Action des trihalogenures de chrome sur le graphite: Synthese et etude des composes graphite-chlorure chromique

Abstract

As indicated by Croft[1], then Leparlier[2] the insertion of chromium trichloride can be carried out with difficulty leading in all cases to inhomogeneous products. Chromium tribromide[3] and chromium tri-iodide[4] cannot be intercalated. Figure 1 shows the quantity of chromium trichloride fixed by natural graphite (250 < θ < 500μ) as a function of the temperature for one week of reaction. The insertion whose effect is to dislocate the large diameter crystallites (Fig. 2) only takes place on the periphery of the pyrographite samples (Fig. 3). Heated in vacuum, the graphite-CrCl 3 products dissociate only at relatively high temperatures (Fig. 4). Study of the X-ray data presented in Table 1 indicates that the products of formulae comprised between C 22CrCl 3 and C 29CrCl 3 are third stage compounds ( I c = 16.15 A ̊ ). The quantity of free graphite increases as that of the inserted halogenide decreases (Fig. 6). The product of average composition C 21CrCl 3 obtained on large crystallites (1 < θ < 2 mm) is in reality a mixture of both second and third stages, although the X-ray diagram carried out on the fine portion obtained by dislocations of the large grains reveals only a second stage of c-axis periodicity I c = 12.80 A ̊ . Contrary to the graphite-FeCl 3 compounds[13,14], the powdered graphite-CrCl 3 products could not be prepared in pure stage states[15]. Both X-ray diffraction (Fig. 7) and electron microdiffraction (Fig. 8) data show that the chromium trichloride layers conserve their own structure. The two hexagonal sublattices, one corresponding to the graphite ( a = 2.46 A ̊ ), the other to the intercalated trichloride ( a = 5.95 A ̊ ) are turned by an angle of 30° as in the case of graphite-FeCl 3 compounds[12], Studies carried out on single crystals allowed us to determine the relative disposition along the c-axis of the carbon layers A and the halogenide layers α. In the first stage graphite-FeCl 3 products, the photographs (Fig. 9) lead unambiguously to the sequence AαAα… In the third stage graphite-CrCl 3 compounds (Fig. 10) analysis of the rotating crystal photographs do not allow us to choose between the two sequences AαABAαA and AαAAAαA… Thermomagnetic curves of the rich products (Fig. 11) show that the magnetic behaviour of the intercalated chromium chloride is close to that of the free chloride: the extrapolated Curie-Weiss temperature (27 ± 3° K) is in the neighbourhood of that the halogenide (32°K); the magnetic moment of the chrome ion (3.60 μ B ) is very similar to that of Cr 3+ ions (3.85 μ B ) measured on CrCl 3.