801
POLYMORPHISM OF CALCIUM CARBIDE
THE POLYMORPHISM OF CALCIUM CARBIDE M. A. BREDIG Vanadium Corporation of America,4.80 Lexington Avenue, New York City, New York Received June 6 , l9@
Variations in the chemical activity of calcium carbide in the Frank-Caro process of making calcium cyanamide have been studied by means of x-ray diffraction methods aa a function of the crystal structure of the carbide. In this investigation, collaborators (6, 7) of A. R. Frank and N.Caro have established the existence of three different crystallized phases of calcium carbide. The most common form of commercial calcium carbide, designated as CaCz I, h w a face-centered tetragonal, deformed rock-salt type of crystal structure (13). CaCz I1 is of lower symmetry, and its occurrence appeared to be connected with the presence of small amounts of calcium cyanamide in solid solution. CaCz 111, possibly of still lower symmetry, was obtained only when neither calcium cyanamide, nor calcium sulfide, nor any “excess of calcium” was present. Several recent industrial applications of calcium carbide other than the manufacture of calcium cyanamide, such as the production of magnesium metal, the polymerization of acetylene obtained from the carbide, or the direct preparation of organic compounds from the carbide (15), have of late revived interest in tlie fundamental properties (IO) of this basic material, of which millions of tons are produced annually the world over. In the following, the results of the continuation of the writer’s x-ray investigation which thus far have only been briefly summarized ( 5 ) are discussed in detail. THE PREPARATION OF CALCIUM CARBIDES I, 11, AND 111 IN THE ELECTRIC-ARC FURNACE
Calcium carbide 111, the phase occurring in the absence of impurities such as calcium sulfide, can be obtained by either the reaction CaO 3C = CaCZ CO or the reaction CaCN, C = CaCz SZin a vacuum furnace, or from calcium metal and carbon in a closed steel tube (7). I t has also been shown that this form, when prepared in one of these ways, is the most active one in th‘e reaction with nitrogen. Therefore the attempt to produce it by the only applicable industrial method of manufacturing calcium carbide, i.e., in the electricarc furnace, appeared to be of practical interest. Since both sulfur and nitrogen are known to suppress the formation of carbide 111, it is necessary to use for the construction of the furnace crucible and electrode a graphite material free from both of these elements. By covering the crucible, the nitrogen of the air must be prevented from entering the melt. The starting material was finely precipitated C.P. calcium carbonate and sugar charcoal, carefully mixed in a mortar and granulated with a solution of sugar in water (size of pellets about 3 mm.). The charge for one experiment amounted to approximately 150 g . , and was added gradually while the electric arc was burning. Samples of the reaction product were handled entirely inside a large
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802
hi. A. BREDIG
glass box containing dried air, where they were pulverized, and filled into thinwalled glass tubes of 0.5-mm. diameter which served to hold the carbide in the center of the Debye-Schemer-Hull x-ray camera. The formation, in the first experiments, of carbide I rather than carbide 111, which was expected in this sulfur-free system, gave rise to the following ronsiderations: According to the previous results (7) not only sulfur, but also an “excess of calcium” leads to the formation of phase I rather than of 111. Consequently, side reactions, such as CaO C = Ca CO, or 2Ca0 CaCl = 3Ca 2C0, which seem entirely possible in the electric-arc furnace and which might cause the formation of a carbide containing an “excess of calcium,” had to be avoided. Prolongation of the melting time appeared to be more suitable than a mere increase in the amount of carbon in the starting mixture: The formation of calcium in excess of the composition CaC2 seemed possible as long as calcium oxide was left in the melt to react according to the above equations. Therefore, the melt was heated a few minutes longer, that is, past the moment a t which the cessation of the violent evolution of carbon monoxide had indicated the end of the main reaction CaO 3C = CaCl CO. During this 1at)erphase of the experiment, the entrance of nitrogen had to be avoided. In the first, stage of the reaction, it had been kept away by the violent evolution of carbon monoxide. Therefore, in the second phase, nitrogen-free carbon monoxide wxs led from the outside into t’he crucible. By this procedure, the format.ion of rarbide I actually was found to be suppressed thenceforth. Howerer, another unexpected observation was made: The diffraction patterns of the pulverized samples invariably now showed the lines of carbide I1 exclusively.’ This was in conspicuous disagreement with the previous results (6, 7), according to which carbide I1 was to be obtained in the presence of calcium cyanamide only. In these latest experiments, however, the nitrogen was carefully prevented from entering the carbide melt; accordingly, the analysis of the pulverized product showed that only 0.05-0.10 per cent of nitrogen was present. In the light of the experience cited, this amount waa entirely insufficient for t>heproduction of carbide 11. From the new results, therefore, the necessity arose of modifying the former assumption that phase I1 was connected with the presence of nitrogen in the carbide. There appeared now to be other factors which likewise were refiponsible for the occurrence of this structure, and which had to be determined. The following results showed that it was during the process of grinding the carbide in the mortar that the carbide suffered a transformation which was responsible for the appearance of phase I1 instead of the expected form 111. In these experiments, the carbide was not used in finely pulverized form, but was only roughly broken into small lumps, of which one of approximately rectangular shape was used for the x-ray exposure in the same manner in which a single crystal would be exposed for a single-crystal rotat,ion x-ray diagram (6, 13). On the resulting x-ray pattern numerous spots were found, the inter-
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1 The mention of phase 111, instead of phase 11, in the summary ( 5 ) on page 1163, the seventeenth line from the bottom, is a misprint.
POLYMORPHISM OF CALCIUM CARBIDE
803
planar spacings of which did not agree with those of a powder x-ray diagram of carbide 11. Especially the spot corresponding to an interplanar spacing of 2.85A., on the equator of the diagram,-by far thc strongest reflection,-could in no way be interpreted as a reflection from phase 11. Furthermore, it corresponded perfectly to thc strongest line of the powder pattern of phase 111 (7), and the remaining reflections, which could not be ascribed to phase 11, corresponded entirely satisfactorily to the lines of thc powder x-ray pattern of phase 111. The conclusion was to be drawn from thesc results that the carbide, made in the electric-arc furnace under the described precautions, originally had structure 111, tts expected, but had changed its crystal lattice, during the process of grinding in the mortar,-that is, by mechanical deformation,-into that of carbide 11. Other cases of crystal lattice transformations by mechanical deformation are known, e.g., in zinc sulfide, lead oxide, silver iodide, and austenitic nickel chromium steel. The transformation cannot be considered as the result of the dispersion of nuclei of crystallization. The mechanism probably is closely related to the formation of twins by- plastic deformation. Tho preparation of carbide I11 in the laboratory electric-arc furnacc indicatcs that it could be produced on an industrial sealc, if only sulfur-free raw materials could he used. In a consecutive investigation, undertaken a t t,he suggestion of and in collaboration with the writcr and published by Franck and Endler (S), it was shown that no part>iculartechnical advantage can as yet be assigned to this specific crystal phaso of calcium rarbide in the manufacture of calcium ryanamide. The reasons which are disrussed in that paper are explained in tho results described on the following pages. T H E RELATION OF THE THREE CRYSTAL PHASES AND T H E DISCOVERY OF A FOURTH O N E
With the tliscovcry of the transformation by mere mechanical deformation, without variation in the chcmical composition, of one crystal form into another, the drtcrniination of the relative thermodynamic stability of these modifications appeared to be possible. -4. Tronsjormation by niechanical di.formataon (grinding) The “purc” carbide, used in the experiments of this invcstigation, was made exclusivcly by the method described above, that is, in the electric-arc furnacc. It was preferred to the methods described formerly (6, 7), since it yielded far greater quantities of carbide, free from sulfur and nitrogen and containing a t least 90 per cent CaCz. According to all previous experience, the slight content of calcium oxide and frce carbon (graphite)had no influence upon the transformations, except as discuhsrd on page 802, and thcrrfore was ncglectcd in the following considerations. The results described in the first section led t o the expectation that a t some temperature between the melting point and room temperature a transition point would be found above which phase I11 would be the stable phase of calcium
804
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M. A. BREDIG
carbide, while below it structure I1 would be the stable one. Actually, a sample of calcium carbide, converted by pulverizing from structure I11 into 11, was found to be retransformed into I11 by heating at red heat for a very short time. This sample, still a loose powder after this thermal treatment, gave a powder x-ray diffraction pattern which was the first to be obtained from carbide I11 made in the electric-arc furnace. Therewith the success in preparing carbide I11 in the electric-arc furnace, indicated in the preceding section by the somewhat uncertain comparison of the “single-crystal” diagram with the powder x-ray pattern of I11 as obtained in the previous investigation, waa now confirmed by direct comparison of two powder diagrams which proved to be identical. For an exact determination of the transitiop point, mere heating at different temperatures did not seem to be a sufficiently safe method. The sluggishness which had prevented form I11 from being transformed spontaneously into form 11, which is more stable at room temperature, might have concealed the actual position of the transition point even a t elevated temperatures. The mechanical treatment in the mortar wm recognized as a means of achieving transformation a t a temperature a t which even years had not been a sufficiently long time for any spontaneous transformation. Following a suggestion by Bloch and Moeller (l), the transition point was therefore determined by pulverizing a sample a t different temperatures, and identifying, after quenching, the resulting structure through a Debye-Scherrer-Hull x-ray pattern. The experiments were carried out in an alumina crucible, 8 in. in length, 22 in. in diameter, and with a rough surface, in which the carbide was ground by means of a pestle attached to a long handle. An inert gas was kept flowing through the crucible, which was held a t constant temperature. The following results were obtained: In samples quenched from temperatures above 450OC f 10’ and held a t that temperature with or without simultaneous mechanical treatment, phase I11 was always obtained. After grinding at lower temperatures, that is, a t 200” and 350°C., neither carbide I11 nor carbide I1 was found to be present in more than minor quantities. Instead, carbide I was obtained which heretofore had been observed only in the presence of calcium sulfide or of an “excess of calcium” (7). The new observation showed that the tetragonal form I, too, was to be considered simply an enantiotropic crystal modification of thc compound CaCz and that the presence of sulfide or “excess of calcium” was not required for its formation. Further extension of these experiments over the whole temperature range up to red heat, including temperatures below normal, led to the approximate determination of two enantiotropic transition temperatures: one, approximately 450°C.,for the transition I S (111) (it will be shown below, that not I11 but another form actually is stable above 45OoC.), and a second one near room temperature, for the transition I1 I. X laboratory ball mill was designed, consisting of a steel cylinder which could be heated from the outside during the grinding by means of a gas burner to various temperatures extending above the transition point of calcium carbide, 450’C. This permitted preparing the three crystal modifications, I, 11, and
POLYMORPHISM OF CALCIUM CARBIDE
805
111, of calcium carbide free from sulfide or cyanamide, in a very convenient way and in much larger quantities than those which could be obtained by the methods described previously (7). Carbide with less than 0.5 mole per cent cyanamide, when ground a t room temperature, took on the structure 11. I t had to be ground a t a temperature above 350” and below 450”C., if a t least 80 per cent of the carbide was wanted in form I. With lower grinding temperatures, the amount of phase I is proportionally smaller, relative to that of phase 11. When ground at 500°C., the carbide is obtained in form 111. B. Experiments with the high-temperature x-ray camera For identification after grinding a t elevated temperature, the carbide samples heretofore always had to be cooled down to room temperature. The uncertainty involved in this procedure was to be avoided by a direct identification, by x-rays, a t the temperature a t which the stable phase was to be determined. A Debye-Scherrer-Hull x-ray camera was used which permitted heating of the sample by a resistance-wire coil with spared-out space for the passing primary and reflected x-ray beams. The carbide was held in a thin-walled glass tube, which was 0.5 mm. in diameter and was open a t its upper end to permit gas expansion during heating. During the exposure of about 1 hr., and a t temperatures below 600°C., oxidation and reaction with the glass were found to be negligible. A platinum, platinum-rhodium thermocouple was placed about 3 mm. below the sample in a small refractory tube which served as a support for the glass tubing. A rough calibration was effected by observing the transition temperature of lithium sulfate a t 575°C. An exact determination of the transition temperature was not attempted, but rather the identification only of the phase present in calcium carbide a t the elevated temperature. Weak lines of calcium oxide and calcium carbonate appeared in all diagrams, and were used for calibration of the films. Any influence of these substances upon the transformation was not observed. The principal result of these experiments was the discovery of a fourth crystal modification of calcium carbide. The interplanar distances of calcium carbide above 470°C. are listed in table 1, and show that the new form, IV, has a facecentered cubic space lattice, with the constant a,, = 5.92 A. at 480°C. This unit cell contains four molecules of CaC2. The calcium ions occupy positions entirely analogous to those of the sodium ions in rock salt, while the positions of the carbon atoms are not exactly known. The centers of the C2 graups are assumed to occupy the places of the chloride ions of sodium chloride. h r t h e r results of the x-ray investigation, a t various temperatures, of “pure” calcium carbide, made in the laboratory electric-arc furnace and containing, besides approximately 80 per cent CaC2, some graphite, 0.45 per cent calcium cyanamide, no sulfide, and no “excess of calcium,” are the following: ( I ) While a small piece, picked from tlle roughly crushed carbide after cooling from the molten state, gives the “single-crystal” rotation diagram of structure I11 at room temperature, the same carbide produces the pattern of structure I1 after grinding at room temperature.
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M. A. BHEDIG
(2) After raising the temperature of this v m pulverized ~ sample in the x-ray camera t o 600°C. within a few minutes, an x-ray pattern of the cubic structure IV is obtained in an exposure of 1 hr. As the temperature was lowered to 48OoC., the pattern of IV was preserved. (3) At 4(jo"C., however, transfomatiqm had taken place. Structure 111, which had been present in the original sample before grinding, again was identified, this time by its powder pattern. The sample retained structure 111 in the exposures a t the lowered temperatures of 440°,420", and 20°C. This experiment showed that calcium carbide, not containing any appreciable quantity of cyanamide, or of sulfide, or "excess of calcium", can adopt each of the four crystal structures without any noticeable variation in its chemical composition. Two of these modifications, IV and 111, are formed spontaneously,
TABLE 1 Powder %-ray patterns of the cubic high-temperature f o r m of the alkaline-earth metal carbides. MeCs CIC. 4Wc.
INDICES
SrCt 370%.
hkl
Carbide Carbide (Oxide (Oxide Carbide Carbide (Oxide Carbide (Oxide CR rbide (Oxide Carbidc Carbide
111 ........... m 200 ........... V . 8 . 111 . . . . . . . . . . . w 20 . . . . . . . . . . . m
m ...........
311 . . . . . . . . . . . 220. . . . . . . . . . . 222. . . . . . . . . . . 311 . . . . . . . . . . .
8
rn
m w w 400.. . . . . . . . . . w 222... . . . . . . . . w
a o .. . . . . . . . . . . . . . . . . . . . . . . . . . Specific gravity... . . . . . . . . . . .
I
BaCt zM%.
-3.42 2.96
2.81 2.44 2.09 1.780 1.720 1.710
3.42 2.96 2.81 2.43 2.09 1.785 1.720 1.705
donM.
dsxptl,
3.60 3.12 3.01 2.59 2.20
3.60 3.12 3.00 2.59 2.205
3.77 3.27
1.885
1.885
1.840 1.800
1.835 1.800
2.77 2.32 1.975 1.975 1.890 1.675 1.640 1.605 1.510 1.465
3.79 3.28 3.19) 2.77) 2.32 1.980 1.980) 1.890 1.675) 1.640 1 .W) 1.505 1.470
~5.92 2.04,
6.24 3.04
I
6.56
3.57
above and below 450°C. f 20",respectively, while the other two forma, I and 11, are obtained in this "pure" carbide by mechanical treatment only, above and below room temperature, respectively, as described on page 805. The statement, on that same page, that above 450°C.phase I is transformed spontaneously into 111, is now to be modified in accordance with the results obtained with the high-temperature x-ray camera: carbide I, when heated above 450OC. and cooled to room temperature, wherc it is found to consist of carbide 111, has undergone not one but two consecutive transformntions,-namely, I 3 IV in heating, and IV + JII in cooling. The structure formed through mechanical deformation in the mortar must naturally be assumed t o be more stable, thermodynamically, a t the temperature of that treatment than the original structure. Since no temperature was found
POLYMOI1L”IShf OF CALCfUhl C.iHUIIJE:
807
a t which structure 111 with&uidS the m ~ ~ c h ~ n ior c dpurely , thermal, treatment, it cannot be considered a stable structure of calcium carbide, but must be a metastable phase only, The causes for the formation of such a metastable phase (111), instead of the stable phase (I), in cooling the high-temperature phase (IV) will be discussed below.
C. The cooling eume of calcium carbide The thermal effect connected with the transformation of the cubic hightemperature phase IV into the metastable phase I11 has been observed thus far with a very simple arrangement only. Approximately 30 g. of carbide was heated and cooled in a porcelain crucible. The time consumed by a certain drop of the temperature was measured with a stop watch. On account of the low heat conductivity of the carbide, the transformation appeared to be extended over a certain small temperature range near 435°C. Therefore, and also in view of the insufficient accuracy in the determination of the transition temperature in the high-temperature camera, a discrepancy remained between the transition temperatures (IV 111), as determined by both methods (470” and 435”C.), which could probably beeliminated by further effort. This was reserved for another investigation in which the specific heats of the various carbide modifications and the heats of transformation were to be determined. From the present cooling curve, the heat of transformation of the cubic phase into form I11 is estimated as of the order of magnitude of 0.6 & 0.2 kilogram-calorie per mole of calcium carbide. The transformation IV + I could naturally not be measured in this way, since it does not take place spontaneously in this carbide, free from sulfur or “excess of calcium.” For the reversed transformation, I -+ IV, in heating, a somewhat less primitive equipment will have to be used. D. Quenching of the cubic phase Attempts were made to obtain the new cubic carbide form I V a t room temperature, by rapid quenching from temperatures above 45OoC., and thus suppressing transformation not only into the stable forms I and 11, but also into the metastable phase 111. Very small quantities (0.005 g.) of calcium carbide were heated briefly a t 600°C. inside a thin glass tube, of 0.5 mm. diameter and 0.01 mm. wall thickness, and quenched in water. The tube was then placed in the x-ray camera. In no case could the cubic form be found. Obviously, the directive force of the anisotropic Csanions, which, for instance, in the tetragonal structure I tends to turn them parallel to each other and which is overcome a t temperatures above the transition point of 450”C., is too strong below that point to permit supercooling. The condition of greater disorder in the orientation of these anions, which must be assumed to exist in the cubic phase and which may include even free rotation, analogous to the cubic forms of sodium and potassium cyanides ( l l ) , cannot be preserved a t lower temperatures. Extension of the measurements of the specific heat of calcium carbide
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M. A. BREDIO
to the temperature range between 400” and 500°C. will furnish more information on the natuw of the cubic phage. It will be the more valuable, as it is the cubic form IV in which calcium carbide exists in all of its reactions above 45oOC. Further experiments with such additions to the carbide as are insoluble in the phases I, 11, and 111,but soluble in IV, may still lead to the stabilization of the high-temperature phase IV. The addition of calcium selenide or calcium telluride appears somewhat more promising than that of sulfide or cyanamide, which were found to be without effect in stabilizing carbide IV. For an understanding of the mechanism of the lattice transformation of form IV into one of the low-temperature phases, the following observation is of interest: Small rectangular pieces of carbide (I, 11, or 111) can be obtained by splitting with a knife. “Single-crystal” rotation x-ray diagrams can be obtained by exposure with simultaneous rotation around any of the three edges. Actually, these diagrams have been shown by Stackelberg (13)and by the writer (6) to be the result of simultaneous rotation around more than one vector of the crystal lattices of form I and form 11. In the case of carbide I, it is the axes c and a of the tetragonal crystal lattice, which, owing to the very finely interwoven lamellar microstructure (14),occur simultaneously in the orientation parallel or nearly parallel to the rotation axis of the x-ray camera (6,13). In the relatively simple case of the tetragonal crystal lattice, the resulting x-ray pattern could be interpreted. In the case of phases I1 and 111,however, not the crystallographic axes, but lattice directions of higher indices, of crystal structures of lower symmetry, are parallel to the cleavage edges of the samples. An interpretation of the x-ray diagrams of such “multiple single crystals” of carbide I1 and I11 has not been possible thus far. The fact that all of the three low-temperature phases occur in a definite orientation, and not as an entirely random aggregate relative to the rectangular coiirdinates of the cubic crystal lattice of the high-temperature form, indicates a comparatively simple structural relation and a very definite transformation mechanism. This explanation supersedes the previous one, by Stackelberg, of the particular orientation in the lamellar crystal aggregate of tetragonal calcium carbide, which was based upon the process of crystallization from the melt (13), rather than upon the formation from the cubic high-temperature phase, unknown to that author.
E . The equilibrium diagram of the one-component wstem CaCl i n the solid state Figure 1 is a schematic presentation of the phase relation in calcium carbide in which the amounts of nitrogen, sulfur, and “excess of calcium” are a t a minimum. There are two enantiotropic transformations: ( 1 ) The transition which can be observed a t room temperature, involving the lowest temperature phase I1 and the tetragonal middle temperature form I. The temperature of this point ( A ) could not be determined accurately. The only applicable method is the identification, by x-rays, of the phase formed by mechanical treatment. An incomplete transformation which prevents the accurate determination may be
POLTVORPHISM OF CALCIU?+f CARBIDE
809
due to the very remarkable extension of the transition point to a transition temperature interval, by the presence of even slight traces of impurities, particularly of cyanamide (cf. page 812), in accordance with the phase rule. It may also be due to the incomplete action of the mechanical treatment upon all carbide particles. ( 2 ) The transition point a t approximately 450°C. ( B ) ,involving the tetragonal middle-temperature phase I and the cubic high-temperature phase IV. The transformation is spontaneous in heating only (I ---f IV), while the reveised transformation IV -+ I can be achieved in two stages: spontaneous formation of I11 from IV, and mechanicd treatment of the resulting carbide I11 which yields I a t temperatures between points A and B. Therefore, by the method of grinding a t various temperatures, this transition point B could also be determined with moderate accuracy only.
- 200
0
200
400 ‘C.
T, FIG. 1. Schematic presentation of the phase relation in calcium carbide in which the amounts of nitrogen, sulfur, and “excem of calcium” are at a minimum.
There are three more transition points, inside the metastable area of the diagram: ( 1 ) The transition IV e I11 a t point C, which is spontaneous in both directions, is so close to the point B a t 450C. that the actual difference thus far has not been established. (2)The transformation I1 111, point D: The real position of this point could not be determined. There was no transformation 111-11 without mechanical treatment. With mechanical treatment above the temperature A , only the phase I, as the most stable phase in this temperature range, could be obtained. However, the transformation I1 I11 was observed, in heating the phafie I1 at various temperatures, at a minimum temperature of approximately 200°C. It must be assumed, therefore, that the curves of the free energies of 111 and I1 intersect in some point (D)between room temperature and 200°C.
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810
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A. BREDIG
The amount of phase I11 formed from I1 increases with increasing temperature above 20O0C., but is independent of the time. Quite similar conditions exist, on cooling, in the transformation, in steel, of austenite into martensite (9). This interesting question needs further investigation. ( 3 ) The transformation I1 -+ I11 is remarkable, because a metastable phase (111) was found to be formed by transformation from a stable phase by raising the temperature. There are numerous cases in which, by cooling, metastable phases are formed instead of stable ones, such as in all instances of monotropy, or in the rase of martensite, or of calcium carbide I11 when formed from IV. There is, however, no theoretical difficulty in understanding the formation of n metastable phase from a stable one by heat. In the first place, it is to be reniembered that a t the temperature (200OC.) a t which formation of the metastable phase I11 takes place, the originally stable phase I1 has itself become metastable relative to the actual stable phase of this temperature range, phase I. Furthermore, the temperature of 20O0C., at rhich the first traces of the transformation I1 -+ I11 are observed, actually may very well correspond not to point D, but to point E , the intersection of the energy curve of phase I1 with that of phase IV. This would mean that the spontaneous formation of phase I11 from I1 may actually depend on reaching the metastable energy level of the cubic phase in which the CZanions were shown to be free to move immediately into the positions which they hold in 111. They cannot move into the positions which they have in phase I, although these correspond to a lower energy level, for the same reasons which are responsible for the formation of phase 111, instead of I, upon the cooling of I V from 45OoC.,and which are discussed later in the section dealing with the influence of the presence of cyanamide ions. THE ISFLUENCE OF THE CHEYICAL COMPOSITION UPOS THE CRYSTAL-LATTICE TRASSFORMATIONS
The previous results (6,7) concerning the influence of small quantities of certain commoii impurities, such as sulfide and cyanamide, can be brought into a satisfactory relation with the new knowledge of the crystal-lattice transformations of Iipure” calcium carbide. The three methods employed in the investigation of the latter, as described on the preceding pages, were again used: (I) identification, by x-ray diffraction at room temperature, after grinding a t various temperatures; ( 2 ) identification by x-ray diffraction a t various temperatures; and ( 3 ) cooling curve.
A . The presence of sulfide Calcium sulfide has been shown in one of the previous papers (7) to be a particularly interesting impurity, since it is one of two factors responsible for the almost exclusive occurrence of the tetragonal structure (I) in commercial calcium carbide. It was further shown that it reduces considerably the reactivity of the carbide (8). Of two samples of carbide containing sulfide, one was a commercial product with 90 per cent calcium carbide and 0.9 per cent sulfur, while the other was
POLTYORPHISM OF CALCIUM C.IRBIDE
811
prepared in the laboratory electric-arc furnace with the addition of 15 per cent calcium sulfate, leading to a sulfide content of the rewlting carbide of approximately 5 per cent calcium sulfide, besides 85 per cent calcium carbide. X-ray patterns of a small rectangular piece of carbide revealed the presence of the tetragonal structure in the mechanically untreated carbide, which persisted through the grinding in the mortar even a t temperatures as low w - 180°C. The powder x-ray pattern a t GOO'C. showed transformation into the cubic phase IV. The cooling curve indicates that the transition point I F ' - + I is located close to the point IV ---f 111. Attention may be directed to the fact that, in wing the cooling curvc for determination of the transition point, a very commoii method was here successfully employed with such a common substance as industrial calcium carbide only after in the course of an investigation a logical demand for it developed. Otherwise, the allotropy of calcium carbide might have been detected during the last fifty years. Seither carbide I1 nor carbide I11 was obtained in the two specimens containing sulfur. The formation of the metastable phase I11 obviously was suppressed by the presence of sulfide, this result being in full agreement with those of the previous paper (7). The same holds for phape 11. In the prrsence of sulfide and the simultaneous absence of cyanamide,--the effect of which will be discussed in the following paragraph,-phase I appears as the only stable phase in the temperature range between 435" and - 180°C. There remains the possibility that, though the transition point I e I1 is lowered considerably below room temperature by the presence of sulfide in solid solution in the carbide, phase I1 still may be the stable phase a t some very low subnormal temperature even in calcium carbide containing sulfide. This can be ascertained only after equipment has been designed which will permit identification of the structure a t the low temperature a t which the carbide is pulverized. This is the only way of avoiding the possible spontaneous retransformation during reheating to room temperature. It is, furthermore, not impossible that the stabilization of the tetragonal phase below room temperature, by sulfide, is due only to a change in the mechanism of the mechanical defbrmation (twin formation, rupture), and not to a true change in thermodynamic stability. By no means can a less stable form always be transformed into the stable one by mechanical deformation (cf., aragonitecalcite). Cases such as silver iodide, zinc sulfide, lead oxide, and austenite, in which transformation in mechanical deformation occurs, seem exceptions rather than the rule.
B. The presence of calcizim cyanamide One peculiar result of the previous work (6, 7) had been the discovery of the influence of the most important industrial reaction product of calcium carbide, namely, calcium cyanamide, upon its crystal structure. Its presence seemed at that time to have caused the formation of structure I1 under any circumstances.
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M. A. BREDIG
X-ray patterns ( a ) of calcium carbide made from peat coke and calcium cyanamide in the electric-arc furnace and containing approximately 5 per cent CaCN2, (a) of industrial carbide which was remelted with the addition of calcium cyanamide,-both Containing sulfide-, and ( c ) of sulfur-free carbide prepared in the laboratory electric-arc furnace with the addition of calcium cyanamide were obtained in the high-temperature x-ray camera a t various increasing temperatures: 3W0, 350", 400", 450°, 500°, and 600°C. In these three specimens, also, phase IV is the stable phase above and a t 450°C. At 3N0, 350°,and 400"C., not structure 1, as in "pure" carbide, but structure IT is the stable phase when calcium cyanamide is present in solid solution, as is indicated by the stability of I1 toward mechanical treatment in that temperature range, In carbide containing cyanamide, the temperature range of stability of phase I1 thus appears to be raised by more than 400OC. above the transition point of pure carbide a t room temperature. In increasing the temperature of the solid solution of cyanamide in carbide 11, the x-ray reflections are shifting slightly, as those of phase I are. In both structures the x-ray lines move together towards those positions a t which, above 450"C., the lines of the cubic phase IV are located. In tetragonal carbide this means a decrease of the ratio of the crystallographic axes c/a, with increasing temperature, from 1.161 a t room temperature, to 1.134 a t 430°C., and to 1.O00, in the cubic form, above 450OC. Two different cases must be distinguished in the behavior of calcium carbide, containing cyanamide, when cooled from the temperature range of the cubic phase: ( 1 ) If the carbide contains cyanamide, but does not contain sulfide a t the same time, form IV is not transformed spontaneously into form 11,actually the most stable phase below 435"C.,but into the metastable form 111. Mechanical deformation is necessary for further transformation into the stable phase 11. This transformation, by grinding, appears t o be much facilitated by the presence of cyanamide. On this basis, former statements that in carbide free from sulfur phase I11 was not obtained in the presence of cyanamide (7) must be revised. The previous result can be understood because of the ease with which transformation from I11 into I1 apparently took place while the sample was ground in the mortar in preparation for the x-ray exposure. Actually, the presenc'e of calcium cyanamide alone does not lead to spontaneous formation of carbide 11; on the contrary it may even promote, and certainly does not prevent, the formation of carbide 111. (S) If the carbide contains both cyanamide and sulfide, it is transformed spontaneously into the stable phase 11 when cooled below 435OC. The effects of calcium cyanamide on the structure of calcium carbide are best represented by a more or less idealized schematic equilibrium diagram (figure 2). While calcium cyanamide appears t o be practically insoluble in tetragonal calcium carbide I, phase I1 can disaolve appreciable quantities of calcium cyanamide in solid solution (6). Since grinding of calcium carbide containing only 1 or less than 1mole per cent of calcium cyanamide leads, in the temperature range between 400' and 435"C., to a two-phase mixture of I and 11, the solubility of calcium carbide in the solid solution I1 can be represented approxi-
POLYMORPHISM OF CBLCIUM CARBIDE
813
mately by the line A E . I t is believed that grinding at an elevated temperature leads to the establishment of a condition not too far from equilibrium between the two phases I and 11, with some slight diffusion probably taking place under these special circumstances. I t is further believed that, even in relatively rapid cooling, carbide with less than 2 mole per cent cyanamide, in the presence of sulfide, suffers spontaneous decomposition of its phase IV into the mixture of phases I and 11, stable below the eutectoid line CDE. Such mixtures were observed in carbide containing, besides sulfide, less than 2 mole per cent of cyanamide (6). Subsequent homogenization and formation of the stable ho-
-
1001
/ VOLE PER CENT C4C2
FIG.2 . Schematic equilibrium diagram representing the effect of calcium cyanamide on the structure of calcium carbide.
mogeneous solid solution I1 by grinding a t room temperature, where diffusion is impossible, can naturally not occur. In the absence of sulfide, the metastable phase I11 is formed spontaneously instead of the stable phases I and 11, when cubic calcium carbide IV is cooled below 435"C., no matter whether calcium cyanamide is present in considerable amounts, or in an amount of 0.15 mole per cent (page 805), or less ( 7 ) . Xo theory can as yet be advanced which might definitely explain the causes of the formation of the metastable phase instead of the equilibrium phases. It is well known, especially to the metallurgists, that the decomposition of a solid solution (e.g., austenite in steel) into two new phases (a-ferrite and iron carbide, in steel) is frequently inhibited by the slowness of diffusion, particularly a t temperatures
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M. A . BREDIG
far below the melting point. Taking into consideration the equilibrium diagram of figure 2, one might feel that the need for diffusion of the cyanamide ions, which is required for the establishment of equilibrium and for the formation of phases I and I1 below 435OC., is the important factor in the formation of the metastable phase (III), just as the trapping of the carbon in the iron space lattice is in the case of the tetragonal martensite in steel. However, the pre3C) in vious results on the formation, from calcium oside and carbon (CaO a vacuum furnace, of carbide I11 in which no noticeable quantities of nitrogen are known to have been present, would not be in agreement with this line of thought, unless estremely small quantities of cyanamide were assunied to be sufficient. Further experimentation is therefore required before any definite cause of the formation of the metastable phase 111 in calcium carbide can be considered established. Each of the two substances, calcium sulfide and calcium cyanamide, widens the temperature range of stability of that crystal modification of calcium carbide which is structurally related to the added substance: The simple tetragonal structure of carbide I is more related to the cubic face-centered crystal lattice of calcium sulfide than is the structure of carbide 11. The tetragonal phase I is therefore stabilized by sulfide below room temperature. On the other hand, carbide I1 is better suited than carbide I to dissolve calcium cyanamide, of rhombohedral crystal structure (2), in solid solution. The identity, in the spacings of thc two strongest x-ray reflections of th$se two substances, calcium carbide I1 and calcium cyanamide, with d = 2.93 A., can be considered as an indication 2f this relation. The corresponding spacing in the tetragonal carbide I is 2.75 A., and in the carbide 111it is 2.85 .&.,that is, it is intermediate between those of I and 11.
+
C . The influence o j calcium Perhaps the most startling and generally most interesting effect upon the crystal-lattice transformations in calcium carbide is the one which is connected with the presence in, or the escape from, calcium carbide of what a t first sight might appear to be an excess of calcium. It has been shown in one of the previous papers (7), as well as in the first section of this paper, that the preparation of calcium carbide under conditions which favor the presence of a calcium vapor pressure higher than the carbide dissociation pressure, leads, a t room temperature, exclusively to the tetragonal phase I, and that forms I1 and 111are entirely suppressed under those conditions. X-ray patterns of calcium carbide, prepared from sugar charcoal and pure calcium metal (in excess of the composition CaC2) in a closed steel tube a t llOo°C., showed with exposures a t various temperatures between 25" and 500°C. that above 345°C. the cubic lattice IV occurred. As in the case of the sulfide addition, cooling below that transition point immediately leads to the stable tetragonal phase I. On the cooling curve, the transition point was found to be at 440°C. In pulverizing a t various temperatures down to - 180°C., the tetragonal structure I was always obtained when the product was identified a t room tem-
POLYMORPHISM OF CALCIUM CARBIDE
815
perature. As in the case of the effect of sulfide, further study will be required to determine whether transformation into carbide I1 actually occurred at some subnormal temperature and was spontaneously reversed in reheating to room temperature for identification by x-rays. No transformation a t subnormal temperatures is indicated in the measurements, by Kelley (lo), of the specific heat of industrial calcium carbide. I t was not reported whether this particular carbide possesses the tetragonal structurp or, owing to the possible presence of cyanamide, structure I1 a t room temperature. Therefore, it is not possible to conclude from these measurements that sulfide or “calcium excess” had suppressed transformation in that subnormal temperature range. The determination of the specific heat of carbide pulverized a t extremely low temperature, in heating to room temperature, might be another way of determining the existence, belo\y normal temperatures, of a spontaneous transformation I1 4 I in calcium carbide containing sulfide or excess of calcium. While the stabilization of the tetragonal phase below normal temperature can easily be understood in the case of sulfide addition, by the formation of a solid solution of calcium sulfide in calcium carbide which wa8 proven to exist with even much larger quantities of sulfide (7), difficulties exist for a similar explanation in the case of the carbide containing an excess of calcium. A solid solution of calcium metal as such in the carbide cannot be assumed. Furthermore, it has not been possible thus far, to find in the acetylene made frcm carbide containing an excess of calcium, other hydrocarbons, such as methane, ethylene, or allylene. Such would be expected if the presumed increase in the ca1cium:carbon ratio in the carbide had resulted in the formation of a solid solution, in calcium carbide, of a “subcarbide”, CaC (4),or of calcium allylide, C&C3,analogous to the known magnesium allylide, Mg& (7). The quantities of such carbides in solid solution in calcium carbide which might be sufficient to produce the described effects may be rather small, certainly less than 1 mole per cent, and may therefore have escaped observation. Consequently, this possibility of explaining the phenomena cannot be entirely ruled out. I t may even be of considerable technological interest, since it implies the possibility of the existence of two varieties of commercial calcium carbide, both of tetragonal structure, due to the presence of sulfur, one of which however could contain very small quantities of carbides such as CaC or C a G . The other one would be free from these carbides, if prepared under conditions similar to those described in the first section on the preparation of carbide 111, e.g., by “overheating” the carbide melt, to the completion of the reactions
+ C = Ca + CO CaC + CaO = 2Ca + CO Ca2C3+ 3Ca0 = 5Ca + 3CO CaO
or, rather, or or
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M. A . BREDIG
+ Ca 2CapCs = 3CaCz + Ca 2CaC = CaC2
or
In another attempt to explain the elimination of the metastable phase I11 as well as the suppression of the transition I + I1 below 25”C., by calcium, these observations may also be considered in the light of the calcium carbide-calcium cyanamide equilibrium diagram (figure 2). Under the conditions under which carbide containing an excess of calcium is prepared, either from calcium metal and carbon in a closed steel tube, or from calcium oxide and carbon (CaO 3C) during a short melting period in the electric-arc furnace without complete decomposition of calcium oxide, traces of cyanamide might be unstable, and be transformed into CY-, N---,or even NH-- (if some hydrogen is present). As a model reaction, the decomposition of pure, white calcium cyanamide by means of calcium metal,
+
2CaCK2
+ Ca = Ca*N2.Ca(CN)9= 2Ca2K(CN)
Le., the formation of a complex nitride-cyanide which, quite differently from pure calcium cyanide (12), was found to be stable even on slow cooling from high temperatures, has been verified by the writer. In calcium carbide containing an excess of calcium, the last traces of cyanamide, which were mentioned above as a possible, though still doubtful, cause for the formation of the metastable phase I11 in “pure” calcium carbide, might thus have been removed from the carbide space lattice. Possibly also, not so much the remmal of cyanamide ions, but the presence of traces of the other forms of nitrogen, such as CN-, N---, or NH--, may be considered as the cause of the suppression of phases I11 and I1 in carbide containing an excess of calcium. Exactly like the sulfide, S--, these may act as a “catalyst” for the formation, a t 435”C.,of the stable carbide form I, and, being components of a solid solution, as stabilizers of this form I against transformation into I1 in grinding below room temperature. It would seem quite reasonable to assume that CN-, N--- or NH--, like S--,fit better into the tetragonal space lattice, of the sodium chloride type, than into the phase 11, of lower symmetry. For this latter, on the other hand, the CNC- ions exhibit a decisive preference, owing to their more anisotropic shape (length of C N T , 4.6 b. (2), as against 4.2 b. (13) for C Y ) . T H E POLYMORPHISM OF STRONTICY AND BARIUM CARBIDES
The crystal structure I1 of calcium carbide has previously (6,7) been found to occur also in strontium carbide, but not in barium carbide. With the electric-arc furnace, described in the second section of this paper, samples of strontium and barium carbides were obtained which contained the oxides and graphite as the only impurities. (a) After grinding a t room temperature, or a t O’C., the strontium carbide prepared in this way consisted of a mixture of the tetragonal form (a0 = 5.81, co =
POLYMORPHISM OF CALCIUM CARBIDE
817
6.68d (13)) with smrtll amounts of the other form analogous in structure to calcium carbide I1 (7). The subnormal transition temperature I e I1 in pure strontium carbide has not yet been determined with certainty; however, grinding at -80°C. led to form 11. In x-ray exposures a t 600",370°, and 320"C., the tetragonal structure was found to be stable a t 320°C. At 370"C., the cubic face-centered lattice with a. = 6.24 b. appeared; this corresponds completely to CaCaIV (table 1). From observation of the cooling curve, the transition temperature cubic S tetragonal strontium carbide was more accurately found to be 37OoC. f 5". In samples containing 62 per cent of strontium carbide, which had been quenched from 600°C. in order to obtain the cubic high-temperature form at room temperature, the cubic strontium carbide structure, with a0 = 6.24 b.,was not obtained, but only the space lattice of the oxide with a0 = 5.16 b. This x-ray diagram is not changed by grinding the quenched sample. More experiments are required for a definite explanation of this rather peculiar observation. It may, however, be recalled that Damiens (3) has obtained solid solutions of cerium carbide and cerium oxide, and that Stackelberg (13) has already pointed to the oriented aggregation of strontium carbide and strontium oxide crystals. ( b ) In barium carbide neither the metastable form I11 nor form I1 of calcium carbide were observed. On slow cooling, the tetragonal form with a. = 6.22 A. and eo = 7.06 b. (13) was always obtained. In x-ray exposures, even a t such a low temperature as 2OO"C., the cubic form with uo = 6.56 b. was found (table 1). This cubic phase can easily be preserved a t room temperature by quenching from 200°C. In quenching from 600°C. the second cubic phase, corresponding to that found in strontium carbide under the same conditions, with the lattice constant of the oxide, a. = 5.52 b.,was also obtained. The fact that a metastable form such as calcium carbide I11 was not observed in strontium or barium carbide may be connected with the higher solubility of strontium and barium cyanamides in the tetragonal forms of these latter two carbides, which makes diffusion for transformation unnecessary. It may also have something to do with the solubility of oxides in solid solution with these carbides. Both these higher solubilities could be explained by the smaller share of the anion in the space lattice dimensions of these heavier carbides, as compared with calcium carbide. In view of the apparent relationship between the transition temperature and the atomic weights (ionic diameter) of the alkaline-earth metal carbides, it seems quite possible that a t an extremely low temperature structure I1 of calcium and strontium carbides might still be found in barium carbide, e.g., by means of the experimental arrangement proposed in section B (page 811). SUMMARY
1. The thermal conditions of stability of various crystal modifications of cal-
cium carbide, prepared by a special procedure in the electric-arc furnace, have been established through the application of mechanical deformation as a means
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M. A. BREDIG
POLYMORPHISM OF CALCIUM
CARBIDC
819
of achieving crystal-lattice transformations, and also by the use of the hightemperature x-ray camera. 2. The polymorphism of the alkaline-earth metal carbides is summarized in table 2. In calcium and strontium carbides, three enantiotropic forms, one of cubic, one of tetragonal, and one of lower symmetry, occur. In barium carbide, the cubic and the tetragonal forms have thus far been observed, but the possibility of the existence of the third form a t very low subnormal temperatures is indicated. All of these three crystal structures are considered typical of the carbides MeCl of the calcium carbide type. 3. A fourth form of calcium carbide, 111,has been proven to be metaatable in the whole temperature range investigated, Le., above 100°K. Possible explanations of the conditions under which it does or does not occur have been outlined. 4. The influence of calcium sulfide, calcium cyanamide, and, particularly, of calcium metal upon the course and the mechanism of the transformations in calcium carbide have been discussed. The author gratefully acknowledges the assistance of Dr. Kin-Hsing Kou in some of the experimental work, as well as the helpful interest of Dr. H. H. Franck and of Dr. Albert R. Frank, in whose research laboratory this investigation waa carried on. REFERENCES
(1) BLOCH,R., AND MOELLER, H.: Z. physik. Chem. A162,245 (1931). (2) BREDIG,M.A.: J. Am. Chem. Soe. 64, in press (1942). (3) DAMIENS, A,: Bull. soc. chim. 16, 370 (1941). R.: Z. Elektrochem. 17, 177 (1911). (4) ERLWEIN,G.,WARTH,C., AND BEUTNER, H.H.:Compt. rend. 17 congr. chink indust., Vol. 2,p. 1160 (1937). (5) FRANCK, (6) FRANCK, H.H., BREDIG,M. A,, AND HOFFMANN, G.: Z. anorg. allgem. Chem. 'dS2, 61 (1937). (7)FRANCK, H.H.,BREDIG,M. A., AND KIN-HSINQKon: 2. anorg. allgem. Chem. 2(M, 75 (1937). (8) FRANCK, H. H., AND ENDLER, K.: Z. physik. Chem. A184, 127 (1939). A. B.: Trans. Am. SOC.Metals SO, 1 (1942). (9) GRENINGER, (10) KELLEY,K. K.: Ind. Eng. Chem. 38,1314 (1941). (11)MESSER,CA. E.,AND ZIEGLER,W. T.: J. Am. Chem. SOC.63,2703 (1941). G.,AND FRANCK, H. H.: Z. anorg. allgem. Chem. 357,l (1938). (12) PETERBEN, M.v.: Z. physik. Chem. BD, 437 (1930). (13) STACKELBERG, (14) WARREN, C. H.:Am. J. Sci. 2, 120 (1921). R. E.: J. Phys. Chem. 46,1179 (1941). (15) WEST,F. B., AND MONTONNA,
