# Energy above hull

Hello,

I am trying to understand temperature correlation with Ehull. If i have 2 phases A and B and both are above the convex hull line. However let’s say A is closer to the hull i.e has lower Ehull value than B. What does this signify? Though both are metastable phases, does it mean that A is more easier to obtain than B? Also is it possible to predict temperature trend of these two phases?

Specifically, is it possible to explain why B forms at a higher temperature than A?

Thank you.

All of the thermodynamic data in MP is calculated at 0K, so there is no information about how the relative stability of phases would change with temperature as that is an entropy-driven effect. The e_above_hull values are just reflections of the enthalpy term in the Gibbs free energy.

However, you can use the “finite temperature estimation” feature of the Phase Diagram app that uses a method described in this paper: Physical descriptor for the Gibbs energy of inorganic crystalline solids and temperature-dependent materials chemistry | Nature Communications

I see, thank you for that, but was just wondering how do we then explain temperature related trends?

You can use the “finite temperature estimation” feature of the Phase Diagram app that uses a method described in this paper: Physical descriptor for the Gibbs energy of inorganic crystalline solids and temperature-dependent materials chemistry | Nature Communications

thank you!

To add to John’s answer, the \Delta E_{hull} value does attempt to quantify the degree of metastability of the phase. People will generally say that this correlates with how difficult that phase is to synthesize. However, this correlation is not exact by any means. Since there are many other energy terms that will (de)stabilize a phase (e.g., surface energy), certain systems will have highly accessible metastable phases while others will not. This concept was thoroughly explored in a fantastic paper: https://www.science.org/doi/10.1126/sciadv.1600225

With the finite temperature estimation on the Phase Diagram app, you may be able to predict why B forms at higher temperatures than A (i.e., it should show that B moves towards the hull while A moves away, which might imply that B is a higher entropy phase or perhaps a more reduced phase). However, again, this is only an approximation assisted by ML fits of solid free energies, so you should do further calculations (such as phonons) to conclude this with more certainty.

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Thank you very much @mattmcdermott . This is very helpful

Hello,

Just to follow up, can phases with positive energy above hull be synthesized? I was able to obtain a certain compound in the lab but when i plot the phase diagram, this particular phase is much above the tieline.

Yes, phases with positive energy above hull may be synthesizable. This is the idea explored in the paper I linked to in my previous reply.

It is still not fully understood when a metastable phase is synthesizable, but generally, it is expected that this phase has an energy below the calculated amorphous energy for that composition. It is also typically expected that for this phase, there exists a set of environmental conditions that locally stabilize the phase during synthesis (think of high pressures stabilizing diamond C, which is normally metastable, instead of the expected ground-state graphite phase).