Crystal Structure of Molybdenum Disulphide

A poorly crystalline rag structure has been described for synthetic product obtained by the reaction between molybdic chloride and lithium sulphide in tetrahydrofuran. The product was purified by repeated washing with tetrahydrofuran to remove the lithium chloride. Heat treatment of the amorphous powder gave a low degree of crystallization. 

It is possible to insert additional atoms or molecules into the inter-lamellar gap of many layer-lattice materials, including molybdenum disulphide, creating what are called intercalation compounds. The intercalated substances may be alkali or alkalyne-earth metals (sodium, potassium, rubidium, caesium, calcium, strontium), salts or organic bases such as ethylene diamine or pyridine . 

Many layer-lattice compounds can intercalate additional metal atoms of the same element as comprised in the original structure (e.g. niobium in niobium diselenide), but molybdenum disulphide will not do so. The behaviour may be determined by the availability of electrons suitably oriented to form bonds with the additional metal atoms, although it seems unlikely that this single factor applies to all intercalation effects. 

The effect of intercalating like metal atoms is of course to change the atomic ratios, and for example it has been reported that niobium diselenide can intercalate additional niobium atoms to a composition of Nb, jSej There will also be corresponding changes in the crystal lattice parameters, and these are discussed in relation to lubrication properties in Chapter 14. 

The physics and chemistry of molybdenum disulphide intercalation compounds have been reviewed by Woollam and Somoano . Perhaps the most interesting of these properties is superconductivity below 6.9°K, ° obtained with either organic bases or alkali metals. Some of the intercalation compounds show high alkali ion diffusivity, and this has led to them being considered for use in electrodes for high energy-density batteries . 

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Figure 4.1 Crystal Structure of Molybdenum Disulphide (Courtesy of E W Roberts)...

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