A new structure model for Ba3Nb2O8: A HREM study

The structural investigations performed by powder X-ray diffraction, select ed area electron diffraction, and high-resolution electron microscopy on Ba 3Nb2O8 clearly show that the palmierite structure previously reported for t his compound is not adopted. The presence of Nb in the B sublattice induces a change in symmetry from rhombohedral to hexagonal,and a 9H unit cell is formed. The two possible layer stacking sequences, (ccchhhhhh) (P3m1) and ( cchhchhhh) (P (3) over bar m1), are discussed in terms of ordering of both anionic and cationic vacancies.