C-Axis Resistivity of Some Stage 1 Donor Compounds
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J.E. FISCHER C.D. FUERST+ AND H.J. KIM+ Laboratory for Research on the Structure of Matter, Philadelphia, Pennsylvania 19104
University of Pennsylvania
ABSTRACT Temperature-dependent c-axis resistivity measurements on LiC6, KC 8 , RbC 8 , CsC8 with and without substitutional boron in the starting material, and KHgC4 are presented. The overall behavior is metallic except for the MC8 compounds below 50-100K where Pc increases with decreasing T. This effect appears to be an intrinsic property of stage 1 materials with commensurate 2x2 superlattices.
INTRODUCTION Intercalation of graphite with electron donors or acceptors reduces Pa, the resistivity parallel to the layers, by a factor of up to 15 relative to pure graphite. This effect has received considerable attention due to its potential practical significance. Much more dramatic changes occur in Pc, the transverse resistivity. These have been less extensively studied and hence are poorly understood. The transverse properties of GIC's are intrinsically interesting for a number of reasons. First, the mechanisms of c-axis conduction should be sensitive to interlayer overlap, as exemplified by the low Pc values of stage 1 donor compounds relative to acceptors [1]. Second, Pc depends more strongly on stage than does Pa; consequently, Pc is a sensitive probe of pressure-induced staging transitions (2]. Third, Pc in high-stage donor compounds should be sensitive to the nonlinear screening of ionized intercalant layers by the delocalized charge on the carbon layers [3]. Two recent papers signal the renewed interest in this important parameter [4,5]. Here we present 4-300K Pc data for the stage 1 binary compounds LiC6, KC8 , RbC 8 , CsC8 and the stage 1 ternary KHgC4. We also describe the effect of 0.5% boron doping on Pc(T) of CsC8 . Resistance measurements were made with four large area gold pressure contacts 3 to 5x5x0.6-0.8 mm HOPG-based specimens, using dc or low frequency ac detection and 10 mA sample current. Resistance data were taken for both cooling and warming at rates 1-3K/min. with the aid of an automatic data acquisition system. Samples were transferred under argon from the growth apparatus to an Air Products Helitran continuous flow cryostat. (009) diffractograms were checked before and after each measurement. RESULTS AND DISCUSSION Table I lists the 300K pc values for the various compounds; variation from sample to sample is -25%. The present results agree with earlier measurements of KC8 [6] and LiC6 [7]. The three MC8 compounds exhibit quite similar Pc values. Surprisingly, 0.5% boron substitutional impurities in the starting material leads to a two-fold lower pc in CsC8 . This is in contrast to its effect One might take on Pa, where we find a 40X higher value relative to no boron. this as presumptive evidence for defect-limited c-axis transport in CsC8 at 300K with or without boron, although the temperature dependences discussed below indicate otherwise. Also of interest is the tenfold higher Pc in KHgC 4 re-
Mat. Res. Soc.
Symp. Proc. Vol. 20 (1983)
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