samarium,' gadolinium,2 and cerium3 dicarbide syslems, extensive reviews of the thermodynamics of both the alkaline earth4 and lanthanides dicarbides have appeared in conjunction with newly obtained results. The vaporization behavior of lanthanide dicarbides appears to fall into two classes-one having only metal vapor in equilibrium with the condensed phase and the other having both the metal vapor and the gaseous metal dicarbide as equilibrium species. The lanthanide metals which commonly exhibit the trivalent oxidation state always exhibit dicarbide vapor species. Together with the alkaline earth dicarbides, both europiuma and samarium' dicarbides exhibit only gaseous metal as significant equilibrium vapor species and consequently vaporize according to the reaction
Preparative Methods. Ytterbium dicarbide was prepared by an adaptation of the technique employed by Spedding, et al.8 Samples were prepared from mixtures of ytterbium metal (Research Chemicals, Inc., Phoenix, Ariz.; 99.9%) and C P graphite (Fisher Scientific Co., Pittsburgh, Pa.) which had been outgassed previously under vacuum at 1800". The reactants were sealed by heliarc welding into previously outgassed tantalum bombs made from 6.35- or 9.52-mm seamless tantalum tubing. Samples were prepared with stoichiometries of YbC,, where 1.5 I x I 2.2. To effect reaction, the bombs were heated by induction for 3-6 hr at 1300-1500" for compositions wherez 2 2 , and at 1000-1300" for those where 1.5 5 x 5 2 . Since helium sealed into the bombs promoted rupture and subsequent loss of metal vapor, samples were packed tightly into the tubes and the remaining volume was eliminated by crimping the tube flat prior to sealing. Analytical Methods. The products were analyzed chemically for metal content by ignition of the oxalate to the sesquioxideg and for unbound graphite which precipitated when the samples were acidified with dilute HCl. X-Ray powder diffraction patterns of the polycrystalline phases were obtained with 114.59-mm Debye-Scherrer cameras using copper K a radiation (Xal 1.54050 -1). Samples were handled in a helium-
-UrC,(s)
----f
T\l(g)
+ 2C(s)
(1)
Europium and samarium, like the alkaline earth metals, exhibit an oxidation state of + 2 . With the anticipation that ytterbium, which is also divalent, would behave similarly, the vaporization of the dicarbide was studied by the target collection Knudsen effusion technique. I n an attempt to obtain better agreement between second- and third-law values of AH029Rfor reaction 1 and to find the source of a slight temperature trend in a preliminary third-law heat, a complete repetition of the measurements was undertaken and constitutes essentially a second independent determination. The analytical procedure used for determining the amount of effusate condensed, the sticking coefficient, the vapor pressure in equilibrium with the dicarbide on the metal-rich side of the YbCz.00 composition, and the procedure used in temperature measurement have, therefore, been examined carefully.
(1) H. A. Eick, J. -M. Haschke, and P. A. Pilato, IUPAC Thermodynamik Symposium, Heidelberg, Sept 1967. (2) C . L. Hoenig, N . D. Stout, and P. C. Nordine, J . Am. Ceram. Soc., 5 0 , 385 (1967).
(3) P. Winchel and N. L. Baldwin, General Atomics GA Project 109, John Hopkins Laboratory Report, San Diego, Calif., 1967. (4) J. Cuthbert, R. L. Faircloth, R. H. Flowers, and F. C. Pummery, Proc. Brit. Ceram. Soc., 8 , 155 (1967). (5) R. L. Faircloth, R. H. Flowers, and F. C. Pummery, United Kingdom Atomic Energy Authority Research Group Report AERE-R 5480, 1967. (6) R. E . Gebelt and H. A. Eick, J . Chem. Phys., 44, 2872 (1966).
(7) P. A. Pilato and H. A . Eick, 152nd National Xeeting of the American Chemical Society, New York, N. Y., Sept 1966. (8) F. H. Spedding, K. Gschneidner, and A. H. Daane, J . Am. Chem. Soc., 80, 4499 (1958).
(9) L. A. Sarver and P. H. Brinton, ibid., 49, 943 (1927).
Volume 72, Number 6
May 1968
JOHN I t . HASCHKE AND HARRY A. EICK
1698 filled glove box and stored under vacuum to prevent hydrolysis. The vaporization mode of ytterbium dicarbide was investigated mass spectrometrically with a Bendix time-of-flight mass spectrometer, Model 12-107. All species emanating from a molybdenum Knudsen cell heated by electron bombardment were subject to analysis with both a high (70 eV) and a low (10 eV) energy ionizing electron beam in the temperature range 1280-1485". Target Collection. The effusion apparatus used for the target collection work is essentially that described by Ackermann.I0 The Pyrex and Vycor system was evacuated to residual pressures of 10-~-10-6 torr. Aluminum targets, which previously had been cleaned with abrasive, washed, and subjected to background count with the X-ray fluorescence spectrometer, were stacked in a liquid nitrogen cooled holder and exposed to the effusate for measured time intervals. To minimize the effects of changes in the fluorescence spectrometer, all targets were accompanied in both background and postexposure counting with an unexposed control target. To ensure reproducible geometry in analysis, the fraction of effusate analyzed was not determined by a knife edge on the target holder but by a knife-edged insert placed inside the rim of the target during analysis. The effective perpendicular distance from orifice to target was obtained by adding the distance from crucible to target rim, as measured with a cathetometer, to the distance from the rim to the target face. Targets were exposed at both successively increasing and decreasing temperatures. Temperature measurements were made with an NBS-calibrated Leeds and Sorthrup disappearingfilament type of pyrometer by sighting via a prism and window into a blackbody hole drilled into the bottom of the Knudsen cell. After exposure of the targets, the holder was replaced by an optical window and the temperatures of the orifice and blackbody hole were checked over the temperature range. Temperatures were corrected for prism and window absorbance, which was measured with a standard lamp. Knudsen cells with knife-edged orifices of both high-density graphite and molybdenum were used in the vaporization experiments. Molybdenum cell orifice areas of 58.0 X 20.8 X and 7.06 x cm2 were determined both by planimeter measurements of micrographic photographs and by microscope measurements of the circular orifices viewed over an American Optical Co. micrometer slide calibrated in 0.01-mm divisions. The cylindrical cells (internal height = internal diameter = 0.795 cm) were charged with 0.3-0.4-g samples. One vaporization experiment with a metal-rich sample (Yb2Ca-YbCJ was conducted at a constant temperature of 928". Targets were exposed at seThe Journal of Phusical Chemistry
lected times to observe the behavior of the Yb pressure as the YbC2 stoichiometry was approached. Since an oxide coat on the aluminum targets might affect the sticking ability of ytterbium vapor, a series of experiments was conducted to check the sticking coefficient. Aluminum targets mounted in the vacuum system were coated with gold by vapor deposition from a Knudsen source at 1400". Targets with an average coating of 8 mg of gold were obtained and exposed to the dicarbide effusate as described previously. The composition study described above and this sticking coefficient investigation were performed with an analytical calibration different from that used for the other data. X - R a y Fluorescence. X-Ray fluorescence analysis was employed for determination of the amount of effusate collected. A Siemens Kristalloflex-IV generator and fluorescence attachment, with LiF analyzing crystal, tungsten tube, and proportional counter were used in initial measurements, while a Norelco generator and Siemens water-cooled scintillation counter were employed in later measurements. The sample holder was modified to accommodate the 2.54-cm 0.d. aluminum targets used in the collection apparatus. Standard curves which allowed analysis of 0.5-10-pg quantities of effusate were obtained for ytterbium LOCIradiation by weighing a standard ytterbium solution onto precounted targets'l and scanning the region 28 = 45.85-49.35" in 4-min preset counts. The initial standard curve (sensitivity equals (662 counts/pg)/4 min) was obtained by integration of detected radiation, while the second (sensitivity equals (274 counts/pg)/4 min) was obtained by discrimination, Le., the maximization of ytterbium to background radiation. The 3-7-pg qdantities of effusate collected per target were analyzable to A0.2 pg by this procedure.
Results Both analytical and X-ray powder diffraction results substantiate the presence of ytterbium dicarbide. The reaction products from stoichiometries YbC,, with x 2 2, produced the dicarbide phase or dicarbide plus graphite. The invariant X-ray powder diffractionpatterns which were indexable on tetragonal symmetry yielded lattice parameters which agree to within 0.01 with those reported previously.8 Faint spurious X-ray diffraction lines were observed occasionally in dicarbide preparations. Analytical results for the crystallographically pure dicarbide samples used in the vaporization experiments, together with standard deviations, are: ytterbium from oxalate ignition, 87.4 =t0.6 wt yo (calcd 87.8) : bound carbon, 12.6 i= 0.5 wt yo (calcd 12.2). The products (10) R.J. Ackermann, U. 8. Atomic Energy Commission Report ANL-5482, 1955. (11) J. M. Haschke, R. L. Seiver, and H. A. Eick, submitted for publication I
THEVAPORIZATION OF YTTERBIUN DICARBIDE from reaction of stoichiometry YbC,, with 1.5 5 z _< 2, produced a mixture of two phases: one the golden dicarbide and the other a silver-gray phase. At the Y b G stoichiometry, the presence of dicarbide was not detectable in the X-ray powder diffraction pattern. Elemental analysis of the silver-gray phase yields: ytterbium by ignition to the sesquioxide, 90.3 It 0.2 wt % (calcd for Yb2C3) 90.58); bound carbon, 9.7 f 0.2 wt % (calcd 9.42); unbound carbon, 0.0 wt %. The only detectable equilibrium vapor species emanating from a Knudsen source charged with dicarbide was gaseous ytterbium. The sensitivity of the mass spectrometer allowed detection of partial pressures of 10-8 atm; thus the equilibrium ratio YbC2(g)/Yb(g) < The high-density graphite Knudsen cells proved unsuitable for equilibrium studies since the ytterbium diffused rapidly through them. X-Ray fluorescence analysis of graphite samples bored from the exterior of the crucibles indicated a high concentration of this element. No difficulties were encountered with the molybdenum cells. The equilibrium pressure above the YbzC3-YbCz two-phase region was found at 928" to be ten times that above the dicarbide-graphite two-phase region. The metal pressure in equilibrium with the sesquicarbide remained essentially constant with time before decreasing sharply to that in equilibrium with the dicarbide. Debye-Schemer X-ray powder diffraction patterns of the sample indicated complete conversion to the dicarbide, which when vaporized to 50% metal depletion exhibited no detectable change in pressure. By comparison with collection on gold, the sticking coefficient of ytterbium on aluminum is unity. Figure 1 (cf. runs 21B (Al) and 22B (Au)) indicates the coincidence of aluminum and gold data for two successive vaporization experiments with the same Knudsen cell. These data are also indicative of the precision attainable with the X-ray fluorescence analytical technique. The equilibrium vapor pressure of ytterbium has been determined in seven vaporization experiments over the temperature range 825-1275'. A graph of log PYb vs. 1/T is presented in Figure 1. The linear least-squares equation describing the 61 data points in the temperature region 1100-1550°K is 2.303 R log P Y b ( a t m ) = [(-50,890
* 640)/T] 4-(18.98 * 0.50)
From this equation, the following thermodynamic data, together with their standard deviations, are calculated for reaction 1 : A H o 1 3 2 5 = 50.89 f 0.64 kcal/gfw; AS"1326 = 18.98 f 0.50 eu. Thesesecondlaw values have been corrected to 298°K by use of published heat content and entropy datalZ-l4 and the
1699
u llB 0
0 128 a 130 m 140
h
\
1
tJ L1
6.0
6.5
ZO
75
I/T
8.0
x
8.5
9.0
10'
Figure 1. Equilibrium pressure of ytterbium vapor over ytterbium dicarbide.
following assumptions. The heat content of YbCz was assumed to be that of CaCz minus AHtrans for the tetragonal to cubic conversion (1.33 kcal/gfw) occurring at 720°K. Although the analogous tetragonal to cubic transition is observed for most of the lanthanide dicarbides, no such transition occurs in the ytterbium case.lS Krikorian, et al., observed "some change of structure" between 700 and 1000" during slow heating in a high-temperature X-ray diffraction apparatus. X-Ray powder diffraction patterns for the dicarbide before and after vaporization experiments, however, imply that tetragonal dicarbide is the equilibrium phase as no change occurred in the sample during vaporization. The results of this data reduction, with the listed error indicating the composite of standard deviation and estimated error in thermodynamic values are: AH"298 = 54.3 f 1.0 kcal/gfw; AS"Zg8 = 24.9 1.2 eu; A G O 2 9 8 = 46.9 1.0 kcal/gfw. The value of AH"298 has been combined with the heat and free energy of formation of gaseous Yb at 298"K12~17~18
*
(12) R. Hultgren, R. L. Orr, P, D. Anderson, and K. K. Kelley, "Selected Values of Thermodynamic Properties of Metals and Alloys," John Wiley and Sons, New York, N. Y., 1963. (13) D. R. Stull and G. C. Sinke, "Thermodynamic Properties of the Elements," Advances in Chemistry Series, No. 18, American Chemical Society, Washington, D. C., 1956. (14) K. K. Kelley, Bureau of Mines Bulletin 584, U. S. Government Printing Office, Washington, D. C., 1960. (15) G. E. More, Ind. Eng. Chem., 35, 1292 (1943). (16) N. H. Krikorian, T . C. Wallace, and M. G. Bowman, Proceedings, Collaque International Sur les Derives Semimetallique du Centre National de la Recherche Scientifique et Universite du Paris, Orsay, 1965.
Volume 72, Number 6 M a y 1968
JOHN M. HASCHKE AKD HARRY A. EICK
1700 to obtain: A.Hf0298(YbC2(s)) = -18.0 & 1.0 kcal/ gfw; AGfo298(YbC2(s)) = -18.6 f 1.2 kcal/gfw. The entropy of ytterbium dicarbide has also been determined from A x 0 2 9 8 and Stull and Sinke's entropy va1uesl3 to be Sozg8(YbCz(S)) = 19.2 f 1.4 eu. A third-law AH029s has been calculated fox reaction 1 using free energy functions approximated for YbCz(s)14,18-z2 as follows: fefYbca(s)= fefcac2(s)- fefCa(s) fefYb(s). To eliminate the effects of the tetragonal to cubic transition, fefCaCz(s) was calculated as follows: fefcaCn(s)= (HOT - ff0298 - AH0t,,d/P' - (SoTAS'trans)], where AHotrans= 1.33 kcal/gfw and ASotrans = 1.84 eu.15 Published free energy functions for gaseous ytterbium and graphite12J3 were combined with the value computed for YbCz(s) to obtain values of Afef for reaction 1. The third-law average = 52.15 f 0.42, which, when combined with estimated = 52.2 f errors in fre eenergy functions, gives 1.2 kcal/gfw. This third-law AH0298 shows no apparent tempem ture trend ; however, a third-law treatment based on free energy functions derived for YbCz(s) using CaC2(s) values uncorrected for enthalpy and entropy of transition had a definite temperature trend of 1.52.0 kcallgfw.
effects in a sevenfold variation of orifice area and the coincidence of the pressure data collected at both ascending and descending temperatures. Although no weight loss data were obtained and a value cannot be calculated, the effective vaporization coefficient seems close to unity. The presence of an oxide coat on the aluminum metal apparently does not affect the sticking ability of an effusing metal vapor. The results of recent investigations of metal surfaces by low-energy electron diffractionzs imply that oxygen is absorbed rapidly on clean metal surfaces, and, therefore, an aluminum oxide or a gold surface with absorbed oxygen might possess similar sticking capacities for effusing metal vapor. The sticking coefficients of nitrogen on tungsten have been measured as a function of temperature, and the sticking probability and capacity were found to be increased greatly as substrate temperature decreased.26 I n light of these considerations, the sticking coefficient of ytterbium vapor on a liquid nitrogen cooled metal target is probably equal to, or at least very close to, unity. The X-ray fluorescence technique which is readily applicable to all elements with 2 > 19 has proved to be a useful method for quantitatively analyzing the effusate collected by Knudsen effusion without transfer Discussion of the effusate from the target. The combined vapor Some metal dicarbides, e.g., uranium d i ~ a r b i d e , ~ ~pressure data in Figure 1, representing tm7o essentially exhibit variation of composition with temperature. independent measurements, show excellent agreement. However, the composition of alkaline earth and most Second-law enthalpy data, Le., relative pressures, lanthanide dicarbides3 is apparently invariant. An may be obtained readily by the fluorescence technique without analytical standardization for the spectromexception may be the europium species which some eter. workers have been unable to preparez4and for which Gebelt and Eick indicated a composition of EuCI.,.~ The usefulness of the third-law enthalpy calculation has been reiterated by the present experiment. PreHowever, Cuthbert, el aL19prepared E U C ~in. ~situ ~ in liminary results (cf.runs llB-lSB, Figure 1)from secondan effusion cell by heating until the europium pressure became constant. The present investigation establaw (AHoZ98 = 56.1 kcal/gfw) and third-law (AHOm= 50.3 kcal/gfw) calculations were not in good agreelished that both YbCz.oo and YbC1.60 represent stable ment, and a temperature trend was evident in the phases in the ytterbium-carbon system. The existhird-law data. From third-law considerations, error tence of a Yb2C3 phase proposed previously8 has been confirmed both analytically and crystallographically, but it is not isostructural with the previously charac(17) C. E. Habermann and A. H. Daane, J . Chem. Phys., 41, 2818 terized sesquicarbides. Although the lanthanide and (1964). alkaline earth dicarbides have many similarities, the (18) J. 12. Berg, F. H. Spedding, and A. H. Daane, U. S. Atomic M2C3phase has been reported for the lanthanides, but Energy Commission Report IS 327, 1961. not for the alkaline earths. Apparently, its existence (19) K. K. Kelley and E. G. King, Bureau of Mines Bulletin 592, U. S. Government Printing Office, Washington, D. C., 1961. requires the presence of a trivalent metal ion. The (20) B. C. Gerstein, J. Mullaly, E. Phillips, R. E. Miller, and F. H. occasional presence of faint diffraction lines in dicarbide Spedding, J . Chem. Phys., 41, 883 (1964). preparations indicates either the existence of other (21) 0. V. Lounasmaa, Phys. Rev., 143, 399 (1966). graphite-rich phases or the reaction of the sample (22) 0. V. Lounasmaa, ibid., 129, 2460 (1963). with the tantalum container. (23) E. K . Storms, U. 8. Atomic Energy Commission Report, LA-DC-6962, 1965. That equilibrium between ytterbium vapor, the (24) A. L. Bowman, N. H. Krikorian, G. P. Arnold, T. C. Wallace, dicarbide, and graphite is achieved very rapidly and N. G. Nereson, Proceedings of the 6th Rare Earth Conference, Gatlinburg, Tenn., 1967. without any apparent diffusion problems is indicated (25) H. E. Farnswof:h and H. H. Madden, "Solid Surfaces and the by the ease of ytterbium diffusion through the graphite Solid-Gas Interface, Advances in Chemistry Series, No. 33, AmeriKnudsen cells. Other evidences that the system can Chemical Society, Washington, D. C., 1961. reaches equilibrium quickly are both the lack of orifice (26) G. Ehrlich, J. Chem. Phys., 34, 29 (1961).
+
The Journal of Physical Chemistry
THEVAPORIZATION OF YTTERBIUM DICARBIDE
1701
into two classes based upon mode, temperature, and enthalpy of vaporization, An examination of the physical properties of the elements in question indicates a marked difference in their vapor pressures. The metal dicarbides which vaporize at low temperatures principally to the elements have high metal vapor pressures. On the other hand, those which vaporize at higher temperatures to both species have low metal vapor pressures. Figure 2 shows a correlation between the enthalpy of vaporization for reaction 1 and the vapor pressure of the metal at I 1500°K.12J7~32 The enthalpies again fall into two 1 -8.0 -6.0 0.0 distinct groups, but even more unusual is the apparently linear relationship existing within each set. L o g Porm. Kinetic or mechanistic implications are unobtainable Figure 2. Correlation of A H O ~ QofS MC*(s) -+ M(g) from equilibrium thermodynamic data, but the correla2C(s) with vapor pressure of M at 1500'K. tionof AH"Zg8, which is actually a measure of change indicarbide decomposition pressure with the metalvaporpressure, may imply that free metal is involved in the vaporicould arise from: (a) inaccuracy in measurement of zation mechanism. In other words, the activity of absolute pressures (i.e., analytical error or sticking metal in equilibrium with the dicarbide is related problems), (b) systematic error in temperature meadirectly to the activity of the free metal at a given surements, or (c) error in the fef approximation for reaction 1. The reexamination of experimental patemperature. rameters eliminated possibilities (a) and (b) and indiAs Cuthbert, et al., have notedj4sorne correlation is cated that free energy functions were in error. Elimialso observed between AH02g8 and the ratio of MC2nation of the temperature trend by deletion of AH (g)/M(g). At the higher values of AH0298, the ratio approaches unity while at the lower values, it apand AS for the tetragonal to cubic transition from the proaches zero. Some examples of hfCz(g)/RI(g) ratios CaC2 data reemphasizes that, although errors in thirdlaw calculations from free energy functions containing for various Rl's which are observed to decrease alsmall enthalpy errors are small, those arising from most in order with decreasing A H o 2 g 8 (Figure 2) are: Ce, 0.S3 or 0.77;33 La, 0.96;34 Y, 0.95;29 Nd, small entropy errors are often large. O.2;35 Sm,
