On crystalline structure of carbyne

70

LETTERS

TO THE

On Crystdline Structure of Carbyne (Received 14 Az~gwt 1972) PREVIOFS~Y [l-3] it was shown that duringoxidativ~ coupiing of acetylene its reaction with the solution of cupric chloride leads to amorphous carbon having the chain structure of carbon macromolecules of the polyyne (--C=%--C-C-),$ or cumulene (=C=C=C=), type. The chain structure was confirmed by comparison of the IR-data of the resulting compound with those of the products obtained during synthesis of polyvinylidene chloride by its dehydrochlorination of the solution of metallic sodium in liquid ammonia[S]. On subsequent heating in vacuum (to 1000%) partial crystallizatio~ takes place to yield a new allotropic crystalline form of carbon retaining the chain structure of carbon macromolecules. The amorphous and crystalline structure of the new form of carbon, which is called ‘carbyne’, was characterized by X-ray and electron diffraction [4]. Goresy and Dannay [5] give the X-ray analysis of the crystalline structure of the new allotropic form of natural carbon, silver-white in colour, isolated from the graphite particles in the meteorite Ries Crater (Bavaria). The crystelline carbon of similar X-ray characteristics was also found in the New Uray meteorite [S]. The ‘white’ crystalline carbon as synthesized by means of sublimation of pyroIytic graphite at a high temperature in vacuum [7]. Recently silver-white carbon was produced by means of sublimation of pyrolytic graphite at the atmosphere pressure [8]. Almost complete coincidence of interplanar spacings dnkl of carbyne and natural ‘white’ carbon measured on the X-ray diffraction patterns of the polycarystalIine specimens (Table 1) testifies to a close similarity of their crystalline structure. Two types of pointed electron diffraction patterns of carbyne monocrystals observed earlier [4] enabled us to draw a conclusion on the existence of the two polymorphous modifications Q!and p. New indication of diffraction lines on the X-ray pattern of the carbyne powder and natural ‘white’ carbon shows good agreement between dcalcd.and dobsvd.on the assumption that there are two hexagonal crystalline modifications with the following cell parameters

(Table 1). Investigations of thermal transformations of carbyne under pressure confirm the

EDITOR

occurrence of the ~-mod~~cation . It was found previously that carbo-chain polymers with triple -C*-bonds in the unit (poiyarylenepolyynes) {lo], as well as carbyne [11 J are not transformed to diamond under pressure both under the conditions of catalysic synthesis and in direct transition. Table 1 shows diffraction lines of the X-ray pattern of carbyne heated for 5 min at 1800°C and at the pressure of 90 kbar. (The Table does not give the lines of graphit formed from the amorphous phase of carbyne.) One can note the disappearance of the lines characteristic of ol-modification and the occurrence of new lines corresponding to interpianar spacings of &modifications of carbyne. This is evidence of the transformation of cw-carbyne, which is less stable under processing conditions, to the more stable P-carbyne in accordance with increasing density. The density for both modifications was determined from the volumes of the elementary cells and the numbers of atoms in them: Z = 144 and Z = 72. The calculated densities were d = 2.68 g/cm3 and d = 3.13 g/cm”. The density of the initial carbyne containing a certain amount of amorphous phase was determined by the gradient liquid method: d,,,,, = 2.48 g/cm”. The vaiue of the parameter ‘a’ that we have taken for the cell of cu-modification is close to that found for natural ‘white’ carbon. n = 8.948 A [5]. Since different crystallographic axes were chosen, this value differs by the factor v/3 from that measured earlier[4,9] on the pointed electron diffraction pattern of the monocrystal (n = 5.15 A). If the atomic chains are arranged along the hexagonal axis of the crystal, the value of the other parameter of the cell of cz-modifications (C = 15.36 A) includes 12 atoms, the length of the 2-atom0 unit of the polymeric carbon chain being 2.56A. As might be suggested a denser &modification corresponds to higher intermolecular order and it is, therefore, characterized by the value of the parameter ‘c’ of the cell, which is, accordingly, two times as low.

V. I. KASATOCHKIN V. V. KORSHAK YU. P. K~DRYAVTSEV A. M. SLADKOV 1. E. STERENBERG Irzstitzcteof orguno-element compounds Academy @Sciences, LLSSR

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LETTERS

72

TO THE

REFERENCES 1. Korshak V. V., Kasatochkin V. I., Sladkov A. M., Kudryavtsev Yu. P. and Usembaev K., Dokl. Akad. Nauk, USSR 136, 1342 (1961). 2. Kasatochkin V. I., Egorova 0. I. and Aseev Yu. G., Dokl. Akad. Nauk, USSR 151, 125 (1963). 3. Sladkov A. M., Kasatochkin V. I., Kudryavtsev Yu. P. and Korshak V. V., Z.zv.Akad. Nauk, USSR, Ser. Khim. 2697 (1968). 4. Kasatochkin V. I., Sladkov A. M., Kudryavtsev Yu. P., Popov N. M. and Korshak V. V., Dokl. Akad. Nauk, USSR 177,358 (1967). 5. Goresy A. El. and Donney G., Science 161, 363 (1968). 6. Vdovykin G. V., Geokhimiya, N.9 (1969).

Carbon, 19’73, Vol. 11, pp. E-73.

Pergamon

Press.

EDITOR

7. Whittaker A. G. and Kintner P. L., Science 165, 589 (1969). 8. Kasatochkin V. I., Kasakov M. E., Savransky V. A., Nabatnikov A. P. and Radimov N. P., Dokl. Akad. Nauk, USSR 201, 1104 (1971). 9. Kasatochkin V. I., Babchinitser T. M., Kudryavtsev Yu. P., Sladkov A. M. and Korshak V. V., Dokl. Akad. Nauk, USSR 184,353 (1969). L. F., Kalashnikov Ya. A,, 10. Vereshtchagin Feklichev E. M. and Nikolskaya I.V, Modern Problems of Physical Chemistry (in Russian), p. 173, State University of Moscow (1968). 11. Kasatochkin I. V., Sterenberg L. E., Slesarev V. N. and Nedochivin Yu. N., Dokl. Akad. Nauk, USSR 194,801(1970).

Printed in Great Britain

Paramagnetic Spin Centers in Amorphous Carbons* (Received 4 August 1972) The mixed character of ESR lines in turbostratic and incompletely graphitized carbons was observed and thoroughly investigated by Mrozowski [l]. With the use of a simple formula based on the temperature dependence of the intensity of the signal he managed to determine the contributions of localized paramagnetic centers and of current carriers to the total signal of the paramagnetic resonance absorption. Amorphous carbons obtained by evaporation according to the method presented in [2], also exhibit resonance lines of mixed character. It was believed that due to the amorphism and high chemical purity of such carbons, investigations of their ESR signal could yield new information as to the nature of paramagnetic centers in carbon. The behaviour of such as-deposited carbons follows almost exactly the Curie law and the value of the ratio of the areas under the absorption curve measured at the temperature of liquid nitrogen and at room temperature (Fig. 1) indicates a contribution of 95% localized centers to the total intensity at room temperature. Carbons heattreated in vacuum up to 500°C exhibit a decrease of the contribution of localized centers to about 45% and what is more significant, a large decrease in the total signal is observed (see Fig. 4,

Ref. [2]. In other words, most localized spins become annealed out by heattreatment to 500°C. However, the contribution of carriers to the resonance line seems not to be altered by heattreatment up to 500°C. This is rather surprising since the value of activation energy as determined from the temperature dependence of electric conductivity, decreases with heattreatment by a factor greater than 2 (from 0.22 eV to 0.09 eV [3]), the electric conductivity at room temperature increasing by one order. The size of the crystallites for such a type of carbon as determined by electron diffraction[3] is not changed by heattreatment up to 500°C and remains equal to approximately 10 A. As shown by Presland and White[4], the crystalhtes in evaporated carbon films do not grow below 1000°C. The only structural change which may occur in heattreatment applied in this work is the filling of

1 0

*Sponsored by Institute of Low Temperature and Structure Research, Polish Academy of Science (contract No. PAN-3.2.02).

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