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Standard enthalpies of formation of some rare earth carbides by direct synthesis calorimetry
Journal of Alloys and Compounds, 205 (1994) 165-168 JALCOM 970
165
Standard enthalpies of formation of some rare earth carbides by direct synthesis calorimetry S.V. M e s c h e l a n d O.J. K l e p p a James Franck Institute, University of Chicago, 5640 S. Ellis Avenue, Chicago, IL 60637 (USA)
(Received September 7, 1993)
Abstract
The standard enthalpies of formation of carbides in the binary systems Sc-C, Y~Z and La-C have been determined by direct synthesis calorimetry at 1473+2 tC The following values are reported: AHf°(Sc2C)= -45.5+ 1.6 kJ (g atom)-~; AH°(Y2C)=-31.8 + 1.3 kJ (g atom)-1 and AHf° (LaC2)=-27.2 + 1.4 kJ (g atom)-1. The results are compared with earlier experimental data and with values predicted from Miedema's semi-empirical model. The enthalpies of formation for the carbides considered are also compared with the corresponding data for silicides and germanides.
I. Introduction
The thermodynamic behaviour of transition metal carbides has received considerable attention since the 1950s, which has resulted in the preparation of some excellent reviews. Reference is made, for example, to the books by Storms [1] and Toth [2], to the early reviews by Elliott and Gleiser [3] and by Richardson [4], and to the recent reviews of Coltters [5] and Berkane [6]. However, most of these studies offer little information regarding the carbides formed by the group IIIB metals in the periodic table. Transition metal carbides generally have high melting points, good resistance to corrosion, exceptional hardness and strength, and good thermal conductivity. Among the carbides formed by the group IIIB metals, the Y-C system is of possible technological interest for the development of containers for nuclear materials, as well as for other high temperature applications [7]. The thermochemical information available for the group IIIB carbides includes an estimated enthalpy of formation for ScC2 based on combustion calorimetry [8], a value for Sc2C calculated from mass spectroscopy [9], as well as derived enthalpy data for phases in the Y-C and La-C systems based on e.m.f, measurements [10] and on mass spectrometric studies [11-15]. In the present work, the standard enthalpies of formation of selected group IIIB carbides were measured by high temperature direct synthesis calorimetry. Specifically, we report information on the heats of formation of Sc2C, Y2C and LaC~. We will compare our results with 0925-8388/94/$07.00 © 1994Elsevier Sequoia. All rights reserved SSDI 0925-8388(93)00970-A
the available literature data and with values predicted from Miedema's semi-empirical model [16].
2. Experimental and materials
The experiments were carried out at 1473_+2 K in a single-unit differential microcalorimeter which has been described earlier [17]. All the experiments were performed under a protective atmosphere of argon gas, which was purified by passing it over titanium powder at about 900 °C. A BN crucible was used to contain the samples. The purities of the elements used ranged from 99.0% for carbon to 99.9% for scandium, yttrium and lanthanum. All the materials were purchased from Johnson-Matthey/Aesar Group; the scandium, yttrium and lanthanum were in ingot form. The particle sizes of the powders used were - 8 0 mesh for the three metals and -300 mesh for carbon. The scandium and yttrium powders were machined from the ingots and tested by X-ray diffraction to detect oxide contamination or other impurities. None was found. Since lanthanum metal is so reactive in air, we prepared the samples immediately after hand filing the ingot without further testing. The carbon was in the crystalline graphite modification. The two components were carefully mixed in the appropriate molar ratio, pressed into pellets 4 mm in diameter, and dropped into the calorimeter from room temperature. In a subsequent set of experiments, the reaction products were dropped into the calorimeter to measure their heat contents. Between the two sets
166
S.V. Meschel et al. / Standard enthalpies o f formation o f some rare earth carbides
of measurements, the samples were kept in a vacuum dessicator to prevent reaction with oxygen or moisture. Calibration of the calorimeter was achieved by dropping weighed Segments of high purity copper wire 2 mm in diameter from room temperature into the calorimeter at 1473 + 2 K. The enthalpy of pure copper at this temperature 46465 J (g atom) -1 was obtained from Hultgren et al. [18]. The calibrations were reproducible to within + 1.2%. The carbide samples were examined by X-ray diffraction to assess their structures and to ascertain the absence of unreacted metals. We observed that, during the preparation of the samples for X-ray diffraction, the carbides all transformed quickly into an amorphous substance. The reaction presumably is a hydrolysis reaction with the moisture in the air, similar to what happens with CaC2. Hajek et al. [19] also observed that LaC2 undergoes rapid hydrolysis during preparation. However, we found that all three compounds studied may be successfully prepared for X-ray diffraction if the samples are thoroughly coated with vaseline and the patterns obtained immediately. In view of their sensitivity to moisture in the air, the samples could not be examined by scanning electron microscopy and X-ray microprobe analysis, since the samples transformed during the mounting process. The phase diagram of the Sc-C system indicates that the most stable phase is Sc2C, which has a wide range of homogeneity [20]. The compound melts congruently at 2270 °C. ScaC was prepared from its elements under conditions similar to those of our calorimeter by Rassaerts et al. [21]. The X-ray diffraction pattern of our preliminary sample largely matched the pattern in the ASTM powder diffraction file. However, while there was no evidence for other carbide phases, we observed 2%-3% unreacted scandium metal. For this reason, we prepared another sample with a small excess of carbon to facilitate completeness of the reaction. The technique of adding an excess of one component to ensure the completeness of the reaction was discussed earlier by Meschel and Kleppa [22]. The sample of composition Sc2C1.1 was subsequently checked by Xray diffraction and found to be free of unreacted metal within the limits of detectability. The phase diagram of the Y--C system shows several compounds which may be stable at the temperature of the calorimeter, these being Y2C3, Y C 2 and Y2C [20]. In our preliminary experiments, we prepared samples with all three compositions. However, X-ray diffraction showed that our samples of YCz and Y2C3 were both mixtures of these two phases. We also tried to prepare YC2 by adding an excess of carbon. This sample also proved to be a mixture of Y2C3 and YC2. Y2C was the only composition which yielded a single phase. This compound has a wide range of homogeneity and melts
congruently near 2000 °C. Since there is no listed pattern for Y2C in the powder diffraction file, we used a computer to generate a pattern using the unit cell parameters and the atomic coordinates from the neutron diffraction study of Atoji and Kikuchi [23]. Our pattern matched well that of the high temperature cubic structure. There was no evidence for the presence of the room temperature trigonal phase. The phase diagram of the La-C system indicates that the most stable composition is LaC2, which melts congruently at 2360 °C [20]. The X-ray diffraction pattern of our sample agreed well with the pattern in the ASTM powder diffraction file. There was no evidence for the presence of unreacted metal or of other carbide phases. However, we observed a small amount of lanthanum oxide, estimated to be 1%-2%.
3. Results and discussion
The standard enthalpies of formation of the transition metal carbides determined in this study were obtained from the difference between the results of two sets of measurements. In the first set, the following reaction took place in the calorimeter: Me(s, 298 K)+rnC(s, 298 K) = MeC,,(s, 1473 K)
(1)
Here, m represents the molar ratio C/Me, Me is the metal considered and s denotes solid. The reacted carbide pellets were reused in a subsequent set of measurements to determine their heat contents, such that MeC,,(s, 298 K ) = MeC,,(s, 1473 K)
(2)
The standard enthalpy of formation is given by Za-/°-- a/-/(1) - AH(2)
(3)
T A B L E 1. Standard enthalpies of formation of some rare earth carbides Compound AH(1)
AH(2)
AHf°"
(kJ (g atom) -t) (kJ (g atom) -1) (kJ (g atom) -1) Sc2C b YzC LaCa
- 13.5 + 1.4 2.0+0.6 5.6 ± 0.9
32.0+ 0.8 33.8+ 1.0 32.8 + 0.9
- 4 5 . 5 + 1.6 - 3 1 . 8 + 1.3 - 27.2 + 1.4
aThe standard deviations given in this column have been calculated from 8=(612+822+623) la. Here ~1 is the standard deviation of AH(1) and 62 is the standard deviation of AH(2), while 63 is the corresponding uncertainty in the calibrations. bThe actual composition studied was Sc2C1.1. The values of AH(1) and AH(2) have been corrected for the excess carbon, using the heat content of graphite at 1473 K (22.595 kJ g-~ a t o m - t ) , taken from Hultgren et al. [18].
S.V. Meschel et al. / Standard enthalpies of formation of some rare earth carbides
167
TABLE 2. Comparison of the AH°data for scandium, yttrium and lanthanum carbides with literature data and predictions from Miedema's semi-empirical model Compound
SczC
2fftf°(exp.) (kJ (g a t o m ) - I ) This study
Literature
- 45.5 + 1.6
- 41.8 (ScC2)
AH°(predicted) (kJ (g atom) -i)
Method
- 6 7 . 2 (ScEC)
Ref. 16
Ref. 24
Combustion [8] Calorimetry Mass spect. [9] 1520 K
- 56
-44
YeC
- 31.8 + 1.3
- 35.0 + 3.5 (YC2) - 3 0 . 3 + 5 . 7 (YC2) - 37.7 + 8.4 (YC2) - 3 8 . 8 (YC2)
E.m.L [10] Mass Spect. [11] Mass Spect. [12] Mass Spect. [13]
- 48
- 37
LaCz
-27.2+1.4
-29.1 - 29.7 _4:_8 -26.5
E.m.f. [10] Mass Spect. [14] Mass Spect. [15]
-69
-60
where AH (1) and AH (2) are the enthalpy changes per gram atom associated with reactions (1) and (2). The experimental results are summarized in Table 1. The heat effects associated with reactions (1) and (2) are given as the averages of 5-7 consecutive measurements with the appropriate standard deviations. The last column shows the standard enthalpies of formation of the phases considered. Table 2 compares the standard enthalpies of formation reported in the present work with experimental values from the published literature and with values obtained using Miedema's semi-empirical model [16]. The published values for the heat of formation in the Sc-C system are not fully compatible with our data. The value of Huber et al. is an estimated value for ScC2 [8]. The value quoted by Gschneider and coworkers refers to 1520 K [9]. We found no experimental data on the heat of formation of Y2C. However, our value compares rather well with the mass spectrometric and e.m.f, enthalpies for YC2 [10-13]. It should be noted that our experimental error is significantly lower than the errors quoted for the e.m.f, and mass spectrometric values. Our heat of formation for LaC2 compares well with values derived from e.m.f, and mass spectrometric studies [10, 14, 15]. In Table 2, the predicted values in column 5 are cited from de Boer et al. [16], while those in column 6 are based on the calculations of Niessen et al. [24]. There is clear a discrepancy between the two sets of predicted values for these carbides published by the Miedema group. The predicted values in column 6 agree reasonably well with our measurements for Sc2C and Y2C. Both the predicted values are considerably more exothermic than are our experimental results for LaCz. In recent theoretical work on transition metal carbides, Zhukov et al. [25] and Guillermet and Grimvali
[26] have predicted correlations between the cohesive energies of transition metal carbides and the average number of valence electrons per atom in the compounds. However, these calculations do not include quantitative predictions for the group IIIB carbides. In Fig. 1 we compare our results for scandium, yttrium and lanthanum carbides with reported results for silicides and germanides. The enthalpies of formation of ScsGe3, YsGe3 and LasGe3 were determined by Jung and Kleppa [27], while the values for ScsSi3 and YsSi3 are cited from Topor and Kleppa [28, 29]. The heat of formation of LaSi was measured by Samsonov et al. and is quoted from Schlesinger [30]. Figure 1 shows that the enthalpies of formation of the group III carbides decrease in magnitude as we move from 3d to the corresponding 4d and 5d compounds. We can see from Fig. 1 that the group IIIB carbides, silicides and germanides exhibit similar behaviors. This is not surprising, since carbon, silicon and -120
i
Sc5Ge3
-100
i
Y5Ge3
E
o
-80
•
Sc5Si3
o'} •~
-60
°"I--
-40
Y5Si3
La5Ge3 © • LaSi
[]
Sc2C
[] Y2C
[] LaC2
I
I
I
Se
Y
La
<3 -20 0
Fig. 1. Comparison of the standard enthalpies of formation for scandium, yttrium and lanthanum carbides with data for corresponding silicides and germanides.
168
S.V. Meschel et al. / Standard enthalpies of formation of some rare earth carbides
germanium are in the same column in the periodic table. Comparing the three sets of data, it is evident that the enthalpies of formation become more exothermic and the compounds more stable as we move from carbides to silicides to germanides.
Acknowledgments This investigation has been supported by the Department of Energy under Grant DE-FG02-88ER4563, and has also benefited from the MRL facilities at the University of Chicago. We are indebted to Dr. Joseph Pluth who generated the X-ray diffraction pattern for Y2C from the reported unit cell parameters and atomic coordinates.
References 1 E.K. Storms, The Refractory Carbides, Academic, New York, 1967. 2 L.E. Toth, Transition Metal Carbides and Nitrides, Academic, New York, 1971. 3 J.F. Elliott and M. Gleiser, Thermochemistry for Steelmaking, Addison-Wesley, Reading, MA, 1960. 4 F.D. Richardson, J. Iron Steel Inst., 175 (1953) 33-51. 5 R.G. Coltters, Mater. Sci. Eng., 76 (1985) 1-50. 6 R. Berkane, Thermodynamic study of Cr, Ti, Zr and Hf carbides by high temperature calorimetry, Thesis, Universit6 de Nancy, 1989. 7 O.N. Carlson and W.M. Paulson, Trans. Metall. Soc. AIME, 242 (1968) 846--852. 8 E.J. Huber, Jr., G.C. Fitzgibbon, E.L. Head and C.E. Holley, Jr., J. Phys. Chem., 67 (1963) 1731-1733.
9 C.T. Horovitz, K.A. Gschneider, Jr., G.A. Nelson, D.H. Youngblood and H.H. Schock (eds.), Scandium, Its Occurance, Chemistry, Physics, Metallurgy, Biology and Technology, Academic, New York, 1975, p. 163. 10 J.S. Anderson and A.N. Bagshaw, Rev. Chim. Miner., 9 (1972) 115-138. 11 F.J. Kohl and C.A. Steams, Z Chem. Phys., 52 (1972) 6310-6315. 12 G. de Maria, M. Guido, L. Malaspina and B. Pesce, J. Chem. Phys., 43 (1965) 4449-4452. 13 E.K. Storms, High Temp. Sci., 3 (1971) 99-122. 14 C.A. Steams and F.J. Kohl,J. Chem.Phys., 54 (1971) 5180-5187. 15 R.L. Faircloth, R.H. Flowers and F.C.W. Pummery, J. Inorg. Nucl. Chem., 30 (1968) 499-518. 16 F.R. de Boer, R. Boom, W.C.M. Mattens, A.R. Miedema and A.K. Niessen, Cohesion in Metals. Transition Metal Alloys, Elsevier, New York, 1988. 17 O.J. Kleppa and L. Topor, Thermochim. Acta, 139 (1989) 291-297. 18 R. Hultgren, P.D. Desai, D.T. Hawkins, M. Gleiser, K.K. Kelley and D.D. Wagman, Selected Values of the Thermodynamic Properties of the Elements, ASM, Metals Park, OH, 1973, p. 154. 19 B. Hajek, V. Kohout and V. Flemr, Coll. Czech. Chem. Commun., 50 (1985) 1153-1160. 20 T.B. Massalski, H. Okamoto, P.R. Subramanian and L. Kacprzak (eds.), Binary Phase Diagrams, ASM International, Materials Park, OH, 2nd edn., 1990. 21 H. Rassaerts, H. Nowotny, G. Vinek and F. Benesovsky, Monat. Chem., 98 (1967) 460--468. 22 S.V. Meschel and O.J. Kleppa, J. Chim. Phys., 90 (1993) 349-354. 23 M. Atoji and M. Kikuchi, J. Chem. Phys., 51 (1969) 3863-3872. 24 A.K. Niessen, F.R. de Boer, R. Boom, P.F. de Chatel, W.C.M. Mattens and A.R. Miedema, Calphad, 7 (1983) 51-70. 25 V.P. Zhukov, V.A. Gubanov, O. Jepsen, N.E. Christensen and O.K. Andersen, J. Phys. Chem. Solids, 49 (1988) 841-849. 26 A.F. Guillermet and G. Grimvall, J. Phys. Chem. Solids, 53 (1992) 105-125. 27 W.G. Jung and O.J. Kleppa, J. Less-Common Met., 169 (1991) 85-92. 28 L. Topor and O.J. Kleppa, MetalL Trans. B, 20 (1989) 879-892. 29 L. Topor and O.J. Kleppa, J. Less-Common Met., 167 (1990) 91-99. 30 M.E. Schlesinger, Chem. Rev., 90 (1990) 607-628.
165
Standard enthalpies of formation of some rare earth carbides by direct synthesis calorimetry S.V. M e s c h e l a n d O.J. K l e p p a James Franck Institute, University of Chicago, 5640 S. Ellis Avenue, Chicago, IL 60637 (USA)
(Received September 7, 1993)
Abstract
The standard enthalpies of formation of carbides in the binary systems Sc-C, Y~Z and La-C have been determined by direct synthesis calorimetry at 1473+2 tC The following values are reported: AHf°(Sc2C)= -45.5+ 1.6 kJ (g atom)-~; AH°(Y2C)=-31.8 + 1.3 kJ (g atom)-1 and AHf° (LaC2)=-27.2 + 1.4 kJ (g atom)-1. The results are compared with earlier experimental data and with values predicted from Miedema's semi-empirical model. The enthalpies of formation for the carbides considered are also compared with the corresponding data for silicides and germanides.
I. Introduction
The thermodynamic behaviour of transition metal carbides has received considerable attention since the 1950s, which has resulted in the preparation of some excellent reviews. Reference is made, for example, to the books by Storms [1] and Toth [2], to the early reviews by Elliott and Gleiser [3] and by Richardson [4], and to the recent reviews of Coltters [5] and Berkane [6]. However, most of these studies offer little information regarding the carbides formed by the group IIIB metals in the periodic table. Transition metal carbides generally have high melting points, good resistance to corrosion, exceptional hardness and strength, and good thermal conductivity. Among the carbides formed by the group IIIB metals, the Y-C system is of possible technological interest for the development of containers for nuclear materials, as well as for other high temperature applications [7]. The thermochemical information available for the group IIIB carbides includes an estimated enthalpy of formation for ScC2 based on combustion calorimetry [8], a value for Sc2C calculated from mass spectroscopy [9], as well as derived enthalpy data for phases in the Y-C and La-C systems based on e.m.f, measurements [10] and on mass spectrometric studies [11-15]. In the present work, the standard enthalpies of formation of selected group IIIB carbides were measured by high temperature direct synthesis calorimetry. Specifically, we report information on the heats of formation of Sc2C, Y2C and LaC~. We will compare our results with 0925-8388/94/$07.00 © 1994Elsevier Sequoia. All rights reserved SSDI 0925-8388(93)00970-A
the available literature data and with values predicted from Miedema's semi-empirical model [16].
2. Experimental and materials
The experiments were carried out at 1473_+2 K in a single-unit differential microcalorimeter which has been described earlier [17]. All the experiments were performed under a protective atmosphere of argon gas, which was purified by passing it over titanium powder at about 900 °C. A BN crucible was used to contain the samples. The purities of the elements used ranged from 99.0% for carbon to 99.9% for scandium, yttrium and lanthanum. All the materials were purchased from Johnson-Matthey/Aesar Group; the scandium, yttrium and lanthanum were in ingot form. The particle sizes of the powders used were - 8 0 mesh for the three metals and -300 mesh for carbon. The scandium and yttrium powders were machined from the ingots and tested by X-ray diffraction to detect oxide contamination or other impurities. None was found. Since lanthanum metal is so reactive in air, we prepared the samples immediately after hand filing the ingot without further testing. The carbon was in the crystalline graphite modification. The two components were carefully mixed in the appropriate molar ratio, pressed into pellets 4 mm in diameter, and dropped into the calorimeter from room temperature. In a subsequent set of experiments, the reaction products were dropped into the calorimeter to measure their heat contents. Between the two sets
166
S.V. Meschel et al. / Standard enthalpies o f formation o f some rare earth carbides
of measurements, the samples were kept in a vacuum dessicator to prevent reaction with oxygen or moisture. Calibration of the calorimeter was achieved by dropping weighed Segments of high purity copper wire 2 mm in diameter from room temperature into the calorimeter at 1473 + 2 K. The enthalpy of pure copper at this temperature 46465 J (g atom) -1 was obtained from Hultgren et al. [18]. The calibrations were reproducible to within + 1.2%. The carbide samples were examined by X-ray diffraction to assess their structures and to ascertain the absence of unreacted metals. We observed that, during the preparation of the samples for X-ray diffraction, the carbides all transformed quickly into an amorphous substance. The reaction presumably is a hydrolysis reaction with the moisture in the air, similar to what happens with CaC2. Hajek et al. [19] also observed that LaC2 undergoes rapid hydrolysis during preparation. However, we found that all three compounds studied may be successfully prepared for X-ray diffraction if the samples are thoroughly coated with vaseline and the patterns obtained immediately. In view of their sensitivity to moisture in the air, the samples could not be examined by scanning electron microscopy and X-ray microprobe analysis, since the samples transformed during the mounting process. The phase diagram of the Sc-C system indicates that the most stable phase is Sc2C, which has a wide range of homogeneity [20]. The compound melts congruently at 2270 °C. ScaC was prepared from its elements under conditions similar to those of our calorimeter by Rassaerts et al. [21]. The X-ray diffraction pattern of our preliminary sample largely matched the pattern in the ASTM powder diffraction file. However, while there was no evidence for other carbide phases, we observed 2%-3% unreacted scandium metal. For this reason, we prepared another sample with a small excess of carbon to facilitate completeness of the reaction. The technique of adding an excess of one component to ensure the completeness of the reaction was discussed earlier by Meschel and Kleppa [22]. The sample of composition Sc2C1.1 was subsequently checked by Xray diffraction and found to be free of unreacted metal within the limits of detectability. The phase diagram of the Y--C system shows several compounds which may be stable at the temperature of the calorimeter, these being Y2C3, Y C 2 and Y2C [20]. In our preliminary experiments, we prepared samples with all three compositions. However, X-ray diffraction showed that our samples of YCz and Y2C3 were both mixtures of these two phases. We also tried to prepare YC2 by adding an excess of carbon. This sample also proved to be a mixture of Y2C3 and YC2. Y2C was the only composition which yielded a single phase. This compound has a wide range of homogeneity and melts
congruently near 2000 °C. Since there is no listed pattern for Y2C in the powder diffraction file, we used a computer to generate a pattern using the unit cell parameters and the atomic coordinates from the neutron diffraction study of Atoji and Kikuchi [23]. Our pattern matched well that of the high temperature cubic structure. There was no evidence for the presence of the room temperature trigonal phase. The phase diagram of the La-C system indicates that the most stable composition is LaC2, which melts congruently at 2360 °C [20]. The X-ray diffraction pattern of our sample agreed well with the pattern in the ASTM powder diffraction file. There was no evidence for the presence of unreacted metal or of other carbide phases. However, we observed a small amount of lanthanum oxide, estimated to be 1%-2%.
3. Results and discussion
The standard enthalpies of formation of the transition metal carbides determined in this study were obtained from the difference between the results of two sets of measurements. In the first set, the following reaction took place in the calorimeter: Me(s, 298 K)+rnC(s, 298 K) = MeC,,(s, 1473 K)
(1)
Here, m represents the molar ratio C/Me, Me is the metal considered and s denotes solid. The reacted carbide pellets were reused in a subsequent set of measurements to determine their heat contents, such that MeC,,(s, 298 K ) = MeC,,(s, 1473 K)
(2)
The standard enthalpy of formation is given by Za-/°-- a/-/(1) - AH(2)
(3)
T A B L E 1. Standard enthalpies of formation of some rare earth carbides Compound AH(1)
AH(2)
AHf°"
(kJ (g atom) -t) (kJ (g atom) -1) (kJ (g atom) -1) Sc2C b YzC LaCa
- 13.5 + 1.4 2.0+0.6 5.6 ± 0.9
32.0+ 0.8 33.8+ 1.0 32.8 + 0.9
- 4 5 . 5 + 1.6 - 3 1 . 8 + 1.3 - 27.2 + 1.4
aThe standard deviations given in this column have been calculated from 8=(612+822+623) la. Here ~1 is the standard deviation of AH(1) and 62 is the standard deviation of AH(2), while 63 is the corresponding uncertainty in the calibrations. bThe actual composition studied was Sc2C1.1. The values of AH(1) and AH(2) have been corrected for the excess carbon, using the heat content of graphite at 1473 K (22.595 kJ g-~ a t o m - t ) , taken from Hultgren et al. [18].
S.V. Meschel et al. / Standard enthalpies of formation of some rare earth carbides
167
TABLE 2. Comparison of the AH°data for scandium, yttrium and lanthanum carbides with literature data and predictions from Miedema's semi-empirical model Compound
SczC
2fftf°(exp.) (kJ (g a t o m ) - I ) This study
Literature
- 45.5 + 1.6
- 41.8 (ScC2)
AH°(predicted) (kJ (g atom) -i)
Method
- 6 7 . 2 (ScEC)
Ref. 16
Ref. 24
Combustion [8] Calorimetry Mass spect. [9] 1520 K
- 56
-44
YeC
- 31.8 + 1.3
- 35.0 + 3.5 (YC2) - 3 0 . 3 + 5 . 7 (YC2) - 37.7 + 8.4 (YC2) - 3 8 . 8 (YC2)
E.m.L [10] Mass Spect. [11] Mass Spect. [12] Mass Spect. [13]
- 48
- 37
LaCz
-27.2+1.4
-29.1 - 29.7 _4:_8 -26.5
E.m.f. [10] Mass Spect. [14] Mass Spect. [15]
-69
-60
where AH (1) and AH (2) are the enthalpy changes per gram atom associated with reactions (1) and (2). The experimental results are summarized in Table 1. The heat effects associated with reactions (1) and (2) are given as the averages of 5-7 consecutive measurements with the appropriate standard deviations. The last column shows the standard enthalpies of formation of the phases considered. Table 2 compares the standard enthalpies of formation reported in the present work with experimental values from the published literature and with values obtained using Miedema's semi-empirical model [16]. The published values for the heat of formation in the Sc-C system are not fully compatible with our data. The value of Huber et al. is an estimated value for ScC2 [8]. The value quoted by Gschneider and coworkers refers to 1520 K [9]. We found no experimental data on the heat of formation of Y2C. However, our value compares rather well with the mass spectrometric and e.m.f, enthalpies for YC2 [10-13]. It should be noted that our experimental error is significantly lower than the errors quoted for the e.m.f, and mass spectrometric values. Our heat of formation for LaC2 compares well with values derived from e.m.f, and mass spectrometric studies [10, 14, 15]. In Table 2, the predicted values in column 5 are cited from de Boer et al. [16], while those in column 6 are based on the calculations of Niessen et al. [24]. There is clear a discrepancy between the two sets of predicted values for these carbides published by the Miedema group. The predicted values in column 6 agree reasonably well with our measurements for Sc2C and Y2C. Both the predicted values are considerably more exothermic than are our experimental results for LaCz. In recent theoretical work on transition metal carbides, Zhukov et al. [25] and Guillermet and Grimvali
[26] have predicted correlations between the cohesive energies of transition metal carbides and the average number of valence electrons per atom in the compounds. However, these calculations do not include quantitative predictions for the group IIIB carbides. In Fig. 1 we compare our results for scandium, yttrium and lanthanum carbides with reported results for silicides and germanides. The enthalpies of formation of ScsGe3, YsGe3 and LasGe3 were determined by Jung and Kleppa [27], while the values for ScsSi3 and YsSi3 are cited from Topor and Kleppa [28, 29]. The heat of formation of LaSi was measured by Samsonov et al. and is quoted from Schlesinger [30]. Figure 1 shows that the enthalpies of formation of the group III carbides decrease in magnitude as we move from 3d to the corresponding 4d and 5d compounds. We can see from Fig. 1 that the group IIIB carbides, silicides and germanides exhibit similar behaviors. This is not surprising, since carbon, silicon and -120
i
Sc5Ge3
-100
i
Y5Ge3
E
o
-80
•
Sc5Si3
o'} •~
-60
°"I--
-40
Y5Si3
La5Ge3 © • LaSi
[]
Sc2C
[] Y2C
[] LaC2
I
I
I
Se
Y
La
<3 -20 0
Fig. 1. Comparison of the standard enthalpies of formation for scandium, yttrium and lanthanum carbides with data for corresponding silicides and germanides.
168
S.V. Meschel et al. / Standard enthalpies of formation of some rare earth carbides
germanium are in the same column in the periodic table. Comparing the three sets of data, it is evident that the enthalpies of formation become more exothermic and the compounds more stable as we move from carbides to silicides to germanides.
Acknowledgments This investigation has been supported by the Department of Energy under Grant DE-FG02-88ER4563, and has also benefited from the MRL facilities at the University of Chicago. We are indebted to Dr. Joseph Pluth who generated the X-ray diffraction pattern for Y2C from the reported unit cell parameters and atomic coordinates.
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