F.L. Vogel

Room temperature electrical conductivity of a highly two dimensional synthetic metal: AsF5-graphite☆

Abstract We report here studies at room temperature of the electrical conductivity of AsF5-graphite, a lamellar intercalation compound. Compounds with composition C8nAsF5 have been synthesized where n is the stage. Preliminary measurements of basal plane electrical conductivities indicating values comparable with OFHC copper have been confirmed. Associated anisotropy ratios α ≡ σ a σ c > 10 6 are observed for n ≤ 3. Data for both the a-axis and c-axis conductivities as a function of stage for low stage compounds is reported.

High electrical conductivity in graphite intercalated with acid fluorides

Abstract Unusually high room temperature electrical conductivity, higher than that of pure copper, has been observed under certain conditions in graphite intercalated with the strong acid fluorides, antimony pentafluoride and arsenic pentafluoride. Since these results are of considerable scientific and technological importance, and likely to affect the course of research in this area, the confirming experiments are reviewed here. The first experiment demonstrating the potential of these compounds was measurements on a composite wire consisting of an SbF5 intercalated graphite powder core with a copper sheath [1]. Results were obtained by a d.c. four point resistance method on samples 1 mm dia. × 10 cm long. Comparison of this composite wire with a control sample of copper demonstrated convincingly the superior conductivity of the intercalated graphite core. More recent experiments of a similar nature have confirmed those early findings. We have also made measurements of highly oriented polycrystalline graphite (HOPG) intercalated with AsF5 [2]. In careful measurements a contactless r.f. (100 kHz) induction technique has substantiated the high electrical conductivity of these materials. While the measured conductivity of the AsF5 compounds has a maximum value only marginally greater than pure copper, this value must be regarded as conservative and any correction made to the r.f. measurements for sample imperfections, etc., would tend to increase the calculated conductivity above the quoted value. These results lend support to the notion that very high electrical conductivities are possible in the acceptor compounds of intercalated graphite and that the intercalated acid is responsible for a marked increase in the density of charge carriers while the mobility remains high.

Measurement of electrical conductivity under conditions of high anisotropy in graphite intercalation compounds

Abstract Recent measurements of room temperature electrical conductivity in AsF5-graphite intercalation compounds have emphasized the need for a re-evaluation of conventional techniques when applied to quasi-two-dimensional materials. We have thoroughly investigated the validity of the classic 4-point bridge measurement under conditions of high anisotropy and find that for parameters appropriate to some intercalation compounds, such techniques are impractical and lead to serious errors in measured electrical conductivities. In attempting to resolve the difficulties inherent in making measurements of electrical conductivity in these materials we have considered the Montgomery d.c. technique and a novel r.f. method. Measurements on AsF5-graphite by the Montgomery technique appear to give qualitatively correct behavior for resistivity versus stage and confirm that the magnitude of the anisotropy is larger than that in any previously measured compound. The technique is, however, critically sensitive to crystal defects and this precludes a definitive measurements of a-axis conductivity in these compounds. The r.f. technique which we have developed is, by contrast, insensitive to the degree of anisotropy and is shown to be well suited to the measurement of basal plane conductivities in quasi-two-dimensional systems.

Electrical transport properties of low-stage AsF5-intercalated graphite

The in-plane resistivity of stage 1 and stage 2 AsF5-graphite intercalation compounds was measured using a contactless r.f. eddy current technique from 1.6 to 290 K. The magnetoresistance of a stage 1 compound was similarly measured from 4.2 K to 290 K. The low temperature stage 2 resistivity data show a well-defined intermediate∞ ∝T2 region in addition to the usual∞ ∝T high temperature region, in qualitative agreement with the Kukkonen theory and indicative of a small, elongated cylindrical Fermi surface. Stage 2 resistivity data also show, for the first time in a graphite-acceptor compound, an apparent low temperature phase transition at ≈ 21 K. Magnetoresistance data were used to determine a stage 1 carrier concentration of ≈ 9×1020 holes cm−3. Resistive anomalies were observed at ≈ 200 K and ≈ 220 K for stage 1 and stage 2 compounds, respectively.

Superconductivity of graphite intercalated with thallium alloys

Abstract Superconductivity in the graphite intercalation compounds (GIC) KTl1.5C4 and KTl1.5C8 is reported with critical temperatures of Tc = 2.7 K and Tc = 2.45K respectively. The critical field behavior of the compound KTl1.5C4 was determined by a.c. susceptibility to be anisotropic ranging from 4 k0e parallel to ≅20 k0e perpendicular to the c -axis. KTl1.5C4 has the highest values of Tc and Hc2 observed to date in a graphite intercalation compound.

Superconductivity of the graphite intercalation compounds KHgC8 and RbHgC8

Abstract Superconductivity was observed in the graphite intercalation compounds, KHgC 8 and RbHgC 8 , using an AC induction technique. The transition temperatures were 1.90K and 1.44K for KHgC 8 and RbHgC 8 respectively. A full Meissner effect was observed for KHgC 8 with a temperature dependant anisotropy in critical field with a value of 25 ± 5 at T c .

Carrier density from 19F nuclear magnetic resonance line shapes for graphite intercalated with SbF5

Abstract High resolution nuclear magnetic resonance measurements were made on the 19 F line of antimony pentafluoride intercalated in graphite. Two types of graphite were used in the synthesis of the compound — natural and synthetic. Two lines, one broad and one narrow, were observed at the same location. It is proposed that the broad line is from the SbF 5 which is polymerized and the sharp line is motionally narrowed and due to SbF 6 − ions. This model can be rationalized with electronic transport data.

Electrical transport properties of natural and synthetic graphite

Resistivity and magnetoresistance measurements were performed on a series of natural graphite crystals and highly oriented pyrolytic graphite (HOPG) samples as a function of temperature from 4.2 K to 293 K using a contactless r.f. technique. Resistance ratios of the natural graphite between 4.2 K and 293 K ranged from about 30 to 49. Carrier mobility of natural graphite was observed to obey at T−3/2 behaviour up to about 35 K. An anomalously high power dependence of μ versus T was observed below 35 K. A new model describing the dispersion of mobility of electrons and holes is presented which gives exact agreement with magnetoresistance results in the low field regime.

Electrical resistivity of graphite intercalated with SbF5

Abstract In this paper we report the synthesis techniques for stage 1–4 SbF5-graphite (highly oriented pyrolytic graphite) and the measurements of the temperature dependence of resistivity for all four stages. The identity distances of pure and homogeneous samples were characterized by Mo Kα radiation. The order-disorder phase transition observed was associated with a cooling rate effect on the resistivity at low temperatures. The temperature dependence of resistivity of these species can be explained qualitatively by electron-phonon scattering of the highly anisotropic Fermi surface.

The effect of extrinsic defects in pyrolytic graphite on the a-axis resistivity

A contactless r.f. resistivity technique is shown to be well suited to the measurement of room temperature a-axis resistivities in lamellar compounds, particularly those of graphite. The method is used to investigate the relative damage induced by different techniques employed in cutting of samples.