M. El Makrini

Graphite lamellar compounds EPR studies

We report X-band EPR results in a range of graphite lamellar compounds with: Li (stages I, II, III), K(I, II, III, IX), Rb (I, III) as donors and with: HNO3 (I), SO3(I, II, III) as acceptors. Line shapes are dysonian and g-anisotropy remains low even in stage IX K-compound. From line parameters, relaxation time T2 and c-axis electronic diffusion coefficient D are deduced. Results are discussed and compared with metal properties: T2 versus temperature, resistivity, stage and intercalant atomic number for donors, c-diffusion coefficient and mean free path, density of states estimations. Particularly KC8 is shown to exhibit more metallic properties than Li-compounds and low density of states are found in acceptors, contrary to donors, in agreement with 13C NMR.

Synthesis and low temperature specific heat of the graphite intercalation compounds KHgC4 and KHgC8

Abstract We have synthesized and characterized the tertiary graphite intercalation compounds (mercurographitides) KHgC4 (stage 1) and KHgC8 (stage 2) based on HOPG. We also report the first measurement of the low temperature specific heat of these compounds between 0.8 and 60 K. Measurements were made using a thermal relaxation technique designed for air sensitive compounds. The specific heat at the lowest temperature is dominated by the electronic contribution, which yields the electronic density of states N(EF), enhanced above KC8 in the stage 1 compound but reduced in the stage 2 compound. In KHgC8 an anomaly at 1.93 K indicates the onset of superconductivity. At temperatures above 2 – 3 K the lattice specific heat dominates, showing Debye temperatures well below that for KC8 and the presence of Einstein type contributions similar to those observed in RbC8 and CsC8.

Structural and electronic study of KHg and RbHg intercalated graphite

Abstract We have shown that the potassium and rubidium amalgams, KHg and RbHg, intercalate into graphite forming very metal-rich ternary compounds of general formula MHgC4 (first stage) and MHgC8 (second stage). The reaction provokes a very high c axis dilation of the host graphite: 203% for KHg and 221% for RbHg in first stage products, attributed to the three-layer structure of the intercalant. Two planes of the alkali metal atoms are epitaxied on the carbon layers on either side of a mercury layer. These latter atoms occupy prismatic sites created by the strictly defined disposition of the former. A study of certain electronic properties has been undertaken. Room temperature values of electrical resistivity are higher than those of the corresponding alkali metal compounds: about 20 μ cm for first stage products and about 12 μ cm for second stage products. The variation with temperature down to 100 K has been measured yielding values smaller by a factor of about five than the ambient temperature values. The compounds possess a Pauli paramagnetic type of susceptibility, practically temperature independent from 4.2 to 300 K. The first stage compounds can give solid solutions, KxRb1−xHgC4 for which the average magnetic susceptibility shows a quite pronounced minimum as a function of composition.

X-ray, e.p.r. and n.m.r. studies and structure of some ternary compounds of graphite

The potassium hydrogenographitides, KH2/3C8 and K3H4/3C16, and potassium and rubidium mercurographitides have been extensively studied. In the hydrogenographitide series 1H n.m.r. is not observed, but e.p.r. shows metallic character in stage I and II compounds. 13C n.m.r. suggests charge transfer, in agreement with a negative hydrogenated ion in the lattice. A structure is proposed using the stability of the H2− ion when the bond is sufficiently lengthened. With mercurographitides, e.p.r. cannot be seen because of the Hg spin-orbit coupling. 13C n.m.r. agrees with a model having negatively charged Hg atoms between two K or Rb layers. The possible structure for this layer is compared with the structure of alloys of the same stoichiometry.

Intercalation of metals and alloys into graphite

Abstract Several aspects of the binary metal graphite compounds are described, i.e. temperature and pressure conditions for the intercalation of pure metals into graphite, structures and stacking of the intercalated layers especially in the first stage compounds, polysynthetic nature of these compounds, and existence of metallic layers which are not epitaxied on graphite sheets. Two kinds of ternary compounds are described: (a) the solid solutions which contain simple metallic layers comparable to those of the corresponding binary compounds, and (b) the ternary compounds which contain multiple layers with different types of structures. The degree of organization of these compounds decreases in going from the G-Hg-M (M = K, Rb) to the G-Ba-Na and to the G-Tl-M compounds. Several aspects of the stability, formation mechanisms, and electronic properties of the two kinds of ternaries are discussed.

On the formula and structure of the first stage graphite-metal compounds

Abstract The formula of a first stage graphite-metal compound is written under the form of MC x . The interaction between the metal and graphite planes is in epitaxy which involves x as an integer: the metal atoms are in the potential wells formed by the hexagonal prismatic sites derived from the AAA… stacking of the carbon layers. If the distribution of the intercalated atoms is locally homogeneous, there are two possible coordinations for the atoms: 6 (hexagonal centered) or 3 (open hexagonal). In the first case, x can only have the values: x = 2, 6, 8, 14, 18… In the second case, the only possible values are x = 3, 9, 12, 21, 27… For each value of x , one has a 2D hexagonal unit cell, characterized by an a x parameter, which varies with x . The investigation of the position of the hk 0 reflections, isolated from a sample prepared with pyrographite, allows the very easy determination of the compound parameter and deduction of the value of x . The crystallographic measurements obtained on many compounds are in excellent agreement with the chemical analysis. Only the MC 6 and MC 8 structures have been observed up to now.

New data on the intercalation of metallic alloys in graphite: Case of alkali metal amalgams

Abstract The metallic alloys are susceptible to intercalation into graphite to form ternary compounds. One can classify these compounds into three types: (1) the solid solutions where the intercalated monoatomic layers are alloys of the two metals; (2) the definite compounds in which intercalated sheets are formed by several superimposed metallic layers of different nature; (3) the binary compounds where small amounts of a third element are dissolved. The study of the systems graphite-mercury-alkali metal is an illustration of these results. We synthesized new lamellar compounds of stage one (MHgC 4 ) and stage two (MHgC 8 ), with M = K or Rb. Each intercalated layer is formed by two alkali metal sheets which surround a mercury layer.

Relation entre l'ordre structural de l'alliage inséré et sa température de fusion dans les composés d'insertion du graphite KHgC4 et KT11,5C4

Relation between structural order of intercolated alloy and the melting temperature in the graphite intercalation compounds KHgC 4 and KTl 1,05 C 4

Ternary compounds of potassium and rubidium graphitides: Neutron, NMR and EPR studies

Abstract Experimental results of X and neutron structural studies, 1H and 13C NMR and EPR measurements are given for two series of alkaline graphitides with H and Hg as third component. Hydrogen structure and position are proposed in KH 2 3 C8 and K3H 4 3 C16 which explain some surprising experimental results such as the lack of 1H NMR signal. Mercury first and second stage compounds were prepared and studied with K and Rb as donor species, respectively, inserted atom positions and charge transfer between them and graphite planes are estimated. The alkaline-Hg bonding seems not to be very different from that observed in bulk amalgams.