Phase diagram comic

One of the more powerful tools in materials screening is the computational phase stability diagram. Unfortunately, it is only utilized at the moment by a few research groups (although I do see its usage increasing), and I thought that a comic book about them might improve the situation.

So here’s that comic book! In addition, this post contains Python examples to create, plot, and analyze phase diagrams using the pymatgen library and Materials Project database. You can now do what earlier took a month of research (computing and generating an entire ternary or quaternary phase diagram) in a few seconds!

This post has three parts:

  1. The comic!
  2. Interactive phase diagram examples
  3. Further resources

The comic!

Click here to download the full high quality PDF version (19MB) of the Phase Diagram comic.

There’s also a small file size version (3MB) for slower connections.

stability_comic_p1stability_comic_p2 stability_comic_p3

stability_comic_p4stability_comic_p5stability_comic_p6

Interactive phase diagram examples

Materials Project Phase Diagram App

MP_PDApp

The Materials Project phase diagram app allows one to access a database of tens of thousands of DFT calculations and construct interactive computational phase diagrams. You can build binary, ternary, and quaternary diagrams as well as open -element diagrams. No programming required!

Python code example: Creating a phase diagram

Python code example: Creating a grand canonical phase diagram

Python code example: Checking to see if your materials is stable with respect to compounds in the MP database

Further resources

“Accuracy of ab initio methods in predicting the crystal structures of metals: A review of 80 binary alloys” by Curtarolo et al.

curtarolo

This (somewhat epic!) paper contains data for 80 binary convex hulls computed with density functional theory. The results are compared with known experimental data and it is determined that the degree of agreement between computational and experimental methods is between 90-97%.

“A Computational Investigation of Li9M3(P2O7)3(PO4)2 (M = V, Mo) as Cathodes for Li Ion Batteries” by Jain et al.

jain

The endpoints of a binary convex hull need not be elements. For example, in the Li ion battery field one often searches for stable intermediate phases that form at certain compositions as lithium is inserted into a framework structure. The paper above is just one example of many computational Li ion battery papers that use such “pseudo-binary” convex hulls.

“Configurational Electronic Entropy and the Phase Diagram of Mixed-Valence Oxides: The Case of LixFePO4” by Zhou et al.

zhou

Incorporating temperature into first-principles convex hulls is often possible, but not always straightforward or easy to do. Here is one example of this process that focuses on electronic entropy.

Wikipedia article on Ternary plots

Ternary_plot_2_(reverse_axis)

Ternary plots are not only for phase diagrams (the most creative usage I’ve ever seen is in Scott McCloud’s Understanding Comics, where it is used to explain the language of art and comics). Wikipedia does a good job of explaining the basics of how to read and interpret compositions on ternary diagrams.

“Li-Fe-P-O2 phase diagram from first principles calculations” by Ong et al.

wang-lfpo

Here is a nice example of the computation of a quaternary phase diagram – sliced into ternary sections – from first principles calculations.

“Accuracy of density functional theory in predicting formation energies of ternary oxides from binary oxides and its implication on phase stability” by Hautier et al.

hautier-accuracy

How accurate are computational phase diagrams? The correct answer, like always, is “it’s complicated”. But based on results from this paper and some experience, colleagues of mine and I have found that an error bar of 25 meV/atom is usually a good estimate. We usually double that to 50 meV/atom when searching for materials to synthesize by conventional methods.

“Formation enthalpies by mixing GGA and GGA + U calculations” by Jain et al.

Jain-mixing

In an ideal world, first principles calculations would live up to their name and require no adjustable parameters. In practice, however, DFT errors do not always cancel when comparing energies of compounds with different types of electronic states. This paper shows how one can mix two DFT approximations along with some experimental data in order to produce a correct phase diagram across a changing landscape of electronic states.

“First-Principles Determination of Multicomponent Hydride Phase Diagrams: Application to the Li-Mg-N-H System” by Akbarzadeh et al.

wolverton-h2

An alternate (but equivalent) approach to the convex hull algorithm for determining phase diagrams is to use a linear programming approach. This is demonstrated by Akbarzadeh et al. in the search for H2 sorbents.

“Thermal stabilities of delithiated olivine MPO4 (M = Fe, Mn) cathodes investigated using first principles calculations” by Ong et al.

ong-safety

 

If Li ion battery cathode materials (generally oxygen-containing compounds) release O2 gas from their lattice, it can lead to runaway electrolyte reactions that cause fire. Thus, a safe cathode material resists O2 release even under extreme conditions. Stated another way, safety is the “price point” (inverse O2 chemical potential) at which a cathode material will give up its oxygen. The higher the price point, the more stable the compound. This paper compares the critical chemical potential for O2 release between MnPO4 and FePO4 cathode materials, finding that similar chemistry and structure doesn’t necessarily imply similar safety.

“CO2 capture properties of M–C–O–H (M.Li, Na, K) systems: A combined density functional theory and lattice phonon dynamics study” by Duan et al.

duan-co2

The CO2 capture problem is to find a compound that absorbs CO2 from an open environment at chemical potentials found in industrial processes, and then releases the CO2 back into some other open environment under sequestration conditions. This paper constructs multi-dimensional phase diagrams to predict how different chemical systems will react with CO2 under different conditions.

 

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11 thoughts on “Phase diagram comic”

  1. I just stumbled upon this blog during my research and I think it is really awesome! I’m very impressed.

    I had a quick question about phase diagrams (I don’t know if this is the right place to ask this). When we plot the phase diagram on using the materials project api, this returns the phase diagram at 0 K and 0 atm, am I correct? This is different from the “Li−Fe−P−O2 Phase Diagram from First Principles Calculations” paper where they use 0.1 MPa as the reference pressure?

    So what I would obtain on the materials project would be somewhat shifted in chemical potential when compared to the paper?

    1. Hi Prateek

      Glad you found it and thanks!

      For the phase diagram question, it’s probably best to contact Materials Project support with such inquiries. But the Materials Project does the same thing as in the Li-Fe-P-O2 paper – in both instances, all solid phases are calculated without any pressure (0 atm), but the gas phase O2 energy is fit (not necessarily calculated) using experimental data for standard conditions (1 atm, 293K). So even in the paper, it is not as if the solid phases are being adjusted for pressure.

      For the solid phases, having 1 atm of pressure versus zero pressure really doesn’t change anything at all – meaning if we (or the paper) did re-calculate all the solid phases at 1 atm, we wouldn’t expect anything different. So solid phase calculations performed at 0 atm (and to a certain extent, 0K) can for practical purposes be assumed to be close to “standard temperature and pressure”.

  2. Dear ANUBHAV, I’ve analyzed your “check_stability.py” code and interest how it can be modified:
    1. what about multi component composition consisting 5-6 and even more elements, is it able?
    2. is it possible for your code to simply provide the compounds of ground states generated by the convex-hull construction, so that user is able to compute them by himself by his own DFT software and thus analyze the stability by his own DFT software. This can be important because some users can not have VASP or not prefer to use it

    1. Hi Maxim,

      1. Yes, it should be able to do any number of elements without any modifications.
      2. Yes, just skip the code about adding your own calculation entry. You can still call pd.stable_entries to get all the stable entries.

      There are many other functions available in the code if you look around the PhaseDiagram and PDAnalyzer classes in pymatgen. If you do give it a try and get stuck, you can always post a question to:

      https://groups.google.com/forum/#!forum/pymatgen

      However, you should give it a try first and send a report/question if you’re unable to get it working after giving it your best shot.

  3. Ok! Glad you got it to work. Yes, the underlying libraries like pymatgen have changed since I first posted the code example. If you want, you could submit a pull request to Github with the updated import statements for future users.

  4. Hi Maxim – thanks! I got your email and updated the code snippet based on the parts that changed (import statements and Py3 compatible print statements)

  5. Dear Anubhav,
    Thank you a lot; using your code I was able to publish in a good journal
    http://pubs.acs.org/doi/abs/10.1021/acs.jpcc.7b01575

    My question is to the code “check_stability.py” and phrase in it: “decomposition energy less than about 0.100 eV/atom might be considered “metastability””
    Could you give a reference for this phrase, if to write it to the “print” code it will be useful. The users will be able to read the corresponding paper or will be able to give a reference in their own papers.

    The second question is to the same 0.100 eV/atom value. For the Li2MnSiO4 compound there is difficulty in obtaining single-phase product (several phases coexist) and the difference in energy between them is within 0.010 eV/atom.
    For ZrO2 only single phase exist in this 0.010 eV/atom range and indeed it is single phase monoclinic form.

    1. I updated the gist, see Sun, W., Dacek, S. T., Ong, S. P., Hautier, G., Jain, A., Richards, W. D., Gamst, A. C., Persson, K. A. & Ceder, G. The thermodynamic scale of inorganic crystalline metastability. (2016).

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