Stability of the metastable phase

The spinodal is not cormected directly with nucleation and is the limit of stability of the metastable phase against infinitesimal changes in the state variables. 

The competitive nucleation and growth of two intermediate phases in volumes of nanometric size is investigated, taking into account the mole fraction depletion of the parent phase. The problem has been solved in the framework of the classical method by representation of the thermodynamic potential in the mole fraction-size space. It is shown that, depending on the particle size and thermodynamic parameters of parent and intermediate phases, there exist the following possible situations (i) total inhibition of separation, (ii) formation and total stabilization of the metastable phase instead of a stable one, (iii) relative stabilization of the metastable phase with the temporary delay of its transformation into the stable phase, (iv) formation and growth of a stable phase when the metastable phase does not appear at all, (v) formation and growth of the stable phase via the metastable phase. This was applied to the coherent precipitation of metastable AhLi ordered phase in supersaturated solid solution Al(Li). 

Previous works [98] have also revealed that (i) the nature of the precursors or (ii) the annealing kinetic parameters play a significant role for the stabilization of the metastable phase. Indeed, for chloride-based precursors, the pyrochlore structure is directly obtained. This is explained by the high affinity of Q anions relative to Ln ", which prevents the formation of the complex with 2,4-pentane-dione and thus the formation of a homogeneous gel. In addition, for the kinetic aspect, the layered perovskite phase is favored for rapid annealing or quenching directly at 950 °C. 

To improve performance as an electrolyte, thin films of 5-Bi203 have been synthesised by Laurent et These authors believe that the small crystallite size (- lO-lS nm) achieved through this preparation route might explain the stability of the metastable phase at low temperature, as they achieve a conductivity of about 0.39 S cm at 440 °C. Although this high conductivity value is very promising, further work is required in order to probe the stability of these thin films. 

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