Solute atoms have the ability to dissolve in a solvent crystal on interstitial sites as well as substitutional sites. These solutes, known as 'hybrid solutes', can then undergo diffusion through interstitial-substitutional exchange mechanisms. Typically, the diffusivity of hybrid solutes in the interstitial arrangement, is significantly greater than their diffusivity in the substitutional arrangement. Under such conditions the incorporation of solute atoms can occur by fast diffusion of interstitial and subsequent change-over to substitutional sites, as shown in Figure .