R. Vangelisti

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

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-CrCl3 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 C22CrCl3 and C29CrCl3 are third stage compounds (Ic = 16.15 A). The quantity of free graphite increases as that of the inserted halogenide decreases (Fig. 6). The product of average composition C21CrCl3 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 Ic = 12.80A. Contrary to the graphite-FeCl3 compounds[13,14], the powdered graphite-CrCl3 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-FeCl3 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-FeCl3 products, the photographs (Fig. 9) lead unambiguously to the sequence AαAα… In the third stage graphite-CrCl3 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 μB) is very similar to that of Cr3+ ions (3.85 μB) measured on CrCl3.

Comparative study of electrical resistivity of metal trichloride intercalated graphite

Abstract We have studied the T and n dependence of ϱ a from 4.2 to 300 K for a variety of metal trichloride-graphite intercalation compounds. The most complete work deals with the gallium trichloride family, for which 1st to 11th stage samples were examined. The results are compared with those concerning intercalated AlCl 3 , FeCl 3 and TlCl 3 . The minimum room temperature ϱ a value for GaCl 3 materials is slightly greater than 3 μω cm at stage 5, consistent with results on other families. The thermal variation of ϱ a has been examined, particularly in the temperature ranges in which DTA and X-ray analyses reveal transitions.

Caracterisations chimique et structurale des composes graphite-trichlorude de gallium

Abstract The reactivity of gallium trichloride towards graphite is such that, even at room temperature, the reaction leads to the blue, saturated, first stage compound of formula C 9.84 GaCl 3.45 . Above 250°C, thermogravimetric data reveal less well defined compounds of 2nd and 3rd stage and confirm the presence of an excess of chlorine in all the products obtained ( Cl / Ga ⋟ 3.45 ). Radiocrystallographic data clearly show a variation in interplanar distance as a function of stage, comparable to that observed for graphite-AlCl 3 products, and the possibility of preparing defined compounds for 5 ≤ n ≤ 10. An in-depth study of the 00 l reflections clearly shows that the gallium trichloride, in the intercalated state, does not respect the classical chlorine-metal-chlorine arrangement observed in the graphite-metal halide systems. The GaCl 3 compounds possess a new chlorine-chlorine-metal-metal-chlorine-chlorine structure ( R = 0.039) obtained through association of Ga 2 Cl 6 dimers.

Transport properties in magnetic acceptor GICs

Abstract Results on the in-plane electrical resistivity, thermopower and thermal conductivity of low stage FeCl 3 and CoCl 2 graphite intercalation compounds (GICs) are reported. Each transport coefficient exhibits anomalies around the magnetic phase transition temperature. As far as the thermal and thermoelectric properties are concerned, it is suggested that these anomalies should be attributed to magnons, in agreement with other experiments previously reported on such magnetic compounds. From electrical resistivity data, it is shown that the CoCl 2 -GIC and the FeCl 3 -GIC systems lead to contrasting behaviors.

Heterocomplex-based graphite lamellar compounds: C22CuAl2Cl8.5 and C10Cd0.2AlCl3.7: Intercalation pathway, structure and transport

Abstract Attempts to intercalate the chloroaluminate complexes CuAl2Cl8 and CdAl2Cl6 into graphite have resulted in the synthesis of two new first stage compounds, C22CuAl2Cl8.5 and C10Cd0.2AlCl3.7. The former possesses an oblique inplane unit cell, the latter a rectangular cell, commensurate with that of the host lattice. The electrical resistivity has been studied within (ϱa) and perpendicular (ϱc) to the basal planes from 295 to 4.2 K. Both d ϱ a dT and d ϱ c dT are positive (metallic) and the anisotropy ϱ c ϱ a lies between 105 and 106 over the full temperature range. The data are analyzed and discussed in terms of the structure and current transport theories.

Thermal-conductivity and Thermopower Enhancement in Magnetic Acceptor Graphite-intercalation Compounds

Abstract Results on the in-plane thermopower and thermal conductivity of first stages FeCl 3 and CoCl 2 graphite intercalation compounds and of a CoCl 2 AlCl 3 biintercalation compound are reported. Around the magnetic phase transition temperature, the thermopowers of the two first stage magnetic compounds are much larger than those observed in non magnetic compounds. The effect is less pronounced in the biintercalation compound. For the FeCl 3 compound, a hump in the thermal conductivity is also observed around 5.5K. We suggest that these anomalies should be attributed to magnons, in agreement with other experiments previously reported on such magnetic compounds.

Transport studies on some metal chloride intercalated graphites and the corresponding biintercalation compounds

Abstract Resistivity measurements seem to be a sensitive method to detect structural or magnetic transitions. For AlCl 3 binary compounds s = 2 ϱ c variation vs T confirms the existence of a structural transition below 220 K. For CoCl 2 containing binary and ternary compounds (GICs - GBICs), ϱ a variation vs T displays an abrupt increase below the T c temperature (10 K).

Sodium tetra­bromo­aurate(III) dihydrate

The structure of the title compound, NaAuBr4·2H2O is isomorphous with that of NaAuCl4·2H2O. The Na, Au and Br atoms lie on the mirror plane. The AuBr4− anions are nearly square planar with Au—Br bond lengths in the range 2.415 (2)–2.433 (2) A.

Transport study of (T, p) phase diagram in PdAl2Cl8 intercalated graphite

We have examined the c-direction variations of resistance, Rc (or resistivity, varrhoc) over the 4.2–295 K. temperature range at ambient pressure, and over the 160–295 K range for pressures up to 1 ...

Potassium tetrabromoaurate(III)

The structure of KAuBr 4 , previously reported to be an isotype of KAuCl 4 , has been determined in the centrosymmetric space group P2 1 /c. The monoclinic cell contains four Au atoms (two Au1 and two Au2). The AuBr - 4 anions are nearly square planar, with Au1-Br bond lengths of 2.4096(15) and 2.4146(15) A, Au2-Br bond lengths of 2.4016(17) and 2.4172 (15) A, and Br-Au-Br angles of 90.31 (5) and 90.87 (6)° for Au1 and Au2, respectively.