RARE EARTH CARBIDES 105 further lowering the temperature to allow the M - C - C M interactions to become important compared to the thermal energy, would the C2 units be able to align in a given orientation.
This model gives a satisfactory explanation for the effect of size difference on the cubic-to-tetragonal transformation (McColm et al. 1972), and has been verified by the neutron diffraction of Ceo.33Uo.67C 2 and Ceo.67Llo.33C2 over 296 to 4 K (Atoji 1980). In the two ternary CaCz-type compounds the cubic NaCl-type structure can be arrested down to 4 K because of the large strain energy of the disordered lattice. A random distribution of Ce and U atoms as well as the orientational disordering of C2 units are also retained down to 4 K. The lattice parameters indicate no valency change over the range 296 4 K, such as Ce 3+ ---> C e 4+.
4.4. Solid solutions (Ca, Y)C2 and (Th, Y)zC3
The solid solutions in the system CaC2-YC2 can be formed over the entire composition range; however, no Vegard's law behavior with respect to the composi- tion dependence of the lattice parameters was found (Hfijek et al. 1971), e.g., the solid solution Cao.33Y0.66C2 has essentially the same lattice parameters (a = 3.62A, c = 6.06 A) as the yttrium dicarbide (a = 3.62 A, c = 6.05 A) (Brozek etoal. 1970). The lattice parameters of the solid solution with 33 mol% YC 2 (a -- 3.64 A, c = 6.16 ~,) are only slightly larger than those of YC2 but markedly lower than those of CaC 2 (a = 3.87 A, c = 6.38 A), indicating that the anomalous valence state of the yttrium ion in these solid solutions leads to an irregular change in lattice parameters.
At high temperatures and under high pressures, Th formed a single-phase solid solution series with Y2C 3 over a wide ternary composition field (Krupka et al. 1969).
The new phase, crystallizing into the body-centered cubic Pu2Ca-type structure, was superconducting over its entire range of homogeneity with a variable transition temperature < 4 ~ 17 K. Annealing at high temperature and ambient pressure destroyed both the bcc structure and the superconductivity.
5. Thermodynamic properties of binary rare earth carbides
106 G. ADACHI et al.
Gingerich 1989) and La2C . (n = 1-6, 8) (Pelino et al. 1984, Pelino and Gingerich 1989), as well as ScC, (n = 2-6) (Haque and Gingerich 1981), have been observed in the equilibrium vapor above the metal-graphite systems at high temperatures. Their thermodynamic characterizations, such as atomization energies etc., have been deter- mined by the Knudsen effusion technique combined with mass spectrometry. The structure of the molecules have been inferred from the assumed models and con- sistency of the thermodynamic results based on various methods of evaluation.
The experimental work on ternary gaseous rare earth carbides has been pioneered by Guido and Gigli (1973) for a molecule containing one metal atom, CeSiC, and by Gingerich (1974) for molecules containing two different metals, RhCeC 2 and PtCeC 2.
Since 1976, these mixed-metal carbide studies have been extended to a number of rare earth metals, including molecules of the type M(R)C, (n = 1-4), (Gingerich and Cocke 1979, Haque and Gingerich 1979, Gingerich et al. 1981a, Pelino et al. 1986).
5.1.1. The species RC,
The high-temperature Knudsen effusion mass spectrometric technique has been used to study equilibria involving gaseous rare-earth-containing molecules. The existence of a large number of stable gaseous rare earth carbides has been established.
The enthalpies AH~ of the reactions
R(g) + nC(graphite) --* RC,(g) (n = 1, 2, 3 . . . . , etc.) (1) were evaluated by the second-law method and the third-law method, as well as the atomization enthalpies, AH] and the standard enthalpies of formation, AH~. Table 7 shows the selected thermodynamic data of RC, (gas) species obtained from the averages, with equal weight, of the second- and third-law values.
Some investigators also reported enthalpy changes for the reactions
RCz(g) + R'(g)--* R'Cz(g) + R(g) (2)
and
RCz(g) + 2C(s) ~ RC4(g) (3)
and further calculated the dissociation energies of the gaseous dicarbides of all lanthanides at 0 K (Filby and Ames 1971a, b, 1972) from the reaction (2) and the atomization energies of RC4(g) (R = Ce, Nd, Dy, Ho, Lu) (table 7) from the reac- tion (3) (Balducci et al. 1968, 1969a, b, Guido et al. 1972).
According to De Maria and co-workers (1972), the dicarbide is always the most abundant gaseous molecular species in the equilibrium vapor over the corresponding rare-earth-carbide-graphite system, followed by the tetracarbide. Table 8 shows examples of equilibrium partial-pressure ratios, P(RC2)/P(R) and P(RC,,)/P(R), at the specified temperatures.
As to the partial pressures of the monolanthanum and monocerium carbides, LaC, and CeC,, in the equilibrium vapor above the rare-earth-metal-platinum-metal- graphite systems, the relative ion currents, which represent more than 90%
ionization from the parent neutral molecules, and thus, the relative partial pressures of the latter, have been measured by means of the mass spectrometric m e t h o d -
RARE EARTH CARBIDES 107
at 2835K for LaC,, LaC +, 2.26x 10-2; LaC2 +, 1.36; LaC3 +, 1 x 10-2; LaC4 +, 7.15 + 10-z; LaC5 +, 9.0x 10-4; LaG6 +, 4 . 8 x 1 0 - 4 ; LaC7 +, 5.7x 10-6; LaC8 +, 4 . 6 x 10 -6 (Gingerich et al. 1981a, b, 1982, Gingerich 1985) and at 2733K for CeCn, CeC +, 1; CeC2 +, 5.3x 10-1; CeC3 +, 9.09; CeC4 +, 1.6x 10-2; CeC5 +, 3.2 x 10-2; CeC6 +, 1.6 x 10-4; CeC7 + 8.1 x 10 -5 (Gingerich et al. 1976a, b). It can be seen from these data that the molecules LaC 2 (Chupka et al. 1958, Stearns and Kohl 1971), LaC 3 and LaC4 (Stearns and Kohl 1971) are the most abundant molecular lanthanum carbides. The additional equilibrium species have a concentration of the order of 0.1% or less of the vapor. In addition, it is also apparent that up to RC5 the molecules with an odd number of atoms, in fact, those containing the C2 radical, have a larger partial pressure than the molecules with the preceding even number of atoms.
The first gaseous rare earth monocarbide, CeC, was observed in the study on the chemical equilibria of the reaction
CeC2(g) + Ce(g) ~ 2CeC(g),
by the mass spectrometer and Knudsen effusion cell method. Its dissociation energy O~ is 301.2 +_ 19.6 kJmol. -1 (Balducci et al. 1967). Subsequently, Kingcade et al.
(1983) studied the reaction CeC(g) --* Ce(g) + C(s)
and obtained the values AH~,298.15 = 694 _+ 12 and AH°0 = 441 _+ 12 kJmo1-1, as listed in table 7. More recently, the monocarbide of lanthanum, LaC, was also found by Pelino and Gingerich (1989); its standard enthalpy of formation AH~, 298.15 = 685 __+ 20 kJmo1-1 and atomization enthalpy, AHa°o = 458 + 20 kJmo1-1 are close to the corresponding values of the cerium monocarbide. However, these monocarbides of rare earths are stable only in the gaseous phase but fail to be found in the solid state as noted earlier, see sect. 2.6.4.
The high-temperature Knudsen effusion mass spectrometric studies of the vaporiza- tion of the rare-earth-carbide-carbon systems also lead to a good knowledge of the bond dissociation energies D~ (R-C2) , Dg (C2-R C2) , Dg (C R) and D~ (R-C3), etc.
(table 9). These values are in good agreement with the dissociation energies of RCz(g ) as determined from the exchange reaction (Filby and Ames 1971a, b), although different thermodynamic functions were used and such a comparison may lead to a misleading conclusion.
The R C2 bond energy for the dicarbides and the tetracarbides are almost equal and the R - C 2 bond strength decreases on going from La to Y to Sc, analogous to the trend observed for the oxides LaO, YO, and ScO (Drowart and Goldfinger 1967).
These findings tempt many investigators to make predictions about the existence and physical stability of the ROz(g ) molecule although no isoelectronic RO 2 molecule has been observed. In addition, from a comparison of the bond energies between R-C, R-Cz, R-C3, R - C a and R - C s , the most preferable structure of the polyatomic carbides could be found; for instance, the linear structures C2-R-C3 and C3-R-C3 for RC 5 and R C 6 (R = Ce, Sc), respectively (Haque and Gingerich 1981).
Of course, it is best to determine the molecular and electronic structures of these carbides directly by optical spectroscopy, however, no information is available.
108 G. A D A C H I et al.
o~ ¢q
~ d
<~
¢)
,z=
L)
+ ~
~ r q
e~
p-. p-.
+1 + l
¢-q
eq
v q ~ q'~
• ,~ ¢ q
~
l +1 + lG~. tt3
+ l + l q'3
+ l
,q. r,q
+~
¢-q
+ l
~-- +l
~ . . . . o o q-I ~ e,i ~ ~ ~ • •
- +
q-I -H - + - I ~ q-I q-I ~ I - H q-I ~ + 1 ~
eq cq ~ " ,.o ~ 7, eq ~ +1 q-I q-I q-I
R A R E E A R T H C A R B I D E S 1 0 9
==
~5
~-~ .~ ~
~ ~ ~~'~5 ~ 5 ~'~
~ ~ ~ ~ =÷t ÷1 ÷1 ÷1 ÷1 ÷1 ÷1 ÷1 ÷1 ÷1 ÷1 ÷1 ÷1 ~ ~ ÷1
/I
÷1 o~ ÷1 ÷1 ÷1 ÷1 ÷1 ÷1 ÷1 c~ ÷1 ÷1 ÷1 ÷1 ÷1
~ ee3 ¢ ' q
÷1 ÷1 ÷1 ÷l
÷1 ÷l
t'v3 0"3
÷1 ÷l ÷1 ÷1 ÷1 ~ ~ ~ ÷1 ÷1 ÷t ÷1 ÷l ÷1 ÷1 ÷1 ÷1 ÷l ÷1 ÷1 ÷ 1 ~ ÷t ÷1
~ tt~ ~ tg3
÷1 ÷1 ÷1 ÷1
÷1 ÷1 ÷1 ÷1 ~ ÷1 ~ ÷1
÷1 ÷1 ÷1 ~ ÷1 ÷1 ÷1 ÷1 ÷ l ~
~ ~ o ÷ 1 ~ ~ ~ ~ ~ ÷1
. . +1 ~ o~
~ +~o~ + ~ + ~ ÷~ ~ ÷~ ÷ ~ ÷ ~
~5
t ' q
+
c- o
o o
.o
110 G. A D A C H I et al.
TABLE 8
Partial-pressure ratios P(RCE)/P(R ) and P(RCg)/P(R ) of rare-earth-metal-carbide-graphite systems.
Rare earth
metal T(K) P(RCz)/P(R) P(RC4)/P(R ) Reference
Sc 2319 2.6x10 -3 6.6x10 -6 Kohl and Stearns (1971b)
Y 2504 1.2x10 -1 1.3x10 a Kohl and Stearns (1970)
La 2561 1.2 2.4x10 -2 Stearns and Kohl (1971)
Ce 2500 1.0 4x10 -2 Balducci et al. (1969a)
Pr 2500 3.7x10 1 1.3x10 4 Balducci et al. (1970)
Nd 2500 1.5x10 ~ 7.5x10 -3 Balducci et al. (1970)
Gd 2500 3.1 x 10-1 Balducci et al. (1970)
Dy 2500 6.2x10 -3 l x l 0 -4 Batducci et al. (1970)
Ho 2500 5.8x10 3 l x l 0 -4 Balducci et al. (1970)
Er 2500 1.1×10 -2 Batducci et al. (1970)
Eu 2320 1.3x10 -3 Balducci et al. (1972)
TABLE 9
Bond dissociation energies" of R-C2, C 2 - R - C 2 and R - C D~(kJ m o l - l ) .
R - C z D~(R-C2) Reference R-Cz D~(R-C2) Reference
Sc-C 2 565+_21 Verhagen et al. (1965) N d - C 2 619_+21 de Maria et al. (1965) 566_+21 Kohl and Stearns (1971b) E u - C 2 539.7_+21 Balducci et al. (1972) Y C z 634_+ 19 Kohl and Stearns (1970) D y - C z 556.5 _+ 12 Balducci et al. (1969b)
653_+21 de Maria et al. (1965) H o - C z 556.5_+9 Balducci et al. (1969b) L a - C 2 664 Gingerich et al. (1981b) E r - C 2 565 + 13 Balducci et al. (1969d)
668_+8 Stearns and Kohl (1971) Lu-C 2 610__21 Guido et al. (1972) Ce C 2 678_+8 Balducci et al. (1969a)
P ~ C 2 661 _+ 25 Balducci et al. (1965)
C 2 - R - C 2 D~(C2-R-C2) Reference C 2 - R - C 2 D ~ ( C 2 - R ~ 2 ) Reference C2-Sc-C 2 1217_+22 Kohl and Stearns (1971b) C2-Ce C 2 1427_+46 Balducci et al. (1965) C 2 - Y - C 2 1271 _+ 2l Kohl and Stearns (1970) C2 Ho C2 1259_+ 80 Balducci et al. (1965) C 2 L a - C 2 1329_+ 16 Gingerich et al. (1981b)
R - C D~ (Ce-C) = 452_+ 29 kJ m o l - 1 Gingerich (1969)
D~(Sc-C) = 439.8_+21 k J m o l i (Haque and Gingerich 1981)
R-Ca, C4, C5 D~(Ce C j - 530 k J m o l - 1 ; D~(Ce-Cs) = 533 k J m o l -~ (Gingerich et al. 1976a) D ~ ( L a - C j = 505 kJ tool-1; D g ( L a - C J = 745 kJmo1-1 (Gingerich et al. 1981) These values are in good agreement with the dissociation energies of RC2(g ) as determined from the exchange reaction (Filby and Ames 1971b), despite the different thermodynamic functions used, but such a comparison may lead to misleading conclusions.
T h e r e f o r e , a s s u m p t i o n s h a v e t o b e m a d e c o n c e r n i n g t h e l o w - l y i n g e l e c t r o n i c s t a t e s a n d t h e i r m u l t i p l i c i t i e s , t h e m o l e c u l a r g e o m e t r y m o d e l a n d t h e f o r c e c o n s t a n t s i n o r d e r t o c a l c u l a t e t h e t h e r m a l f u n c t i o n s n e e d e d i n t h e e v a l u a t i o n o f t h e b o n d e n e r g i e s f r o m t h e m a s s s p e c t r o m e t r i c d a t a . I n g e n e r a l , i n d i r e c t i n f o r m a t i o n a s t o t h e p r o b a b l e
RARE E A R T H C A R B I D E S 111
favorable geometry of a new molecule can be obtained if reliable second- and third- law evaluations can be performed. F o r example, for the molecules LaC 3 and LaC 5, the linear chain structure with the lanthanum atom at the end of the carbon chain appears to be favored on the basis of the best agreement between the second and third laws, and also in terms of the smallest standard deviation in the third-law reaction enthalpy.
Concerning the possible structure of the higher complex carbides (RC,), the best agreement between the two thermodynamic treatments of the data and the smallest associated standard deviation were achieved with the linear symmetric structure for RC 2 and for R C 4 (Pelino et al. 1988a, b, Gingerich et al. 1981b, Kohl and Stearns 1971a, b), confirming the assumptions made by De Maria et al. (1965) and Stearns and Kohl (1971) about the structure of these molecules. In the same way with the linear asymmetric structure ( R - C , ) for RC 3, RC 5 and R C 6 (R = La, Y), one finds that the R C, structure is preferred to the linear structure C, R C m with the R atom in the center of the carbon chain (Pelino et al. 1984, 1988a, b, Gingerich et al. 1981b).
However, for RC 3, RC 5 and R C 6 ( R - - C e , Sc), the linear symmetric structures C 1 R C2, C 2 - R - C 3 and C 3 - R - C 3, respectively, were found to be the ones that are most preferred (Kingcade et al. 1983, Haque and Gingerich 1981).
Finally, it should be pointed out that for the molecular series LaC, (Gingerich et al.
1981b), CeC, (Kingcade et al. 1983) and YC, (Pelino et al. 1988a, b), the experimental atomization energies show an alternation of bond energies for each added carbon, with the molecules containing an even number of carbon atoms being more stable and, correspondingly, the relative abundances of these gaseous carbides that contain an even number of carbon atoms are higher than those of the corresponding species with an odd number of carbon atoms in the molecule. These facts further confirm the assumption made by Chupka et al. (1958) on the pseudo-oxygen character of the C 2 radical in the gaseous rare earth carbide molecules.
5.1.2. The species R 2 C n
The high-temperature Knudsen effusion mass spectrometric technique has also been used to identify and characterize the L a z C . (n = 2-6, 8) (Pelino et al. 1984), Ce2C, (n = 1-6) (Kingcade et al. 1984) and Y2C, (n = 2 8) (Pelino et al. 1988a, b) molecules in the equilibrium vapor above the L a - I r - g r a p h i t e system at 2220-2831 K, the cerium-graphite system at 2100-2800 K and the Y - I r - A u - g r a p h i t e system at 2532-2792 K, respectively. Recently, the stable gaseous dilanthanum monocarbide La 2 C has also been observed (Pelino and Gingerich 1989). The enthalpies AH~ of the reaction 2 R ( g ) + n C ( g r a p h i t e ) ~ RzCn(g ) were evaluated using the third law, and only for two dilanthanum carbides, La 2 C a (g) and La 2 C4(g), a second-law evaluation was also performed, indicating the linear symmetric structure, R - C , - R , as being the most probable.
The selected values for the reaction enthalpies were combined with ancillary literature data to yield the atomization energies AH°,0 and the standard heats of formation AH~,z98.15 of the gaseous di-rare-earth carbides (table 10).
In the equilibrium vapor above the rare-earth-metal-graphite systems described above at high temperature, the concentration of the molecules R 2C4 is greater than
112 G. ADACHI et al.
TABLE 10
Selected reaction enthalpies AH~, AH~98, for the reaction 2R(g)+ nC(graphite)~ R2C,(g ) and the atomization energies, A H~ °, 0 and AH]. 298, and the standard heats of formation, A H~' 218 of gaseous R 2 C n
(kJ mol- 1 ).
Species AH~ AH~98 A H ° , o /~Ha.298 AH~,298 Reference
La2C La2 C2 La2 C3 La2 C4 La2Cs La2C6 LazC8
Ce 2 C
Ce2 C2
Ce2C 3
Ce 2 C4 Ce2C 5
Ce2 C 6 Y2C2
Y2C3 Y2C4 Y2Cs Y2C6 Y2C7 Y2C8
945+30 625_+30 Pelino and Gingerich (1989) -247.2+40 -247.9_+40 1670_+40 1682_+40 621_+40 Pelino et al. (1984) -205.5_+27 -203.9_+27 2339+27 2355_+27 658___27 Pelino et al. (1984) -214.5_+21 -213.4_+21 3060_+21 3081-+21 648-+21 Pelino et al. (1984) -146.8-+50 -144.9_+50 3703_+50 3728_+50 717+_50 Pelino et al. (1984) -127.7+50 -125.1_+50 4395_+50 4425_+50 737_+50 Pelino et al. (1984) 31.0_+60 3 5 . 8 - + 6 0 5658_+60 5697_+60 898-+60 Pelino et al. (1984) 336_+4 339 1047_+26 1 0 5 6 506+26 Kingcade et al. (1984) 268_+7 269 1690___25 1702 576__+25 Kingcade et al. (1984) 198_+ 1 198 2332_+ 28 2348 647_+ 28 Kingcade et al. (1984) 231_+8.6 230 3076_+25 3 0 9 7 6 1 5 _ + 2 5 Kingcade et al. (1984) 98_+9 95 3654_+32 3 6 7 9 7 5 0 _ + 3 2 Kingcade et al. (1984) 79_+4 75 4346_+36 4 3 7 5 4770_+36 Kingcade et al. (1984)
t h a t of the R 2 C 3 species a n d is a p p r o x i m a t e l y e q u a l to t h a t of the p e n t a c a r b i d e s a n d h e x a c a r b i d e s . T h e r e m a i n i n g d i m e t a l c a r b i d e s h a v e a c o n c e n t r a t i o n a p p r o x i m a t e l y a n o r d e r of m a g n i t u d e less t h a n t h a t o f the d i m e t a l t e t r a c a r b i d e s . F o r e x a m p l e , the relative i o n c u r r e n t s o v e r the L a - I r - g r a p h i t e system m e a s u r e d with 20 V e l e c t r o n s at 2 8 2 9 K are: 1 . 5 × 1 0 - s ( L a 2 C + ) , 4 . 6 × 1 0 - 5 ( L a z C 2 + ) , 3 . 9 × 1 0 - s ( L a z C 3 + ) , 3.4 × 10 - 4 ( L a z C 4 ) , 6.5 × 10 - 5 ( L a z C s + ) , 1.3 × 10 - 4 ( L a z C 6 + ) , 3.0 x 10 - 6 ( L a z C 7 ) a n d 4.6 x 10 - 6 ( L a z C 8 ) ( G i n g e r i c h 1985). Therefore, for these d i m e t a l c a r b i d e s it is i n t e r e s t i n g to n o t e t h a t the ones with a n even n u m b e r of c a r b o n a t o m s a p p e a r to be favored; in p a r t i c u l a r , the R 2 C 4 molecules, w h i c h m a y be c o n s i d e r e d as the d i m e r of the very s t a b l e ( a b u n d a n t ) R C 2 molecules, are m o s t a b u n d a n t . I n c o n c l u s i o n , it can be said t h a t the p s e u d o - o x y g e n c h a r a c t e r of the C 2 g r o u p b o n d e d with an e l e c t r o n d o n o r a t o m forms a s t r o n g e r b o n d for a n even n u m b e r of c a r b o n a t o m s while the c o v a l e n t b o n d s in a c a r b o n c h a i n are m o r e stable for a n o d d n u m b e r of a t o m s (Pelino et al.
1988a, b).
5.1.3. Ternary gaseous rare-earth-containing carbides
T h e t e r n a r y r a r e - e a r t h - m e t a l - p l a t i n u m - m e t a l c a r b i d e s h a v e been o b s e r v e d in the K n u d s e n effusion m a s s s p e c t r o m e t r i c i n v e s t i g a t i o n s of the g a s e o u s species o v e r the (La, Ce, Y, S c ) - ( P t , Rh, Ir, Ru, e t c . ) - g r a p h i t e systems. T h e existence of R h C e C , R u C e C 2, R u C e C , R u C e C 2 a n d P t C e C 2 ( G i n g e r i c h et al. 1976b, G i n g e r i c h 1974),
RARE EARTH CARBIDES TABLE 11
Atomization energies, AH °, o, of ternary gaseous rare-earth-containing carbides (kJ tool-1 ).
113
Molecule ASa, o Reference
CeSiC 1046_+42 Guido and Gigli (1973)
LaIrC 1027_+ 30 Pelino et al. (1986)
YIrC 996_+33 Gingerich et al. (1981a)
CeRhC 1031 _+40 Gingerich and Cocke (1979)
ScRhC 1010_+40 Haque and Gingerich (1979)
CeRuC 1109 _+ 40 Gingerich and Cocke (1979)
S c R h C 2 1629_+ 50 Haque and Gingerich (1979)
YRhC 2 1672_+ 50 Haque and Gingerich (1979)
CePtC 2 1695 _+ 50 Gingerich (1974)
YIrC 2 1678_ 33 Gingerich et al. (1981a) LaIrC 2 1779_+ 25 Pelino et al. (1986)
LaIrC 3 2344+ 35 Pelino et al. (1986)
LaIrC 4 2966_+ 35 Pelino et al. (1986)
RhScC, R h S c C 2 and RhYC2 (Haque and Gingerich 1979), etc., have been established.
The relative c o n c e n t r a t i o n s (relative ion currents) of the molecules, L a I r C +, L a I r C 3 + and L a I r C 4 + over the L a - I r graphite system m e a s u r e d with 20 V electrons at 2829 K have been reported to be 2 . 2 x 10 -4, 7 . 9 x 10 -6 and 3 . 9 x 1 0 -6, respectively (Gingerich 1985). O w i n g to the presence of a C - C b o n d ( ~ 600 kJ tool. - 1 ) a n d a R - C 2 b o n d ( ~ 650 kJ tool. -~) a n d multiple R - M b o n d s (400-500 kJ t o o l . - 1), these c o m p o u n d s are highly stable. Their a t o m i z a t i o n energies, AH°,o, have been deter- mined (table 11).
5.2. Thermodynamic properties of the solid state rare earth carbides
The v a p o r i z a t i o n studies of the solid rare earth carbides by using the mass spectrometry K n u d s e n effusion m e t h o d have succeeded in determining the s t a n d a r d enthalpy of formation, AH~, 29s, for some of the solid rare earth dicarbides t h r o u g h the reaction
RC2(s ) ~ R(g) + 2C(s),
but it is necessary to use estimated thermal functions when reducing the equilibrium d a t a taken at high temperatures u p t o 298.15 K.
The e n t r o p y of f o r m a t i o n at 298 K for several R C 2 phases a n d a few R2C 3 phases were calculated by G s c h n e i d n e r and C a l d e r w o o d (1986) in their progress report using, respectively, the k n o w n values of 70.29 J m o l . -1 K -1 for CaC2 a n d of 47.46 J m o l . - 1 K - 1 for H o 2 C 3 (Wakefield et al. 1965), as well as assuming the validity of the following relationships:
SRC 2 = SCaC2 ~- S R -- Sca
114 G. A D A C H I et al.
and
SR2C3 = SHozC 3 -1- 2 S R - - 2SHo.
Values of SR, SCa and Sno were taken from Hultgren et al. (1973).
On the basis of the experimental high-temperature mass spectrometric thermody- namic data and the estimated free-energy functions and heat content data for the dicarbides of the rare earth metals, the enthalpies of formation of solid R C 2 have been reported by a number of scientists (table 12). It should be noted that the data reported by Anderson and Bagshaw (1970, 1972) were obtained from a study by using a solid state galvanic cell with a calcium fluoride electrolyte. The reduction of their data by the second law gave erroneous results but that by the third law gave values for CeC 2, LaC 2 and YC 2 that compared favorably with those obtained by mass spectro- metry. In addition, the data from Baker et al. (1971) were obtained by using bomb calorimetry.
Anderson and Bagshaw (1970) reported high-temperature e.m.f, data for La2C 3, Ce2C3, Pr2C 3 and NdzC 3. Gschneidner and Calderwood (1986), utilizing free-energy function estimates and the AG~, T equations presented by Anderson and Bagshaw, calculated AGE, 298 values, and thus the AH~' 298 values (AH~, r = AG~, r - ASp, r T).
TABLE 12
Enthalpy of formation of solid RC 2 (kJ mol-1).
Species Anderson and Faircloth et al.
Bagshaw (1972) (1968) Other references
L a C 2 - 87.4 - 108 (second law) - 89_+ 24 (Stearns and Kohl 1971)
- 7 5 (third law)
CeC 2 --88.7 -- 104.6 - 9 7 - + 5 . 4 (Baker et al. 1971), - 8 1 . 6 (Winchell and Baldwin 1967), - 6 3 _ + 2 0 (Balducci et al. 1969a, b,c,d)
PrC2 - 84.5
N d C 2 - 88.7 - 52.3 _+ 10.5
SmC2 - 65.2 _+ 6.7
EuC 2 - 6 6 . 9 _+ 5.4
G d C 2 - 10i.2
DyC2 HoC2
- 9 4 . l
ErC 2 - 110.8
T m C z YbC 2 L u C 2
YC 2 - 105.0
- 9 7 . 9 (Stout et al. 1969) - 3 8 . 3 (Gebelt and Eick 1966a, b),
67.5 (Cuthbert et al. 1967) - 8 2 (Jackson et al. 1963), - 125.5 (Hoenig et al. 1967) - 8 8 (Balducci et al. 1969b),
110_+15 (Huber et al. 1973) - 4 6 . 2 (Balducci et al. 1969b)
- 8 8 + 4 (Balducci et al. 1969b),
- 8 2 (Wakefield et al. 1965) - 8 6 . 4 (Balducci et al. 1969d) - 9 8 . 7 (Seiver and Eick 1971) -75.3___4 (Haschke and Eick 1968) - 1 1 7 _ 2 1 (Guido et al. 1972) - 9 1 + 17 (Kohl a n d Stearns 1970),
- 119 (Storms 1971), - 9 8 (de M a r i a et al. 1965)
RARE EARTH CARBIDES TABLE 13
Entropy, free energy and enthalpy of formation of solid R2C 3 at 298 K.
ASf,298 -- AGf,298 - - A H f , 2 9 8 ( T A S - AG)
Species (J tool- i ) (kJ tool- i ) (kJ tool - i ) Reference
115
La2C 3 - 24.18 a 108.69 ~ 115.89
Ce2C 3 40.77 a 111.72" 105.99
PrzC 3 46.14" 97.65 ~ 83.91
Nd2C 3 42.72" 114.99" 102.27
Sm2C 3 40.83" 123.18 111
Ho2C 3 47.40 48.93 34.8
Anderson and Bagshaw (1970) Anderson and Bagshaw (1970), Baker
et al. (1971)
Anderson and Bagshaw (1970) Anderson and Bagshaw (1970) Haschke and Deline (1982) Wakefield et aI. (1965) These data are obtained from Gschneidner and Calderwood (1986).
The calculated values are listed in table 13, together with a tew experimental values for SMC1.43 (Haschke and Deline 1982) and Ho2C 3 (Wakefield et al. 1965).
For other binary rare earth carbides, a few thermodynamic data have also been determined. F o r example, Storm (1971) reported the free energy of formation at 1700 K, - 6 4 . 3 kJ mol -* for Y2C; - 112.5 kJ tool -1 for Y5C6; - 125.44 kJ tool -1 for Y2C3 and - 1 4 3 . 2 2 kJ mo1-1 for YC2, but these values have not been reduced to 298.15 K. In addition, several investigators reported only high-temperature AHv values for the RC 2 phases. These values are: AHv, 220o = 543.0 kJ m o l - 1 for LaC 2 (Jackson et al. 1963), AHv,2,4s = 149.1 + 13.8 kJmo1-1 for N d C 2 (de Maria et al.
1967), and AHv, 169o = 272.7 __ 16.8 k J m o 1 - 1 for SmC2 (Avery et al. 1967) for the reaction RC2(s)--*R(g ) + 2C(s); AHv, 2~50 = 596.7 + 2 . 4 k J m o 1 - 1 for GdC2 (Jackson et al. 1963) for the reaction G d C 2 ( s ) ~ G d C 2 ( g ) ; AHv, 16o7 = 509.0 _+ 33.5 kJ mol -~ for Sm2C 3 (Avery et al. 1967) for the reaction Sm2C3(s) --* 2Sm(g) + 3C(s).
In addition to the experimental thermodynamic values, Niessen and de Boer (1981) calculated the enthalpies of formation at 0 K according to a semiempirical model proposed by Miedema et al. (1980) for the carbides R2C , RC and RC2, where R = La, Sc, Y, AHf . . . temperature = 99, 132 and 111 for La2C, SczC and Y2C; 100, 132 and 112 for LaC, ScC and YC; 180, 210 and 192(kJmo1-1) for LaCz, ScC2 and YC2, respectively. Comparison of the predicted heats of formation for LaG 2 and YC 2 with the experimental values shows these predicted values to be inaccurate by about 100%.
6. Ternary rare-earth-X-carbon phase diagrams and ternary carbides 6.1. Phase diagrams and formation of ternary carbides in R - B - C systems
6.1.1. Ternary R - B - C phase diagrams
Although only a few phase diagrams of the R - b o r o n - c a r b o n systems [R = Y (Bauer and N o w o t n y 1971), Eu (Schwetz et al. 1979), Gd (Smith and Gilles 1967), and Ho (Bauer et al. 1985)] have been published, several studies on the formation of the
116 G. A D A C H I et al.
rare earth borocarbides have been carried out by researchers. In ternary R B-C systems, there are nine compounds, i.e. the RBzCz, RBzC, RBC3, RBC, RB2C 4,
R s B 2 C s , R s B z C 6 , R15BaC17 and R z B C 2 compounds. Their existences and crystal structures have been reported.
6.1.1.1. The yttrium-boron carbon phase diagram.
Bauer and Nowotny (1971) investigated the ternary system Y-B-C. The arc-melted alloys were quenched without additional annealing. Four ternary phases Y B z C 2 ,
YBaC, YBC and YBo.sC were found, all of which melt congruently. The phase diagram of the ternary system Y B-C in the as-quenched condition is shown in fig. 15.
According to their X-ray diffraction data, three of the four ternary compounds have been identified (table 14), the crystal structures have been determined and one of these structures is shown in fig. 16.
¥
"Y 8C~Y15Ci9
B 13C2
Fig. 15. Phase diagram of the Y - B - C system in the as-quenched condition (Bauer and N o w o t n y 1971).
(Reprinted by permission of the publisher, Institut f/it Anorganishe Chemie, Inc.)
TABLE 14
Crystal structures o f t e r n a r y yttrium borocarbides.
C o m p o u n d Crystal structure Lattice parameters
(A)
Interatomic distance (~,) Y Y Y - B Y - C B - C C - C B - C YB2C2
YB2C
YBC
Tetragonal
Tetragonal
O r t h o r h o m b i c
a = 3.79(6) c = 7.12(4) c / a = 1.876 a = 6.76(9) c = 7.430 c / a ~ 1.096 a = 3.38(8) b = 13.69(3) c = 3.62(7)
3.68 a 2.75 2.68 1.76 1.28 1.62
3.60" 2.73 2.55 1.75 - 1.64
3.71 a 2.70" 2.55 1.98 - 1.65
" The average value.
RARE EARTH CARBIDES 117
~
Y [Q~N Z=OB IN Z=l/4 0 C 1N 0 Z=l/4
©
[3 1N Z=3/4
©
C IN 7=3/4
Fig. 16. Crystal structure of the YB2C 2 phase with a D2a(P42c ) space group (Bauer and Nowotny 1971). (Reprinted by permission of the publisher, Institut f'tir Anorganische Chemie, Inc.)
As for the YBo.sC compound, they found that this compound was present in the mixture with a composition of 40: 20: 40 at. % (Y: B : C) as a homogeneous form, and its X-ray diffraction pattern was similar to that of the Y15 C19 carbide. More recently, Hfijek et al. (1984a-d) have determined the composition to be Y15BzC17.
6.1.1.2. The europium-boron-carbon phase diagram. The ternary system E u - B - C has been studied, with special emphasis on the technologically important sections EuB6-C and EuB6-B4C (Schwetz et al. 1979). Samples were synthesized from the pure elements at 1500°C in sealed molybdenum capsules and annealed for 20 h. The X-ray diffraction pattern of EuB2C2 has been indexed on the basis of a tetragonal unit cell with a = 3.77 ~. and c = 4.03 ~..
The pseudobinary sections of EuB 6 C and EuB6-B4C have been studied. The solubility of carbon in europium hexaboride was measured by using equilibrate samples annealed at various temperatures. Schwetz et al. found that the lattice parameters of the EuB 6 _xC x solid solutions decrease linearly with increasing carbon substitution x, in excellent agreement with the results of Kasaya et al. (1978). The solid solubility limits are 0.82 wt.% C (x = 0.15) at 1400°C, 0.87 (x = 0.16) 1600°C, 1.04 (x = 0.19) 1800°C and 1.08 (x = 0.20) 2000°C, respectively, and the maximum solubil- ity 1.4 wt.% C (x = 0.25) at the eutectic temperature was obtained from the sample containing more than 50 at.% C and quenching from the melt. In carbon-rich ( > 3 wt.% C) samples equilibrated at temperatures higher than 2000°C, a new phase coexists with Eu(B, C)6.
Along the EuB 6 B4C section again the formation of the solid solution EuB6_xC x occurs, but no ternary compound was found. The isothermal section of the tentative phase diagram of the e u r o p i u m - b o r o n - c a r b o n system at 1500°C has been deduced (see fig. 17). To give a better overview, the Eu(B, C)6 and BIz+xC2_ x solid solution has not been included within the diagram.
6.1.1.3. The gadolinium-boron-carbon phase diagram. The G d - B - C system in the range 2000-3000°C was determined (Smith and Gilles 1967, fig. 18). The upper portion of this diagram is less well-established than the lower part. This diagram shows only the approximate composition of the phases and does not necessarily show