Sign change in c-axis thermal expansion constant and lattice collapse by Ni substitution in transition-metal zirconide superconductor Co1-xNixZr2

Yuto Watanabe; Hiroto Arima; Hidetomo Usui; Yoshikazu Mizuguchi

doi:10.1038/s41598-023-28291-y

Sign change in c-axis thermal expansion constant and lattice collapse by Ni substitution in transition-metal zirconide superconductor Co_1-xNi_xZr₂

Sci Rep. 2023 Jan 18;13(1):1008. doi: 10.1038/s41598-023-28291-y.

Authors

Yuto Watanabe¹, Hiroto Arima¹, Hidetomo Usui², Yoshikazu Mizuguchi³

Affiliations

¹ Department of Physics, Tokyo Metropolitan University, 1-1, Minami-Osawa, Hachioji, 192-0397, Japan.
² Department of Physics and Materials Science, Shimane University, Matsue, Shimane, 690-8504, Japan.
³ Department of Physics, Tokyo Metropolitan University, 1-1, Minami-Osawa, Hachioji, 192-0397, Japan. mizugu@tmu.ac.jp.

Abstract

Recently, c-axis negative thermal expansion (NTE) was observed in a CoZr₂ superconductor and related transition-metal zirconides. Here, we investigated the structural, electronic, and superconducting properties of Co_1-xNi_xZr₂ to achieve systematic control of c-axis NTE and switching from NTE to positive thermal expansion (PTE) by Ni substitution. At x ≤ 0.3, c-axis NTE was observed, and the thermal expansion constant α_c approached zero with increasing x. At x = 0.4-0.6, c-axis thermal expansion close to zero thermal expansion (ZTE) was observed, and PTE appeared for x ≥ 0.7. On the superconducting properties, we observed bulk superconductivity for x ≤ 0.6, and bulk nature of superconductivity is suppressed by Ni heavy doping (x ≥ 0.7). For x ≤ 0.6, the evolution of the electronic density of states well explains the change in the superconducting transition temperature (T_c), which suggests conventional phonon-mediated superconductivity in the system. By analyzing the c/a ratio, we observed a possible collapsed transition in the tetragonal lattice at around x = 0.6-0.8. The lattice collapse would be the cause of the suppression of superconductivity in Ni-rich Co_1-xNi_xZr₂ and the switching from NTE to PTE.