Philippe Lagrange

Superconductivity of Bulk CaC 6

We have obtained bulk samples of the graphite intercalation compound, CaC6, by a novel method of synthesis from highly oriented pyrolytic graphite. The crystal structure has been completely determined showing that it is the only member of the MC6, metal-graphite compounds that has rhombohedral symmetry. We have clearly shown the occurrence of superconductivity in the bulk sample at 11.5 K, using magnetization measurements.

Intercalation of the amalgams KHg and RbHg into graphite: Reaction mechanisms and thermal stability

Abstract The intercalation of the amalgams MHg (M = K, Rb) allows the synthesis of the first stage MHgC4 and the second stage MHgC8 ternary compounds. The intercalant is formed by two layers of alkali metal surrounding an intermediate plane of mercury atoms. We have shown that the intercalation occurs in two steps. (i) A quasi-selective intercalation with a continuous change of stage where only the alkali metal penetrates between the graphite planes. This step leads to the MC8 compound with some mercury atoms in the metallic sheets. (ii) A simultaneous cooperative intercalation: alkali metal and mercury penetrate together in the occupied interlayers with the ratio Hg:M = 2:1 to build the three layered intercalant MHgM. The formation of the second stage compounds from the MC8 binaries involves an addition of mercury atoms with a reorganization of the metallic layers in the graphite interval. On the other hand, the intercalation isobar and isotherm curves show that the MHgC4 compounds have a very small stability domain when they are in presence of an excess of free amalgam in a temperature gradient. When this gradient exceeds a few degrees, these ternary compounds decompose in the binary MC8 compounds.

Etude structurale du graphiture I de cesium

Abstract The metallic yellow compound CsC8 belongs to the 1st stage. The formula CsC8 involves the occupation, in each interlayer, of only one in four possible sites the compact structure would entail the composition CsC2 (Fig. 1). From single crystal photographs, one can determine the c parameter as three times the identity period along the c axis: Ic. The structural determination is based on separate study of each family: 00l (Table 1, Fig. 2), hk0 and hkl (Table 2, Fig. 3) and powder diagrams (Table 3, Fig. 4). The hk0 family allows to determine the symmetry of the unit cell and the parameter; the systematic extinctions of the hkl reflections permit to determine the space group. The stacking of the cesium atoms: three in the four possible sites involves a screw axis 62 or 64. The position of the carbon and cesium atoms takes into account all the symmetries of the cells corresponding to the space group P6222 (or P6422) (Fig. 5). The foregone extinctions of those unit cells are respected, in particular, the 201, 202, 204…, reflexions are missed (Fig. 6). The two domains built on αβγ and αγβ are actually symmetric to a mirror and not superimposable. It is shown on Fig. 8 that there are two enantiomorphous structures: the stacking αβγ corresponds to an obverse screw axis for the cesium atoms. The presence of 6 different stacking zones even in a simple crystal involves a disorder in the compound which can explain the variations of intensity of the reflexions hk0 and hkl. The CsC8 compound is described by an hexagonal unit cell belonging to the space group P6222 (or P6422) with 24 carbon atoms in position 12k (x = 1 6 , y = 1 3 , z = 1 3 ) and 6i (x = 1 6 , z = 0 and x = 1 3 , z = 0) and the cesium atoms in 3b. The parameters are a = 4.945 ± 0.01 A and c = 17.76 ± 0.03 A . Two unit cells are mentioned, according to the direction of the screw axis. In fact, there are 6 different possible unit cells. We tried to imagine the different possibilities of stacking for the cesium atoms between the graphite layers. From an ideal graphite plane, one places the 1st cesium layer in position α. The second metallic layer can be placed on the β, γ or δ sites. Let us imagine a part in β position, an other in γ and the last one on δ. In the same graphite matrix, one can define three distinct domains spotted by the successions αβ, αγ and αδ. While the filling of the last cesium layer, each domain is divided in two and it appears 6 zones, which correspond to the stacking αβγ, αβδ; αγβ, αγδ; αδβ, αδγ (Fig. 7). Each of these domains is defined by an hexagonal unit cell. They differ one from each other.

Synthesis and superconducting properties of CaC6

Abstract Among the superconducting graphite intercalation compounds, CaC6 exhibits the highest critical temperature Tc=11.5 K. Bulk samples of CaC6 are obtained by immersing highly oriented pyrographite pieces in a well-chosen liquid Li–Ca alloy for 10 days at 350 °C. The crystal structure of CaC6 belongs to the space group. In order to study the superconducting properties of CaC6, magnetisation was measured as a function of temperature and direction of magnetic field applied parallel or perpendicular to the c-axis. Meissner effect was evidenced, as well as a type II superconducting behaviour and a small anisotropy. In agreement with calculations, experimental results obtained from various techniques suggest that a classical electron-phonon mechanism is responsible for the superconductivity of CaC6. Application of high pressure increases the Tc up to 15.1 K at 8 GPa.

KC4, A New Graphite Intercalation Compound

Abstract New stage 1 graphite-potassium intercalation compounds have been synthesized. They are very rich in metal, as each intercalated sheet consists of two superimposed potassium planes. The chemical formula is close to KC4. Five different phases were observed. They were studied by X-Ray diffraction. Electrical measurements were carried out on two phases.

A new graphite intercalation compound containing sodium associated with oxygen

Abstract A new second stage blue phase of donor-type with an interplanar distance of 745 pm has been synthesized by reaction of graphite with partly oxidized sodium. It contains oxygen in form of peroxide ions.

On the great difficulty of intercalating lithium with a second element into graphite

Abstract Lithium is able to intercalate into graphite leading to various binary graphite intercalation compounds, that are well defined by their stage. Concerning the ternaries, there is little literature on the subject. Thermodynamical and structural data, that differ largely from those of the other alkali metals, lead one to foresee some serious difficulties in synthesising such ternary compounds. Many experiments have attempted to synthesise ternary graphite intercalation compounds with lithium, using successively very electronegative elements, then fairly electronegative species and lastly electropositive metals. Numerous results, that are wholly negative, are described in this paper. The calcium–lithium system only allows one to prepare a novel intercalation compound, that is a first stage ternary phase exhibiting a large interplanar distance. This latter suggests that the intercalated sheets consist of several superimposed atomic layers. The synthesis of this ternary is not easy, because it needs reagents of very high purity. It possesses the brightness of metals and its strong hardness is very unusual among graphite intercalation compounds. On the other hand, the charge transfer between the graphene planes and the intercalated sheets, that just allows the intercalation, is especially high, and much higher than the LiC 6 compound.

Effect of an alkyl chain substituent on the kinetics and thermodynamics of complexation of 8-hydroxyquinolines with Ni2+ and Co2+ in methanolic solutions

Abstract 7-(4-ethyl-1-methyloctyl)-8-hydroxyquinoline (C11ue5f8HQ) is the active component of the industrial extractant Kelex 100. The thermodynamic as well as the kinetic parameters for the complexation of this ligand molecule with( Ni2+ and Co2+are compared with that of 8-hydroxyquinoline (HQ) in identical conditions (methanolic solutions with up to 0.4% of water, 0.1 M NaClO4, 0.1 M triethanolamine buffer). From stopped-flow experiments, the alkyl-chain substituent is shown to be responsible for a seven-fold decrease of the rate constant for the 1 : 1 complex formation between the neutral form of the ligands and the Ni2+ ion. A similar behaviour is observed for the Co2+ ion, for which quantitative data have only been obtained with the alkylated ligand. Two different mechanisms are considered to explain the pH-dependence of the observed rate constant, which is attributed to the metal hydrolysis rather than to the ligand deprotonation.

Superconductivity in Li3Ca2C6 intercalated graphite

In this paper, we report the discovery of superconductivity in Li{sub 3}Ca{sub 2}C{sub 6}. Several graphite intercalation compounds (GICs) with electron donors, are well known as superconductors [T. Enoki, S. Masatsugu, E. Morinobu, Graphite Intercalation Compounds and Applications, Oxford University Press, Oxford, 2003]. It is probably not astonishing, since it is generally admitted that low dimensionality promotes high superconducting transition temperatures. Superconductivity is lacking in pristine graphite, but after charging the graphene planes by intercalation, its electronic properties change considerably and superconducting behaviour can appear. Li{sub 3}Ca{sub 2}C{sub 6} is a ternary GIC [S. Pruvost, C. Herold, A. Herold, P. Lagrange, Eur. J. Inorg. Chem. 8 (2004) 1661-1667], for which the intercalated sheets are very thick and poly layered (five lithium layers and two calcium ones). It contains a great amount of metal (five metallic atoms for six carbon ones). Its critical temperature of 11.15 K is very close to that of CaC{sub 6} GIC [T.E. Weller, M. Ellerby, S.S. Saxena, R.P. Smith, N.T. Skipper, Nat. Phys. 1 (2005) 39-41; N. Emery, C. Herold, M. dAstuto, V. Garcia, Ch. Bellin, J.F. Mareche, P. Lagrange, G. Loupias, Phys. Rev. Lett. 95 (2005) 087003] (11.5 K). Both CaC{sub 6} and Li{sub 3}Ca{submorexa0» 2}C{sub 6} GICs possess currently the highest transition temperatures among all the GICs.«xa0less

Complex formation of peroxouranyl (and uranyl) with polyaminocarboxylate ligands

Abstract Complexation of the uranyl ion (UO 2 2+ ) and of the peroxouranyl species (UO 4 ) by some polyaminocarboxylate ligands has been investigated in solution (3M NaClO 4 ) at 25°C. The logarithms of the cumulative formation constants of the UO 2 2+ chelates formed are: UO 2 edta 2− (15.65), UO 2 Hedta − (18.59), (UO 2 ) 2 edta (20.24); UO 2 edda (16.02); UO 2 Hnta (12.19); UO 2 ida (9.63), UO 2 H 2 (ida) 2 (23.80). The equilibrium UO 2 2+ + H 2 O 2 ⇌ UO 4 + 2H + has a stability log K = −3.99. The peroxocomplexes formed are UO 4 Hedda − (14.81, expressed from UO 2 2+ and H 2 O 2 ) and UO 4 Hnta 2− (8.50). Solution structures of the chelates are proposed.