Frustrated and Unfrustrated Magnetic Orders in the 10H Perovskite Ba₅Sb_1−xMn_4+xO_15−δ

A 10-layer hexagonal perovskite Ba 5 Sb 1− x Mn 4+ x O 15−δ solid solution (0.24 ≤ x ≤ 0.36; space group P 6 3 / mmc, a = 5.7095(1) Å, and c = 23.4866(3) Å for x = 0.36) has been synthesized by solid state reaction and studied using powder X-ray and neutron diffraction and magnetization measurements. This 10H polytype structure contains one corner-sharing (Sb,Mn)O 6 octahedron and a tetramer of four face-sharing MnO 6 octahedra per formula unit. Two magnetic transitions are observed at 129−167 K ( T M ) and 13−22 K ( T F ). The frustration index |θ|/ T M, where θ is the Weiss temperature, is very close to unity, but considerable magnetic frustration is evidenced through the long-range order of a small ∼0.5 μ B antiferromagnetic moment below T M, and the formation of ferro- or ferrimagnetic clusters. These freeze at T F with the |θ|/ T F ≈ 7−10 index representing high magnetic frustration. T F increases directly with the excess of Mn, x, in Ba 5 Sb 1− x Mn 4+ x O 15−δ rather than with the overall Mn content, showing that the occupation of the corner-sharing Sb/Mn sites is key to the extended magnetism.