Key Role of Bismuth in the Magnetoelastic Transitions of Ba_3BiIr_2O_9 and Ba_3BiRu_2O_9 As Revealed by Chemical Doping

摘要

The key role played by bismuth in an average intermediate oxidation state in the magnetoelastic spin-gap compounds Ba_3BiRu_2O_9 and Ba_3BiIr_2O_9 has been confirmed by systematically replacing bismuth with La~(3+) and Ce~(4+). Through a combination of powder diffraction (neutron and synchrotron), X-ray absorption spectroscopy, and magnetic properties measurements, we show that Ru/Ir cations in Ba_3BiRu_2O_9 and Ba_3BiIr_2O_9 have oxidation states between +4 and +4.5, suggesting that Bi cations exist in an unusual average oxidation state intermediate between the conventional +3 and +5 states (which is confirmed by the Bi L_3-edge spectrum of Ba_3BiRu_2O_9). Precise measurements of lattice parameters from synchrotron diffraction are consistent with the presence of intermediate oxidation state bismuth cations throughout the doping ranges. We find that relatively small amounts of doping (～10 at%) on the bismuth site suppress and then completely eliminate the sharp structural and magnetic transitions observed in pure Ba_3BiRu_2O_9 and Ba_3BiIr_2O_9, strongly suggesting that the unstable electronic state of bismuth plays a critical role in the behavior of these materials.