In plain words
A helper chemical, such as iodine, that turns a solid into gas at one end of a sealed glass tube and lets it crystallise again at the other end, growing crystals along the way.
Going deeper
A shuttle for atoms
Many layered compounds cannot be grown from a melt: they decompose before melting, or one element evaporates first. Chemical vapour transport sidesteps melting. A small amount of a reactive gas – the transport agent – converts the solid into a volatile compound at one temperature and gives it back at another, so material migrates through the gas phase from a powdered source to growing crystals. Because the agent is released each time a crystal grows, a few milligrams per cubic centimetre can move grams of material.
Which way the material travels
The direction follows from the thermodynamics of the transport reaction. If forming the volatile compound absorbs heat (endothermic), it is favoured at high temperature, so the solid is picked up in the hot zone and deposited in the cooler one. If it releases heat (exothermic), the reverse holds and crystals grow at the hot end. A temperature difference of some tens of degrees is typically enough, and growth runs from days to weeks. Too steep a gradient or too much agent gives many small crystals; a gentle gradient favours fewer, larger ones.
Choosing an agent, and what it leaves behind
Iodine is the classic agent for sulfides, selenides and tellurides; bromine, chlorine, TeCl4 and various metal are used where they form more suitable volatile species. The choice sets growth rate, crystal habit and sometimes the that forms. It also leaves traces: halogen atoms can be incorporated into the crystal as impurities, and vapour-transport crystals of many carry more than crystals grown slowly from a flux. For the lowest defect densities is often preferred, at the cost of longer growth and removing the flux afterwards.
For specialists
A volatile reactant (I2, Br2, TeCl4 and others) that carries a solid through the gas phase in chemical vapour transport.