Giant magnetoelasticity at a spin gap transition in the 5d oxide Ba₃Bilr₂O₉

The 4d and 5d transition metals have much more diffuse valence orbitals than their 3rd (first-row) transition metals. Quantum cooperative phenomena that arise due to changes in the way these orbitals overlap and interact, such as magnetoelasticity, are therefore relatively rare in 4d and 5d compounds. However, we recently discovered a 6H-perovskite Ba₃Bilr₂O₉, containing 5d lr⁴⁺ (S = 1/2) dimerised into isolated face-sharing lr₂O₉ bi-octahedra, which exhibits a giant magnetoelastic effect - the largest of any known 5d compound - upon the opening a spin-gap at T* = 74 K. The first-order transition observed by neutron powder diffraction is characterised by a remarkable 4% increase in Ir-Ir distance and 1% negative thermal volume expansion. The transition is driven by a dramatic change in the interactions among Ir 5d orbitals, and represents a crossover between two very different, competing, ground states: one that optimises direct lr-lr bonding (at high temperature); and one that optimises lr-O-lr magnetic superexchange (at low temperature).