Why are solid-solid zone-refining methods limited by slow diffusion?

Solid-solid zone-refining methods are limited by the slow rates of diffusion that occur within solid materials.1

In standard zone melting, purification relies on the liquid-solid transformation, where impurities are redistributed based on their preference for the liquid phase over the solid phase at the interface. Because atoms and molecules move significantly faster in a liquid state than they do within a rigid crystal lattice, the liquid-solid process allows for efficient impurity transport. In contrast, solid-solid transformations lack this liquid phase, meaning the movement of impurity atoms through the solid structure is governed by the much slower process of solid-state diffusion, which restricts the speed and effectiveness of the purification.1