title: Complex 5d magnetism in a novel S = 1/2 trimer system, the 12L hexagonal perovskite Ba4BiIr3O12.
abstract: The 12L hexagonal perovskite Ba4BiIr3O12 has been synthesized for the first time and characterized using high-resolution neutron and synchrotron X-ray diffraction as well as physical properties measurements. The structure contains Ir3O12 linear face-sharing octahedral trimer units, bridged by corner-sharing BiO6 octahedra. The average electronic configurations of Ir and Bi are shown to be +4(d(5)) and +4(s(1)), respectively, the same as for the S = 1/2 dimer system Ba3BiIr2O9, which undergoes a spin-gap opening with a strong magnetoelastic effect at T* = 74 K. Anomalies in magnetic susceptibility, heat capacity, electrical resistivity, and unit cell parameters indeed reveal an analogous effect at T* ≈ 215 K in Ba4BiIr3O12. However, the transition is not accompanied by the opening of a gap in spin excitation spectrum, because antiferromagnetic coupling among S = 1/2 Ir(4+) (d(5)) cations leads to the formation of a S = 1/2 doublet within the trimers, vs S = 0 singlets within dimers. The change in magnetic state of the trimers at T* leads to a structural distortion, the energy of which is overcompensated for by the formation of S = 1/2 doublets. Extending this insight to the dimer system Ba3BiIr2O9 sheds new light on the more pronounced low-temperature anomalies observed for that compound.
authors.affiliation: School of Chemistry, The University of Sydney , Sydney 2006, Australia.
authors.forename: Wojciech
authors.initials: W
authors.lastname: Miiller
authors.forename: Matthew T
authors.initials: MT
authors.lastname: Dunstan
authors.forename: Zixin
authors.initials: Z
authors.lastname: Huang
authors.forename: Zakiah
authors.initials: Z
authors.lastname: Mohamed
authors.forename: Brendan J
authors.initials: BJ
authors.lastname: Kennedy
authors.forename: Maxim
authors.initials: M
authors.lastname: Avdeev
authors.forename: Chris D
authors.initials: CD
authors.lastname: Ling