ORDERING AND DEFECTS IN BAMNO3-Y ESS-THAN-OR-EQUAL-TO-Y-LESS-THAN-OR-EQUAL-TO-0.40)

An electron diffraction and high resolution electron microscopy study shows that nonstoichiometry in BaMnO3-y (0.22 less than or equal to y less than or equal to 0.40) perovskite-related materials is accommodat ed either by means of disordered intergrowths of hexagonal BaMnO3-y po lytypes for 0.22 less than or equal to y < 0.25 or by random distribut ion of anionic vacancies for 0.25 < y less than or equal to 0.40. For y = 0.25, an ordered 4H-Ba4Mn4O11 phase is obtained with stacking sequ ence ... hchc .... Structural models for the 4H type showing ordering of vacancies along the cubic layer are discussed. (C) 1995 Academic Pr ess, Inc.