6.1% Volumetric Negative Thermal Expansion Induced by the Reversible Reconstructive Phase Transition in Pb‐Free Bi 1− x Ln x CoO 3 ( Ln : Lanthanoid)

Negative thermal expansion (NTE) materials, which can compensate for the thermal expansion of structural materials, have attracted much attention in the field of nanoscale electronics and optical devices requiring precise positioning. The present paper demonstrates a reversible colossal NTE in the perovskite‐type oxide lanthanoid‐substituted BiCoO 3. The 6.1% volume shrinkage in Bi 0.82 Nd 0.18 CoO 3 is the largest ever observed in Pb‐free NTE materials. Synchrotron X‐ray diffraction and Co L‐edge soft X‐ray absorption spectroscopy measurements and machine learning force field molecular dynamics simulations confirm that the origin of the NTE is the coordination change from the CoO 5 pyramid with high‐spin Co 3+ to the CoO 6 octahedron with the LaCoO 3 ‐type Co 3+ spin state (intermediate‐spin state or mixture of high‐spin and low‐spin states) due to melting of the d xy orbital ordering in the d 6 electron configuration. The NTE properties as functions of ionic radius and concentration of substituting Ln ions are well explained by the nucleation mechanism of the reconstructive martensitic phase transition. The present results offer a new strategy for developing large‐NTE materials.