BaMnO_3-x Revisited:  A Structural and Magnetic Study

The synthesis and structural characterization of 15R-, 8H-, 6H‘-, and 10H-BaMnO_3-x and 9R-Ba_0.875Sr_0.125MnO₃ is described. Neutron powder diffraction data reveal a strong preference for the location of anion vacancies within the face-shared hexagonal layers of these phases. At high anion vacancy concentrations, Mn₂O_9-x dimer units become prevalent. Magnetic characterization indicates antiferromagnetic order in all phases. Ordering temperatures are in the range 250 K < T < 270 K and show a dependence on the fraction of cubic layers present in the stacking sequence.