Calcite (CaCO3 )
Note1: If you want to read the coordinates directly from an external file, the Ad option shows you the same initial screen of page 1
11. For this tutorial, we will add the atoms and their coordinates manually.
When you enter At, tetr asks you to specify the chemical element and then the coordinates.
Note2: In the rhombohedral representation, there are two formulae units of CaCO3 with a total of 6 atoms. However, you need to specify only the position of one Ca, C and O atoms because the others are generated by tetr automatically on the basis of the symmetry operations of the group.
12. Let us start with the Ca at the (0 0 0) position. Then enter At again and add C at the (1/4 1/4 1/4) position (in fractional coordinates). An extract of the corresponding Wyckoff positions from the International Tables is given below at the bottom of the page.
13. By definition, the position of one of the O atoms is at (x, -x+1/2, 1/4). Here, we use approximately x~0. Again, we enter At and then add O at (0, 1/2, 1/4).
14. We are now ready to generate the whole unit cell with the command Gn.
tetr has generated correctly all other positions. As a double check, we can compare with the positions reported in the International Table for Crystallography (Vol.A, 2002)
15. Lets now check the geometry created by tetr with the Co command.
O positions
USED to generate the atomic positions
C positions
Ca positions
