Synthesis and crystal growth

 Key point: d-Metal disulfides are synthesized by the direct reaction of the elements in a sealed tube and purified by using chemical vapour transport with iodine. Compounds of the chalcogens with d metals are prepared by heating mixtures in a sealed tube (to prevent the loss of the volatile elements). The products obtained in this manner can have a variety of compositions. The preparation of crystalline dichalcogenides suit able for chemical and structural studies is often performed by chemical vapour transport (CVT), as described below. It is possible in some cases simply to sublime a compound, but the CVT technique can also be applied to a wide variety of nonvolatile compounds in solid-state chemistry. In a typical procedure, the crude material is loaded into one end of a borosilicate or fused quartz tube. After evacuation, a small amount of a CVT agent is introduced and the tube is sealed and placed in a furnace with a temperature gradient. The polycrystalline and possibly impure metal chalcogenide is vaporized at one end and redeposited as pure crystals at the other (Fig. 24.37). The technique is called chemical vapour transport rather than sublimation because the CVT agent, which is often a halogen, produces an intermediate volatile species, such as a metal halide. Generally, only a small amount of transport agent is needed because on crystal formation it is released and diffuses back to pick up more react ant. For example, TaS₂ can be transported with I₂ in a temperature gradient. The reaction with I₂ to produce gaseous products

is endothermic, so the equilibrium lies further to the right at 850ºC than at 750ºC. Consequently, although TaI₄ is formed at 850ºC, at 750ºC the mixture deposits TaS₂. If, as occasionally is the case, the transport reaction is exothermic, the solid is carried from the cooler to the hotter end of the tube.

Figure 24.37 Vapour transport crystal growth and purification of TaS₂. A small quantity of I₂ is present to serve as a transport agent.

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