Review on Uniaxial Negative Thermal Expansion of <i>Tr</i>‐Zr Superconductors

ABSTRACT Thermal expansion is one of the most fundamental properties of materials, and control of thermal expansion is a key technology to provide productions such as electronic devices with high performance and high robustness to thermal shock. Studying negative thermal expansion (NTE) materials has been a hot issue in modern physics and material science because mechanisms of NTE cannot be understood by general thermal expansion theory, and NTE materials have the potential to fabricate zero thermal expansion materials by combination with positive thermal expansion (PTE) materials. NTE materials with a wide working temperature range are mainly ceramics or insulators. However, in 2022, uniaxial NTE along the c ‐axis in a wide temperature range was observed in CuAl 2 ‐type tetragonal CoZr 2 superconductor. Since superconductors with a wide temperature range of NTE are quite rare, the discovery has attracted attention and plays an important role in leading further studies. In this article, we review the recent progress of the study on transition metal ( Tr ) zirconide Tr Zr 2 with its unique NTE phenomena. We discuss how to control the uniaxial NTE along the c ‐axis and the mechanisms of the NTE for Tr Zr 2 . We also introduce similar compounds Tr Zr 3 exhibiting uniaxial NTE along the c ‐axis even though crystal structures are different from Tr Zr 2 .