The Pourbaix energy data

#1

Hi,
I am a new user of pymatgen pourbaix analysis. I was trying to get the E-pH diagram for Ga. I am curious to know how the energy for a Pourbaix entry is calculated. I have this entry:
Pourbaix Entry : Ga1 O3 with energy = 0.5484, npH = -6.0, nPhi = -3.0, nH2O = 3.0, entry_id = ion-2
It would be nice, if I get to know how the energy is calculated here.

Thank you very much,
Naiwrit.

#2

Hi Naiwrit,

The description of the Pourbaix ion energies can be found in this paper:

Briefly, one can take the dissolution energy of a compound and add it to a reference oxide. For example, if I know the energy of dissolving Ga2O3 in water to result in GaO3^3-(aq), I can add that reaction energy to the DFT energy Ga2O3, to produce a ‘DFT-referenced’ aqueous energy for the GaO3^3-(aq) ion.

Best,
Wenhao Sun

#3

Thanks Wenhao,

I have another doubt. I am trying to calculate the aqueous stability of GaP at pH 9 and potential 1.5 vs RHE as given in the paper by Singh et. al. (Chem. mater. 2017, 29, 10159-10167)
From the pourbaix entry I got the entry as
Multiple Pourbaix Entry : with energy = -3.7451, npH = -10.0, nPhi = -8.0, nH2O = 5.5, entry_id = [‘mp-886’, ‘ion-13’], species: Ga2O3(s) + PHO4[2-]
The energy per atom for GaP, I got -0.406 eV/atom (MP database)
So following the given eqn, G = G0 + 0.0591 log10(conc) - nO mu_H2O + 0.059*(nH - 2nO) pH + phi (-nH + 2nO + q)
at conditions pH 9 and pot. 1.5 and conc 10e-6. The formation energy for water is -2.46 eV. I guess G0 would be -3.7451, q = -2 and using phi = -1.5
Then Decomposition energy should be
(-7.8791)(from eq) - (-0.81)(from GaP) = -7.0791 eV
So delGpbx per atom would be 3.53 eV/atom. But the given value is 8.15 in the paper.

I am not able to understand, what I am missing or where is the wrong I am doing.
Thank you very much.

Regards,
Naiwrit.