Tausonite, a mineral composed of Sr, O, and Ti, has the cubic unit cell and the chemical formula is SrTiO3.....

Ti is coordinated by 6 O atoms, because the Ti is located at the center of the unit cell. To see the full coordination environment of the other ions, we have to consider neighboring unit cells. How many oxygens coordinate to Sr?

Apparently Steve can't copy links. Here it is.

http://dissertations.ub.rug.nl/FILES/faculties/science/2004/o.f.goebel/appendix.pdf

In a cubic unit cell of Tausonite, the Ti atoms are indeed located at the center of the unit cell. To determine the coordination of Sr, we need to consider the arrangement of neighboring unit cells.

For a cubic unit cell, there are 8 corner atoms that are shared with adjacent unit cells, and there is 1 atom located at the center of each face, which is shared with 2 neighboring unit cells. Since Sr atoms are larger and occupy the face center sites, they are not only surrounded by O atoms within the same unit cell but also by O atoms from neighboring unit cells.

To find the coordination of Sr, we can consider how many corner oxygen atoms are shared with each Sr atom and how many face-centered oxygen atoms are shared.

For the corner atoms, each Sr atom in the unit cell shares one corner oxygen atom with 8 adjacent unit cells. Therefore, each Sr atom is coordinated by 1/8 of each of the 8 corner oxygen atoms, resulting in a coordination of (1/8) × 8 = 1.

For the face-centered atoms, each Sr atom in the unit cell shares one face-centered oxygen atom with 2 adjacent unit cells. Therefore, each Sr atom is coordinated by 1/2 of each of the 6 face-centered oxygen atoms, resulting in a coordination of (1/2) × 6 = 3.

Combining the coordination from the corner and face-centered oxygen atoms, we find that Sr is coordinated by a total of 1 + 3 = 4 oxygen atoms in Tausonite.

To determine the coordination environment of Sr in the Tausonite crystal structure, we need to consider the neighboring unit cells.

In the cubic unit cell of Tausonite, the Ti atom is located at the center of the unit cell. Since Ti is coordinated by 6 O atoms, it forms an octahedral coordination geometry.

When we consider the neighboring unit cells, we can see that each Sr atom is surrounded by 12 O atoms. This is because in a cubic lattice, each atom at the center of the unit cell is surrounded by atoms from its own unit cell and the 8 neighboring unit cells. Each of these 12 O atoms is coordinated to the Sr atom.

Therefore, in Tausonite, Sr is coordinated by 12 oxygens.

Looks like 12

do a web search on "tausonite coordinate oxygen" and pick the "Crystal Structures" link for a good diagram.