desublimation

Desublimation, also called deposition, is the phase transition in which a gas transforms directly into a solid without passing through a liquid. It is the reverse of sublimation and occurs when the vapor is cooled or compressed to conditions where the solid is thermodynamically stable, or when the vapor is supersaturated with respect to the solid. Desublimation is typically exothermic, releasing latent heat as the unordered gas molecules arrange into an ordered solid lattice.

Natural examples include frost formation on surfaces when moist air comes into contact with cold objects, and the deposition of carbon dioxide or water ice in planetary atmospheres and polar environments. In interstellar space and on dust grains, gases such as water, carbon monoxide, and methane can desublimate to form icy mantles.

Desublimation proceeds via nucleation, either on existing surfaces (heterogeneous nucleation) or within the gas (homogeneous nucleation). The rate increases with greater supersaturation and lower temperature and depends on surface area and the availability of nucleation sites. Conditions must favor the solid phase over the gas phase in the relevant region of the phase diagram.

In materials and surface science, desublimation is used to describe the formation of solid films from vapor in deposition processes on substrates. The term is sometimes used interchangeably with deposition, though some contexts distinguish gas-to-solid deposition from other forms of deposition.