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.

负热膨胀（Negative thermal expansion, NTE）在无铅铋酸钴（BiCoO3）衍生物中成功实现，其超大体积收缩率达4.4%，可与钛酸铅基钙钛矿中已报道的最强负热膨胀（体积变化ΔV = −4.8%）相媲美。钙钛矿型（ABO3）铋酸钴本身存在巨大晶格畸变（c/a = 1.27），通过与钛酸钡（BaTiO3）形成固溶体可有效抑制该畸变；钛酸钡具备较大的容限因子与较小的晶格畸变（t = 1.06，c/a = 1.01）。此举提升了铋酸钴钙钛矿结构的热稳定性，最终诱发了由温度驱动的极性四方相至非极性立方相的相变，该相变正是产生负热膨胀效应的核心诱因。综合实验与理论研究证实，该显著负热膨胀特性源于A/B位阳离子与氧的轨道杂化作用所弱化的铁电性。本研究通过调控容限因子的策略，为在结构畸变程度较高的无铅化合物中实现大尺寸负热膨胀提供了设计指引。