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I am trying to simulate the properties of FeF3(H2O)3, so I download its crystal structure file from Crystallography Open Database, but it seems that in the lattice, the O atom and F atom exist at the same coordinate position. It also shows the F atom and H2O are at the same coordinate in the original paper, which measures the structure of FeF3(H2O)3,

Here is the chemical formula information of FeF3(H2O)3. it seems the H atoms would be lost when forming a solid crystal.

Chemical name   Fe F3 (H2 O)3
Formula F3 Fe H6 O3
Calculated formula  F3 Fe O3

My question is:
Is it possible that two atoms exist at the same coordinate in the lattice?

enter image description here

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    $\begingroup$ That's what we call "disorder". In some unit cells, there is an O, and in some an F. $\endgroup$
    – Ivan Neretin
    May 8, 2022 at 16:16

2 Answers 2

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The paper states that four ligands of the iron are occupied by a 50:50 statistical mixture of water and fluoride (the double circles). They could have added the partial occupancy to the coordinates if the file format had allowed it:

enter image description here

Because fluoride and water have distinct net charges, the stoichiometry is fixed, and there is probably some local order to maintain local charge neutrality as well, but not at the atom-by-atom level.

The hydrogen atoms might not be resolved using the method employed here, but they certainly are part of the crystal.

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Ivan Neretin sums it up well in his comment. There can be either one type of atom or molecule or another at the relevant position in the unit cell.

It's actually fairly commonplace. Consider an ordinary mild steel, which consists of the metallic ferrite phase reinforced by a hard, semi-ceramic cementite (carbide) phase. Typically the steel will contain manganese alloyed with the iron. Then in the body-centered ferrite phase each lattice point could have either an iron atom or a manganese atom (or an atom of any other metal added to the alloy), and the same holds true for the metal-atom positions in the orthorhombic cementite phase. We could render this by labeling each metal position as $\ce{(Fe,Mn)}$ (with additional elements as necessary for the alloy).

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