What characterizes a metastable phase?

A metastable phase is one that is thermodynamically unstable yet remains in that state due to kinetic hindrances. This means that, although the phase has a higher free energy compared to the stable phase, it does not spontaneously convert to the more stable state because the energy barrier for that transition is too high or the process is slow. Such phases can persist for considerable amounts of time and may even be observed under certain conditions where the transition to the stable state is impeded.

For example, in crystallization processes, certain structures may form that are not the most energetically favorable, but due to the conditions of formation (like rapid cooling or specific environmental factors), they remain trapped in these metastable states until a perturbation occurs that allows them to transition to the more stable structure.

The other options do not accurately reflect the nature of metastable phases; they either suggest conditions that don't apply to metastability or describe characteristics of phases that are either stable or in equilibrium. Understanding the concept of metastability is crucial in fields such as materials science and chemistry, as it impacts how substances behave under various conditions.