Will a material evolve to a higher energetic phase (space group) under pressure? Or it will continue in the same crystal structure or transform to lower energetic phase? As applying pressure means giving more energy to the material should it evolve to a higher energy phase?
It depends on the material. Under pressure materials will become denser. If the material has the possibility to transform into a denser phase (crystal structure), a transformation may occur, but whether it happens depends on the precise thermodynamic conditions of stability for the different phases involved. Under pressure the energy of a material will increase following p x delta V (pressure times volume change of the system)
Hi Santanu,
As said above it depends on the material and there is no unique and simple answer. However a general tendacy can be summarized as follows. When one heats a compound one increases the entropy so a tendency is that the crystal structure evolve to higher symmetry. Simple considerations would lead to a converse effect. But... it is much more complicated, and better understandings need experimental analyses, thermodynamicsderivations and electronic structure calculations, owing that the phases transformations could be either 2nd order or 1st order. A simple exemples see the attached document. on well known unary (Fe) and binary systems (MgH2)
Good Luck and Happy New Year
Daniel
Small addition to what has been said above:
In principle some thermodynamic considerations can allow to make some prediction. Very extended phase diagrams have been successfully calculated for (very) simple systems. As an example the "Uspex" project by A. Oganov comes to my mind.
However, in real life there is also kinetics playing a role, making it very difficult to predict what will actually occur.
Even if there is no phase transition, the material under pressure is in a higher energy state compared to ambient pressure (whatever is the aggregation state of the compound at ambient conditions).
Predicting a phase transition is possible and, typically, reliable. However, generalizing the prediction is not. Bridgeman predicted that all materials should transform into metals at high pressure. This is likely correct, but we cannot say so easily at which pressure, not even the order of magnitude, without doing precise and sometime very costly calculations. Moreover, we cannot easily anticipate if the initial compound or the products of its decomposition into simpler compounds, perhaps elemental, will undergo such a transformation. In addition, it is now well established that metals transform into non-metals at high pressure, localizing the valence band electrons. This is of course an unusual, very high energy, dielectric state. So Bridgeman's prediction is not "complete". There is a post-metallic phase for all substances. Again we cannot say at which P and which composition exactly.
Last but not least, as Arianna wrote, the thermodynamics predictions may not be experimentally observable due to kinetic effects, which are of course very difficult to predict, and even to detect.