Pronounced Negative Thermal Expansion in Lead-Free BiCoO₃-Based Ferroelectrics Triggered by the Stabilized Perovskite Structure

Negative thermal expansion (NTE) with a giant volume shrinkage of 4.4%, comparable to the strongest NTE (ΔV = −4.8%) in PbTiO3-based perovskites, was achieved in a lead-free BiCoO3 derivative. The giant lattice distortion of perovskite-type (ABO3) BiCoO3 (c/a = 1.27) was effectively suppressed by making solid solution with BaTiO3, which exhibits a large tolerance factor and a small lattice distortion (t = 1.06 and c/a = 1.01). As a consequence, the thermal stability of the perovskite structure of BiCoO3 was improved, and temperature-induced polar-tetragonal to nonpolar-cubic phase transition resulting in NTE occurred. The pronounced NTE is attributed to the weakened ferroelectricity caused by hybridization between the A/B-site cations and oxygen, as evidenced by comprehensive experimental and theoretical investigations. The present study of tuning the tolerance factor provides guidelines for realizing large NTE in lead-free compounds with a large structure distortion.