Structural chemistry and magnetic properties of 6H and 15R hexagonal perovskites BaIrxFe 1−x O 3−δ

The crystal structures and magnetic properties of three compounds within the system BaIrxFe1−xO3−δ have been investigated using a combination of X-ray and neutron powder diffraction, high resolution transmission electron microscopy and d.c. SQUID magnetometry. BaIr0.2Fe0.8O2.932(5) and BaIr0.6Fe0.4O3 adopt the 6H structure (space group P63/mmc, a = 5.72 Å, c = 14.15 Å), whilst BaIr0.3Fe0.7O2.949(7) exists in the rare 15R modification (space group Rm, a ≈ 5.73 Å, c ≈ 35.55 Å). Both structures comprise dimers of face-sharing octahedra as well as octahedra which share only vertices with their neighbours. For 6H BaIr0.2Fe0.8O2.932(5) and 15R BaIr0.3Fe0.7O2.949(7) neutron diffraction experiments show an antiferromagnetically ordered state at 2 K. TN has been identified through variable temperature neutron diffraction to be 200(4) K for 15R BaIr0.3Fe0.7O2.949(7). In both of these samples a proportion of the spins remain decoupled from the magnetic backbone and there is evidence for a spin-glass freezing transition at low temperatures. Susceptibility data for 6H BaIr0.6Fe0.4O3 indicate a single transition to a spin-glass state.