Axis thermal expansion switching in transition-metal zirconides superconductors TrZr₂ by tuning the c/a ratio

CuAl₂-type CoZr₂ (space group #140) has been reported to exhibit superconducting transition at T_C ~ 5.2 K and c-axis negative thermal expansion (NTE) from up to 570 K[1]. To explore CuAl₂-type superconducters with NTE, we synthesized NiZr₂, (Co,Rh,Ir)Zr₂, and (Co,Ni,Cu,Rh,Ir)Zr₂ and conducted X-ray powder diffraction and magnetization mesurement for them. The heat expands lattice constants a and c of NiZr₂, which is positive thermal expansion (PTE). Conversely, although a-axes of (Co,Rh,Ir)Zr₂ and (Co,Ni,Cu,Rh,Ir)Zr₂ exhibit PTE, those of c-axes shrink with incresing temperature indicating NTE. We made the plot of the c-axis thermal expansion amplitude as a function of the ratio of lattice c and a (c/a) for the examined compounds and related TrZr₂, and we reveal that the c-axis thermal exoansion is controllable by tuning c/a throhgh the Tr-site substitusion.

[1]Y. Mizuguchi, et al., J. Phys. Soc.Jpn, 91, 103601 (2022)