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L.E. DELONG, V. YEH, V. TONDIGLIA AND P.C. EKLUND Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506, USA, and S.E. LAMBERT AND M.B. MAPLE Department of Physics, University of California, San Diego, La Jolla, CA 92093, USA.

ABSTRACT Measurements of the pressure (P) dependence of the superconducting transition temperature Tc of KHgC 4 , KHgC8, KTZI. 5 C4 and KC 8 are reported. KC 2 4 was found to be nonsuperconducting to a limiting temperature T z 0.07 K for P < 10 kbar, and RbC8 was found to be nonsuperconducting to T z 1.30 K for P • 22 kbar. The Tc of KHgC 8 was found to monotonically decrease at a rate dTc/dP = -6.5 x 10-5 K/bar from a zero pressure Tc = 1.850 K. The Tc of KTk 1 . 5 C4 initially decreases at a rate dTc/dP ý -4.5 x 10-5 K/bar from a zero pressure Tc = 2.54 K. KHgC4 displayed a broad zero pressure superconducting transition with an onset at T : 1.42 K, followed by an abrupt narrowing of the transition and rapid decrease of Tc at a rate dTc/dP = -5 x 10-5 K/bar at P ý 1 kbar. The Tc of KC8 was observed to jump from a zero pressure value of 0.13 K to a broad transition with an onset at - 1.7 K for P > 2 kbar.

INTRODUCTION The first search for superconductivity in a graphite intercalation compound (GIC) was conducted some time ago [1]. During an approximate fifteen year period following the discovery of superconductivity in stage-one alkali GIC's [2], very little was done to explore their superconducting properties in any detail. Very few superconducting GIC's are presently known, and those that have been studied all contain alkali metal atoms within the intercalant layers. Recently, interest in the superconducting properties of these materials has increased, and conflicting values for the superconducting transition temperature Tc of several compounds have been reported [2-6]. A prime characteristic of these lamellar materials is the marked anisotropy ("quasi-two-dimensionality") of many of their physical properties [7]. Hannay et al implied that the early failure to detect superconductivity in stage-two and higher stage GIC's indicated that "two-dimensional" superconductivity was not a viable model for these materials [2]. Numerous other experimental observations suggest that the intercalant species strongly perturbs only the two adjacent or "bounding" carbon layers, whereas the properties of the more distant "interior" carbon layers (relevant for stage number n > 2) are approximately those of pristine graphite, as Research supported by USDOE Contract #DE-AS05-81ER10936 Research supported by USDOE Contract #DE-AT03-76ER70227.

Mat. Res. Soc. Symp. Proc. Vol.

20 (1983) OElsevier Science Publishing Co.,

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196 modified by "zone folding" and charge transfer effects [7]. Qualitative arguments for the dominant role of the properties of the pure intercalant species (e.g., elemental K) in determining the superconducting behavior of GIC's have been advanced many times [6,8]. The large c-axis compressibilities [9,10], high anisotropy of Hc 2 [2,5,6, 11-13], and the obser