For layered cathodes in Li-ion batteries, such as LiCoO2, it is considered that they have the structure of R-3m space group.
In some articles, it is represented that Li ions occupy at 3a sites and TM at 3b sites. However, there exist some papers depicting that Li ions are in 3b sites and TM in 3a sites.
which one is correct ? As we know, in R-3m space group, the coordinates of 3a and 3b sites are different. the former is (0 0 0) and the later is (0 0 0.5).
Both are technically correct, these two descriptions can be obtained from each other by the shift of the origin. But you have to take into account that this shift will also affect oxygen position: it will be approximately (0; 0; 0.24) if Li is in 3a and (0; 0; 0.26) if Li is in 3b.
ICSD uses Li in 3a as a standard choice. This choice is based on J. Appl. Cryst. (1987) 20, 139-143. One of the parameters of standardization there is the minimal value of Σi(xi2+yi2+zi2)1/2. Obviously, this condition requires oxygen in (0; 0; 0.24).
thanks for your answering, but there still exists some confusions for me.
For layered cathodes, the cation mixing is a crucial issue. It seems to lead to different results that whether Li in 3a sites or in 3b sites
3a site coordinates: （0,0,0），（2/3,1/3,1/3），（1/3,2/3,2/3）
3b site coordinates: （0,0,1/2），（2/3,1/3,5/6），（1/3,2/3,7/6）
J. Am. Chem. Soc. 1954, 76 (6), 1499-1503.
Neglecting oxygen's effect, if Li is in 3b sites, the structural factors as follow
F(003)=3fTM-3fLi , F(104)=3fTM+3fLi .
When Li is in 3a sites, F(003)=3fLi-3fTM , F(104)=3fLi+3fTM .
It seems, only when Li is in 3b sites, that the ratio of F(003)/F(104) decreases as the cation mixing increases.
whether there are some mistakes above, I am not sure.
May you give some advice ?
Thanks a lot
What you really measure is not the structure factor, but intensity, which is proportional to F2. (3fTM-3fLi)2 = (3fLi-3fTM)2 so nothing will change when you choose different origin.
Thanks for your response.
You have mentioned that choosing different origins would change the coordinates of oxygen. Are there any references stating this issue？
In some papers, their XRD refinement results of layered cathodes show that the oxygen positions is appromixately (0; 0; 0.26) when Li is in 3a. Some cif files I find also exhibit the same results. It confuses me.
I am a novice in crystallography field, there exists some difficulty for me to fully understanding this issue. Would you like to give some advice, recommanding some articles ?
Thanks a lot !
Some trivial results are never published. Try to plot the structure in whatever program you have for that and find how the origin changes when you switch from one description to another and which ions are used for the asymmetric unit in both cases.
As for your second question, not all published results are correct. Li-O distance should be always larger than Co-O distance because Li ion is bigger.
About the general references. It is always better to start a new field from the textbooks, trying to look at the same time at your material and find how each idea works there. As for the articles, the classical studies of LiCoO2 were performed by J.B. Goodenough, J.R. Dahn, T. Ohzuku and C. Delmas. Try to look through their publications between 1980 and 1995.
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