Phase diagram
When f assumes a double-well form a miscibility gap which is a situation where the systems cannot take any arbitrary concentrations is observed. Figure 1 is a phase diagram which relates to the final form of f. The common tangent f defines the points of concentration with the ce1 and ce2 as the values, which is used to establish the coexistence curve in the Tc-plane as illustrated in the bimodal. The positions of concentrations are defined by equal chemical potentials which are the slope of the tangent.
The stable single phase is above the bimodal area, the supplement of this area is known to states that are unstable thermodynamically. The curve indicated by spinodal is determined by inflection points cs1, cs2 of f. Metastable (f”> 0 ) and unstable (f”< 0)subareas of the curve are separated with the spinodal.
If the systems is reduced below a critical temperature, it undergoes a split into two phases this is known as a homogeneous state. The path to phase division conventionally is referred as spinodal decomposition if it is an inner state of the spinodal and nucleation if it is in a metastable area or region.
Phase transition
The qualitative changes that occur in equilibrium state of a system and those changes act as externally imposed constraints is referred to as phase transition. The constraints may be physical quantities such as pressure, temperature, concentration or any other. In the subsequent section, I will bear in mind that a transition as a function of temperature. It is possible to change the quantity observed in these transitions in a qualitative manner as a function of temperature. It is evident that something is happening in the transition phase and it is possible that more than one quantity can be observed.
There are many parameters such as temperature and pressure that can drive a transition phase. The high temperature phase is more or less at all times disordered because of the higher symmetry compared to low-temperature phase, in cases where temperature is the driving force.