7.1. Rare earth oxycarbides
7.1.1. Preparation and formation of rare earth oxycarbides
The existence of ternary carbon-oxygen--rare-earth-metal phases, R 2 0 2 C 2 (R = La, Ce, Pr, Nd) (Seiver and Eick 1976, Pialoux 1988, Butherus et al. 1966, Karen and Hfijek 1986), Ce402C2 (Clark and McColm 1972, Anderson et al. 1968), R 4 0 3 C ( R = L a , Nd, Gd, Ho, Er) (Butherus and Eick t968), CeO1.4s(C2)o.o7 and Ce01.27(C2)o.34 (Clark and McColm 1972), R z O C (R = Sc, Y, Dy), and their corres-
154 G. ADACHI et al.
ponding NaCl-type vacant rare earth oxycarbide phase R 1 (O, C, [~) (Karen et al.
1986, Hfijek et al. 1984a-d, Bro~ek et al. 1985, Haschke and Eick 1970b, Haschke and Deline 1980) has been established. The La402C 2 oxycarbide reported by Clark and McColm (1972) has a different structure and different hydrolysis product for the R z O C oxycarbide, although an identical stoichiometric composition. Thus, it is possible that more than five types of rare earth oxycarbides may be present in ternary rare-earth-oxygen-carbon systems.
The rare earth oxycarbides are commonly prepared by carboreduction of the oxides, and are found to coexist with the rare earth carbides or oxides in the samples.
For example, the preparation of ~-CezOzC 2 by a progressive carbon-reduction of cerium dioxide was always accompanied by the formation of Ce203 or [3-CEC2, C, CO (Pialoux 1988). In the reaction of carbon with the rare earth sesquioxide (SczO3, Y203 and Oy203), the products contained not only R2OC (R = Sc, Y, Dy) but also Sca5C19 , RCz(R = Y, Dy) (H/tjek et al. 1984b). In some cases, the carboreduction conditions were modified (carbon deficit, lower temperature, high CO pressure) to obtain samples richer in the methanide-type oxycarbide RzOC (Hfijek et al.
1984a, b, d), where R represents the rare earth elements with smaller atomic radii such as Sc, Y, Dy.
Other techniques were also used to prepare these rare earth oxycarbides, such as arc melting under a carbon monoxide atmosphere for the formation of the R 2 0 2 C 2 phase alone (Butherus et al. 1966, Achard 1957) and zone-melting techniques for pure compounds (Butherus and Eick 1968), as welt as direct reaction of the rare earth elements with carbon in oxygen at high temperatures (Karen and Hfijek 1986).
However, in many cases the formation and the yield of the rare earth oxycarbides depend strictly on the starting materials and synthesis conditions (temperature, stoichiometry and CO partial pressure) (Hfijek et al. 1984a), e.g. the oxycarbide N d 2 0 2 C 2 could be prepared only in the presence of a CO partial pressure (Butherus et al. 1966).
7.1.2. Phase diagrams and thermodynamics
With respect to the phase relationships of the formation of the rare earth oxycarb- ides, three R - O - C system phase diagrams have been reported for R = Ce (Pialoux 1988, Clark and McColm 1972), Y (Bro~ek et al. t985) (fig. 29) and Sm (Haschke and Define 1980). The phase diagrams of the cerium and the yttrium systems can be regarded as typical of the light and the heavy rare earth systems, respectively, and that of the samarium system is similar to the latter, showing that only one ternary phase SmOo.5 Co.4 is present in this system. Comparing the two types of phase diagrams, it is apparent that no CezOzCz-type ternary phase was found to be present in the yttrium-oxygen-carbon system, while in the cerium-oxygen carbon system the homogeneity region of C e 4 0 2 C 2 (corresponding to the YzOC phase) seems to be uncertain. The existence of the rare earth oxycarbides, R403C, was reported for five lanthanide elements, La, Nd, Gd, Ho and Er, and was considered to exist possibly throughout the entire lanthanide series (Butherus and Eick 1968). However, the reported phase diagrams of either the C e - C - O or Y C - O system gave no indication of the presence of this phase.
RARE EARTH CARBIDES 0
_w
Y Y2 C Y4C3 YI5CI9 YC 2 C
155
Fig. 29. Regions of homogeneity of the NaCl-type vacant yttrium oxy- carbide Y-(O, C, ~) showing the redistribution of the initial com- position of the pellets among the phases formed after thermal treat- ment at 900-1200°C (Bro~ek et al.
1985). i,, initial elemental composi- tion of the pellet; 0, elemental composition of the Y-(O,C, D) phase; x, additional phase exhibi- ting rather complex X-ray diffrac- tion patterns.
Corresponding to the studies on the preparation of the rare earth oxycarbide, the thermodynamics of the formation of the compounds N d 2 0 2 C2 (Butherus et al. 1966), Ce202C2 (Pialoux 1988) and Y b 2 O C (Haschke and Eick 1970a, b) have been investigated in detail. F o r the C e - C - O system the equilibrium pressures and standard free energies of formation for the various c o m p o u n d s that appear in the course of progressive carboreduction of cerium dioxide were determined from the two large m o n o v a r i a n t fields [ C e 2 0 3 , ~3-Ce202C2, C, C O ] and [-~-Ce202C2, [~-CeC2, C, OJ by high-temperature (1600 2000 K) X-ray diffraction under a controlled carbon monoxide pressure (between 10 -6 and 1 bar). Corresponding to each equilibrium monovariant,
(i)
(ii)
C e 2 0 3 + 3C ~ ~ - C e 2 0 2 C 2 + CO, [3-Ce2 02 C2 + 4C ~ ' 2 ~-CeC 2 + 2CO,
the equilibrium pressures of C O can be expressed by the following equations:
(i)
(ii)
l n P l ( b a r ) = - 5.62x 1 0 4 / T + 26.67 (1610 ~< T~< 1993K), l n P 2 ( b a r ) = - 4.97x 1 0 4 / T + 22.35 (1738 ~< T~< 2043K).
The standard free energies of formation were determined to be as follows:
AG~(13-Ce202C2) = - 1 . 2 1 7 x 106 -F 2 3 2 . 3 T - 24.5Tlog T ( J m o l 1), AG}'(f3-CeC2) = - 8 0 x 103 + 16.9T-- 12.5Tlog T (J m o l - 1 ) ,
156 G. ADACHI et al.
and at 1883 K
AG~(~-Ce2O2C2) = - 9 3 0 kJ mo1-1, G~(I~-Ce202.1C1.9) = - 9 6 3 kJ m o l - 1, AG~(13-CeC2) = - 1 2 5 kJ mo1-1, G~(~3-CeC1.95Co.5) : 142 kJ tool
These data have been used to construct the phase diagram of the Ce O C system at about 1800 K (Pialoux 1988).
7.1.3. Structure and homogeneity region of rare earth oxycarbides
According to the different hydrolysis products, the rare earth oxycarbides reported so far have usually fallen into one of two categories. The first category is one in which acetylide ions, C 2-, randomly replace oxygen ions in an oxide lattice, such as in Ce01.48(C2)0.07, Ce01.27(C2)0.34 (Clark and McColm 1972) and R 2 0 2 C 2 (R = La, Ce, Nd) (Butherus et al. 1966, Butherus and Eick 1973, Seiver and Eick 1976, Pialoux 1988), or in an unknown and unlikely oxide, e.g. C e 4 0 2 C 2 (Clark and McColm 1972, Anderson et al. 1968). The second is that in which the methanide ions, C 4 , and oxygen ions are randomly distributed at the anion sites of an NaC1 lattice. This category includes the R403 C series (R = La, Nd, Gd, Ho, Er) (Butherus and Eick 1968), the compound R z O C , R = Yb (Haschke and Eick 1970a, b), Y, Sc, Dy (Bro~ek et al. 1985, Hfijek et al. 1984a d) and their vacant phases R 1 (O, C, []) (Karen et al.
1986). In addition, an allylene type of carbide was found as an intermediate product of the yttrium oxide carboreduction. This phase is probably a partially oxygen- substituted compound, Y15 (O, C)19 (Hfijek et al. 1984a) and the stability of this phase increases in the direction from Dy, Ho towards Lu (Karen and Hfijek 1986).
The cubic oxycarbides generally exhibit a significant stoichiometry range through a simple substitution to parent oxide, although the monoxide, which is the basis for methanide substitution, is probably only stable in the presence of carbon or nitrogen contamination. It has been shown that between the hitherto known cubic phases of the Sc2OC carbide oxide (a = 4.5612~) and the Sc2-3C carbide there exists a continuous region of mixed crystals of NaCl-type structure (Fm3m, Z = 2). In the oxygen-rich region the homogeneity range extends as far as ScCo.3Oo.7. This com- position is approximately in equilibrium both with the metal and with the Sc203 at 900°C. The lattice parameter of the oxycarbide is 4.536 ~ and the substance can be regarded as a mixed crystal of Sc20C and the nonexistent ScO. It is apparent that the homogeneity region of the NaCl-type phase will expand with increasing temperature, at least in the direction towards ScO (Karen et al. 1986). In the carbon-rich range, ScCxOy is in equilibrium with the Scl 5 C19 carbide. The homogeneity region gradu- ally extends from ScCo.5 0o.5 to the binary-defect carbide Sc2_3C. This behavior is consistent with the tendency of the Y (O, C, [] ) phase (fig. 29) (Bro;~ek et al. 1985). In the Y - O C ternary diagram at 900-1200°C, there exist two homogeneity regions, one adjacent to the YI-(O, C)1 component in the Y201 +xC1 -x region with x = 0 -0.4 and directed towards Y2 C up to YO0.31C0.51 D0.18 , and the other adjacent to the
RARE EARTH CARBIDES 157 YI-(C, D)I component in the region between YCo.3 and YCo. 5 and directed towards Y2OC up to YO0.04Co. 5 U10.46.
Corresponding to the deviation in composition from the stoichiometry, the com- pounds R2OC (R = Sc, Y, Dy, Sc + Dy, Y + Sc) can contain, in the nonmetal sublattice, as much as 4% vacancies at the expense of carbon (Hfijek et al. 1984d) and the SmOo.sCo. 4 has 10% vacancies (Haschke and Deline 1980). Their behavior as good electrical conductors is consistent with an inherent conduction-band popula- tion. On the other hand, the ytterbium compound YbOo.soCo.47, closely approaching the ideal ratio and with a low defect-induced conduction population, behaves like a semiconductor (Haschke and Eick 1970a, b). F r o m these facts, it is likely that for the NaCl-type rare earth oxide carbides the apparent increase in the C : R ratio and the accompanying decrease in the defect concentration and conductivity would occur across the lanthanide series.
As reported by Butherus and Eick (1968), the NaCl-type oxide carbides probably exist for all the lanthanides except europium. The compound with the stoichiometry R 4 0 3 C could also be regarded as a carbon-stabilized monoxide of rare earth elements, like R 20C. In the europium system, neither R 2 0 C and R 4 0 3 C was found.
The lattice parameter (5.14 ,~) reported for Eu 2 0 C (Darnell 1977) does not correlate with the parameters of adjacent oxide carbides (5.066 • for SmOo.5Co.4) and is identical with that of EuO (Haschke and Deline 1980). Haschke and Eick (1970a, b) suggested that the greater stability of the monoxide prevents formation of the europium oxide carbide.
In addition to the fcc NaCl-type rare earth oxycarbides with the methanide ions, C 4-, the orthorhombic Ce402C2, hexagonal CEO1.48(C2)0.07 , with a = 3.902 A, c = 6.027 ~ (Anderson et al. 1968) and CeO1.27(C2)o.34 , as well as monoclinic and hexagonal R 2 0 2 C 2 , definitely belong to the acetylide compound, which have no homogeneity range (for Ce402C2) or a narrow one.
The crystal structure of La2OzC 2 determined from three-dimensional X-ray diffrac- tion on a twin crystal is of monoclinic symmetry, space group C/2m (Seiver and Eick 1976). The lattice parameters are a = 7.069(8) A, b = 3.985(4) A, c = 7.310(9) ~ and [3
= 95.70(6)°; the calculated density is 5.41 g cm -3. In this structure, the lanthanum atom has four oxygen and four carbon atoms situated in a distorted bicapped trigonal prismatic arrangement. Interatomic L a - O distances range from 2.392(8) to 2.823(9) A and L a - C distances from 2.86(1) to 3.11(1)/~. The carbon atoms are present as C 2 units with an interatomic C C distance of 1.21(3) ~. Oxygen atoms are tetrahedrally coordinated, as in the sesquioxide.
In recent work (Pialoux 1988) it has been shown that C e 2 0 2 C 2 has two modifica- tions, ( x - C e 2 0 2 C 2 isotypic with L a 2 0 2 C 2 and the high-temperature form [3-Ce202C 2. The structure of ~ - C e 2 0 2 C 2 (P312/m) can be derived from that of
~-Ce203 (P32/ml) by substitution of the O 2- ions at the octahedral sites of the hexagonal crystalline network by C~- pairs. Moreover, there is a close structural relationship between the ~ and 13 modifications: their anisotropic behavior becomes more emphasized between 273 and 1333 K and then more so from 1333 up to 1923 K (7.294 >~ c ~> 7.279 ~, 7.029 ~< a ~< 7.094 ~, 1.039 ~>
c/a
~> 1.026). A displacive cz monoclinic ~ [3 hexagonal transition occurs at 1333 K.158 G. ADACHI et al.
In summary, the tendency is obvious for the formation of the oxycarbides of the rare earth elements; La, Ce, Pr and N d form the carbide-oxides R 2 0 2 C 2 o r C e 4 O 2 C 2
of an acetylide nature and with complex crystal structures, while the "smaller" rare earth elements Sm, Dy, Ho, Lu, Y and Sc form the fcc methanide-type carbide-oxides, M-(C, O, [] ).
7.2. Rare earth nitride carbides
The formation of the rare-earth-nitrogen carbon compounds has been investi- gated only for the light lanthanide systems with the emphasis on the existence of the
"monocarbide" of La, Ce, P r and Nd, in the presence of nitrogen. It is well known that no evidence for the existence of a face-centered cubic phase, assumed to be the monocarbide, was found in the detailed work on the light-lanthanide-carbon systems (Spedding et al. 1958, Dancy et al. 1962, Anderson et al. 1969).
Anderson et al. (1969), Colquhoun et al. (1975) and McColm et at. (1977) prepared samples containing unstable and highly nonstoichiometric nitride-carbides of cerium, praseodymium and lanthanum with a NaC1 structure by high-temperature reactions between RC2 and RN, RN and C, as well as R and H C N etc. The methods employed to produce alloys in the l a n t h a n u m - n i t r o g e n - c a r b o n , cerium-nitrogen carbon and praseodymium-nitrogen carbon systems only lead to nonequilibrium phases.
The composition ranges of the nitride-carbide have been determined. F o r the cerium system and the praseodymium system, two series of fcc nitride-carbide phases were identified.
One series is found in the presence of small amounts of rare earth metals and the total nonmetal content (x + y) is close to and not lower than unity. For CeN~Cy, e.g.
CeN0.74Co.6o (a = 5.111 ~ ) or CeNo.65Co.s6 (a = 5.032 ~), owing to hydrolysis products, the Ce compounds could be regarded as the acetylide. Although Colquhoun et al. (1975) found an acetylide fcc praseodymium nitride-carbide PrNo.73
Co.69
with a large lattice parameter of 5.180 ~ and an x + y value much greater than unity, this phase seems particularly unstable for vacuum annealing at 1300°C and rapidly decomposes to a high-carbon methanide with a = 5.136 ~. Therefore, it is apparent that Pr has a greater ability to stabilize the methanide phase than Ce. In the lanthanum system, this type of compound is not found (McColm et al. 1977).The second series of the fcc nitride carbides were found to coexist with the sesquicarbides of the light lanthanides, La, Ce and Pr. The composition ranges of this type of rare earth nitride carbides are different for different systems, e.g. for LaNxCy, this range is very narrow, x + y = 0.3-0.4, x ~> 0.04, in contrast to x + y ~> 0.8 1.0, y = 0.08 0.4, x = 0.4-0.7 for CeNxCy and x + y ~> 0.53-0.92, y = 0.05-0.55, x = 0.55-0.72 for PrNxCy. These compounds contain methanide C1 carbon units and are also nonequilibrium phases. The lattice parameters vary over a wide range depending on the carbon and nitrogen contents in the compounds, e.g. for LaN~Cy, the lattice parameter a = 5 . 2 9 5 - 0.171y + 0.186x (McColm et al. 1977) and for PrN~Cy a parameter increases on increasing the nitrogen content from 5.123 A up to 5.166 ~(. Thus, this type of compound might be thought of as a nitrogen-substituted defective monocarbide phase.
RARE EARTH CARBIDES 159
McColm et al. (1977) considered that the composition ranges seem to be related to the amounts of R(IV) found in the respective nitrides and carbides (Lorrenzelli et al.
1970, Atoji 1962). In the cerium systems, Ce(IV) is present up to 70%, while in the praseodymium case the amount is less and in the lanthanum system the higher oxidation state is absent. This suggests that Ce(IV) and Pr(IV) do assist in preventing the catenation that leads to acetylide ion formation. However, the reviewers suggest that this factor is probably minor while the size factor of the rare earth atom is essential.
In conclusion, it should again be emphasized that the presence of nitrogen is shown to be essential to the formation of methanide carbide in the light-lanthanide- n i t r o g e n - c a r b o n systems.
7.3. Ternary rare-earth-hydrogen carbon compounds
Studies concerning the preparation and properties of the rare earth carbide hydrides showed that only two or three ternary compounds, hexagonal RC0.sH (R = Y, La, Yb), fcc RCH0. 5 (R = Yb) and possibly YbC2H exist in the rare- e a r t h - c a r b o n hydrogen system; for Yb (Haschke 1975), for La and Y (Peterson and Rexer 1962, Rexer and Peterson 1964). The fcc phase of variable composition with lattice parameters ranging from 4.88 to 4.96/~ obtained by treating ytterbium dihydride with graphite (Lallement 1966) has also been regarded as the carbide hydride YbCHo. s (Haschke and Eick 1970a, b). The phase diagram for the Y b - C - H system at 1173 K and 0.5 atm of hydrogen are shown in Fig. 30 (Haschke 1975).
X-ray diffraction data for hexagonal YbCo.sH prepared under different hydrogen pressures show larger variations in lattice parameters with a ranging from 3.574 to 3.593 ~ and c ranging from 5.723 to 5.852 ~. The composition limits for this phase
C
H Yb
Y b H 2
Fig. 30. Phase diagram for the Yb C H system at 1173K and 0.5 atm of hydrogen (Haschke 1975). (Reprinted by permission of the publisher, The American Chem- ical Society, Inc.)
160 G. A D A C H I et al.
have not been determined (Haschke 1975). For this type of compound for La or Y, no data on composition were reported and thus these compounds were assigned the RCH x formula (Samsonov et al. t970). The fcc YbCHo. 5 hydrocarbide has a lattice parameter a = 4.974 +_ 0.001 •, and the exact composition limits of this phase are not known.
Reference to fig. 30 shows that YbCo.5 H and YbCHo.5 lie on lines of constant C: Yb ratio connecting the known carbide composition with hydrogen. These phases rela- tionships suggest the possibility of preparing these and other carbide hydrides by hydrogen substitution into binary carbides. Thus, YbC 2 H, an acetylide hydride with the conduction electron bound as a hydride ion would be expected to form by the low- temperature hydrogenation of YbC2 (Haschke 1985) and structural similarities be- tween the carbide and hydrocarbide phase might be expected, as well as regions of nonstoichiometry might be observed.
Ionic models have been proposed for explaining the observed nonstoichiometry and properties of these hydrocarbides on the basis of magnetic, electrical and hydrolysis data (Haschke 1975). The structure of the hexagonal YbCo.sH phase was considered to be composed of the metal occupying the closest-packed positions with the methanide carbons occupying half of the octahedral interstices, and hydrogens occupying half of the .tetrahedral voids. Occupancy of the remaining octahedral and tetragonal voids by hydrogen essentially accounts for the wide composition range of this type of compound. The fcc YbCHo. 5 phase was proposed as a methanide acetylide hydride, the anion-to-cation ratio is 1.25 : 1; this phase also has a NaCl-type structure.
Carbide ions presumably occupy three-quarters of the octahedral sites, and hydride ions fill one quarter of the tetrahedral holes. This model is similar to that proposed for the hexagonal carbide hydride, and the same processes for nonstoichiometry are expected to be operative. An immeasurably large resistivity was found for the hexagonal phase, and a value of t012 f~cm was observed for the fcc phase.
It has been shown that in the presence of oxygen, a metastable fcc quaternary phase (with lattice parameters in the range 4.85-4.96 •) forms between the YbOo. s Co.s and YbCHo. 5 compositions. This phase is a nonequilibrium one and disproportionates into YbCHo. 5 and YbCo.500.5 at equilibrium.
The preparative reaction with europium dihydride failed to produce analogous carbide hydrides. Obviously, the failure is consistent with the high stability of divalent europium. In YbCo.sH, YbCHo. s and YbOo.5 C0.5, ytterbium is clearly trivalent, as shown by magnetic data (Haschke 1975), and for La and Y the hexagonal phases have also been found (Lallement and Veyssie 1968). Therefore, it can be expected that the hexagonal RCo. 5 H phases probably exist across the lanthanide series, which exhibit a trivalent state as their stable valence state, with the exception of europium.
7.4. Ternary rare-earth-carbon-halide compounds
There is another series of cluster-type ternary carbides that contain not only metal and carbon atoms but also the group VIIA elements, C1, Br and I (Simon et al. 1981, Warkentin et al. 1982, Simon and Warkentin 1983, Schwanitz-Sch/iller and Simon 1985, see also chapter 100, this volume). Electronic structure calculations have been
RARE EARTH CARBIDES 161 performed on GdloCllsC 4 (Satpathy and Anderson 1985), GdloCl17C4, Gd12117C 6 (Bullett 1985) and Gd2C2C12 (Miller et al. 1986). The carbon atoms in these com- pounds also form pairs. For Gd~oCI~sC~, GdloC117C 4 and Gd12117C6 which contain short metal-metal contacts the emphasis has been on the extent of metal-metal bonding. Other chloride carbides, R 2 C12 C (R = Sc, Y) with the 1 T type structure (Hwu et al. 1986), Gd6ClsC 3 (Simon et al. 1988), GdsC19C 2 (Simon et al.
1981) and the quaternary CszLuTCllaC compound have been synthesized and their structures have been determined. In addition, the Se compounds Sc 7 X 12 C (X = I, Br), S c 6 I l l C 2 and Sc416C 2 have also been studied with respect to their synthesis and structure (Dudis et al. 1986, Dudis and Corbett 1987).