New barium niobium sulfides, types 12H and 18R

A new barium niobium sulfide was prepared with the composition of BaNb3.5S7.6. The sulfide has two crystal structures of types 12H and 18R. They are probably polytypic in relation. The X-ray powder diffraction patterns of the structures are indexable with the lattice parameters of a = 0.33315(4) nm and c = 3.6944(4) nm for 12H, and a = 0.33333(4) nm and c = 5.5646(9) nm for 18R in hexagonal setting. Electron diffraction patterns of these structures exhibit supercells of 3a × 3a. Single phases of 12H and 18R were prepared by employing iodine as a mineralizing agent.