Excess antimony induced 12R Ba4Sb0.85Mn3.15O12 phase and Mn3O4 exsolution from 10H Ba5Sb0.7Mn4.3O15: Crystal structure and magnetic properties

Hexagonal perovskite manganite has attracted much attention due to its rich structural chemistry and magnetic properties . The transformation between Ba M x Mn 1− x O 3 polymorphs can be caused by the change in substitution level or oxygen vacancies . In this study, we find a new polymorph change from 10 H Ba 5 Sb 0.7 Mn 4.3 O 15 ( P 6 3 / mmc ) to 12 R Ba 4 Sb 0.85 Mn 3.15 O 12 ( R 3 ‾ m ) via a ‘topochemical’ reaction with excess Sb 2 O 3 . The 10 H Ba 5 Sb 0.7 Mn 4.3 O 15 structure contains BaO 3 layers in a ( cchhh ) 2 sequence and face-shared MnO 6 tetramers (Mn 4 clusters), while the 12 R Ba 4 Sb 0.85 Mn 3.15 O 12 structure consists of BaO 3 layers in a ( cchh ) 3 stacking and face-shared MnO 6 trimers (Mn 3 clusters). The 10 H → 12 R phase conversion accompanies the exsolution of the Mn 3 O 4 phase in trace amounts. For both 10 H Ba 5 Sb 0.71 Mn 4.29 O 15 and 12 R Ba 4 Sb 0.85 Mn 3.15 O 12 , the Mn 3+ cation on the corner-shared octahedra at M 1 sites mediated competing ferromagnetic and antiferromagnetic exchanges between the magnetic Mn 3 and Mn 4 clusters, leading to spin-freezing transition at T F  ≈ 11–15 K. The 10 H → 12 R phase change leads to an abrupt change of 2 K saturation moment from M s ≈ 0.1 μ B of 10 H Ba 5 Sb 0.71 Mn 4.29 O 15 to M s ≈ 1.75 μ B of 12 R Ba 4 Sb 0.85 Mn 3.15 O 12 .