Experimental observation and computational study of the spin-gap excitation in Ba3BiRu2 O9

Ba3BiRu2O9 is a 6H-type perovskite compound containing face-sharing octahedral M2O9 (M=Ir, Ru) dimers, which are magnetically frustrated at low temperatures. On cooling through T∗=176 K, it undergoes a pronounced magnetostructural transition, which is not accompanied by any change in space group symmetry, long-range magnetic ordering, or charge ordering. Here, we report the first direct evidence from inelastic neutron scattering that this transition is due to an opening of a gap in the excitation spectra of dimers of low-spin Ru4+ (S=1) ions. X-ray absorption spectroscopy reveals a change in Ru-Ru orbital overlap at T∗, linking the emergence of this spin-gap excitation to the magnetostructural transition. Ab initio calculations point to a geometrically frustrated magnetic ground state due to antiferromagnetic interdimer exchange on a triangular Ru2O9 dimer lattice. X-ray total-scattering data rule out long-range magnetic ordering at low temperatures, consistent with this geometrically frustrated model.