Thermal evolution of metastable phases

Although the formation of a large number of metastable materials that are far from equilibrium cannot be explained thermodynamically, thermodynamics predicts that they will with time transform to the stable phase or phase mixture, often via intermediate phases. More than one hundred years ago, Ostwald pointed out that 

Let us initially look at a semiconducting binary oxide Ai gO in a chemical gradient an oxygen potential gradient. Reduction takes place on the low oxygen 

The segregation or demixing is a purely kinetic effect and the magnitude depends on the cation mobility and sample thickness, and is not directly related to the thermodynamics of the system. In some specific cases, a material like a spinel may even decompose when placed in a potential gradient, although both potentials are chosen to fall inside the stability field of the spinel phase. This was first observed for Co2Si04 [39]. Formal treatments can be found in references [37] and [38], 

---