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The cerium-oxygen-carbon system
J. inorg,nucl.Chem., 1972,VoL34, pp. 117-123. PergamonPress. Printedin GreatBritain
THE
CERIUM-OXYGEN-CARBON
SYSTEM
N. J. C L A R K
School of Physical Sciences, Flinders University of South Australia, Adelaide, South Australia, and I. J. McCOLM
School of Materials Science, University of Bradford, Bradford, U.K. (Received 13 May 1971)
Abstract- For the C e - O - C system it is established by X-ray analyses and hydrolytic studies that no discrete cerium oxide-carbides exist in argon arc melted preparations. At lower temperatures the existence of a series of oxide-carbides of cerium based on isomorphous replacement of oxide ions by C~~- ions in the reduced cerium oxides is established. One hexagonal oxide-carbide of composition CeOl.~(Ca)o.0~ has been isolated from the products of the cerium metal plus carbon monoxide reaction at 1300-1600 ° and is described. A discrete orthorhombic oxide-carbide phase Ce40~C~ has also been isolated from the Ce + CO reaction which cannot be described on an isomorphous oxide ion replacement model. It is established by hydrolysis that neither of these phases contains C1 units in its structure. INTRODUCTION
THE LANTHANON-CAgaON binary systems have been studied in some detail and for the lighter lanthanons are well understood. However, most light rare earth metals as prepared contain oxygen as an impurity and many reports of binary systems probably refer to ternary systems. The present study was undertaken to determine the effect of oxygen contamination in the nominally binary cerium/ carbon system and in particular to investigate the possibility of oxygen stabilizing a cerium monocarbide phase with the rock salt structure, a situation known to occur in the cerium-nitrogen-carbon system [1]. The existence of discrete oxycarbide phases of the rare earths containing either methanide or acetylide carbon units has been established by Eick and coworkers. They report L~OoC (Ln = La, Nd, Gd, Ho, Er)[2], and YbzOC [3], as facecentred cubic structures containing methanide carbon units, whereas La202C2 [4] and NchOzCz[5] contain C2 carbon units and are of unknown symmetry. The parameters that determine whether C2 or C1 units exist in the carbon-containing phases of the lighter lanthanons are interesting and complex [ 1,7], and form a continuing subject of investigation for the authors. Thus the effect that oxygen ions might have on the coexistence of these two types of carbon unit was of considerable importance, particularly in view of the constancy of the charge and size of the 02- ion in the solid state. I. J. S. Anderson, N. J. Clark and 1. J. McColm,J. inorg, nucl. Chem. 31,1621 (1969). 2. A. Duane Butherus and H. A. Eick, J.Am. chem. Soc. 90, 1715 (1968). 3. J. M. Haschke and H. A. Eick, lnorg. Chem. 9, 851 (1970). 4. A. Duane Butherus and H. A. Eiek, U.S. Atomic Energy Commission COO-716-027. 5. A. Duane Butherus, R. B. Leonard, G. L. Bucbel and H. A. Eick, lnorg. Chem. 5, 1567 (1966). 6. Le Prince Ringuet, C.r. A cad. Sci. Paris. Ser C 264, 1597 (1967). 117
118
N . J . C L A R K and I. J. M c C O L M
Le Prince Ringuet [6], in a short note, reported two oxycarbide phases in the C e - O - C system and suggested these to be of composition CeOC and CeO~C~ (x and y unknown); he referred to these as red and grey oxycarbide respectively, and while reporting no detailed properties suggested upper temperature limits to their stability of 1200° for CeO~Cu and the fusion point of CeOC. If one thinks of the C~~- ion as being able to replace an O z- ion in an oxide, then in view of the several oxide phases that exist in the cerium system [8] the C e - O - C system may be unrepresentative of the L n - O - C series and may contain a whole range ofoxycarbides not analogous to those listed above. EXPERIMENTAL Several preparative methods were used beginning with cerium metal (-I: 99.9% purity, Rare Earth Products Limited) carbon (impurities :1, 10 ppm, Morganite Limited) and cerium dioxide (JohnsonMatthey Limited). Carbon monoxide (British Oxygen Research grade) was dried over phosphorus pentoxide before use. Samples were prepared by heating reactant mixtures of the desired overall stoichiometry either by arc melting in argon or carbon monoxide or at lower temperatures in vacuum or atmospheres of oxygen and carbon monoxide. The reaction of carbon monoxide with Ce metal at temperatures up to 1450 ° was studied using a Stanton Mass Flow thermal balance. The effect of the preparative method upon the product is described later. All specimens were characterized after heat treatment. Analysis for cerium was by ignition of the sample in air at 800°C and weighing of the cerium dioxide residue. Carbon was weighed as barium carbonate after burning the sample in oxygen and absorption of carbon dioxide in barium hydroxide. After extensive experience with cerium-carbon binary samples had shown the precision of these methods, oxygen was estimated by difference. X-Ray powder diffraction data was collected using Debye Scherrer and Guinier techniques with nickel-filtered Cu(Ka) radiation. Hydrolysis examinations used the techniques described elsewhere [9]. RESULTS AND DISCUSSIONS
High-temperaturepreparations Argon arc-melted specimens showed large changes in stoichiometry in a direction consistent with oxycarbide preparations having high equilibrium carbon monoxide pressures at or near the temperature reached in the arc. Such was this effect that regardless of the initial composition of the melt, the final composition of these specimens always fell on the cerium-rich side of the Ce~Cs-Ce~O3 tie line on the ternary diagram. The powder diffraction patterns for these specimens could be indexed completely assuming that they were three-phase mixtures of cerium, cerium sesquicarbide and cerium sesquioxide. The lattice parameters for these three phases coexisting were f.c.c, a = 5.10-5.12.A, b.c.c, a = 8.57-8.64 A, and hexagonal a = 3.88 .~, c = 6.06 ~ respectively. The lattice parameters for cerium and cerium sesquicarbide in equilibrium in the C e ' C binary system are 5.130 A and 8-433-~ respectively[9] and for CeO,.5~2 in the C e - O binary system a = 3-889 .~, c = 6.054 .~ [8] Hydrolysis products from these samples were typically those of a carbide containing C2 units and the percentage methane observed did not rise above background levels. 7. 1. J. McColm, N. J. Clark and B. M. Mortimer, J. inorg, nucl. Chem. 33, 49 (1971). 8. D . J . M . Bevan, J. inorg, nucl. Chem. 1, 49 (1955). 9. J. S. Anderson, N. J. Clark and I. J. McColm, J. inorg, nucl. Chem. 30, 105 (1968).
The cerium-oxygen-carbonsystem
119
At temperatures existing in the arc furnace, and in the absence of any significant carbon monoxide partial pressure, no discrete oxycarbide phases exist in the C e - O - C system. The expanded lattice parameter for cerium sesquicarbide suggests that this phase contains some oxygen. By comparison the change in the sesquioxide lattice parameter is not large, even though some carbon solution occurs as C22-. This oxygen ion replacement was more clearly shown by hydrolysis results for lower temperature preparations discussed later. The hydrolysis data give no indication of any methanide carbon species in any of the arc preparations and there is no evidence for stabilization of a rock salt monocarbide. Using carbon monoxide as the gas in the arc furnace reduced the loss of carbon monoxide from the sample and changes in stoichiometry on melting were much reduced, so that sample compositions lying near the CeC2-CeO2 tie line were readily produced, in contrast to argon arc samples.
Low-temperaturepreparations At 1000°C the carbon monoxide partial pressure over CeO2-CeC2 pellets was not appreciable, but as the temperature was increased changes in stoichiometry during heating in vacuo became larger. To obtain samples with high (C + O)/Ce ratios it was necessary to heat in a CO atmosphere. However, samples prepared by these methods were always polyphasic, with X-ray patterns which could not be completely indexed, but again no simple rock salt lattice was present. Reaction of carbon monoxide with cerium below 700°C was slow and the rate increased with temperature, becoming very rapid above 1200 °. At 1300° reaction apparently ceased after a weight increase equivalent to the absorption of one mole of carbon monoxide for each gram atom of cerium. The empirical formula, CeOC, is not believed to be significant. Although the metal billets retained their original outward appearance after reaction they consisted of shells of varying colour and crystallinity, and equilibrium between the interior and exterior was clearly not established. A thin grey outer skin, rich in carbon, covered a layer of translucent orange to red crystals surrounding a silver grey crystalline centre. By careful use of a probe and binocular microscope it was possible to isolate samples of the red crystals and silver-grey crystals, relatively free of other phases, beneath dry benzene or CCl4. These hand-picked samples were characterized by analysis for cerium and carbon, by hydrolysis and by X-ray diffraction. The red crystals analysed as 84.6 per cent cerium, 0.73 per cent carbon or CeOx.~C0.15 and the powder pattern could be indexed on a hexagonal cell, a = 3.902 A, c = 6.027 ,~. The hydrolysis product distribution for this phase is shown in Table 1. The very large percentage of acetylene and only trace amounts of other C2 hydrocarbons is not typical of cerium carbide phases, and the hydrolysis products could not arise from small amounts of carbide impurity. This hydrocarbon distribution is similar to that observed for calcium carbide and is consistent with the existence of acetylide units in a non-conducting solid. The composition is more correctly represented as CeO1.4s(C2)0.0r and is apparently derived from CeO~.56 (hexagonal a = 3.894, c = 6.051) [8] by random replacement of 02- by C22-. This isomorphous replacement can occur to a considerable extent. A sample prepared by heating a (CeO2 + CeC2) pellet in a sealed silica tube at 1050°C for 200 hr analysed as CeOv2r(C2)0.a4. It was X-ray single phase with a hexagonal
120
N. J. CLARK
and I. J. McCOLM
Table 1. Hydrolysis product distribution for two cerium oxycarbide phases
% Product
CeOI.&O.IS
CH,
0.44 0.11 0.07 99.30 0.04
C&6 C&4 C,H, GH,
Ce,O& 1.8 10.4 3.8 84.0
unit cell, a = 3.874 A; c = 6.038 A. Samples of the same composition obtained by arc melting in carbon monoxide were metallographically two-phase and X-ray data gave one of these as the hexagonal oxide. A fragment of CeO,.,&,,.,, was mounted and studied using a precession camera. It contained a number of crystallites of similar orientation with about 70 per cent of the mass a single crystal. The precession data confirmed the hexagonal indexing and did not reveal any superstructure. On the basis of this data the oxycarbide is assigned the P6322 space group with two cerium atoms on twofold special positions. The anions must then randomly occupy four, six or 1Zfold positions. The accommodation of the larger CZ2- anion in the cell may then occur at one of these positions when an adjacent anion site is vacant. The Ln202C2 phase observed for La and Nd would represent the limit of this replacement process, as the unit cell is too small to contain more than two 02- and one CZ2- anion. In the cerium-oxygen system Bevan[8] observed b.c.c. and rhombohedral oxides in addition to the hexagonal phase. Replacement of 02- by C22- probably occurs in these oxides also, but the incorporation of C22- units could not be proved from hydrolysis data, because samples containing these reduced oxides were never single-phase and always occurred together with phases known to contain hydrolyzable carbon. The silvery crystals separated from the 1300” preparation had a high metallic lustre and were extremely moisture sensitive. The analysis figures of 91.2 per cent Ce, 3.48 per cent C, fit closely C%OC (calculated 90.9 per cent Ce, 3.98 per cent C). The hydrolysis product distribution given in Table I is typical of that observed for a lanthanide carbide containing C, units and the formula should be written as Ce,O,C,. Metallographic examination of the silver oxycarbide confirmed this material was single-phase (see Fig. 1). The powder pattern of the Ce,O,C, phase can be indexed on an orthorhombic cell with a, = 9.46 A, b0 = 7.73 A , co = 6.54 A;, as shown in Table 2. It is interesting to note that while the red crystals can be seen to arise from isomorphous replacement of 02- units by Cz2- ions this cannot be true of the shiny grey phase, where an analogous oxide would be the unknown Ce,O,. It is difficult to imagine such a hypothetical precursor containing anything other than Ce3+ and 02- ions, with a high concentration of electrons in a band system. Such a low formal cationic charge would hardly be able to polarise C22- units and stabilise any resultant methanide units, while the high concentration of electrons and
Fig. 1.Metallograph of Ce,O,C, specimen X500.
120
The cerium-oxygen-carbon system
121
Table 2. Observed and calculated sin~0 values for the first 20 lines of a Debye Scherrer film of Ce40~C~ Observed
Calculated
Index
w m-m w vw vvw m+ s+
0.0263 0.0402 0.0460 0.0555 0.0615 0.0685 0.0726 0.0830
0.0266 0.0398 0.0462 0.0556
w
0.0964
0,0970
s vvw s w+
0.1004 0.1169 0.1246 0'1558
m+
0.1661
0.0997 0-1162 0.1251 0.1553 0.1658~ 0-1660J
200 020 120 002 102 310 112 311 130 320 230 003 322 140~ 500J
w vvw mm--
0.1734 0.1862 0.2062 0.2186
s(b)
0.2491
w
0.2694
0.1731 0.1858 0.2052 0.2187 0"2484) 0.2490[ 0-2488J 0.2695
041 240 332 340 6101 204[ 050J 531
0,0622
0.0697 0.0723 0,0835
a = 9.46,~; b --- 7-73 A; c = 6-54,~.
highly ionic nature would give rise to rapid and easy hydrolysis of the type and product distribution observed. Cerium reacted at 1600°C in carbon monoxide to produce a material of gross composition Ce~OsCz (Ce = 88.6 per cent, C = 3.80 per cent). The sample was not obviously inhomogeneous, but the X-ray pattern showed the hexagonal sesquioxide-related phase and other unassigned lines. Other discrete oxycarbides probably exist in this ternary system in addition to the compounds described above. In one preparation, a silvery crystalline material was separated in an amount sufficient only for an X-ray powder investigation and subsequent hydrolysis. The hydrolysis product distribution was similar to Ce40~C~ but the diffraction pattern was quite different and has not been indexed. In addition the X-ray data for many polyphasic preparations from 1050°C to arc temperatures contained reflections which did not belong to any of the phases definitely established to date. A tentative ternary diagram for the cerium-oxygen-carbon system at temperatures below 1600 ° and in the presence of carbon monoxide is shown in Fig. 2. It contains a discrete oxycarbide compound and also considerable extention of the existence fields of binary oxides and carbides into the ternary region due to the replacement of oxide ions by acetylide ions. In Fig. 2 the regions I, II, IV, V, IX, X have been shown unambiguously to
122
N . J . CLARK and I. J. McCOLM ¢e
•
C
Cez
03
CO
0
Fig. 2. Possible ternary equilibrium diagram for Ce-O-C system at temperatures in the range 1150-1600°C. Area I: Ce + Ce2Cs(O)+ Ce40~C2. Area II: Ce + Ce~Os+ Ce402C~. Area III: Single-phase Ce4OsCz. Area IV: C~Ca(O)+ CeC2(O). Area V: Ce, C3(O)+ CeC~(O) + Ce402C2. Area VI: CeC2(O)+ Ce4OzC~+ Ce~O2C~. Area VII: Ce, Os-x(C2)z + Ce402C~. Area IX: C+CeC2(O). Area X: C+CeC2(O)+Ce~O2C2. Area XI: C + Ce202C2 + CO. Area V I I I: See text. Area XI I: See text.
be ternary or binary equilibrium regions containing the phases expected from the diagram. Ce40:C2 is the only single-phase product established and is denoted as the small region III, which carries the implication that the material has a small range of stoichiometry, a fact not definitely established as yet but implicit in the ideas of isomorphous O ~- ion replacement by C22- ions. Region VI has not been definitely fixed, and is shown here as being bounded by Ce~O~C~+ CeC2(O)+ the point corresponding to Ce20~C2 which is the limiting composition for O~replacement by C~z- in the hexagonal Ce203 structure. Region VII represents a two-phase area where equilibrium is achieved between Ce40~C2 and a hexagonal phase of variable composition. Area VIII is drawn on the assumption that the oxides identified in the binary Ce-O system above 600°C can also undergo isomorphous replacement. Area XI represents Ce~O~C2 and carbon in equilibrium with CO, and region XII is the limit of compositions of the oxycarbides of area VIII in equilibrium with CO. The present work shows that at arc temperatures and low carbon monoxide pressures there are no stable oxycarbide phases in the Ce-O-C system and cerium, cerium sesquicarbide and cerium sesquioxide exist in equilibrium. At 1600°(2 and lower a new cerium oxycarbide phase, Ce402C2, has been characterized. An oxycarbide phase with wide composition limits exists based on the replacement of oxide ions by acetylide ions in cerium sesquioxide. There is also some evidence that analogous phases exist for the other reduced cerium oxides.
The cerium-oxygen-carbon system
123
There is no evidence for the existence of any phases containing methanide carbon units, and oxygen does not stabilize any f.c.c, monocarbide in this ternary system. The low stability of C1 units relative to C2 units in cerium-based systems has been illustrated previously. It is probably because of this inherent instability of C1 units in cerium that for a rare eartbto-non-metal ratio of one the f.c.c. "monoxide-like" compound known for La, Nd, Gd, Ho, Er and Yb does not exist. Acknowledgements-The authors wish to thank Dr. M. R. Taylor for the space group determination. NJC wishes to acknowledge the support of the Australian Research Grants Committee and the Australian Institute of Nuclear Science and Engineering. IJM wishes to acknowledge an equipment grant from the Royal Society.
THE
CERIUM-OXYGEN-CARBON
SYSTEM
N. J. C L A R K
School of Physical Sciences, Flinders University of South Australia, Adelaide, South Australia, and I. J. McCOLM
School of Materials Science, University of Bradford, Bradford, U.K. (Received 13 May 1971)
Abstract- For the C e - O - C system it is established by X-ray analyses and hydrolytic studies that no discrete cerium oxide-carbides exist in argon arc melted preparations. At lower temperatures the existence of a series of oxide-carbides of cerium based on isomorphous replacement of oxide ions by C~~- ions in the reduced cerium oxides is established. One hexagonal oxide-carbide of composition CeOl.~(Ca)o.0~ has been isolated from the products of the cerium metal plus carbon monoxide reaction at 1300-1600 ° and is described. A discrete orthorhombic oxide-carbide phase Ce40~C~ has also been isolated from the Ce + CO reaction which cannot be described on an isomorphous oxide ion replacement model. It is established by hydrolysis that neither of these phases contains C1 units in its structure. INTRODUCTION
THE LANTHANON-CAgaON binary systems have been studied in some detail and for the lighter lanthanons are well understood. However, most light rare earth metals as prepared contain oxygen as an impurity and many reports of binary systems probably refer to ternary systems. The present study was undertaken to determine the effect of oxygen contamination in the nominally binary cerium/ carbon system and in particular to investigate the possibility of oxygen stabilizing a cerium monocarbide phase with the rock salt structure, a situation known to occur in the cerium-nitrogen-carbon system [1]. The existence of discrete oxycarbide phases of the rare earths containing either methanide or acetylide carbon units has been established by Eick and coworkers. They report L~OoC (Ln = La, Nd, Gd, Ho, Er)[2], and YbzOC [3], as facecentred cubic structures containing methanide carbon units, whereas La202C2 [4] and NchOzCz[5] contain C2 carbon units and are of unknown symmetry. The parameters that determine whether C2 or C1 units exist in the carbon-containing phases of the lighter lanthanons are interesting and complex [ 1,7], and form a continuing subject of investigation for the authors. Thus the effect that oxygen ions might have on the coexistence of these two types of carbon unit was of considerable importance, particularly in view of the constancy of the charge and size of the 02- ion in the solid state. I. J. S. Anderson, N. J. Clark and 1. J. McColm,J. inorg, nucl. Chem. 31,1621 (1969). 2. A. Duane Butherus and H. A. Eick, J.Am. chem. Soc. 90, 1715 (1968). 3. J. M. Haschke and H. A. Eick, lnorg. Chem. 9, 851 (1970). 4. A. Duane Butherus and H. A. Eiek, U.S. Atomic Energy Commission COO-716-027. 5. A. Duane Butherus, R. B. Leonard, G. L. Bucbel and H. A. Eick, lnorg. Chem. 5, 1567 (1966). 6. Le Prince Ringuet, C.r. A cad. Sci. Paris. Ser C 264, 1597 (1967). 117
118
N . J . C L A R K and I. J. M c C O L M
Le Prince Ringuet [6], in a short note, reported two oxycarbide phases in the C e - O - C system and suggested these to be of composition CeOC and CeO~C~ (x and y unknown); he referred to these as red and grey oxycarbide respectively, and while reporting no detailed properties suggested upper temperature limits to their stability of 1200° for CeO~Cu and the fusion point of CeOC. If one thinks of the C~~- ion as being able to replace an O z- ion in an oxide, then in view of the several oxide phases that exist in the cerium system [8] the C e - O - C system may be unrepresentative of the L n - O - C series and may contain a whole range ofoxycarbides not analogous to those listed above. EXPERIMENTAL Several preparative methods were used beginning with cerium metal (-I: 99.9% purity, Rare Earth Products Limited) carbon (impurities :1, 10 ppm, Morganite Limited) and cerium dioxide (JohnsonMatthey Limited). Carbon monoxide (British Oxygen Research grade) was dried over phosphorus pentoxide before use. Samples were prepared by heating reactant mixtures of the desired overall stoichiometry either by arc melting in argon or carbon monoxide or at lower temperatures in vacuum or atmospheres of oxygen and carbon monoxide. The reaction of carbon monoxide with Ce metal at temperatures up to 1450 ° was studied using a Stanton Mass Flow thermal balance. The effect of the preparative method upon the product is described later. All specimens were characterized after heat treatment. Analysis for cerium was by ignition of the sample in air at 800°C and weighing of the cerium dioxide residue. Carbon was weighed as barium carbonate after burning the sample in oxygen and absorption of carbon dioxide in barium hydroxide. After extensive experience with cerium-carbon binary samples had shown the precision of these methods, oxygen was estimated by difference. X-Ray powder diffraction data was collected using Debye Scherrer and Guinier techniques with nickel-filtered Cu(Ka) radiation. Hydrolysis examinations used the techniques described elsewhere [9]. RESULTS AND DISCUSSIONS
High-temperaturepreparations Argon arc-melted specimens showed large changes in stoichiometry in a direction consistent with oxycarbide preparations having high equilibrium carbon monoxide pressures at or near the temperature reached in the arc. Such was this effect that regardless of the initial composition of the melt, the final composition of these specimens always fell on the cerium-rich side of the Ce~Cs-Ce~O3 tie line on the ternary diagram. The powder diffraction patterns for these specimens could be indexed completely assuming that they were three-phase mixtures of cerium, cerium sesquicarbide and cerium sesquioxide. The lattice parameters for these three phases coexisting were f.c.c, a = 5.10-5.12.A, b.c.c, a = 8.57-8.64 A, and hexagonal a = 3.88 .~, c = 6.06 ~ respectively. The lattice parameters for cerium and cerium sesquicarbide in equilibrium in the C e ' C binary system are 5.130 A and 8-433-~ respectively[9] and for CeO,.5~2 in the C e - O binary system a = 3-889 .~, c = 6.054 .~ [8] Hydrolysis products from these samples were typically those of a carbide containing C2 units and the percentage methane observed did not rise above background levels. 7. 1. J. McColm, N. J. Clark and B. M. Mortimer, J. inorg, nucl. Chem. 33, 49 (1971). 8. D . J . M . Bevan, J. inorg, nucl. Chem. 1, 49 (1955). 9. J. S. Anderson, N. J. Clark and I. J. McColm, J. inorg, nucl. Chem. 30, 105 (1968).
The cerium-oxygen-carbonsystem
119
At temperatures existing in the arc furnace, and in the absence of any significant carbon monoxide partial pressure, no discrete oxycarbide phases exist in the C e - O - C system. The expanded lattice parameter for cerium sesquicarbide suggests that this phase contains some oxygen. By comparison the change in the sesquioxide lattice parameter is not large, even though some carbon solution occurs as C22-. This oxygen ion replacement was more clearly shown by hydrolysis results for lower temperature preparations discussed later. The hydrolysis data give no indication of any methanide carbon species in any of the arc preparations and there is no evidence for stabilization of a rock salt monocarbide. Using carbon monoxide as the gas in the arc furnace reduced the loss of carbon monoxide from the sample and changes in stoichiometry on melting were much reduced, so that sample compositions lying near the CeC2-CeO2 tie line were readily produced, in contrast to argon arc samples.
Low-temperaturepreparations At 1000°C the carbon monoxide partial pressure over CeO2-CeC2 pellets was not appreciable, but as the temperature was increased changes in stoichiometry during heating in vacuo became larger. To obtain samples with high (C + O)/Ce ratios it was necessary to heat in a CO atmosphere. However, samples prepared by these methods were always polyphasic, with X-ray patterns which could not be completely indexed, but again no simple rock salt lattice was present. Reaction of carbon monoxide with cerium below 700°C was slow and the rate increased with temperature, becoming very rapid above 1200 °. At 1300° reaction apparently ceased after a weight increase equivalent to the absorption of one mole of carbon monoxide for each gram atom of cerium. The empirical formula, CeOC, is not believed to be significant. Although the metal billets retained their original outward appearance after reaction they consisted of shells of varying colour and crystallinity, and equilibrium between the interior and exterior was clearly not established. A thin grey outer skin, rich in carbon, covered a layer of translucent orange to red crystals surrounding a silver grey crystalline centre. By careful use of a probe and binocular microscope it was possible to isolate samples of the red crystals and silver-grey crystals, relatively free of other phases, beneath dry benzene or CCl4. These hand-picked samples were characterized by analysis for cerium and carbon, by hydrolysis and by X-ray diffraction. The red crystals analysed as 84.6 per cent cerium, 0.73 per cent carbon or CeOx.~C0.15 and the powder pattern could be indexed on a hexagonal cell, a = 3.902 A, c = 6.027 ,~. The hydrolysis product distribution for this phase is shown in Table 1. The very large percentage of acetylene and only trace amounts of other C2 hydrocarbons is not typical of cerium carbide phases, and the hydrolysis products could not arise from small amounts of carbide impurity. This hydrocarbon distribution is similar to that observed for calcium carbide and is consistent with the existence of acetylide units in a non-conducting solid. The composition is more correctly represented as CeO1.4s(C2)0.0r and is apparently derived from CeO~.56 (hexagonal a = 3.894, c = 6.051) [8] by random replacement of 02- by C22-. This isomorphous replacement can occur to a considerable extent. A sample prepared by heating a (CeO2 + CeC2) pellet in a sealed silica tube at 1050°C for 200 hr analysed as CeOv2r(C2)0.a4. It was X-ray single phase with a hexagonal
120
N. J. CLARK
and I. J. McCOLM
Table 1. Hydrolysis product distribution for two cerium oxycarbide phases
% Product
CeOI.&O.IS
CH,
0.44 0.11 0.07 99.30 0.04
C&6 C&4 C,H, GH,
Ce,O& 1.8 10.4 3.8 84.0
unit cell, a = 3.874 A; c = 6.038 A. Samples of the same composition obtained by arc melting in carbon monoxide were metallographically two-phase and X-ray data gave one of these as the hexagonal oxide. A fragment of CeO,.,&,,.,, was mounted and studied using a precession camera. It contained a number of crystallites of similar orientation with about 70 per cent of the mass a single crystal. The precession data confirmed the hexagonal indexing and did not reveal any superstructure. On the basis of this data the oxycarbide is assigned the P6322 space group with two cerium atoms on twofold special positions. The anions must then randomly occupy four, six or 1Zfold positions. The accommodation of the larger CZ2- anion in the cell may then occur at one of these positions when an adjacent anion site is vacant. The Ln202C2 phase observed for La and Nd would represent the limit of this replacement process, as the unit cell is too small to contain more than two 02- and one CZ2- anion. In the cerium-oxygen system Bevan[8] observed b.c.c. and rhombohedral oxides in addition to the hexagonal phase. Replacement of 02- by C22- probably occurs in these oxides also, but the incorporation of C22- units could not be proved from hydrolysis data, because samples containing these reduced oxides were never single-phase and always occurred together with phases known to contain hydrolyzable carbon. The silvery crystals separated from the 1300” preparation had a high metallic lustre and were extremely moisture sensitive. The analysis figures of 91.2 per cent Ce, 3.48 per cent C, fit closely C%OC (calculated 90.9 per cent Ce, 3.98 per cent C). The hydrolysis product distribution given in Table I is typical of that observed for a lanthanide carbide containing C, units and the formula should be written as Ce,O,C,. Metallographic examination of the silver oxycarbide confirmed this material was single-phase (see Fig. 1). The powder pattern of the Ce,O,C, phase can be indexed on an orthorhombic cell with a, = 9.46 A, b0 = 7.73 A , co = 6.54 A;, as shown in Table 2. It is interesting to note that while the red crystals can be seen to arise from isomorphous replacement of 02- units by Cz2- ions this cannot be true of the shiny grey phase, where an analogous oxide would be the unknown Ce,O,. It is difficult to imagine such a hypothetical precursor containing anything other than Ce3+ and 02- ions, with a high concentration of electrons in a band system. Such a low formal cationic charge would hardly be able to polarise C22- units and stabilise any resultant methanide units, while the high concentration of electrons and
Fig. 1.Metallograph of Ce,O,C, specimen X500.
120
The cerium-oxygen-carbon system
121
Table 2. Observed and calculated sin~0 values for the first 20 lines of a Debye Scherrer film of Ce40~C~ Observed
Calculated
Index
w m-m w vw vvw m+ s+
0.0263 0.0402 0.0460 0.0555 0.0615 0.0685 0.0726 0.0830
0.0266 0.0398 0.0462 0.0556
w
0.0964
0,0970
s vvw s w+
0.1004 0.1169 0.1246 0'1558
m+
0.1661
0.0997 0-1162 0.1251 0.1553 0.1658~ 0-1660J
200 020 120 002 102 310 112 311 130 320 230 003 322 140~ 500J
w vvw mm--
0.1734 0.1862 0.2062 0.2186
s(b)
0.2491
w
0.2694
0.1731 0.1858 0.2052 0.2187 0"2484) 0.2490[ 0-2488J 0.2695
041 240 332 340 6101 204[ 050J 531
0,0622
0.0697 0.0723 0,0835
a = 9.46,~; b --- 7-73 A; c = 6-54,~.
highly ionic nature would give rise to rapid and easy hydrolysis of the type and product distribution observed. Cerium reacted at 1600°C in carbon monoxide to produce a material of gross composition Ce~OsCz (Ce = 88.6 per cent, C = 3.80 per cent). The sample was not obviously inhomogeneous, but the X-ray pattern showed the hexagonal sesquioxide-related phase and other unassigned lines. Other discrete oxycarbides probably exist in this ternary system in addition to the compounds described above. In one preparation, a silvery crystalline material was separated in an amount sufficient only for an X-ray powder investigation and subsequent hydrolysis. The hydrolysis product distribution was similar to Ce40~C~ but the diffraction pattern was quite different and has not been indexed. In addition the X-ray data for many polyphasic preparations from 1050°C to arc temperatures contained reflections which did not belong to any of the phases definitely established to date. A tentative ternary diagram for the cerium-oxygen-carbon system at temperatures below 1600 ° and in the presence of carbon monoxide is shown in Fig. 2. It contains a discrete oxycarbide compound and also considerable extention of the existence fields of binary oxides and carbides into the ternary region due to the replacement of oxide ions by acetylide ions. In Fig. 2 the regions I, II, IV, V, IX, X have been shown unambiguously to
122
N . J . CLARK and I. J. McCOLM ¢e
•
C
Cez
03
CO
0
Fig. 2. Possible ternary equilibrium diagram for Ce-O-C system at temperatures in the range 1150-1600°C. Area I: Ce + Ce2Cs(O)+ Ce40~C2. Area II: Ce + Ce~Os+ Ce402C~. Area III: Single-phase Ce4OsCz. Area IV: C~Ca(O)+ CeC2(O). Area V: Ce, C3(O)+ CeC~(O) + Ce402C2. Area VI: CeC2(O)+ Ce4OzC~+ Ce~O2C~. Area VII: Ce, Os-x(C2)z + Ce402C~. Area IX: C+CeC2(O). Area X: C+CeC2(O)+Ce~O2C2. Area XI: C + Ce202C2 + CO. Area V I I I: See text. Area XI I: See text.
be ternary or binary equilibrium regions containing the phases expected from the diagram. Ce40:C2 is the only single-phase product established and is denoted as the small region III, which carries the implication that the material has a small range of stoichiometry, a fact not definitely established as yet but implicit in the ideas of isomorphous O ~- ion replacement by C22- ions. Region VI has not been definitely fixed, and is shown here as being bounded by Ce~O~C~+ CeC2(O)+ the point corresponding to Ce20~C2 which is the limiting composition for O~replacement by C~z- in the hexagonal Ce203 structure. Region VII represents a two-phase area where equilibrium is achieved between Ce40~C2 and a hexagonal phase of variable composition. Area VIII is drawn on the assumption that the oxides identified in the binary Ce-O system above 600°C can also undergo isomorphous replacement. Area XI represents Ce~O~C2 and carbon in equilibrium with CO, and region XII is the limit of compositions of the oxycarbides of area VIII in equilibrium with CO. The present work shows that at arc temperatures and low carbon monoxide pressures there are no stable oxycarbide phases in the Ce-O-C system and cerium, cerium sesquicarbide and cerium sesquioxide exist in equilibrium. At 1600°(2 and lower a new cerium oxycarbide phase, Ce402C2, has been characterized. An oxycarbide phase with wide composition limits exists based on the replacement of oxide ions by acetylide ions in cerium sesquioxide. There is also some evidence that analogous phases exist for the other reduced cerium oxides.
The cerium-oxygen-carbon system
123
There is no evidence for the existence of any phases containing methanide carbon units, and oxygen does not stabilize any f.c.c, monocarbide in this ternary system. The low stability of C1 units relative to C2 units in cerium-based systems has been illustrated previously. It is probably because of this inherent instability of C1 units in cerium that for a rare eartbto-non-metal ratio of one the f.c.c. "monoxide-like" compound known for La, Nd, Gd, Ho, Er and Yb does not exist. Acknowledgements-The authors wish to thank Dr. M. R. Taylor for the space group determination. NJC wishes to acknowledge the support of the Australian Research Grants Committee and the Australian Institute of Nuclear Science and Engineering. IJM wishes to acknowledge an equipment grant from the Royal Society.