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I have a beta Ti (Space Group 229) structure with 16 atoms, I prepared 15 disordered TiNb structures with 5 atoms of Nb. I did it using VASPKIT. However, before the geometry optimization, I found out that the symmetry of all multicomponent structures was decreased to monoclinic. Is it ok?

Moreover, what is better to do the relaxation in VASP, using ISIF=2? If the symmetry was decreased how may I calculate the correct elastic constants for my cubic TiNb structure? I am a bit confused.

This is my POSCAR file for pure Ti

   1.00000000000000     
     6.6399998665000002    0.0000000000000000    0.0000000000000000
     0.0000000000000000    6.6399998665000002    0.0000000000000000
     0.0000000000000000    0.0000000000000000    6.6399998665000002
   Ti
    16
Direct
  0.0000000000000000  0.0000000000000000  0.0000000000000000
  0.0000000000000000  0.0000000000000000  0.5000000000000000
  0.0000000000000000  0.5000000000000000  0.0000000000000000
  0.0000000000000000  0.5000000000000000  0.5000000000000000
  0.5000000000000000  0.0000000000000000  0.0000000000000000
  0.5000000000000000  0.0000000000000000  0.5000000000000000
  0.5000000000000000  0.5000000000000000  0.0000000000000000
  0.5000000000000000  0.5000000000000000  0.5000000000000000
  0.2500000000000000  0.2500000000000000  0.2500000000000000
  0.2500000000000000  0.2500000000000000  0.7500000000000000
  0.2500000000000000  0.7500000000000000  0.2500000000000000
  0.2500000000000000  0.7500000000000000  0.7500000000000000
  0.7500000000000000  0.2500000000000000  0.2500000000000000
  0.7500000000000000  0.2500000000000000  0.7500000000000000
  0.7500000000000000  0.7500000000000000  0.2500000000000000
  0.7500000000000000  0.7500000000000000  0.7500000000000000

This is one of the substituted Ti with Nb POSCAR prepared by VASPKIT This file is generated by VASPKIT code

   6.6399998665000002    0.0000000000000000    0.0000000000000000
   0.0000000000000000    6.6399998665000002    0.0000000000000000
   0.0000000000000000    0.0000000000000000    6.6399998665000002
  Nb   Ti
    5    11
Direct
   0.0000000000000000    0.5000000000000000    0.0000000000000000
   0.5000000000000000    0.5000000000000000    0.0000000000000000
   0.2500000000000000    0.2500000000000000    0.2500000000000000
   0.2500000000000000    0.2500000000000000    0.7500000000000000
   0.7500000000000000    0.2500000000000000    0.7500000000000000
   0.0000000000000000    0.0000000000000000    0.0000000000000000
   0.0000000000000000    0.0000000000000000    0.5000000000000000
   0.0000000000000000    0.5000000000000000    0.5000000000000000
   0.5000000000000000    0.0000000000000000    0.0000000000000000
   0.5000000000000000    0.0000000000000000    0.5000000000000000
   0.5000000000000000    0.5000000000000000    0.5000000000000000
   0.2500000000000000    0.7500000000000000    0.2500000000000000
   0.2500000000000000    0.7500000000000000    0.7500000000000000
   0.7500000000000000    0.2500000000000000    0.2500000000000000
   0.7500000000000000    0.7500000000000000    0.2500000000000000
   0.7500000000000000    0.7500000000000000    0.7500000000000000
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  • $\begingroup$ I have couple of questions. what is composition of alloy? Is TiNb (50%Nb) or Ti11Nb5. why your initital random structure is monoclinic. Did you allow to change box during random alloy generation? $\endgroup$ Aug 20 at 12:22
  • $\begingroup$ Is it monoclinic before relaxation or after relaxation. $\endgroup$ Aug 20 at 12:32
  • $\begingroup$ @pranavkumar, I used the different compositions of TiNb (35, 40, 45, and 50 % of Nb). I did the relaxation of pure beta Ti with the 16 atoms. I've checked the symmetry, and it was ok, because the system is stable. After that, I used "Advanced Structure Models" of VASPKIT for the preparation of the substituted structures (TiNb) (I don't know how to fix the symmetry during the substitution in VASPKIT, I think it is impossible. Due to the fact that this substitution has broken the symmetry, it became lower without any calculation. Then I used ISIF=2 for the optimization. $\endgroup$
    – Irina
    Aug 20 at 14:29
  • 1
    $\begingroup$ This is not correct way to create random disordered solid solution as mentioned by Brandon in answer also. The best way to use ATAT to create special quasi random structure.Use this link cniu.me/2017/08/05/SQS.html to constrain your geometry so box size don't change. For further chat use vasp chat box chat.stackexchange.com/rooms/109983/vasp $\endgroup$ Aug 20 at 15:30
  • 1
    $\begingroup$ I have created one random SQS for test,kindly check chat.stackexchange.com/rooms/109983/vasp $\endgroup$ Aug 20 at 15:50
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A few things:

  1. If you start with your supercell of pure bcc Ti, the spacegroup symmetry will be Im-3m, as expected. As soon as you replace one or a few atoms of Ti with Nb, or any different species, you are breaking symmetries and the symmetry will be lowered. Marker et al. computed elastic properties for bcc Ti-X (X=Mo,Nb,Ta,Zr,Sn) and handled the lower symmetry of the special quasirandom structures (SQS) that they used by a Voigt-Reuss-Hill approach.

  2. bcc Ti is not stable at 0K (it has imaginary phonon modes). I would expect the symmetry of bcc will break if you relax it. Whether that's okay for you depends on what you are trying to study, but you may not be studying the bcc structure that you intend.

  3. This is not typically the way you model a random alloy, if that is your goal. Some recent work from Skripnyak et al. has some nice calculations for bcc Ti-V alloys. Ti-V should be similar to your Ti-Nb and you can see both the effect of bcc Ti instability and a few different methods for calculating energies for random alloys.

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