Frustrated and Unfrustrated Magnetic Orders in the 10H Perovskite Ba5Sb1-xMn4+xO15-delta

A 10-layer hexagonal perovskite Ba5Sb1-xMn4-1-xOj5-delta solid solution (0.24 <= x <= 0.36; space group P6(3)/mmc, a = 5.7095(1) angstrom, and c = 23.4866(3) angstrom 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 MnO6 octahedra per formula unit. Two magnetic transitions are observed at 129-167 K (T-M) and 13-22 K (T-F). The frustration index vertical bar theta vertical bar/T-M, where theta is the Weiss temperature, is very close to unity, but considerable magnetic frustration is evidenced through the long-range order of a small similar to 0.5 mu B antiferromatznetic moment below T-M, and the formation of ferro- or ferrimagnetic clusters. These freeze at T-F with the vertical bar theta vertical bar/T-F approximate to -10 index representing high magnetic frustration. T(F)increases directly with the excess of Mn,x, in Ba5Sb1-xMn4+xO15-delta rather than with the overall Mn content, showing that the occupation of the corner-sharing Sb/Mn sites is key to the extended magnetism.