Dear All, thank you very much for your comments.
In fact my material is hexagonal. A study on the systematic absences did not allow a unique assignment of the space group and six possible solutions were determined: P63mc (No. 186), P-62c (No. 190), P63/mmc (No. 194), P3c1 (No. 158) and P-31c (No. 163). I tried to solve the structure in all these space groups but the only successful solution appeared in P63mc (No. 186). The structure can be drawn and all atomic distances and angles look ok. Stanislav