I have a question about the crystallographic information of Fe2B

Materials Project
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1

I’m writing to have some questions about the crystalographic information of Fe2B(ID:mp-1915).
It is shown that the Hermann Mauguin of Fe-B is I4/mcm[140],which is also conforms to the fact that the crystal system shown is a tetragonal crystal system. But it is also shown that the lattice parameters is a = b= c and α=β=γ≠ 90°, which does not conform to the definition of a tetragonal crystal system .and it seems that the model given is not a tetragonal crystal system.
So I would like to ask if there are any errors in the lattice parameters or if there are errors in the Hermann Mauguin and crystal system? Or is it due to my insufficient knowledge reserve that there are some problems in understanding the information?

I would greatly appreciate it if I could receive a reply.

2

Hi @qjy, the computational determination of space groups is dependent on the tolerances used to determine the positions / angles of ionic sites within a lattice (basically the distance and angle thresholds for determining when two position vectors are numerically identical).

To see this, you can pull the structure from MP and adjust the tolerance used to get the space group:

from mp_api.client import MPRester

with MPRester(<your API key>) as mpr:
     structure = mpr.materials.summary.search(
         fields = ["structure"], material_ids = ["mp-1915"]
     )[0].structure

for symprec in (1.e-2, 1.e-3, 1.e-4, 1.e-5, 1.e-6,):
    print(symprec, structure.get_space_group_info(symprec=symprec))

which should print something like this (can depend on your installed python package versions):

0.01 ('I4/mcm', 140)
0.001 ('I4/mcm', 140)
0.0001 ('I4/mcm', 140)
1e-05 ('Fmm2', 42)
1e-06 ('P1', 1)

However, if we lightly symmetrize the structure by looking at its conventional cell, we get something that is very clearly I4/mcm:

from pymatgen.transformations.standard_transformations import ConventionalCellTransformation

conventional = ConventionalCellTransformation().apply_transformation(structure)

Which gives conventional.a = conventional.b = 5.09, conventional.c = 4.23, and conventional.alpha = conventional.beta = conventional.gamma = 90.

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