Giant Magnetoelastic Effect
at the Opening of a Spin-Gap
in Ba<sub>3</sub>BiIr<sub>2</sub>O<sub>9</sub>
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Abstract
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
Ba<sub>3</sub>BiIr<sub>2</sub>O<sub>9</sub>, which contains 5d Ir<sup>4+</sup> (<i>S</i> = 1/2) dimerized into isolated face-sharing
Ir<sub>2</sub>O<sub>9</sub> 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)