2.1. Two prototypes of binary rare-eart~carbon phase diagrams
Only two nearly complete and one partial binary rare-earth carbon phase dia- grams have been reported. Almost complete Phase diagrams were proposed by Spedding et al. (1959) for the lanthanum-carb0n system, and by Calson and Paulson (1968), refined by Storms (1971), for the yttrium-carbon system. The partial cerium-carbon phase diagram reported by Stecher et al. (1964) included only the data on the existence and crystal structure of the phases from thermal analysis and metallographic observation. The entire liquidus and the peritectic and eutectic reactions were not determined and are thus shown as dashed lines in their diagram. As is well known, the lanthanides can be divided into two groups, the light rare earths and the heavy rare earths. Lanthanum behaves much like cerium, praseodymium and neodymium, and yttrium behaves like dysprosium and holmium. Both lanthanum and yttrium could be regarded as the representative elements of the light rare "earths and the heavy rare earths, respectively. Therefore, both the phase diagrams of the
64 G. ADACHI et al.
l a n t h a n u m - c a r b o n and y t t r i u m - c a r b o n systems may be regarded as the prototypes of the phase diagrams of the light-rare-earth-carbon and heavy-rare-earth-carbon systems, respectively. As is described later, the l a n t h a n u m - c a r b o n phase diagram is available for all the light lanthanide systems with carbon, but the y t t r i u m - c a r b o n phase diagram may be available for only some of the heavy-rare-earth-carbon systems, because the types, compositions and structures of the rare earth carbides in each of the heavy-rare-earth carbon systems change as the atomic number of the rare earth increases. In particular, at about holmium the rare earth sesquicarbides with the Pu2C3-type body-centered cubic structure disappear in the systems and are substitu- ted by the tetragonal Sc15C19-type carbides in the systems of holmium, erbium, thulium and lutetium with carbon (Bauer and N o w o t n y 1971). In addition, it was shown (Krikorian et al. 1967) that the lutetium dicarbide has a different stable room temperature modification from the other heavy lanthanide dicarbides.
2.1.1. Phase diagram of the lanthanum-carbon system
A phase diagram of the L a - C system has been proposed on the basis of thermal metallographic, X-ray, dilatometric and electrical resistance analyses (Spedding et al.
1959), as is shown in fig. 1. Using La metal, which was prepared by the metallothermic reduction of the fluoride with Ca metal, and high-purity graphite, alloys were prepared by arc melting under an atmosphere of purified helium or argon. Photo- micrographs of quenched low-carbon alloys indicated that the solid solubility of C in La lies between 1.6 and 3.4 at.% at 775°C, and slightly below 1.6 at.% at 695°C. The eutectic between La and LazC3 occurs at 806 _ 2°C at a composition of 20.6 at.% C.
The solubility limit of C in J3-La at the eutectic temperature is 3.4 _ 0.6 at.% C. The solid solution range of LazC 3 at room temperature was found to extend from 56.2 to 60.2 at.% C. The C content at the L a - L a z C 3 phase boundary was found to vary from 54 +_ 0.7 at.% at 800°C to 57.1 + 1.2 at.% at 1000°C. LazC 3 forms peritectically at 1415 __ 3°C on cooling from the melt and LaC 2. The crystal structure of this compound was found to be body-centered cubic of the PuzC 3 type with the I743d space group (Atoji et al. 1958). The melting point of LaC2 is 2356 _+ 25°C. This compound is dimorphic, with tetragonal CaC2-type structure at room temperature and cubic CaFz-type structure at high temperatures. The polymorphic transformation occurs at 1800 _+ 100°C, which is in good agreement with the value of 1750°C reported by Bredig (1960). However, more recent investigators have reported considerably lower temperatures for this transition, Krikorian et al. (1967) gave a value of 1060°C. They speculated that the definite change in slope of the electrical resistance curves between 1750 and 1800°C observed by Spedding et al. (1959) may have been related to some change in carbon content at that temperature. In fig. 1 this temperature is shown as 1060°C.
The temperature of the eutectie reaction between LaC 2 and C is also shown in fig. 1 at 2250°C, which is the average of the two reported values by Spedding et al. and by Krikorian et al.
e-LaC2 contains less than the stoichiometric amount of C at room temperature, and the lattice parameter of the alloy around the LaC 2 composition varies slightly with C
RARE EARTH C A R B I D E S 65
L A N T H A N U M - C A R B O N 2 4 0 0
o 2000
1600
I
900 I000 i
875 °
700
8 0 6 ° 20.6 % ) + (vLo)
(,SLo) + (Lo2C 3)
,zfj 600
1200
¢w o..
b--
8 0 0
4°0 k
c~La~_
O ' 0
La
0 10 20 30
AT. % C
8 0 6 ° 20.6 %
510"
J o m I I
1060 °
I I b',%~ I
20 4 0 60 80
ATOMIC PERCENT CARBON
Fig. 1. Assessed La-C phase diagram (Gschneidner and Calderwood 1986).
content, indicating a solid solubility range from about 2.3 to 3.4 at.%. It reaches the stoichiometric dicarbide composition at higher temperatures.
The microscopic examinations showed that a 64.4 at.% C specimen revealed a single-phase structure at 1385 + 25°C, from which La2C3 precipitated during furnace cooling to room temperature, and a 62.5 at.% C sample consisting of two phases on quenching from 1400 + 25°C, but on slow cooling the amount of the La2C 3 phase increased further at the expense of the LaC2 phase.
The study of the La C phase diagram showed that only two carbides, LaC2 and
La2C3,
exist, which are the compounds with a solid solubility range instead of a line compound. La2C 3 forms b y a peritectic reaction while LaC 2 melts congruently.66 G. A D A C H I et al.
2.1.2. Phase diagram of the yttrium-carbon system
The phase diagram of the yttrium-carbon system was proposed by Carlson and Paulson (1968) and refined by Storms (1971), is shown in fig. 2.
Carlson and Paulson used Y of 99.9 wt. % purity prepared by Ca reduction of YC13.
Samples were prepared by arc melting a Y sponge with pieces of high-purity graphite in a purified argon atmosphere. The solidus was determined by observations with an optical pyrometer focussed on a small hole in the specimen. Thermal analysis was employed in the Y-rich end of the system. Specimens were also examined under the microscope after quenching in an oil bath.
Storms (1971) studied the phase relationship at temperatures between 1027 and 1827°C over a wide composition range in the Y-C system using a combination of mass spectrometric and thermal analysis techniques. Samples were prepared by arc melting purified crystal bar yttrium metal and a spectroscopically pure graphite rod.
Y T T R I U M - C A R B O N
1 7 0 0
1600 2 oo
2 2 0 0 1300(
?
z )
o,_
i I
i /.4 I
L i I ( L ) + ( y )
1522 ° / 156o ° 2415 °
~ e . e % ( , S v ) + ( Z )
1520° - [~,~\ /I
# !~.28.,~-,,/ 11~'7.'~"v ' / 2 2 9 0 ° '.6V!'{17;'
ATOMIC PERCENT CARBON
20 4 0 60 80
ATOMIC PERCENT CARBON
100 C
Fig. 2. Assessed Y - C phase dia- gram (Gschneidner and Calder- wood 1986).
RARE EARTH CARBIDES 67 The as-supplied yttrium metal was purified by heating the metal in vacuum just below the melting point to reduce the YF+/Y ÷ ratio from 10 to 10 -4. The phase boundaries were obtained from a plot of the logarithm of Y activity at various temperatures against the C to Y ratio by observing the intersections of the curve through single- phase regions with the horizontal line through two-phase regions.
According to Carlson and Paulson, the Y-C system consists of three compounds;
Y2C, Y/C 3, and YC2. At high temperatures, Y2 C and YC2 are face-centered cubic and join in a single,phase region above 1645°C. On the other hand, 13-Y2C 3 decomposes congruently into this phase. The phase diagram proposed by Storms is in good agreement with that of Carlson and Paulson but with two major exceptions.
Storms found a higher C concentration at the low-C phase boundary of the hypo- carbide phase and the YsC6 phase, which exists in at least two crystal forms and decomposes at about 1527°C into [3-Y2C 3 and Y2 C. On the basis of the review of Gschneidner and Calderwood (1986), this new phase was regarded as Y15Ca9 with the ScasC19-type tetragonal structure (Bauer and Nowotny 1971).
Figure 2 shows the finally refined Y-C phase diagram. The solvus data for the Y- rich end are taken from Carlson et al. (1974). The low-C phase boundary for the 7-(Y3C ) phase is midway between the boundary presented by Carlson and Paulson, and Storms. Above 900°C, the Y-rich 7 phase has a cubic structure of the F%N type.
Below about 900°C, the ordering of C atoms creates the CdC12-type trigonal structure for Y2C. The solidus line of the 7 phase exhibits a maximum at 2000 + 15 ° C at about 30 at.% C, and a minimum at 1805 _+ 5°C near 54at.% C. The Y15C19 phase decomposes into the cubic Fe4N-type Y3 C phase and [3-YzC3 phase at 1527°C, and undergoes a polymorphic transformation at 1304°C. The ~ phase, for which no structural data are available, has a tetragonal Scl 5 C19-typ e structure. The Y2C3 phase forms from the 7 phase solid solution at 1645°C and the ~ J 3 transformation occurs at 1186°C. In addition, a metastable yttrium sesquicarbide with the bcc PuzC3-type structure formed at a high temperature under a high pressure (Krupka et al. 1969a).
The YC 2 phase melts at 2415°C, and the eutectic between YC 2 and C is placed at 2290°C, the mean of the acceptable data (Carlson and Paulson 1968, Storms 1971, Krikorian et al. 1967, Kosolapova and Makarenko 1964). The ~-YC2 phase has been observed to have tetragonal CaCz-type structure. The high-temperature form was stated by Storms (1971) to be cubic, assuming that [3-YC/ has cubic CaFz-type structure similar to the ~-RC 2 phase for R = La, Ce, Eu, Tb, Lu. The ~ [3 trans- formation occurs at 1324°C, the average of the acceptable values (Krikorian et al.
1967, Adaehi et al. 1976, Carlson and Paulson 1968, Storms 1971).
2.2. General characteristics of phase diagrams of the light-lanthanide-carbon systems
Except for the phase diagram of the lanthanum-carbon system, no complete phase diagram is available for the light-lanthanide-carbon systems. Only some data on the formation of the carbides have been reported. However, on the basis of these data, the general characteristics of the binary light-lanthanide-carbon phase diagram can be deduced.
68 G. ADACHI et al.
In these systems, two types of rare earth carbides have been confirmed to exist; the rare earth dicarbides and the rare earth sesquicarbides. The existence of the mono- carbides of cerium and praseodymium (Warf 1955, Brewer and Krikorian 1956, Dancy et al. 1962) has been discredited. It was found on the basis of an X-ray study (Spedding et al. 1958) that the earlier reported cerium monocarbide was most probably a solid solution of carbon in cerium; the lattice parameter reported by Brewer and Krikorian (1956) for the cerium monocarbide was identical with that of cerium metal saturated with carbon.
The sesquicarbides of cerium, neodymium and praseodymium were shown as forming peritectically from the melt, the corresponding rare earth dicarbide just like the lanthanum sesquicarbide (see fig. 1) at 1505°C [the average of the values reported by Stecher et al. (1964), Anderson et al. (1969), Paderno et al. (1969) and Kosolapova et al. (1971), discarding 1800°C of Stecher et al. (1964)], 1560°C of Kosolapova et al.
(1971) and 1620°C of Paderno et al. (1969a, b). These compounds, except for the cerium sesquicarbide, exhibit a solid solubility range (Spedding et al. 1958) with different lattice parameters for the metal-rich and the carbon-rich phases. All of the lighter lanthanide sesquicarbides have the cubic Pu3C3-type structure with the I43d space group (for Ce, Spedding et al. 1958, Atoji and Williams 1961, Anderson et al.
1968, Baker et al. 1971; for Pr and Nd, Spedding et al. 1958).
The dicarbides of cerium, neodymium and praseodymium as well as lanthanum crystallize congruently from the melt and form a eutectic mixture with carbon. The melting points of the dicarbides were given by Kosolapova and Makarenko (1964) as 2540 _+ 100°C for the cerium dicarbide, and 2535 + 100°C for the praseodymium dicarbide. For the neodymium dicarbide, the reported melting point temperatures range from 2207 to 2280 _+ 15°C, whereas Krikorian et al. (1967) found the eutectic temperature to be 2275 _+ 20°C. Because of this discrepancy and the lack of the available data, the melting temperature of the neodymium dicarbide would be certainly estimated to be greater than 2260°C. The analogous situation also occurs for the praseodymium dicarbide. The reported melting temperatures by Paderno et al.
(1966) and Makarenko et al. (1965) were in the range from 2147 to 2160°C, which is about 100°C below the PrC2-C eutectic temperature found by Krikorian et al. (1967).
Thus, these data were discarded in the case of determining a reasonable value. The compositions for the eutectic of the light lanthanide dicarbide with carbon were not found in the literature. The temperatures of the LaC 2, CeC2, PrC2 and N d C 2 - C eutectic reactions are at 2250, 2260 [the average of 2270 + 20°C (Winchell and Baldwin 1967) and 2245 _+ 20°C (Krikorian et al. 1967)], 2255 and 2075 _+ 20°C (Krikorian et al. 1967), respectively.
Like the lanthanum dicarbide, the cerium dicarbide exists in two types of modifica- tion, the high-temperature [3-CeCz having a cubic CaF2-type structure with the space group Fm3m, and the room-temperature ~-CeC 2 having a tetragonal structure of CeC 2 type with the space group I4/mmm. The ~ [ 3 transformation occurs at 1100 _+ 20°C (Winchell and Baldwin 1967). Other reported values for this transformation temperature range from 1090 to 1107°C (Krikorian et al. 1967, McColm et al. 1973, Loe et al. 1976, Adachi et al. 1978). The transformation temperatures for the praseodymium dicarbide and the neodymium dicarbide have also been
RARE EARTH CARBIDES 69 reported to be at 1130 _ 30°C and 1150 _ 20°C (the latter being the average of 1120 + 10°C to 1164 _+ 6°C), respectively, and their room-temperature modifications have been found to be of the tetragonal CeCz-type structure. However, no information is available on the structure of their high-temperature modifications. Because the known high-temperature modifications of the dicarbides of lanthanum and cerium have the cubic CaFz-type structure, with the Fm3m space group, it seems reasonable to expect 13-PrC2 and [3-NdC2 to follow this pattern. In addition, E1-Makrini et al. (1980) prepared the compounds R C 6 (R = Ce, Pr, Nd, Sm, Eu, Yb) by compression of powder. However, the structures were not determined at that time.
According to the general characteristics of the light-lanthanide (La, Ce, Pr, Nd) carbon phase diagrams, the hypothetical ideal phase diagram for misch metal- carbon has been drawn (Gschneidner and Calderwood 1986, fig. 3). They assumed that the ideal misch metal contains 27 at. % La, 48 at. % Ce, 5 at. % Pr, 16 at. % Nd, and 4 at. % other rare earths that behave as 2 at. % Gd and 2 at. % Y. The sesquicar- bide of each of these lanthanides decomposes peritectically at temperatures that increase systematically with atomic number. By applying the method of Palmer et al.
(1982) to calculate the misch metal properties from the reported data for the light- lanthanide-carbon systems involved, a peritectic reaction is calculated to occur at 1510°C, where the misch metal sesquicarbide decomposes to the melt and 13-misch metal dicarbide, which melts at about 2425°C and undergoes an ~ 13 transformation at 1100°C. Below this temperature the misch metal dicarbide exists in the tetragonal CaCz-type structure and above this temperature it transforms to the cubic CaFz-type structure. The misch metal-carbon eutectic composition is not known, because no composition was given for any of the individual rare-earth-carbon systems. A eutectic temperature of about 2260°C was also estimated by Gschneidner and Calderwood (1986). As for the solid solubility region for the dicarbide and sesquicarbide of misch metal, the sesquicarbides of lanthanum, praseodymium and neodymium exhibit solid solubility while the cerium sesquicarbide does not, and the cerium is 48 at.% of the misch metal. The solid solubility region for the misch metal sesquicarbide was narrowed from that of the lanthanum carbon phase diagram. The solid solubility for the misch metal dicarbide was suggested to be lower on the basis of only the data of the lanthanum dicarbide because no information is available about the Ce-C, Pr-C, and Nd-C systems.
2.3. General characteristics of phase diagrams of the heavy-lanthanide carbon systems
Here, the so-called heavy lanthanides include the elements from samarium to.
lutetium, except for ytterbium and europium which behave like bivalent metals and have unique properties. For these heavy-lanthanide-carbon systems, no complete phase diagram was found, only some information about the formation and the crystal structure of the carbides is available. On the basis of these data the general characteristics of the phase diagrams of the heavy-rare-earth carbon systems can be summarized. In this case the yttrium carbon phase diagram may be regarded as the best prototype available for compounds of the heavy lanthanide systems with carbon.
70 G. ADACHI et al.
The existence of the hypocarbides of the heavy lanthanides has been established (Spedding et al. 1958, Huber et al. 1973, Atoji 1981, Aoki and Williams 1979, Bacchella et al. 1966). Like the yttrium hypocarbides, the cubic tri-rare-earth carbides
~-R3C with the Fe4N-type structure are the high-temperature forms of the heavy lanthanide hypocarbides, which can exist at room temperature in a metastable state and have a solid solubility range as reported for R = Gd (Spedding et al. 1958), Dy (Aoki and Williams, 1979) and Er (Atoji 1981). This form was obtained immediately from the melt. The stable room-temperature form has the approximate composition R2C and the rhombohedral CdClz-type structure for R = Gd, Tb, Dy, Ho. For the erbium hypocarbide ErxC, if x is close to 2, the cubic form transforms at lower temperatures to a trigonal CdC12-type structure, and if x is much different from 2, the cubic structure is retained at all temperatures (Atoji 1981). For the hypocarbide of thulium and lutetium, no report of a trigonal compound with the CdC12-type structure was found. There is some difference of opinion concerning the existence of the tri-samarium carbide between Haschke and Deline (1980) and earlier investi- gators. The latter reported the existence of Sm3C (Spedding et al. 1958, B6rner et al.
1972, Hackstein et al. 1971), while Haschke and Deline insisted that binary Sm3C was not formed on the basis of their reinvestigation of phase equilibria in the Sm-C system, and suggested that the phases described by Hackstein et al. (1971) and B6rner et al. (1972) actually were oxycarbides, because the colors, the C-to-Sm ratio, the structure, and the lattice parameters reported for the Sm3C phase were nearly identical with the corresponding properties observed by them for SmO0.sC0.,.
However, they could not explain the reason why the lattice parameter reported by Spedding et al. (1958) for Sm3C is about 0.01 A larger than that observed by Haschke and Deline for the oxycarbide.
In addition to the hypocarbides, the dicarbides of the heavy lanthanides were reported to exist in all the heavy-lanthanide-carbon systems. Until now, three forms of the dicarbide have been reported, the tetragonal CaCz-type room-temperature structure, the cubic CaFz-type high-temperature structure and the orthorhombic LuCz-type structure. These structures formed under high pressure or during long annealing at high temperatures from ~ - R C 2 with the CaCz-type structure for the systems of R = Tb, Dy, Y or R = Ho, Er, Tm (Krupka et al. 1968). From the viewpoint of phase equilibrium, these compounds of holmium, erbium and thulium with such an orthorhombic LuC2-type structure should be the more stable phase with respect to the [~-RC 2 phase at least at about 1150, 1305 and 900 to 1250°C, respectively.
The room-temperature forms of the heavy-lanthanide dicarbides possess the body- centered tetragonal CaCz-type structure (Spedding et al. 1958) with the exception of the lutetium dicarbide, which has the low-symmetry orthorhombic structure with a large cell volume (Krupka et al. 1968) but was also designated as the ~ - R C 2
compound. These a-RC2 compounds transform to the high-temperature cubic struc- ture. These temperatures have been assessed by Gschneidner and Calderwood (1986) and a plot of the average transition temperature against the atomic number of the lanthanides has been given in combination with the data for the light rare earth dicarbides. As shown in fig. 3, it can be seen that the transition temperatures of the
RARE EARTH CARBIDES 71 1500
W
130C
r ~ U J f l .
I I I t l l l l l i l l l l l
O
1
I t I I I i I I I I t ~ I I
57 59 61 63 65 67 69 71 Fig. 3. The transformation temperatures of Ce Nd Sm C,~ 13/ Er gb the rare earth dicarbides (Gschneidner and
ATOMIC NUMBER Calderwood 1986).
RC2 compounds increase with increasing lanthanide atomic number, roughly follow- ing two straight lines which intersect at gadolinium, with the exception of the dicarbides of europium, ytterbium (with a mixed-valence state) and lutetium (~-LuC2 has a different stable crystal structure from the other ~-RC 2 compounds). The distinction between the light rare earths and the heavy rare earths may be noted from the YC2 transformation temperature, which may be placed between ErC2 and H o C 2 in fig. 3.
Krikorian et al. (1967) systematically investigated the phase relationship of the high-carbon portion of the lanthanide-carbon systems, including the ~ J 3 trans- formation of the rare earth dicarbides, and provided structural data on the 13 form, which has the cubic KCN-type structure with Fm3m space group (Bowman et al.
1966). The lattice parameters of the ~3-dicarbides, LaC2, TbC2, LuC2 and YC 2 (Bowman et al. 1967) have been determined. However, the existence of the 13 struc- tured form for the J3-dicarbides of Sm, Gd, Dy, Ho, Er, and Tm were established mainly by thermal analysis data on the ~ [3 transformation and their structures were reasonably assumed to be the same as the KCN-type cubic structure.
Krikorian et al. also reported the temperatures of the J3-RC2-C eutectic reactions for all the rare earths except for Eu but no eutectic composition was given (table 1). In addition, there are few other data to confirm or contradict their results.
Gschneidner and Calderwood (1986), in a systematic investigation with respect to the melting points of the rare earth dicarbides and the RC2-C eutectic temperatures, treated the values reported by Russian investigators (Yupko et al. 1974, Kosolapova et al. 1971) as the melting points of the dicarbides of gadolinium, dysprosium and erbium, or as the eutectic temperatures of the corresponding RC2-C eutectic reaction.
Including these values in their calculation of the average values, the average eutectic temperatures of the RC2-C eutectic (with R = Gd, Dy, Er) were given as is listed in table 1 together with the values reported by Krikorian et al. (1967). Compared with the data of Krikorian et al., the average values of Gschneidner and Calderwood vary in a rather regular manner; as a function of atomic number, however, the data treatment they made was based on some not very strict assumptions.
According to the data in table 1, the eutectic temperature increases with increasing atomic number of the light rare earth in the dicarbide, and reaches a maximum at