Miscibility gap using emc2-test run discussion

Alloy Theoretic Automated Toolkit (ATAT)
1

Dear Axel and ATAT users,

I have a rather trivial querie to share with you, even though I have not fully understood if what I am doing is right using the emc2 code.

Assume we have a bcc phase with a miscibility gap with a critical temperature at 1000 K. Now, in an existing phase diagram where miscibility gap exists like U-Zr, Fe-Cr etc., . If we suspend all other phases except the bcc phase, and calculate the phase diagram, we get a bcc metastable phase boundary (shape of a parabola) across the composition range with a maximum at x(A)=0.5. The excess Gibbs energy can be obtained from the Redlich-Kister parameter
G_ex=[L0+L1*(xA-xB)+L2*(xA-xB)^2]xA*xB
where L0, L1 and L2 will determine if the miscibility gap is symmetric or assymmetric toward A or B.

Now, coming to ATAT, if we do a CE of the bcc phase, we do not get any other ground states except those at x(A)=0 and x(A)=1. Of course, taking into consideration the structures which may relax too far from an ideal lattice (which has already been discussed before in this forum).
The eci.out shows
0.151098
-0.016639
-0.047684
0.013492
0.001585

CE was obtained with about 15-21 structures (with only volume relaxation) and the plot is given as seen in the attachment.
Please suggest on the eci.out if it is correct and needs some further correction. Even if, 100 structures are used, CE will not change as there are no ground states.

Now, my querie begins here.

Can emc2 be used to get the metastable phase boundary of this bcc phase? That is, getting a symmetric/asymmetric shape parabola with a critical temperature at 1000K +/- 20%

I wish to calculate the phase boundary with the following command (as given in Axel’s tutorial)
emc2 -q -gs=1 -keV -er=20 -dx=3e-3 -T0=300 -dT=50 -T1=2000 -mu0=1.5 -mu1=2.5 -dmu=0.05 -innerT -o=mc1.out &

Am I doing this right Axel? Or this is a totally wrong line of thought…Please correct me.

Secondly, I also see htedat.out, mfdat.out and ltedat.out as output files. Can you please explain these files?

Thank you and regards

Suddhasattwa

2

Yes, emc2 can be used to get the metastable phase boundary of this bcc phase, even if asymmetric.
I can’t tell from just your ecis if everything is ok! Check the CV score, check the fit.out. Check if you have triplets (if not, it won’t be asymmetric).

Your emc2 command seems roughly right, but the precise values of -mu0 and -mu1 may need to be adapted to span the right composition range.

You may want to give the phb command a try to get a cleaner plot, but emc2 is fine to try first.

You may not get the right critical temperature, unless you include lattice vibration effects.

BTW, getting the right GS is not the only objective of the CE, it needs to fit the energies well (see cv score). So adding more structure can improve accuracy even if GS don’t change.