Halide Graphite Compounds

A glass tube of about 2 cm. I.D. is used. At least 5 g. of anhydrous FeClg is sublimed onto 2 g. of coarse or fine crystalline graphite. The tube is sealed off on both sides and heated for 24 hours in an electric tubular furnace at a constant temperature of 200-300°C. One end of the tube is then withdrawn from the furnace, and the heating is continued at the same temperature until no further uptake of the desublimed FeClg can be observed. 

Preparation is similar to that presented above imder (A). About 3-4 g. of AICI3 is sublimed in a stream of dry CI3 onto 1 g. of graphite. The sealed reaction tube (capacity about 40 crrfi) must 

Structure of A and B. Graphite lattice expanded in the o direction with a layer of metal halide in each layer vacancy. 

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Many properties of residue compounds are also consistent with the assumption that the separate crystals of a graphite compound are not homogeneous after decomposition, and still contain undecomposed compound in their interiors. This is surrounded by a more or less thick outer layer of graphite which protects it from further decomposition. This idea has been discussed already in Section HID in connection with the behavior of metal halide-graphite compounds towards water. It will be necessary to wait for further experiments in order to explain more precisely the mode of decomposition of graphite compounds. 

Structural data of halide-graphite compounds. n = stage number... 

First metal halide-graphite intercalation compound made with FeCl.i. 

The similarity of the two structures led Croft (15) to attempt the preparation of boron nitride intercalation compounds. He used the metal halides SbCls, SbCU, AsCU, CuCl, CuCh, FeCU, Aids, and also BF3, BCls, Br2, ICl, liquid ammonia, and N2H4. The same procedure was used as in the preparation of metal chloride-graphite compounds. 

A quite different sort of graphite intercalation compound is formed by the halides of many elements, particularly those halides which themselves have layer structures or weak intermolecular binding. The first such compound (1932) was with FeCl3 chlorides, in general, have been the most studied, but fluoride and bromide intercalates are also known. Halides which have been reported to intercalate include the following ... 

The graphite-metal halides constitute the most populous group of intercalation compounds. Most of the investigative efforts have been directed towards the metal chlorides, particularly FeCls, whereas considerably less is known about the metal bromides S21). Compounds... 

At this time, no all-inclusive rule can be given that will predict whether a given compound will intercalate or not. Most of the information available seems to have been obtained empirically. Such analogies as similar chemical properties have been helpful. The many factors that infiuence the intercalation process have been surveyed by Herold (H14). In Tables II-VI are listed metal halides considered to intercalate into graphite, together with some structural information (S2J, i 9). Several general characteristics have been ascribed to intercalat-... 

Typically, Be-containing alloys and intermetallic phases have been prepared in beryllia or alumina crucibles Mg-containing products have been synthesized in graphite, magnesia or alumina crucibles. Alloys and compounds containing Ca, Sr and Ba have been synthesized in alumina , boron nitride, zircon, molybdenum, iron , or steel crucibles. Both zircon and molybdenum are satisfactory only for alloys with low group-IIA metal content and are replaced by boron nitride and iron, respectively, for group-IIA metal-rich systems . Crucibles are sealed in silica, quartz, iron or steel vessels, usually under either vacuum or purified inert cover gas in a few cases, the samples were melted under a halide flux . 

Synthesis by means of volatile compounds A number of halides (especially chlorides) of the transition metals display a high volatility and in the gaseous state they are easy to mix. They can be synthesized from oxides, or scrap metals, and chlorine they are highly reactive and can be utilized for the preparation of various compounds, either as powder or a coherent solid or as coatings. Mixtures, for instance, of TiCLj + CH4 + H2 have been used to prepare ultrafine TiC powder, to deposit TiC on graphite (at 1200-1300°C), etc. 

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