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I. INTRODUCTION The first graphite-donor compounds, which have been synthesized, were the heavy alkali metals. 1-3 They are still currently widely studied by physicists because they appear as a generic model for graphite-donor compounds. More recently the intercalation of other metals has allowed synthesis of some new binary graphite-donor compounds, but their synthesis is often much more difficult, and the diversity of stages is generally narrower. Other metals able to intercalate into graphite include sodium,4 lithium,5 strontium, and barium,6 and also various lanthanides (europium and ytterbium). 7 The first ternary compounds containing two metals were obtained by action on the graphite of two heavy alkali metals8 or of sodium and a heavy alkali metal.9 They have a structure similar to that of the corresponding binary compounds: the metallic intercalated sheets are mbnolayers: both metals can be statistically distributed in the same graphitic intervals, creating a solid solution,8 but they can aiso lead to intercalated layers, alternatively constituted by one metal, then by the other (along the c axis), thus creating a heterostructure.10 The sodium-barium alloys are also able to intercalate into graphite11; however, they behave very differently. They lead to neither solid solutions nor heterostructures, but rather they form relatively complicated intercalated polylayered sheets. A barium plane, which is surrounded by two sodium planes, is situated in the center, but numerous substitutions can occur in the central layer and the flanking layers, so that the chemical formulas of these ternaries can vary widely. Unexpected ternary compounds have been obtained by intercalation into graphite of binary alloys containing a heavy alkali metal and another metal, entirely unable to intercalate by itself into graphite. We can thus intercalate mercury,12 thallium,13-14 and bismuth 15 in the form of an alloy. These ternary comJ. Mater. Res. 2 (6), Nov/Dec 1987

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pounds also possess intercalated sheets, which are polylayered.16"18 In this paper we present an overview concerning the synthesis and the structure of these ternaries, and additionally we present some discussion of the superconductivity of these compounds.

II. SYNTHESIS AND CHEMICAL APPROACH A. Experimental methods Binary alloys are prepared12"15 by mixing both pure liquid metals under vacuum, which were previously weighed to obtain a suitable composition. The pristine graphite (a single-crystal sample or a small plate of HOPG) and the alloy are then put together in a glass tube and sealed under vacuum. This tube is subsequently heated until the alloy melts: thus the reaction occurs between the graphite and liquid alloy. It is necessary to use a large excess of alloy in order to avoid any variation of its composition during the intercalation. After the reaction the sample is carefully cleaned to remove any excess alloy that adheres to its surface. The samples are then subjected to x-ray examination for (001) reflections: this measuremen