Giant Magnetoelastic Effect at the Opening of a Spin-Gap in Ba₃BiIr₂O₉

As compared to 3d (first-row) transition metals, the 4d and 5d transition metals have much more diffuse valence orbitals. Quantum cooperative phenomena that arise due to changes in the way these orbitals overlap and interact, such as magnetoelasticity, are correspondingly rare in 4d and 5d compounds. Here, we show that the 6H-perovskite Ba₃BiIr₂O₉, which contains 5d Ir⁴⁺ (<i>S</i> = 1/2) dimerized into isolated face-sharing Ir₂O₉ bioctahedra, exhibits a giant magnetoelastic effect, the largest of any known 5d compound, associated with the opening of a spin-gap at <i>T</i>* = 74 K. The resulting first-order transition is characterized 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 optimizes direct Ir–Ir bonding (at high temperature), and one that optimizes Ir–O–Ir magnetic superexchange (at low temperature)